SEARCH

SEARCH BY CITATION

Keywords:

  • Aerosol hygroscopic growth factor;
  • f(RH);
  • Direct aerosol radiative forcing;
  • Aerosol optical properties;
  • Climate forcing;
  • Vertical profiles

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Methods
  5. 3. Results and Discussion
  6. 4. Summary
  7. Appendix A.
  8. Appendix B.
  9. Acknowledgments
  10. References

[1] In situ measurements of aerosol optical and hygroscopic properties were made over the Indian Ocean onboard the National Center for Atmospheric Research (NCAR) C-130 aircraft as part of the 1999 Intensive Field Phase (IFP) of the Indian Ocean Experiment (INDOEX). Research flights were conducted primarily in the Northern Hemisphere to the west and southwest (i.e., downwind) of the Indian subcontinent, although several flights crossed the Intertropical Convergence Zone (ITCZ) into the much cleaner Southern Hemisphere air. The optical and hygroscopic properties of submicrometer aerosols were measured most of the time, although measurements on aerosol particles smaller than ∼3 μm have also been reported. Low-altitude (0–1 km altitude) measurements of the submicrometer aerosol light scattering coefficient (σsp, adjusted to standard temperature and pressure) in the INDOEX pollution aerosol showed a median value of 53 Mm−1, which is a factor of ∼2–5 higher than the median values during polluted periods at remote North American marine/coastal sites but similar to those observed at rural U.S. stations during high-aerosol periods. Submicrometer light absorption coefficients were even higher compared with the North American measurements, at 4–23 times those median values. Single-scattering albedo (ω0) measurements showed that the Indian Ocean pollution aerosol was highly absorbing, with mean values at ambient relative humidity between 0.84 and 0.87 for low-altitude flight segments conducted in the Northern Hemisphere. The aerosol hygroscopic growth factor, defined as f(RH) = σsp (RH=85%)sp (RH=40%), averaged 1.58 for flight segments below 1 km altitude and north of 5°N. This is a substantially lower f(RH) than typically observed at midcontinent Northern Hemisphere sites, although it is nearly identical to that observed at these sites when aerosols were influenced by agricultural burning or dust episodes in the surrounding area. Aerosol optical properties over the Indian Ocean also showed significant variability in both the horizontal and the vertical on very short spatial and temporal scales. Cloud-free level flight segments of only ∼10–30-min duration showed a mean variability (standard deviation/mean) in the 1-min average ambient aerosol extinction coefficient of ∼18% and a mean difference between the lowest and the highest segment extinction measurement of 39%. Vertical profiles conducted over the Northern Hemisphere Indian Ocean showed two major types of profiles. One type showed nearly constant to decreasing aerosol scattering with increasing altitude, while the other type displayed elevated aerosol layers that did not appear to be associated with boundary layer aerosols and were occasionally separated from them by relatively clear layers. These intense elevated layers, which were present in 52% of all profiles, often showed scattering coefficients several times as large as those measured near the surface. A change in synoptic-scale circulation patterns halfway through the project may have caused more of these elevated decoupled layers to be observed during the second half of the IFP and may have caused them to be observed at higher altitudes.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Methods
  5. 3. Results and Discussion
  6. 4. Summary
  7. Appendix A.
  8. Appendix B.
  9. Acknowledgments
  10. References

[2] In order to better predict the forcing of climate, the perturbations of climate caused by humans must be understood and quantified. One of the major objectives of the 1999 Indian Ocean Experiment (INDOEX) Intensive Field Phase (IFP) was to accurately measure the radiative effects of anthropogenic pollution aerosols over a wide region (J. A. Coakley Jr. et al., General overview of INDOEX, submitted to Journal of Geophysical Research, July 2000). The intention in doing this was to better understand how anthropogenic aerosols can influence the radiative balance and to lessen the uncertainties associated with radiative forcing, one of the major components of climate forcing.

[3] The INDOEX project has focused on one area of the world, the Indian Ocean, which annually experiences a dramatic atmospheric transition from intensely polluted to nearly pristine [Ramanathan et al., 2001]. The yearly oscillations of the Intertropical Convergence Zone (ITCZ), the atmospheric region separating polluted Northern Hemisphere (NH) air from clean Southern Hemisphere (SH) air, and the dramatic shift in atmospheric circulation patterns to a polluted northeasterly flow in winter and spring make this area ideal for studying the effects of predominantly anthropogenic aerosols on the regional radiation balance and comparing them with those from aerosols of natural origin. While other components of the INDOEX project have consisted of year-round measurements, the IFP was conducted in February and March 1999 during the northeast (dry) monsoon season. These strong monsoon winds push vast amounts of anthropogenic aerosols from the Indian subcontinent and surrounding countries out over the northern and central Indian Ocean, including industrial and automobile emissions, aerosols from biomass burning, and biofuel and fossil fuel emissions, along with aerosols of natural origin. Aircraft, ship, ground-based, and satellite sensors were positioned in and over this area to make the complete suite of aerosol and radiation measurements necessary to determine the aerosol contribution to radiative forcing of climate.

[4] Aerosol physical, chemical, and optical properties were measured in situ on up to 3 aircraft during the INDOEX IFP; the largest of these was the National Center for Atmospheric Research (NCAR) C-130 research aircraft (A. D. Clarke et al., An overview of the C-130 flight missions and measurements during INDOEX, submitted to Journal of Geophysical Research, September 2000). The aerosol optical property (AOP) measurements described in this paper were all made on the C-130. This large aircraft was based at the Hulule International Airport near Malé (latitude 4.0°N, longitude 73.0°E), the capital city of the Republic of Maldives. The C-130 had sufficient flight capabilities to stay airborne for 9–10 hours, to climb to over 7 km altitude, and to reach ∼17°N and 9.5°S. This facilitated the comparison of aerosols from clean and anthropogenically perturbed regions, the comparison of anthropogenic aerosols of different types from various populated regions, and the investigation of vertical aerosol variability. The major scientific questions we hoped to answer by participating in the INDOEX IFP are the following:

  1. What are typical values of AOPs measured in clear-sky conditions over regions of the Indian Ocean at several latitudinal distances from the Indian subcontinent during the 1999 northeast monsoon season?
  2. What is the spatial (horizontal and vertical) and temporal variability of AOPs observed during the INDOEX IFP?
  3. Are there major aerosol layers aloft that may not be adequately sampled or detected by surface aerosol instruments, and if so, how do the aerosol properties of these layers differ from those of the surface aerosols?
  4. How do the aerosol optical and hygroscopic properties measured over the Indian Ocean compare with those from other rural and remote sites, particularly those in North America?

[5] In this paper we present statistical analyses of aerosol optical and hygroscopic properties at various locations and altitudes over the tropical Indian Ocean during the INDOEX IFP. We then compare these aerosol properties to determine if significant latitude or altitude gradients were observed, and also compare them with aerosol properties measured at several North American aerosol monitoring stations.

2. Experimental Methods

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Methods
  5. 3. Results and Discussion
  6. 4. Summary
  7. Appendix A.
  8. Appendix B.
  9. Acknowledgments
  10. References

2.1. Measurements

[6] The primary AOP measurements made by the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory (NOAA/CMDL) during the INDOEX IFP were (1) the aerosol light scattering coefficient (σsp) at 3 visible wavelengths and at low (typically <45%) and high (usually 80–85%) relative humidity (RH), (2) the aerosol hemispheric backscattering coefficient (σbsp) at the same wavelengths and RHs, and (3) the aerosol light absorption coefficient at a low RH and at a wavelength of 550 nm. The aerosol instruments that comprised the NOAA/CMDL airborne aerosol measurement system to measure these quantities consisted of two integrating nephelometers (TSI Model 3563, St. Paul, MN, USA) connected in series and separated by a humidity control system, and a filter-based light absorption photometer (Radiance Research Model PSAP, Seattle, WA, USA). Figure 1 shows a schematic of the aerosol measurement system deployed on the NCAR C-130 aircraft. Aerosols were sampled from the C-130 community aerosol inlet (CAI) and traveled at a flow rate of ∼30 L min−1 into our main sampling line. It is important to note that in this paper we are reporting the properties of aerosol particles that have passed through an aircraft-mounted, high-speed aerosol inlet, and passage through the inlet may have modified these properties in some way. Any sampling bias or sampling artifact in the CAI could influence our reported aerosol results. The sampled aerosols then passed through a drying tube followed by a line heater that were used to lower the sample line RH to ∼40%. If the ambient RH was below 40%, the drying tube and heater would have little effect, because the drier would not remove much additional moisture and the heater would not be active. Following the sample line heater was a sealed and insulated impactor box, which contained a switched dual-impactor system that permitted us to change the cut size of our sampled aerosols when desired from particles smaller than 10 μm aerodynamic diameter (Dp < 10 μm) to particles smaller than 1 μm aerodynamic diameter (Dp < 1 μm). Most of the airborne measurements (∼90%) during INDOEX were conducted with a Dp < 1 μm size cut, because the upper-limit size estimate of particles passed by the CAI and the uncertainty in this estimate were unknown at the time and not quantified until a later study [Blomquist et al., 2001]. Rather than measuring aerosols with an unknown (and potentially broad and variable) upper size limit, we decided that it would be better to eliminate that with a firm and reproducible 1 μm (aerodynamic) size limit. For this reason, we focused most of our sampling efforts on the submicrometer particle fraction, which was thought to have a high passing efficiency through the inlet. Based on the results of the study by Blomquist et al. [2001] which are discussed below, particles larger than ∼3 μm diameter were not transmitted efficiently through the CAI. Therefore, we have redefined our Dp < 10 μm size fraction to be Dp < 3 μm, with the caveat that any errors in the determination of the CAI cutpoint by Blomquist et al. [2001] would change our upper particle size limit.

image

Figure 1. Schematic of the NOAA/CMDL aerosol measurement system onboard the NCAR C-130 aircraft. All tubing in the system, with the exception of the low-flow pickoff line to the PSAP instrument, was nominal 1.59-cm (inner diameter) stainless steel or flexible conductive tubing.

