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Keywords:

  • reactive nitrogen;
  • nitrate;
  • sulfur dioxide;
  • sulfate;
  • transport;
  • East Asia

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Measurements
  5. 3. Study Area and Emission Data
  6. 4. Approach
  7. 5. Results and Discussion
  8. 6. Summary
  9. Acknowledgments
  10. References
  11. Supporting Information

[1] Measurements of gaseous and particulate reactive nitrogen and sulfur species, as well as other chemical species, were made using the P-3B and DC-8 aircraft over the western Pacific during the NASA Transport and Chemical Evolution over the Pacific (TRACE-P) experiment, conducted between February and April 2001. These measurements provide a good opportunity to study the extent to which anthropogenic NOx and SO2 emitted over the East Asian countries remain as NOy and SOx (=SO2 + nssSO42−) in the form of gas or fine particles when an air mass is transported into the western Pacific region. In this paper a method to estimate transport efficiencies, ε(NOy) and ε(SOx), in an air mass that has experienced multiple injection, mixing, and loss processes is described. In this analysis, CO and CO2 are used as passive tracers of transport, and the emission inventories of CO, CO2, NOx, and SO2 over the East Asia region are used. Results from the P-3B presented in this study indicate that 20–40% and 15% of NOx emitted over the northeastern part of China remained as NOy over the western Pacific in the boundary layer (BL) and free troposphere (FT), respectively. In the FT, PAN is found to have been the dominant form of NOy, while only 0.5% of emitted NOx remained as NOx. The transport efficiency of SOx is estimated to have been 25–45% and 15–20% in the BL and FT, respectively. Median values of the nssSO42−/SOx ratio are 0.4–0.6 both in the BL and FT, however large variability is found in the FT. These results are generally consistent with those derived using DC-8 data. The results obtained in this study indicate that more than half of NOy and SOx were lost over the continent and that the vertical transport from the BL to FT further reduced their amounts by a factor of 2, likely due to wet removal. Budgets of NOy and SOx were also studied for air masses, which we sampled during TRACE-P and the flux out from the continent in these cases is estimated to be 20% of the emissions. Flux in the BL and FT is found to have a similar contribution.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Measurements
  5. 3. Study Area and Emission Data
  6. 4. Approach
  7. 5. Results and Discussion
  8. 6. Summary
  9. Acknowledgments
  10. References
  11. Supporting Information

[2] Anthropogenic emissions of nitrogen oxides (NOx = NO + NO2) and sulfur dioxide (SO2) in East Asian countries are of great concern because of their impact on the atmospheric environment on regional and intercontinental scales. These two compounds play critical roles in controlling the oxidizing power of the atmosphere (primarily by NOx) and air quality, causing acid rain, and changing radiative forcing, through gas-phase chemistry and particle formation. Rapid economic growth and increasing fossil fuel energy consumption in the People's Republic of China in the past 2 decades has resulted in large emissions of reactive nitrogen and sulfur compounds into the atmosphere. China is expected to undergo further economic growth and modernization in the next several decades. Emissions of SO2 from China, however, are estimated to have reached a peak level in 1996, followed by a slow decline, due to a combination of environment regulations and other factors, although the primary energy source still relies on coal burning [Streets et al., 2000; Streets et al., 2003]. Emissions of NOx due to commercial energy use in China are also estimated to have been the greatest in 1996, with a subsequent declining trend, mainly due to the reduction of coal consumption [Hao et al., 2002]. Consequently, future emissions of NOx and SO2 from China will be less than those anticipated in the early to mid-1990s. Nevertheless, current emissions from China are still large and dominant in Asia for NOx (42%) and SO2 (59%) [Streets et al., 2003], and a further increase in NOx emissions is anticipated in Southeast Asia and the Indian subcontinent [van Aardenne et al., 1999].

[3] Anthropogenic emissions of NOx and SO2 occur predominantly at the continental surface, and chemical conversion and loss processes take place within the boundary layer. Eventually, these air masses leave the source region via horizontal transport within the boundary layer and free troposphere. The concentrations in the free troposphere are usually smaller than in the boundary layer because a large fraction of the species is removed in the boundary layer before transported to the free troposphere. Wet deposition of soluble species during vertical transport further reduces the influences from anthropogenic emissions. On the other hand, long-range horizontal transport in the free troposphere is more efficient due to an increasing westerly wind speed with altitude. Thus it is not immediately obvious whether transport through the boundary layer or free troposphere is more efficient in bringing emitted material to the western Pacific. Processes of transport and the regional budget of anthropogenic reactive nitrogen and sulfur compounds over the East Asia region have been studied by many investigators using three-dimensional model simulations [e.g., Bey et al., 2001; Xiao et al., 1997; Chin et al., 2000; Tan et al., 2002]. In these studies, observations such as aircraft measurements made during the Pacific Exploratory Mission in the Western Pacific Ocean (PEM-West) were used to evaluate the validity of the model calculations. However, measurements which can be used for quantitative evaluation are still limited.

[4] Measurements of gaseous and particulate compounds of reactive nitrogen and sulfur species, as well as other chemical species, were made from the NASA P-3B and DC-8 aircraft over the western Pacific during the NASA Transport and Chemical Evolution over the Pacific (TRACE-P) experiment, conducted between February and April 2001 [Jacob et al., 2003]. The major goal of the TRACE-P experiment is to evaluate the impact of outflow from Asia, especially that of anthropogenic emissions over the Asian continent, on the atmospheric environment.