Download figure to PowerPoint

[7] The separate reporting of the properties of the fine aerosol fraction is a common practice and reflects the distinct sources, sinks, and atmospheric lifetimes of these smaller particles [Seinfeld and Pandis, 1998]. One pre-INDOEX study was performed in 1996 that measured atmospheric size distributions during the winter monsoon season over the Indian Ocean and Arabian Sea [Jayaraman et al., 1998]. This study showed average mass-size distributions with minima in the 1–2 μm diameter range, which suggests that the submicrometer aerosol mode of INDOEX aerosols should be reasonably well resolved.

[8] The aerosols passed from the impactors into the reference nephelometer, which always measured aerosol light scattering at the lower RH. This RH was typically below 40%, but because of the varying cabin temperature in the aircraft, the RH in the reference nephelometer occasionally increased to >45%. Upon exiting the reference nephelometer, the sample passed through a humidifier that was set to maintain a much higher RH, usually in the 80–85% range. Aerosols exiting this humidity control unit then entered the second nephelometer where the humidified aerosol scattering measurements were made. Just upstream from the inlet of the reference nephelometer was a pickoff for the PSAP instrument that sampled aerosols at a flow rate of 1–2 L min−1.

[9] In Table 1, detailed descriptions of all primary and derived measurements are presented. Formulas used to calculate the hemispheric backscatter fraction (b), Ångström exponent (å), single-scattering albedo (ω0), and aerosol hygroscopic growth factor (f(RH)) are shown. Of these, by far the most complicated calculation is that for f(RH). The calculation of f(RH) requires that the reference and humidified scattering coefficients (which are typically not exactly at 40% and 85% RHs) be adjusted to their respective values at 40% and 85% RH. The adjustment of the scattering coefficient to that at a different RH is done using a multiple step process. First, the hygroscopic fit parameter γ is determined using a nonlinear least squares fit of the scattering coefficients from both the reference and humidified nephelometers (σsp (dry) and σsp (wet), respectively) over the averaging period of interest

  • equation image

where RH(wet) and RH(dry) are the relative humidities measured inside both nephelometers and the exponent γ depends on the hygroscopic nature of the aerosol [Kasten, 1969]. A similar strategy has been used in other recent airborne humidified nephelometry studies [e.g., Gassó et al., 2000]. Humidity-scanning nephelometer measurements conducted by our group during the INDOEX IFP [Clarke et al., 2002] at the Kaashidhoo Climate Observatory (KCO) (J. Lobert et al., Kaashidhoo Climate Observatory (KCO): A new site for observing long-term changes in the tropical Indian Ocean, submitted to Journal of Geophysical Research, June 2000) on the Maldivian island of Kaashidhoo were used to confirm the functional form of this equation as opposed to more complicated multiparameter fits [e.g., Kotchenruther and Hobbs, 1998]. The assumption has been made implicitly here that the hygroscopic nature of the aerosols measured by the humidograph system at KCO [Clarke et al., 2002] was similar to that for aerosols measured on the C-130 aircraft. Once γ has been determined, σsp at any other RH (σsp(RH)) and f(RH) can be determined using the similar equations

  • equation image

and

  • equation image

Determination of the correct scattering coefficient at an RH other than that measured in the reference or humidified nephelometer using the 2-point statistical fit method of equations (1) and (2) requires that the scattering varies smoothly with RH between RH(wet) and RH(dry). Fortunately, the polluted marine aerosols measured at KCO showed scattering to be a smooth function of RH (<2% of all nephelometer humidity scans displayed potential evidence of deliquescence). For aerosols such as these, σsp can be retrieved via the 2-point fit method with little error. A typical mean value for γ, this for all (N = 74) low altitude (0–1 km) level flight segments over the northern Indian Ocean, is ∼0.33, with a standard deviation of 0.10. This compares well with mean γ value of 0.368 found for our measurements at KCO during the INDOEX IFP [Clarke et al., 2002].

Table 1. In Situ Aerosol Optical Property Measurements and Instruments Onboard the NCAR C-130 During the INDOEX IFP
InstrumentPrimary measurementsDerived measurements
  • a

    PSAP instrument uses 565 nm incident radiation to determine the absorption coefficient. These data are corrected to 550 nm through the use of the calibrations found by Bond et al. [1999].

TSI Model 3563 three-wavelength, backscatter/total scatter integrating nephelometers, operated at both low (<40%) and high (∼80–85%) relative humidityTotal scattering and hemispheric backscattering coefficients from particles (σsp and σbsp) at 450, 550, and 700 nm wavelengthHemispheric backscatter fraction, b = σbspsp Ångström exponents, å = −log[σsp1)/σsp2)]/log[λ12] Single scattering albedo (550 nm), ω0 = σsp/(σspap) Hygroscopic growth factor, f(RH) = σsp (RH=85%)sp (RH=40%)
Radiance Research Model PSAP particulate light absorption photometerLight absorption coefficients from particles, σap, at 550 nmaSingle scattering albedo, ω0, at 550 nm

2.2. Data Analysis

[10] Aerosol optical property data were collected at 1-s resolution and averaged over appropriate time periods. Prior to averaging, the data were quality-checked and edited to remove spikes caused by electronic glitches, PSAP filter changes, system pressure changes, etc. Light absorption data from the PSAP often contain spikes, especially during ascents and descents. These are thought to occur because of flexing and/or settling of the filter substrates due to the internal system pressure changes experienced through changing altitudes. These spikes are often quite large, are relatively easy to identify, and are of short duration (typically <10 s duration), so that few of the vertical profile data are affected.

[11] Other edits and corrections were applied to the raw data from INDOEX. A sample line heater that malfunctioned at low altitude (<1000 m above sea level, asl) for part of the project caused some particle volatilization in our system. All time periods when this heater was active were flagged and the data rigorously checked. If the volatilization artifact was found, these questionable data were removed from the data set. Dilution corrections were applied when appropriate to correct for the presence of several small leaks in our system that were each present for part of the project. The leaks for the most part showed zero particle penetration efficiency, so a straight dilution correction was deemed appropriate. These heater edits and leak corrections are discussed in more detail in section 2.3, and a complete presentation of them can be found on the NOAA/CMDL website (contact authors for current website address). An empirically derived correction to the humidified nephelometer scattering data was applied to account for particle losses in transit through the humidity control system. The light scattering data from both nephelometers were initially adjusted to conditions of standard temperature and pressure for comparison with the light absorption measurements (which use that reference), and also for comparison with scattering measurements at other locations and altitudes. Both the light scattering and absorption coefficients were also adjusted to conditions of temperature, pressure, and (for scattering) RH that were representative of the ambient atmosphere. Light absorption data from the PSAP instrument were corrected for filter spot size differences from the calibration standard and scattering artifacts using the calibration methods of Bond et al. [1999]. Additionally, PSAP data were removed from the data set if the filter transmittance dropped below 0.5, and they were left in but flagged if the transmittance dropped below 0.7. Finally, the TSI nephelometer data were corrected for angular nonidealities including truncation effects as detailed by Anderson and Ogren [1998], who used a procedure similar to that proposed by Rosen et al. [1997].

[12] For each vertical profile, processed 1-s data were sorted into bins that represented each 100 m of altitude. The vertical profile plots, therefore, show one data point for each parameter every 100 m in the vertical, except where data were missing or edited from the data set. Typical ascent and descent rates for the C-130 during INDOEX were ∼150–300 m min−1, so the 100-m bin averages represent 20–40 s of data.

[13] A slightly different data processing strategy was employed for the processing of horizontal flight segments. Much of the data presented herein are from level flight segments conducted at various locations and altitudes. Segment averages were generated for aerosol properties that reflect the duration of these flight segments. Level flight segments were not included in the compilations if they lasted <5 min. For higher altitude horizontal flight segments, segment averages were typically computed for segments >15 min in duration. This was not only because most higher-altitude flight segments were >15 min in duration, but also because the very low aerosol concentrations observed on some of these segments required averaging over longer periods to increase the signal-to-noise ratio in both the σsp and σap measurements to acceptable levels.

2.3. Discussion of Measurement Uncertainties

[14] Uncertainties in the measurements of aerosol optical properties onboard the C-130 aircraft can be broken down into three major categories: those related to getting the ambient aerosols into our instruments, those related to normal instrument operation and those related to instrument sampling or operational problems. For the first category, the major uncertainties are involved with transporting aerosol particles through the Community Aerosol Inlet (CAI) on the C-130 and into our aerosol instruments. The CAI was designed as a shrouded, multiple-diffuser inlet with a boundary layer suction vent located just behind each diffuser. Its internal flow characteristics are not completely laminar, and turbulence intensities at the sampling plane of 7–9% have been noted in the recent CAI evaluation study by Blomquist et al. [2001]. This study also concluded that the 50% cut size for particles in the CAI was near 3 μm diameter, but that the size cut was not a sharp one. A small fraction of particles larger than 5 μm in diameter were likely to pass through the inlet, while a few particles smaller than 1 μm diameter probably would be unable to pass. This is why we have estimated the upper size limit of particles reaching our instruments as 3 μm, even though we sampled through a 10-μm impactor.

[15] Ram heating also occurs to a small extent, although the relatively low turbulence intensities and attention to isokinetic flow considerations minimize this concern. Given these findings, we expect that for submicrometer particles (which were exclusively sampled by our instruments ∼90% of the time through the use of the switched impactor system), the inlet loss effects are thought to be minimal (i.e., no more than a few percent) and the uncertainty in those measurements due to that effect very small. A thorough analysis of the wing probe size distribution data during the INDOEX IFP is necessary before we can determine the CAI effect on the uncertainty associated with our Dp < 3 μm diameter aerosol data. We did not use the wing tip optical particle counter data to attempt to determine the fraction of extinction caused by the unsampled supermicrometer particles. The sizing accuracy of optical particle probes in the supermicrometer range is subject to large uncertainties in the absence of reliable information on particle composition. As the aerosols measured in INDOEX have been shown to be highly variable in composition (and hence their refractive index) and contained significant fractions of light absorbing particles such as carbon soot (J. R. Anderson, P. Crozier, and S. Howell, Electron microscopy of sulfate particles north and south of the ITCZ during INDOEX, submitted to Journal of Geophysical Research, personal communication, 2000), the large uncertainties in particle size derived from these instruments minimizes the usefulness of these data for anything other than qualitative assessments of supermicrometer concentrations (D. Baumgardner, personal communication, 2000). As discussed below, however, several independent measurements suggest that light scattering by supermicrometer particles over the Indian Ocean was typically less than that from submicrometer particles.

[16] For our nephelometers under normal operating conditions, total measurement uncertainties can be calculated from the individual uncertainty components associated with instrument accuracy, calibrations and corrections, and adjustment of instrument RH to ambient RH. Calculation of the total measurement uncertainty associated with the nephelometers can be calculated from the major sources and is expressed as a combination of the following terms

  • equation image

where δσp designates the uncertainty in σsp associated with the parameter p. For parameters where a percentage number is given for the uncertainty, these represent either 95% confidence intervals of the uncertainty data or commonly accepted uncertainties (e.g., the Rayleigh correction uncertainty). These individual uncertainties are instrument-specific and represent (1) instrument noise (∼10% at σsp = 1 Mm−1 and ∼0.4% at σsp = 50 Mm−1 for 10-min averaging times), (2) drift in the calibrations based on repeated measurements of calibration gases (∼3%), (3) uncertainty in the calibrations due to uncertainties in the measured Rayleigh scattering of air and CO2 (7%), and (4) uncertainty in the truncation or blocking of near-forward scattered light (∼2%), and (5) the uncertainty associated with adjusting σsp to standard temperature and pressure (<1%).Table 2 shows the approximate uncertainties and the calculation method used for each component of the total uncertainty for various magnitudes of σsp. A 10-min averaging time was used in the noise calculation because these uncertainties apply to all level C-130 flight segments in INDOEX, and nearly all of these exceeded 10 min in duration. Therefore, the noise uncertainty reported in Table 2 is actually an upper limit to that experienced during most of the level flight segments. To estimate the noise uncertainty component at a 1-min averaging period (more appropriate for vertical profiles), multiply the noise uncertainty in Table 2 by 3.16.

Table 2. Estimated Uncertainties in σsp at 550 nm for 10-min Averaging Times and Submicrometer Particles (Mm−1)
σsp (550 nm)NoiseaDriftbCalibrationcTruncationcSTPdTotale
10.100.020.070.020.000.13
100.120.240.700.210.040.79
200.150.481.400.410.081.56
500.201.203.501.030.213.89
1000.272.417.002.060.427.77

[17] The angular sensitivity uncertainty δσtrunc includes corrections for both the truncation of forward-scattered light and for the slightly non-Lambertian distribution of illumination intensity. The original angular sensitivity measurements made on this nephelometer were made at 550 nm (T. Anderson, personal communication, 2000), and recent angular sensitivity measurements at 700 nm showed excellent agreement with the earlier data (N. Ahlquist, personal communication, 2000). Little or no wavelength dependence for the non-Lambertian effect is expected in a fairly thick, multiple scattering environment like the TSI opal glass diffuser, and assumption is corroborated by the closure tests performed by Anderson et al. [1996], where the difference between measured and modeled scattering was within 7% and showed essentially no wavelength dependence. In any case, the angular sensitivity uncertainty is small for submicrometer particles, which were predominantly sampled by our system during INDOEX.

[18] An additional uncertainty is present when adjusting values of σsp at a given RH to those appropriate at another RH. This uncertainty depends primarily on the magnitude of the RH to which the measurement is being adjusted. Scattering measurements taken at a typically low instrument RH and adjusted to another low RH (≲50%) show very small additional uncertainties, usually a few percent or less. This is because the nonlinear fits are quite good at lower RH where the scattering coefficient is not changing rapidly. Adjustment of σsp to higher RHs (e.g., 80–90%) shows uncertainties several times larger than for the adjustment to lower RHs. For low RH conditions (<50%) and moderate aerosol levels (σsp = ∼50 Mm−1), total measurement uncertainties in σsp (including the RH adjustment) of 9% were realized for 10-min average data, while for ambient humidities near 90% the uncertainty increased to ∼14%.

[19] For the light absorption measurement, total uncertainties are calculated by combining the major individual uncertainties associated with instrument accuracy, instrument precision, instrument noise, and adjustment of the detected absorption signal to account for a contribution from light scattering. Uncertainty in the PSAP-derived σap results from uncertainty in the following components of the measurement [Bond et al., 1999]: (1) instrument accuracy (estimated at ∼20%); (2) instrument precision (∼6%); (3) instrument noise (fixed at 0.88 Mm−1 and 0.28 Mm−1 for 1-min and 10-min averaging times, respectively); (4) uncertainty in the calibration that converts the wavelength to 550 nm and corrects for filter-based scattering that is sensed as absorption by the instrument (∼4%). Added in quadrature, these components yield total uncertainties of δσap ∼ 49% for σap = 2 Mm−1, δσap ∼ 28% for σap = 5 Mm−1, and δσap ∼ 23% for σap = 10 Mm−1 for 1-min average data. The propagated uncertainty in σap for 10-min averaging periods was ∼26% for σap = 2 Mm−1, ∼ 22% for σap = 5 Mm−1, and ∼21% for σap = 10 Mm−1. The uncertainties are rather large for our absorption measurements, which unfortunately are typical of the current state of the art in filter-based aerosol light absorption measurements.

[20] The uncertainties reported below for the derived parameters ω0, b, and å were calculated based on 10-min averaging periods using the methods described by Anderson et al. [1996, 1999] and Anderson and Ogren [1998], from the uncertainties in the appropriate σsp, σbsp, and σap measurements. A typical uncertainty for each parameter is provided below for a high-scattering case (e.g., low altitude segments north of the equator) and a low-scattering case (high altitude segments or those in the Southern Hemisphere). The specific flight segments we used to obtain the scattering and absorption values for the high-scattering uncertainty calculations are the lowest-altitude segments conducted north of 5° north latitude (0–1 km altitude, Dp < 1 μm). The flight segments used for the low-scattering uncertainty determinations were the lowest-altitude segments conducted south of 5° south latitude (0–1 km altitude, Dp < 1 μm). For the high-scattering case (σsp = 58 Mm−1), the absolute uncertainties in ω0, b, and å were 0.036, 0.012, and 0.49, respectively. For the low-scattering case (σsp = ∼6 Mm−1), the uncertainties in ω0, b, and å were 0.049, 0.018, and 0.53, respectively. These values compare with the uncertainties estimated by Anderson et al. [1999] for ω0, b, and å during a polluted marine aerosol episode (with a mean σsp value of 18 Mm−1) of approximately 0.024, 0.018, and 0.33, respectively.

[21] The uncertainty in f(RH) was calculated by computing the increased uncertainties in the scattering coefficients from the reference and humidified nephelometer after adjustment to 40% and 85% RH, respectively. For the case of low-altitude flight segments conducted north of 5° north latitude (0–1 km altitude, Dp < 1 μm), the relative uncertainty in our f(RH) parameter was ∼21%.

[22] During the INDOEX IFP, two operational problems contributed at various times to the measurement uncertainty in the light scattering and absorption measurements. One of these problems was that the sample intake line heater was not being controlled properly and stayed on longer than it should have, resulting in higher temperatures than planned in the intake line. The malfunctioning heater caused a periodic volatilization of the aerosol particles and loss of aerosol mass. This malfunction caused pronounced drops in σsp of up to 40% that were not evident when the heater was off. This heater was only activated when sample line RH was >40%. Given that a drying tube upstream of the sample line heater removed some of the moisture from the air stream, the heater only came on when flying through the highest RH air. This only occurred (1) in clouds, and (2) at times in the moist marine boundary layer (MBL) below 1000 m asl. Cloud data have been screened from this data set, so they have not been reported. Based on comparison of the CMDL nephelometer signal with concurrent condensation nucleus and 180°-nephelometer measurements, data periods influenced by the heater malfunction were identified. When the aircraft was flying below 1000 m asl, the condition that allowed the malfunction to occur was found to be present ∼55% of the time. These data are considered unrecoverable, because the particle size distribution, chemistry, and thermal gradient in the sample line are not known accurately enough to correct for the volatility losses of aerosol species. Therefore, all data from the identified malfunction periods were removed. Thus, no additional uncertainty in the aerosol optical property measurements reported in this paper was realized through the heater malfunction.

[23] Over the course of the INDOEX IFP, several leaks in the CMDL aerosol system were identified and corrected. A detailed discussion of the all leak problems, tests results that quantify the individual leaks, and corrections applied to the data are available on the NOAA/CMDL web site. The air leaks included a leak in the humidified nephelometer, a leak in the sample inlet line, a connector ferrule leak, and a leak in the impactor box. The humidified nephelometer leak was discovered on Flight 12 (13 March 1999) and immediately fixed; the duration of the leak could have been from Flights 1–12 (16 February–13 March 1999). The sample inlet line leak was also discovered on Flight 12. This was in a part of the line that was frequently disassembled and reassembled. We cannot rule out the possibility that this leak was present on Flights 1–12. The connector ferrule leak was only present on Flight 15 (19 March 1999), and the impactor box leak was only present on Flight 16 (21 March 1999).

[24] Leaks of cabin air into our system were present whenever the cabin pressure exceeded the pressure inside our instrument. The cabin pressure above 1800 m was maintained around 830 mbar, although not consistently. The consequence of a leak on the scattering signal was that the sample air was diluted with cabin air and potentially contaminated with cabin aerosol. A formula that quantifies this correction is:

  • equation image

Here Sa is the ambient signal, Sm is the measured signal, Sc is the cabin air signal, X is the leak rate and E is the aerosol penetration efficiency. Post flight tests of the leak rate and aerosol penetration efficiency as a function of pressure were performed to determine X and E for each of the known leaks. Cabin aerosol scattering signals were serendipitously measured throughout the campaign during in-flight PSAP filter changes, which allowed cabin air to flood the nephelometers.

[25] Leaks had little impact on the nephelometer signal above ∼3500 m altitude, as both the cabin and ambient aerosol scattering signals were near the instrument detection limit. The altitude range of maximum uncertainty in the contribution of leaks on the nephelometer signals usually fell between 2 and 3 km, when the cabin pressure exceeded the instrument pressure by more than 50 mbar. Fortunately, particles in the size range detected by the nephelometers were only able to penetrate through one of the leaks (the connector ferrule leak), which was present on only one flight (Flight 15), so that the primary effect of the leaks was a 1–10% dilution of the sample with filtered cabin air. All scattering and absorption data were corrected for dilution by filtered air from each leak as a function of the cabin/instrument pressure differential. Uncertainties in σsp and σap associated with errors in the measurement of the leak rate or in the measurement of particle penetration efficiency through the leaks are difficult to estimate, but probably constitute another 5–10% uncertainty for scattering and absorption data for all flights except Flight 15. For Flight 15, where the particle penetration was significant through the ferrule leak, uncertainties are estimated to be in the 10–15% range. Combining these leak uncertainty estimates with the other uncertainty components (added in quadrature) increases the total uncertainty in σsp presented above for 10-min averaging times to ∼10% and ∼14% for low and high RH measurements, respectively. The σap total measurement uncertainty for 10-min average data increased to ∼23% for σap = 5 Mm−1 when accounting for the effect of leaks.

3. Results and Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Methods
  5. 3. Results and Discussion
  6. 4. Summary
  7. Appendix A.
  8. Appendix B.
  9. Acknowledgments
  10. References

3.1. In Situ Aerosol Measurements

3.1.1. Latitude and Altitude Dependence of Aerosol Optical Properties

[26] Mean values of aerosol optical properties measured during INDOEX level flight segments, along with standard deviations and the number of samples used in the calculations, are reported in Appendices A and B. A graphical representation of the variability in AOPs from low-altitude (0–1 km asl) level flight segments from different regions is shown in Figure 2. In the tabulation in Appendix A and in the left hand panels of Figure 2, all light scattering data have been adjusted from instrument to ambient conditions of temperature, pressure, and RH, for ease of use in model comparisons and for comparison with other INDOEX measurements. All light absorption data have been adjusted to ambient temperature and pressure, but were not adjusted from the usually low instrument RH because the RH-dependence of aerosol light absorption is not known. Thus, the ambient ω0 values in Appendix A probably should be viewed as upper limits, because the scattering coefficients that were used for the calculations were usually adjusted upward by some amount (because the ambient RH was typically higher than the RH inside the reference nephelometer) while the absorption coefficients were left unchanged. In Appendix B and in the right hand panels of Figure 2, a similar compilation is presented, except that the AOPs have all been adjusted to a standard set of temperature (0 °C), pressure (1013.25 mb), and RH (40%) conditions. These adjusted data are more useful for determining real differences in the aerosol properties between different locations or altitudes, since much of the observed variability could be caused by changes in RH. As in Appendix A, the light absorption coefficient was not adjusted to another RH, but rather was held at instrument RH. Since instrument RH was often close to 40%, this makes little difference in the σap values.

image

Figure 2. Plots of the mean values of aerosol optical properties from low-altitude (0–1 km) flight segments in four different latitude bands (NIO, latitude north of 5°N; CIO, latitude between 5°N and 1°S; ITCZ, latitude between 1°S and 5°S; and SH, latitude south of 5°S). The four panels show aerosol properties adjusted both to ambient conditions of temperature, pressure, and RH (left panels), and also to a set of standard conditions (0°C, 1013.25 mb, 40% RH, right panels). The top panels show data flight segments sampling for Dp < 3 μm particles, while the bottom panels show submicrometer particle data.

Download figure to PowerPoint

[27] In both appendices, measurements have been categorized by altitude (4 levels) and latitude (4 bands). Comparison of aerosol properties in this paper is based on the comparison of average aerosol properties from one latitude/altitude group with those of another. Discrete aerosol layers have not been compared with one another. The choice of altitude levels was made after observing that the height of the MBL was typically around 1 km (based on INDOEX field meteorology briefings), and the altitude at which elevated aerosol layers was observed was most often 1–3 km asl. The choice of latitude bands was based on analysis of the aircraft location data and on the fact that the northernmost extent of the ITCZ during the mission was approximately 1°S latitude. The region denoted Northern Indian Ocean (NIO, north of 5°N latitude) showed air mass backward trajectories (from the aircraft) and modeled winds (at several altitudes below ∼2500 m) predominantly from across northern and central India during the first half of the IFP and down the western coast of India during the second half [Rasch et al., 2001]. The Central Indian Ocean region (CIO, between 1°S and 5°N latitude) showed airflow from the Bay of Bengal and/or Southeast Asia that occasionally passed over the southern third of India during the first half of the IFP and which weakened considerably during the second half at lower altitudes [Rasch et al., 2001]. With the exception of the 0–1 km level measurements, all values listed in the appendices are for segments measuring submicrometer (Dp < 1 μm) particles. For the lowest altitude level where there were numerous flight segments, averages are reported separately for segments measuring Dp < 1 μm particles, for segments measuring Dp < 3 μm particles, and for all segments.

[28] Data from all level flight segments of at least 5 min duration (and usually longer than 10 min) on one size range were included in this tabulation. Population outliers for the parameters derived from our primary light scattering and absorption measurements (i.e., ω0, b, å, and f(RH)) were removed in one of two ways. First, if the mean aerosol light scattering coefficient for a given flight segment was <1 Mm−1, the derived parameters, which often show large variability at very low aerosol concentrations, were not calculated for that segment. This criterion removed many of the questionable data points at high altitudes or in the Southern Hemisphere, where the aerosol burdens were usually much lower. Of the remaining data, population outliers for each parameter were removed using Chauvenet's criterion [Holman, 1978], which targets outliers based on population variability and the number of observations. At σsp values less than a few Mm−1, unreasonable or even physically unrealistic values of these parameters were sometimes calculated. The use of Chauvenet's criterion removed most of these questionable data points.

[29] Figure 2 shows that for NIO and CIO flight segments conducted at 0–1 km asl over the Indian Ocean in polluted air from the NH, mean aerosol light extinction coefficient (σep) values adjusted to standard conditions for Dp < 1 μm particles were ∼80 Mm−1. For these same flight segments, mean σsp values for Dp < 1 μm particles ranged from 58 to 64 Mm−1. These σsp values, measured during the INDOEX IFP over the ocean 1000–2000 km away from major aerosol sources in India and southeastern Asia, are significantly higher than similar measurements made during high-aerosol (i.e., polluted) periods at our surface stations in remote coastal and marine areas of North America. Comparisons of these INDOEX measurements with those from other platforms and sites are discussed in more detail in section 3.2. Mean σsp and σap values were much lower in areas south of the CIO region. The low number of level flight segments at altitudes above ∼1 km in the ITCZ and SH make comparisons with those data difficult. Mean σsp and σap values also generally decreased with increasing altitude in all regions, consistent with surface sources. Appendix B shows that the variability in the AOPs (shown by relatively large standard deviations) was highest at the intermediate altitudes (e.g., 1–3 and 3–5 km), because of the sporadic presence of elevated strong aerosol layers.

[30] Comparison of the aerosol extinction, scattering and absorption coefficients from low-altitude Dp < 1 μm and Dp < 3 μm categories requires some caution. First, all aerosol samples were collected through the CAI, so the true upper size limit of the Dp < 3 μm aerosol measurements may be uncertain. Second, it is important to remember that the reported flight segments are all discrete segments (i.e., we did not obtain both Dp < 1 μm and Dp < 3 μm aerosol data from the same segment). With the exception of the NIO region, the average values from Dp < 3 μm segments were found to be less than the average from the segments sampling submicrometer particles. This is because several low-aerosol flight segments were sampled for Dp < 3 μm particles, and these segments lowered the overall averages dramatically. Finally, the measurements being reported here are possibly for a truncated marine aerosol size distribution. A recent study by Kleefeld et al. [2002] reported that a significant portion of the aerosol light scattering at Mace Head, Ireland, could be attributed to Dp > 10 μm particles. It is not known whether the aerosol size distribution at the beach at Mace Head is similar to that in the lowest 1 km of the MBL over the Indian Ocean. If similar, our instruments would not have been able to sample these larger particles and the results presented herein would not necessarily be representative of the full marine aerosol distribution.

[31] Aerosol single-scattering albedos were very consistent in the NH MBL, ranging from 0.84 to 0.87 at ambient conditions and 0.79 to 0.81 at standard conditions over both aerosol size ranges. At higher altitudes in the NIO region, the mean ω0 values were 0.79 to 0.83. The distributions of ω0 at all altitudes closely approximated normal distributions, so t-tests were used to test whether the differences in the mean values were significant. The t-tests showed that the difference in the mean ω0 values from the 0–1 km segments (0.81) and the 3–5 km segments (0.79) was statistically significant at a significance level of 0.43. Given the low significance of this result and the fact that the uncertainties in the ω0 measurement are ∼0.04 and ∼0.05 for high-concentration and low-concentration aerosol conditions, respectively, the variability of ω0 with altitude in NIO flight segments reported in Appendix B can be considered not statistically significant. This suggests that while the amount of aerosols in the atmosphere decreased at higher altitudes, the nature (e.g., the ability of the particles to scatter versus absorb light) of these aerosols did not change substantially. At low altitudes over the ITCZ, mean ω0 values were very close to those measured to the north. Mean ω0 values in SH air below 3 km altitude were usually substantially higher than those in the NH or ITCZ, although the mean value for submicrometer particles on 0–1 km flight segments was much lower than we expected. The high ω0 value of 0.98 (standard conditions) for low-altitude SH flight segments sampling Dp < 3 μm particles may be caused by a dominant supermicrometer sea salt mode, at least some of which appears to have passed through the C-130 CAI, and a diminished submicrometer pollution mode. The trend toward higher SH ω0 values observed in this study is in agreement with ω0 measurements made by Quinn et al. [2002] during the INDOEX IFP. It is possible that our definition of SH air (beginning at 5°S latitude) did not start far enough south to be completely out of the range of influence of NH aerosols for the entire IFP period.

[32] The hemispheric backscatter fraction did not change much over all sampled latitudes and altitudes. At latitudes north of 5°S, mean b values were 0.10–0.13 over all altitude ranges. Submicrometer and total aerosol b values for any altitude and location were not significantly different from one another. In contrast, the Ångström exponent shows some interesting variability. Values of å in Figure 2 are typically 1.7–2.1 in Northern Hemisphere air, although å appears to decrease slightly at high altitudes (>5 km asl) in the NIO region (Appendix B). The ITCZ region shows å values at low altitudes that are not significantly different from those in the NH. As expected, the average å values for Dp < 3 μm aerosol segments are significantly lower than for the corresponding Dp < 1 μm segments. In the SH region, the mean value of å for Dp < 3 μm aerosols is observed to be much lower than those in the other regions at the lowest altitude levels. Again, this could be due to a reduced fine-particle pollution mode in the SH region, with the remaining aerosols composed largely of supermicrometer sea salt particles.

[33] The mean aerosol hygroscopic growth factor showed its lowest values in the NIO region, and increased with movement to more southerly regimes. Since the distributions of f(RH) observations approximated normal distributions in all regions, students t-tests were used to determine whether the means of these populations were statistically different. The vertical variability of f(RH) in the NIO region was found not to be statistically significant at a confidence level of 95%. This was the only region where enough valid f(RH) measurements were collected at altitude to perform such an analysis. The latitudinal variability of f(RH) at the lowest altitude level was greater than the vertical variability. The mean f(RH) of the NIO region (0–1 km, Dp < 1 μm) was found to be statistically different from the means of the ITCZ and SH regions at significance levels of 0.06 and 0.07, respectively. For Dp < 3 μm aerosols, the NIO mean of 1.68 was statistically different from the mean of the ITCZ region at a significance level of 0.11. The mean of the 0–1 km SH region, although much higher than the NIO mean at 2.61, was not statistically different from that of the NIO region, presumably because of the high variability and small number of observations in the SH region. The observed latitudinal gradient would be consistent with moving farther away from the biomass burning source regions (which often produce hydrophobic aerosols) [Kotchenruther and Hobbs, 1998; Sheridan et al., 2001] on the Indian subcontinent and into cleaner oceanic areas with larger fractions of hygroscopic (e.g., sea salt) aerosols.

3.1.2. Horizontal Inhomogeneity of Indian Ocean Aerosols

[34] One observation made frequently over the duration of the field campaign was the large variability in aerosol light scattering, absorption, and extinction coefficients at a given altitude over relatively short horizontal distances. Figure 3 shows a time series of 1-min average ambient RH and σep data from a typical cloud-free, horizontal flight segment conducted in NH air. During this 22-min flight segment, the ambient aerosol optical properties were observed to track very closely with the changing RH, which is not surprising since the scattering component of σep was adjusted from instrument to ambient RH. The percentage difference between the lowest and highest extinction coefficients measured during this segment was 78%. A difference of 45% was observed between σep values only 2 min (or approximately 15 km based on the typical C-130 research velocity) apart.

image

Figure 3. Time series plot of aerosol extinction coefficient over a typical cloud-free, constant-altitude, Northern Hemisphere flight segment on 25 February, 1999. The σep is observed to track closely with RH and shows a 78% difference between the lowest and highest 1-min average reading during the segment and a 45% difference between readings only 2 min apart.

Download figure to PowerPoint

[35] In order to investigate the degree of inhomogeneity in NH aerosols sampled within discrete flight segments during INDOEX, we grouped all horizontal flight segments for analysis that met the following criteria. First, the segments had to be conducted in NH air (NIO or CIO regions) in the 1–3 km asl altitude band. This altitude band was chosen for analysis because it was usually above the MBL but in most cases still contained significant amounts of lower tropospheric aerosols. It was also above the altitude at which aerosol volatilization problems caused by the malfunctioning heater occurred. Next, each segment had to contain at least 10 minute-average observations on the Dp < 1 μm size cut. We then limited the segments to those that showed segment-average ambient σep values >10 Mm−1 to minimize variability based on instrument noise. Finally, segment aerosol data which were potentially influenced by proximity to clouds were eliminated from these analyses if ambient RH was >95% for any data point during the segment. Table 3 shows the 12 flight segments that met all of these criteria.

Table 3. Variability in Submicrometer Aerosol Light Extinction Coefficient Measured Under Ambient Conditions Observed During Relatively Short, Level Flight Segments
Date/Start Time (GMT)Number of ObservationsaPressure Altitude (m asl)RH Range (%)Mean σep, (Mm−1)Std. Dev. σep (Mm−1)Min. σep (Mm−1)Max. σep (Mm−1)Segment Variability (%)b
  • a

    Usually equivalent to the segment duration in minutes, except during times when data editing has decreased the number of observations. A rough estimate of the path length (in km) is found by multiplying the number of observations by a factor of 6 or 7, because the aircraft research velocity was typically ∼100–120 m s−1 (6–7 km min−1).

  • b

    Expressed as (standard deviation/mean) * 100.

  • c

    σsp was used to determine aerosol variability instead of σep on this flight segment because absorption measurements were not valid at this time.

18-2-99/083619238052–59923.089983
20-2-99/090922114183–931732113321012
20-2-99/100522185181–9397147912414
25-2-99/075222191530–5988165910618
25-2-99/09254728476–5111141.854127
25-2-99/114115161238–50119139313711
27-2-99/125910116377–9482255313130
09-3-99/095022164539–621531812218612
13-3-99/050315128877–93925.0831005
13-3-990650/31222741–7968383216756
13-3-99/101118183832–79102138312513
21-3-99/0856c21194728–425712417221

[36] A conservative way to express the variability in percent of σep is the standard deviation for each segment divided by its mean value, with the result multiplied by 100. This is shown in the last column of Table 3. The highest-altitude segment, conducted at 2847 m asl, shows tremendous variability in σep. During this segment the RH increased from 6% to 51%, and the σep increased from 1.8 to 54 Mm−1. This clearly indicates a transition from clean, dry, free tropospheric air into the more humid pollution aerosol. Since we were flying a level flight path at this time, the boundary between the two air masses was probably a gradually sloping one. Since this segment is not representative of a sample taken entirely in the pollution aerosol, its variability is expected to be greater than that of the other samples. Eliminating this segment from the analysis, a mean variability of σep of ∼18% was obtained for the other 11 level flight segments, with two segments showing 30% or greater variability. Expressed another way, this time as the percentage difference between the maximum and minimum σep values (σep (max) − σep(min)ep(max)) for each of the 11 flight segments, the mean variability was ∼39%.

[37] The degree of aerosol variability observed during these relatively short, level flight segments (where minimal variability would be expected) indicates that additional caution should be used when attempting to compare fixed-site measurements with aircraft measurements. These comparisons include aircraft-ship, aircraft-ground station, and aircraft-stationary lidar (e.g., the vertical lidar extinction measurements made at Malé-Hulule airport). The comparisons performed to date between the C-130 aircraft and surface measurements (ships and ground stations) have shown relatively good agreement [Clarke et al., 2002], suggesting that the methodology of comparing similar sampling path lengths is a valid one for flyby intercomparisons and that the aerosols are not dramatically different at the elevations of aircraft flying at their lowest sampling altitudes and surface samplers. The comparison of C-130 aircraft and Hulule lidar aerosol extinction data has shown less favorable agreement and may be due to significant spatial and temporal offsets between the compared measurements. A detailed uncertainty analysis and discussion of possible reasons for the discrepancies between the C-130 and lidar data sets are provided by Masonis et al. [2002].

[38] By analyzing timelines of the reference scattering coefficient measured at low RHs over level flight segments we calculated the variability (i.e., variance) caused by changes in aerosol concentration. The difference between this and the variability in the ambient light scattering coefficient is due to a combination of changes in RH and f(RH). For the four atmospheric regions that routinely contained pollution aerosols (NIO and CIO, 0–1 km and 1–3 km altitude), the median fraction of the variance in the ambient σsp that is accounted for by changes in the reference σsp was typically in the 0.1–0.2 range. In these regions, the vast majority of variability in σsp (ambient) during most segments appears to have been caused by variations in RH and/or f(RH), rather than by changes in σsp (reference).

3.1.3. Aerosol Vertical Profiles

[39] During every C-130 research flight of the INDOEX IFP, several vertical profiles from high altitude to near the surface were conducted. These were usually slant-path profiles, which could show different vertical aerosol distributions than true vertical profiles because of the horizontal aerosol inhomogeneities mentioned above. Two major types of vertical aerosol profiles were observed during the campaign. Type I profiles typically showed a deep aerosol layer (most easily observed in σsp and σap) extending from the surface up to an altitude of between 2 and 3.5 km and dropping off sharply above that altitude. Elevated aerosols in this type of profile appeared to be coupled to the surface layers, without major aerosol peaks or valleys at the intermediate altitudes. Type II profiles showed elevated aerosol layers with scattering maxima centered at between ∼1.5 and 3.5 km that were sometimes decoupled (i.e., separated by clean layers of low aerosol concentrations) from the boundary layer aerosols. Large aerosol peaks at higher altitudes were usually observed in these profiles. Quantitatively, a vertical profile was classified as a Type II vertical profile if the ambient σsp (550 nm, Dp < 1 μm) average for a 100-m layer at some point in the profile above 1000 m asl was ≥50% higher than at some other point lower in the profile, and if the σsp measurement at that higher point was ≥20 Mm−1. The 20 Mm−1 threshold was used to eliminate most profiles being classified as Type II based on noise-dominated signal variability of the measurements at high altitudes. Using this definition for all of our valid NH vertical profiles during INDOEX ranging from altitudes higher than 2500 m to within a few hundred meters of the ocean surface, 34 of the 65 profiles (52%) were classified as Type II profiles. Figure 4 shows examples of typical Type I and Type II vertical profiles from the INDOEX IFP. The Type II profiles indicate that scattering coefficients (and thus aerosol mass concentrations) in the elevated layers were often several times larger those observed at or near the surface.

image

Figure 4. Examples of the two major types of vertical aerosol profiles observed during the INDOEX IFP. (Top) Type I vertical profile showing fairly steady or decreasing aerosol light scattering above the marine boundary layer (∼1 km altitude). (Bottom) Type II vertical profile showing an intense layer of elevated light scattering. The σsp values at the peak of these layers were often several times that at lower altitudes.

Download figure to PowerPoint

[40] Table 4 shows a breakdown of the scattering coefficient maxima of the 34 INDOEX elevated aerosol layers (from Type II profiles) and at what altitudes these were observed. For the flight on 16 March 1999, we could not correct the scattering coefficients to ambient RH because we were not operating a humidified nephelometer at this time. For these profiles, we report “low-RH” (RH ∼ 40% or less) σsp measurements. It is evident from Table 4 that elevated aerosol layers were observed more frequently later in the program (the midpoint of the 6-week C-130 aircraft deployment was 7 March 1999). Figure 5 shows a frequency distribution of the peak aerosol layer altitudes for all valid NH vertical profiles. The median altitude for these layer maxima was 2200 m asl, although most of the higher altitude maxima were observed later in the program (on or after 13 March 1999). This may be related to the observed synoptic-scale changes in lower tropospheric wind patterns that occurred on or around 9 March [Rasch et al., 2001].

image

Figure 5. Frequency distribution of the altitude at which the scattering maximum was observed for vertical profiles conducted over the Northern Hemisphere Indian Ocean. The most frequently observed altitude for the layer maximum was 1500–1600 m, with another large population of observations between ∼2400 and 3000 m. More of the higher altitude observations occurred in the second half of the campaign.

Download figure to PowerPoint

Table 4. Maximum Observed Scattering Coefficient (Dp < 1 μm particles) for 100-m Average Altitude Bins at Ambient Conditions and Peak Altitude for All NH Elevated Aerosol Layers Observed During INDOEX
Date/Profile Start Time (UTC)Maximum σspAltitude (m asl)a
  • a

    Represents the weighted average of the 100-m altitude layer with the largest average σsp value.

16-2-99/09051631551
16-2-99/09501241552
18-2-99/07411622948
18-2-99/1113802750
18-2-99/1207861749
25-2-99/0414661450
25-2-99/11011201549
27-2-99/0813881848
27-2-99/1210601350
28-2-99/0523231356
07-3-99/0740661954
09-3-99/07541031552
09-3-99/10161401547
13-3-99/04551572050
13-3-99/05271432450
13-3-99/07581362351
13-3-99/08461732549
13-3-99/11061152142
13-3-99/11421141353
16-3-99/06512612945
16-3-99/07291522865
16-3-99/07472503249
16-3-99/1109431855
18-3-99/0913243958
19-3-99/05581482451
19-3-99/08301192251
19-3-99/10591152151
21-3-99/06381443250
21-3-99/08481152845
21-3-99/11531102749
24-3-99/0335261555
25-3-99/0335763050
25-3-99/11321112548
25-3-99/13001052532

[41] The Type II vertical aerosol profiles we present are in good qualitative agreement with two categories of shipboard micropulse lidar (MPL)-derived extinction profiles presented by Welton et al. [2002]. These two lidar extinction profile classifications were made using air mass back-trajectory analysis, and represent periods with continental aerosols arriving at the 2.5-km altitude over the NOAA Research Vessel Ron Brown from (1) the southern portion of India, and (2) southern Saudi Arabia and Yemen. Both lidar profile classes showed an increase in extinction from the surface to near the top of the MBL, followed by an extinction minimum at 1.2–1.5 km (depending on the profile class). Extinction then increased with altitude up to a mean extinction maximum at 2.4 km for both classes before finally decreasing until reaching the top of the aerosol layers, which were near 3.5 km altitude. The overall magnitudes of the extinction values in these lidar-derived profiles may be incorrect due to assumptions (e.g., constant S-ratio with altitude) used in the algorithms [Welton et al., 2002]. However, the position of peaks and minima within a profile is believed to accurately represent the overall shape of the vertical distribution of the aerosol. While the elevated maxima in the lidar-derived extinction profiles were not larger than the MBL extinction peaks (as was the case in our Type II profiles), the position of the extinction peaks and minima corroborate our findings of elevated aerosol layers during much of the INDOEX IFP.

3.2. Comparisons With Other Platforms and Sites

[42] The aerosol optical property measurements made from the C-130 aircraft were compared with those from other instruments on the C-130 and with similar measurements made on several other platforms in the region during the INDOEX IFP. Since some of these comparisons are presented in more detail by Clarke et al. [2002], only a brief discussion is given here. Aerosol measurements made during low-altitude flybys of KCO showed good agreement (typically within 15%) with surface aerosol measurements made there by CMDL. Nearly as good agreement was observed between aerosol properties measured on the NOAA Research Vessel Ron Brown by the NOAA/Pacific Marine Environmental Laboratory (PMEL) and the C-130 during similar low-altitude flybys. Airborne intercomparisons of σsp with another research aircraft (the Citation jet operated by several European universities and institutes) showed less favorable agreement. Mean Citation σsp (the only measured aerosol optical property on that aircraft) values were sometimes less than half of those measured on the C-130 during side-by-side runs.

[43] When compared with pollution-period measurements at other stations, the pollution aerosols sampled over the Indian Ocean during the INDOEX IFP showed relatively large values for the scattering, and especially the absorption, of visible light. Table 5 shows median values of several aerosol optical properties for both low-altitude (0–1 km asl), NIO C-130 flight segments and several CMDL surface aerosol-monitoring sites at rural continental and remote coastal locations. Aerosol data are reported at standard temperature and pressure and low instrumental RH to facilitate comparison of intrinsic aerosol properties between sampling platforms at different locations and altitudes. All data are for Dp < 1 μm particles and partially dried aerosols (sample RH typically between 30% and 40%), and for the CMDL stations represent the median values of each parameter for the highest-aerosol month of 1999, with median values for the three-year period of 1997–1999 also shown for comparison (in parentheses). It was not possible to adjust the data from all stations to a consistent RH because we have no long-term aerosol hygroscopic growth measurements at most of the sites. At these low RHs, however, this range of RH for these measurements causes very minor variability (a few percent or less) in the observed AOPs. High-aerosol months such as April at Barrow represent periods when the anthropogenic component of the aerosol (i.e., Arctic haze) is thought to be largest.

Table 5. Comparison of Median INDOEX Aerosol Optical Propertiesa Measured on the C-130 With Those Measured During the Highest-aerosol Monthb of 1999 (and the Three-year Period 1997–1999) at Rural Continental and Remote Coastal/Marine CMDL Surface Stations
LocationNorthern Indian Ocean, C-130 Aircraft, 0–1 km altitudeSouthern Great Plains, Lamont, OklahomaBondville, IllinoisSable Island, Nova ScotiaNorth Slope of Alaska, Barrow, Alaska
 
Station CodeC-130SGPBNDWSANSA
 
Polluted PeriodFebruary–March 1999August 1999July 1999August 1999April 1999
 
3-Year Period (1997–1999)(1997–1999)(1997–1999)(1997–1999)
  • a

    All aircraft and station data are for Dp < 1 μm particles and are reported at standard temperature and pressure, and instrument relative humidity (typically 30–40%).

  • b

    The highest-aerosol month is defined as the one having the highest median σsp at 550 nm for Dp < 1-μm particles.

  • c

    SGP f(RH) data are for 1999 only.

  • d

    f(RH) data for WSA are reported by McInnes et al. [1998] and represent polluted air masses from North America (and clean marine air masses) during a two-week period in 1996.

σsp5343 (27)49 (39)29 (9.6)10 (4.7)
σap143.0 (1.3)3.5 (3.3)2.6 (1.0)0.6 (0.2)
ω00.800.93 (0.94)0.93 (0.92)0.90 (0.90)0.94 (0.96)
b0.110.12 (0.12)0.12 (0.12)0.11 (0.13)0.10 (0.11)
å2.022.49 (2.34)2.37 (2.38)2.29 (2.29)1.71 (1.62)
f(RH)1.561.75 (1.87)cNo data1.7 (2.7)dNo data

[44] The median INDOEX aerosol optical property data show marked differences from measurements at the other surface locations. The median σsp value measured on the C-130 aircraft was >80% higher than at the marine WSA site during the highest-aerosol month, and was over 5 times higher than at the coastal NSA site. Like the C-130 in INDOEX, both of these stations sample aerosols during these months that are advected from populated areas over extensive stretches of ocean. The median σsp during INDOEX was slightly higher than those measured during the highest aerosol months at our rural continental stations BND and SGP. Aerosol light absorption over the Indian Ocean showed even higher values relative to the CMDL sites. The median σap value of 14 Mm−1 for submicrometer aerosols measured on the C-130 during the INDOEX IFP was ∼5 times higher than the WSA value during the highest-aerosol month (August) of 1999, where efficient transport from the northeastern U.S. was observed, and over 23 times the median NSA value for the highest-aerosol month. The median INDOEX σap value is also ∼4 times higher than that observed during the high-aerosol months at the rural U.S. stations. The median ω0 was much lower over the Indian Ocean, at 0.80, than at any of the North American CMDL stations. The hemispheric backscatter fraction showed little variability between all locations, while the Ångström exponent was highest at the continental sites and lowest at NSA. The magnitude of the differences in å was not large, suggesting that significant differences in the ambient aerosol size distributions among most stations were not apparent. The aerosol hygroscopic growth factor was lower than those observed during high-aerosol periods at the two North American sites equipped to measure it, suggesting that the highly absorbing particles over the Indian Ocean were less efficient at growing through the absorption of atmospheric water than the North American aerosols.

[45] The aerosol optical depth (τa) and σep were calculated for the vertical profiles described above, using the scattering and absorption coefficient primary measurements. During descent profiles, 100-m layer average extinction measurements were integrated in the downward direction to provide a τa for aerosols above the aircraft. These in situ aerosol optical depths were compared with optical depths determined by an upward-looking, total direct/diffuse radiometer (TDDR) that was mounted on the C-130 aircraft and operated by the Scripps Institute of Oceanography, for 8 of their valid descent profiles. The agreement was excellent (typically within 5 percent) at altitudes above ∼1.5 km, with steadily diverging agreement (varying from approximately 5% to 40%) at the lowest altitudes where the nephelometer-derived τa measurements were consistently lower than the TDDR values (A. Bucholtz et al., A comparison between direct airborne radiometric measurements of visible aerosol optical depth and nephelometer derived values during INDOEX, submitted to Journal of Geophysical Research, May 2001). Other recent studies have also observed that airborne nephelometer-derived τa estimates were lower than those derived from other methods, principally sunphotometers [Ross et al., 1998; Hartley et al., 2000; Schmid et al., 2000]. Large particles removed by the 1-μm cutpoint impactor or by aircraft inlets may explain a portion of this discrepancy at lower altitudes. Sub-1 μm to sub-10 μm scattering ratios of between 0.53 and 0.92 (average of ∼0.74 for all NH regions) were observed from shipboard measurements in the NH Indian Ocean by Quinn et al. [2002], indicating a significant, but not dominant, supermicrometer aerosol component near the surface. Similar values of the submicrometer scattering fraction were obtained during our IFP measurements at KCO (mean value of ∼0.7 for the entire experiment), and the mean submicrometer absorption fraction was higher there at ∼0.84. In situ τa values from C-130 vertical profiles also agree well with the range of optical depths produced by an aerosol assimilation procedure, which merges modeled aerosol distributions with satellite aerosol estimates, performed by Rasch et al. [2001].

4. Summary

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Methods
  5. 3. Results and Discussion
  6. 4. Summary
  7. Appendix A.
  8. Appendix B.
  9. Acknowledgments
  10. References

[46] Aerosol optical property measurements made during level flight segments onboard the NCAR C-130 during the 1999 INDOEX IFP showed substantial spatial and temporal variability over all sampled areas of the Northern Hemisphere Indian Ocean. Submicrometer aerosol light scattering and absorption coefficients decreased substantially as the aircraft moved to more southerly latitudes, which are farther from the major aerosol source regions. Scattering and absorption coefficients (σsp, corrected to ambient temperature, pressure, and relative humidity, and σap, corrected to ambient temperature and pressure but held at instrument RH) in the MBL were similar for the NIO and CIO regions. Aerosol single-scattering albedos adjusted to ambient conditions of temperature, pressure, and RH were very consistent in the NH, with mean measured values for both size ranges of between 0.79 and 0.87 over all altitudes. Single-scattering albedos were generally higher on flight segments conducted south of the ITCZ, with the SH showing ω0 measurements of between 0.92 and 0.99. These measurements suggest some “spillover” of polluted NH air into the SH, and it is quite possible that our definition of a pristine SH region south of the ITCZ (defined as all points south of 5°S) should have been moved even farther south. The number of samples collected well to the south of 5°S was, however, very small, and statistical analysis of the data from this region would have been difficult. Hemispheric backscatter fractions showed little variability in both the horizontal and vertical, with mean values for all regions in the 0.10–0.13 range. Ångström exponents for segments sampling submicrometer particles under ambient conditions in the NIO region decreased with altitude, averaging 2.05 for flight segments below 1 km altitude and decreasing to 1.37 (suggesting a shift to larger particles) above 5 km altitude. Valid Ångström exponent data are not available at higher altitudes for the other regions, because of the very low σsp values observed there. The aerosol hygroscopic growth factor was similar on both high and low altitude segments NH flight segments. The lowest mean boundary layer f(RH) values were observed in the NIO region, and these values steadily increased for regions farther south, which is consistent with the notions of aerosol aging in the atmosphere and of moving away from sources of hydrophobic (e.g., biomass combustion, dust, or black carbon) aerosols.

[47] Aerosols over the NH Indian Ocean also showed substantial small-scale variability in the horizontal. All clear-sky, level flight segments were analyzed, and of those segments 12 met certain duration and aerosol loading criteria. These flight segments showed a mean variability in σep, expressed as the segment standard deviation divided by the mean, of 18% over the duration of each segment. Expressed as (σep (maximum) − σep (minimum))/σep (maximum) for each segment, the mean variability was 39%. Horizontal variability of this magnitude means that the NH Indian Ocean aerosol is not homogeneous on the scale of 10–20 km, and that comparisons with fixed-site sensors will require caution.

[48] Approximately 65 vertical profiles of over 2500 m vertical range were conducted in the NH during the INDOEX IFP. Two major types of profiles were observed. Type I profiles, which represent ∼48% of all profiles, displayed an aerosol layer near the surface that diminished with increasing altitude. Type II profiles showed an elevated aerosol layer centered in the ∼1.5–3.5 km altitude range with σsp values at least 50% larger than at some point lower in the profile; these higher layers were sometimes decoupled from the surface or boundary layer aerosols. The observation of the presence of strong elevated aerosol layers during INDOEX agrees with the in situ aircraft-based and shipboard lidar findings of other investigators. These higher aerosol layers often showed peak σsp values several times or more than those observed near the surface. The elevated NH aerosol layers were more frequently observed during the latter half of the IFP, a period that experienced a substantial change in meteorology from that observed during the first half [Rasch et al., 2001]. The scattering maximum in each profile also shifted to higher altitudes during the second half of the IFP. Aerosol optical depths above the aircraft calculated from the in situ aerosol measurements during descent profiles showed excellent agreement above ∼1.5 km with the total direct/diffuse radiometer on the C-130. Below ∼1.5 km the agreement was not as good, with the two sets of τa measurements diverging to 5–40% agreement at the lowest altitudes on all compared profiles. Differences below 1.5 km are not currently well understood, but may be related to the prevalence of supermicrometer particles in the MBL [Clarke et al., 2002; Quinn et al., 2002] and the Dp < 1 μm cutpoint used in the CMDL sampling system during vertical profiles. The range of τa values produced from the C-130 vertical profiles agreed well with the average values produced from the aerosol assimilation procedure of Rasch et al. [2001].

[49] Comparison with NH CMDL sites using the same equipment to measure aerosol optical and hygroscopic properties showed that the INDOEX aerosols displayed median σsp and σap values that were ∼80% and ∼5 times larger, respectively, than those observed during the highest-aerosol month of 1999 at Sable Island, Nova Scotia, a site that frequently experiences pollution from the eastern U.S. In general, the NH Indian Ocean aerosol, with mean near-surface, ambient ω0 values in the 0.84–0.87, is a much darker aerosol at this time of year than at any time of the year at any of the rural or remote North American stations we monitor. The lower aerosol hygroscopic growth factors observed during INDOEX also identify these Indian Ocean aerosols as less likely to absorb atmospheric water than aerosols over much of North America.

Appendix A.

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Methods
  5. 3. Results and Discussion
  6. 4. Summary
  7. Appendix A.
  8. Appendix B.
  9. Acknowledgments
  10. References
Table Appendix A. Level Flight Segment Averages, {1σ Standard Deviations}, and [Number of Valid Observations] of Aerosol Optical Properties Measured on the C-130 During INDOEX Adjusted to Ambient Conditions of Temperature, Pressure, and Relative Humidity
  • a

    Average extinction and scattering values are much lower for ambient as compared to low-RH conditions because the ambient pressure, temperature, and dew point temperature were not recorded on the aircraft during the 2 aerosol segments with the highest aerosol concentrations (therefore the scattering/extinction at ambient conditions for these segments could not be calculated).

  • b

    Note: Higher scattering than extinction for NIO (1–3 km) because 2 of the highest scattering episodes did not have valid absorption readings.

Parameter, wavelengthPressure Altitude (km asl)NIO (lat. > 5°N)CIO (lat. 1°S–5°N)ITCZ (lat. 5°S–1°S)SH (lat. > 5°S)
σep, 550 nm>5 km0.65 {0.65} [20]a0.37 {0.24} [13]0.22 {0.06} [2]0.12 {0.07} [3]
 3–5 km5.1 {14.1} [11]0.43 {0.25} [4]0.0 {–} [1]0.0 {–} [1]
 1–3 km77 {42} [18]70 {47} [14]no data [0]3.6 {2.9} [4]
 0–1 km (all segments)101 {45} [65]102 {44} [41]22 {21} [19]9.7 {6.6} [19]
 0–1 km (Dp < 1 μm)93 {41} [47]109 {48} [29]27 {25} [12]9.8 {7.3} [15]
 0–1 km (Dp < 3 μm)121 {50} [18]84 {26} [12]14 {10} [7]9.2 {3.7} [4]
σsp, 550 nm> 5 km0.41 {0.48} [29]a0.31 {0.17} [13]0.20 {0.04} [2]0.12 {0.07} [3]
 3–5 km21 {47} [14]b0.33 {0.18} [4]0.0 {–} [1]0.0 {–} [1]
 1–3 km66 {34} [20]60 {42} [14]no data [0]3.4 {2.9} [4]
 0–1 km (all segments)82 {39} [74]87 {38} [43]19 {18} [19]8.6 {5.6} [19]
 0–1 km (Dp < 1 μm)76 {36} [54]94 {42} [30]23 {21} [12]8.5 {6.1} [15]
 0–1 km (Dp < 3 μm)100 {43} [20]70 {21} [13]12 {8} [7]8.8 {4.3} [4]
σap, 550 nm>5 km0.08 {0.15} [26]0.06 {0.11} [13]0.02 {0.02} [2]0.0 {0.0} [3]
 3–5 km0.73 {2.25} [14]0.10 {0.07} [5]0.0 {–} [1]0.0 {–} [1]
 1–3 km13 {7} [20]9.8 {6.0} [15]no data [0]0.28 {0.32} [4]
 0–1 km (all segments)13 {7} [92]14 {7} [43]3.4 {4.0} [19]1.1 {1.4} [19]
 0–1 km (Dp < 1 μm)12 {6} [64]14 {8} [31]4.4 {4.6} [12]1.3 {1.5} [15]
 0–1 km (Dp < 3 μm)14 {7} [28]13 {5} [12]1.6 {1.8} [7]0.39 {0.65} [4]
ω0, 550 nm>5 km0.81 {0.14} [4]– [0]– [0]– [0]
 3–5 km0.79 {0.03} [4]– [0]– [0]– [0]
 1–3 km0.85 {0.06} [18]0.85 {0.04} [14]no data [0]0.96 {0.5} [2]
 0–1 km (all segments)0.86 {0.04} [65]0.86 {0.05} [41]0.85 {0.12} [19]0.93 {0.07} [17]
 0–1 km (Dp < 1 μm)0.86 {0.04} [47]0.87 {0.06} [29]0.83 {0.13} [12]0.92 {0.07} [14]
 0–1 km (Dp < 3 μm)0.86 {0.02} [18]0.84 {0.03} [12]0.89 {0.10} [7]0.99 {0.01} [3]
b, 550 nm>5 km0.11 {0.01} [3]– [0]– [0]– [0]
 3–5 km0.13 {0.05} [5]– [0]– [0]– [0]
 1–3 km0.11 {0.01} [20]0.11 {0.01} [14]no data [0]0.10 {0.03} [2]
 0–1 km (all segments)0.11 {0.01} [74]0.10 {0.01} [43]0.11 {0.02} [19]0.12 {0.04} [19]
 0–1 km (Dp < 1 μm)0.11 {0.01} [54]0.10 {0.01} [30]0.11 {0.02} [12]0.13 {0.04} [15]
 0–1 km (Dp < 3 μm)0.11 {0.01} [20]0.10 {0.01} [13]0.12 {0.03} [7]0.11 {0.02} [4]
å, 550/700 nm>5 km1.37 {1.30} [2]– [0]– [0]– [0]
 3–5 km2.02 {0.98} [6]– [0]– [0]– [0]
 1–3 km1.83 {0.52} [20]2.24 {0.77} [14]no data [0]2.36 {–} [1]
 0–1 km (all segments)1.99 {0.31} [74]1.90 {0.17} [43]1.67 {0.48} [19]1.88 {0.48} [14]
 0–1 km (Dp < 1 μm)2.05 {0.30} [54]1.93 {0.19} [30]1.88 {0.46} [12]2.03 {0.34} [11]
 0–1 km (Dp < 3 μm)1.82 {0.31} [20]1.83 {0.11} [13]1.30 {0.23} [7]1.35 {0.58} [3]
f(RH), 550 nm>5 km1.58 {0.10} [3]– [0]– [0]– [0]
 3–5 km1.63 {0.51} [4]– [0]– [0]– [0]
 1–3 km1.42 {0.16} [20]1.87 {0.49} [15]no data [0]2.38 {–} [1]
 0–1 km (all segments)1.58 {0.21} [74]1.68 {0.21} [43]1.91 {0.46} [19]2.16 {0.87} [12]
 0–1 km (Dp < 1 μm)1.55 {0.22} [54]1.69 {0.18} [30]1.82 {0.45} [12]2.07 {0.80} [10]
 0–1 km (Dp < 3 μm)1.68 {0.14} [20]1.64 {0.26} [13]2.06 {0.47} [7]2.61 {1.40} [2]

Appendix B.

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Methods
  5. 3. Results and Discussion
  6. 4. Summary
  7. Appendix A.
  8. Appendix B.
  9. Acknowledgments
  10. References
Table Appendix B. Level Flight Segment Averages, {1σ Standard Deviations}, and [Number of Valid Observations] of Aerosol Optical Properties Measured on the C-130 During INDOEX Adjusted to Standard Conditions of Temperature (0°C) and Pressure (1013.25 mb), and to a Low Reference Relative Humidity (40%)
  • a

    Average extinction and scattering values are much lower for ambient as compared to low-RH conditions because the ambient pressure, temperature, and dew point temperature were not recorded on the aircraft during the 2 aerosol segments with the highest aerosol concentrations (therefore the scattering/extinction at ambient conditions for these segments could not be calculated).

  • b

    Note: Higher scattering than extinction for NIO (1–3 km) because 2 of the highest scattering episodes did not have valid absorption readings.

Parameter, wavelengthPressure Altitude (km asl)NIO (lat. > 5°N)CIO (lat. 1°S–5°N)ITCZ (lat. 5°S–1°S)SH (lat. > 5°S)
σep, 550 nm>5 km10 {31} [27]a0.74 {0.46} [13]0.45 {0.12} [2]0.25 {0.14} [3]
 3–5 km8 {22} [11]0.76 {0.44} [4]0.0 {–} [1]0.0 {–} [1]
 1–3 km89 {48} [18]60 {38} [14]no data [0]2.8 {2.6} [4]
 0–1 km (all segments)83 {35} [67]78 {30} [40]18 {20} [19]6.8 {6.5} [18]
 0–1 km (Dp < 1 μm)76 {32} [49]81 {32} [29]24 {23} [12]7.1 {7.0} [15]
 0–1 km (Dp < 3 μm)102 {37} [18]69 {22} [11]9.3 {5.0} [7]5.1 {1.7} [3]
σsp, 550 nm>5 km7 {24} [30]a0.62 {0.34} [13]0.42 {0.08} [2]0.25 {0.14} [3]
 3–5 km23 {46} [14]b0.58 {0.31} [4]0.0 {–} [1]0.0 {–} [1]
 1–3 km74 {38} [20]45 {30} [15]no data [0]2.4 {2.4} [4]
 0–1 km (all segments)63 {29} [76]62 {22} [42]15 {15} [19]5.3 {5.0} [19]
 0–1 km (Dp < 1 μm)58 {26} [56]64 {24} [30]19 {18} [12]5.6 {5.6} [15]
 0–1 km (Dp < 3 μm)78 {32} [20]55 {17} [12]7.5 {3.0} [7]4.4 {2.0} [4]
σap, 550 nm>5 km1.7 {5.7} [28]0.11 {0.22} [13]0.03 {0.04} [2]0.0 {0.0} [3]
 3–5 km1.1 {3.4} [14]0.18 {0.13} [5]0.0 {–} [1]0.0 {–} [1]
 1–3 km16 {10} [20]12 {8} [15]no data [0]0.36 {0.41} [4]
 0–1 km (all segments)14 {7} [92]16 {8} [44]3.9 {4.5} [19]1.3 {1.7} [18]
 0–1 km (Dp < 1 μm)14 {7} [64]17 {9} [32]5.1 {5.1} [12]1.5 {1.7} [15]
 0–1 km (Dp < 3 μm)16 {8} [28]15 {5} [12]1.9 {2.1} [7]0.08 {0.12} [3]
ω0, 550 nm>5 km0.82 {0.10} [7]0.93 {–} [1]– [0]– [0]
 3–5 km0.79 {0.04} [5]– [0]– [0]– [0]
 1–3 km0.83 {0.06} [18]0.79 {0.03} [14]no data [0]0.96 {0.05} [3]
 0–1 km (all segments)0.81 {0.04} [67]0.80 {0.06} [40]0.83 {0.11} [18]0.88 {0.09} [10]
 0–1 km (Dp < 1 μm)0.81 {0.05} [49]0.81 {0.07} [29]0.82 {0.11} [11]0.84 {0.07} [7]
 0–1 km (Dp < 3 μm)0.81 {0.02} [18]0.79 {0.02} [11]0.84 {0.13} [7]0.98 {0.03} [3]
b, 550 nm>5 km0.10 {0.01} [6]0.10 {0.10} [2]– [0]– [0]
 3–5 km0.13 {0.04} [6]– [0]– [0]– [0]
 1–3 km0.11 {0.01} [20]0.11 {0.01} [15]no data [0]0.13 {0.09} [4]
 0–1 km (all segments)0.11 {0.01}[76]0.10 {0.01} [42]0.11 {0.02} [18]0.13 {0.03} [13]
 0–1 km (Dp < 1 μm)0.11 {0.01} [56]0.10 {0.01} [30]0.11 {0.02} [12]0.12 {0.03} [9]
 0–1 km (Dp < 3 μm)0.11 {0.01} [20]0.10 {0.01} [12]0.12 {0.03} [6]0.13 {0.03} [4]
equation image, 550/700 nm>5 km1.39 {0.96} [5]– [0]– [0]– [0]
 3–5 km1.80 {1.15} [6]– [0]– [0]– [0]
 1–3 km1.90 {0.46} [20]2.03 {0.15} [15]no data [0]2.28 {0.52} [4]
 0–1 km (all segments)1.88 {0.41}[76]1.95 {0.21} [42]1.85 {0.45} [19]1.49 {0.77} [13]
 0–1 km (Dp < 1 μm)1.95 {0.42} [56]2.04 {0.09} [30]2.00 {0.35} [12]1.70 {0.23} [9]
 0–1 km (Dp < 3 μm)1.69 {0.30} [20]1.71 {0.23} [12]1.60 {0.51} [7]0.99 {1.33} [4]

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Methods
  5. 3. Results and Discussion
  6. 4. Summary
  7. Appendix A.
  8. Appendix B.
  9. Acknowledgments
  10. References

[50] This research was funded through the National Science Foundation Climate Dynamics Program (Grant #ATM-9612888). Additionally, we thank the National Science Foundation for providing C-130 flight hours for the INDOEX IFP and the National Center for Atmospheric Research (NCAR) Research Aviation Facility for conducting a safe and well-organized field deployment. Thanks are also due Sarah Masonis for running our airborne system on certain flights, and to Tad Anderson, Dave Covert, Bob Charlson, Tony Clarke, and Sarah Masonis for helpful discussions about the data.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Methods
  5. 3. Results and Discussion
  6. 4. Summary
  7. Appendix A.
  8. Appendix B.
  9. Acknowledgments
  10. References