[5] During the TRACE-P experiment, air masses that had been influenced by anthropogenic emissions over China and other East Asian countries, were occasionally encountered. Using these data, we estimate the extent to which anthropogenic reactive nitrogen and sulfur compounds emitted over the northeastern part of China were transported into the western Pacific region (in the form of gas + fine particles). The method uses measured concentrations of CO and CO2 as indicators for an amount of material injected into the sampled air mass. The background levels of CO, NOy, and SOx (= SO2 + nssSO42−) are estimated for individual air masses, and the increases in their concentrations above the background levels, ΔCO, ΔNOy, and ΔSOx, are derived from the measurements. Ratios of increases, ΔNOy/ΔCO and ΔSOx/ΔCO, are then compared with emission ratios ENOx/ECO and ESO2/ECO. We define a transport efficiency ε(NOy) and ε(SOx), as follows:

  • equation image
  • equation image

The denominators are the expected increase in NOy or SOx in the absence of removal processes deduced from the observed ΔCO value, while the numerators are the observed increases, ΔNOy or ΔSOx. In this paper, a method to estimate the transport efficiencies, ε(NOy) and ε(SOx), in an air mass that has experienced multiple injection, mixing, and loss processes, is described. Vertical profiles of ε(NOy) and ε(SOx) are presented and possible processes resulting these efficiencies are discussed. Most of the analyses are made using the P-3B data set because NOy measurements are available. The DC-8 data set is used to compare the results from the two platforms to confirm the results from the P-3B.

2. Measurements

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Measurements
  5. 3. Study Area and Emission Data
  6. 4. Approach
  7. 5. Results and Discussion
  8. 6. Summary
  9. Acknowledgments
  10. References
  11. Supporting Information

[6] In this study, gas and aerosol data obtained on board the NASA P-3B and DC-8 aircraft were used. Techniques used for these measurements are listed in Table 1, as well as their references.

Table 1. Measurements Onboard the NASA P-3B and DC-8 Aircraft During TRACE-P
SpeciesAircraftTechniqueReference
COP-3B, DC-8Differential absorption technique using a tunable diode laserSachse et al. [1987]
CO2P-3B, DC-8Nondispersive infrared (NDIR) analyzersVay et al. [1999]
NO, NO2, NOyP-3BChemiluminescence/photolysis converter/gold catalytic converterKondo et al. [1997]
NO, NO2DC-8Two-photon laser-induced fluorescence (TP-LIF)Bradshaw et al. [1999]
PANP-3BGC/ECD 
PANDC-8GC/ECDSingh et al. [1996]
HNO3P-3BCIMSMauldin et al. [1998]
HNO3DC-8Mist chamber/ion chromatographyTalbot et al. [1997]
SO2P-3BAtmospheric pressure ionization mass spectrometry (APIMS)Thornton et al. [2002]
SO2DC-8Mist chamber/ion chromatography 
SO42− and NO3P-3BParticle-into-liquid sampler (PILS)Weber et al. [2001]
SO42−DC-8Mist chamber/ion chromatographyScheuer et al. [2002]
NMHCs/halocarbonsP-3B, DC-8Whole air samples/GC/FID/ECD/MSBlake et al. [2001]

[7] The treatment of aerosol measurements is briefly described here. In the measurements of NOy onboard the P-3B, NOy compounds were catalytically converted to NO on the surface of a heated gold tube with the addition of CO, and NO was subsequently detected by an NO-O3 chemiluminescence technique [Kondo et al., 1997; Koike et al., 2002]. Although the inlet tube for air sampling faced rearward, particles with diameters smaller than about 1.5 and 1.1 μm are considered to have been sampled by the NOy instrument at altitudes of 0.5 and 6 km, respectively (B. Liley, personal communication, 2003). Because of the operating temperature of the NOy converter (300°C), ammonium nitrate (NH4NO3, boiling point 210°C) would be expected to thermally dissociate, while sodium nitrate (NaNO3, boiling point 380°C) and calcium nitrate (Ca(NO3)2, melting point 561°C) would not be converted.

[8] Aerosol particle chemical composition, such as sulfate (SO42−) and nitrate (NO3), were measured onboard the P-3B, using a particle-into-liquid sampler (PILS) [Weber et al., 2001]. The collection efficiency of particles with diameters smaller than 0.7 μm was estimated to be about 90%, decreasing to 50% for particles with diameters of 1.2 μm. The efficiency for particles larger than 3 μm in diameter was negligible [Ma et al., 2003]. This collection efficiency was similar to that of the NOy instrument, and both instruments are considered to sample particles in a similar size range, i.e., fine particles.

[9] Sulfate aerosol measurements onboard the DC-8 were made using a mist chamber and ion chromatography (MC-IC) technique. The median upper particle size cutoff was estimated to be 2.7 μm in diameter [Scheuer et al., 2003]. Because of this larger cutoff size as compared with that of the PILS instrument, a greater number of coarse mode dust particles including calcium sulfate (CaSO4), is expected to have been sampled. During the three P-3B/DC-8 intercomparison flights made during TRACE-P, agreements within 30–50% were found between P-3B/PILS and DC-8/MC-IC SO42− measurements [Ma et al., 2003]. The MC-IC values were systematically lower during one of the intercomparison flights in spite of larger cutoff size, suggesting that the observed differences were resulted from the error in the measurements.

[10] For the P-3B data set, amounts of non-sea-salt sulfate (nssSO42−) were calculated from measurements of SO42− and Na+ using an empirical relationship in seawater. Because Na+ data in the MC-IC SO42− size range is not available for the DC-8 data set, SO42− values are used as nssSO42− values. In this study, we denote SO2 + nssSO42− as SOx, which is considered to be a product of anthropogenic SO2 emissions.

[11] For analyses using the P-3B data set, all data were averaged to the PILS aerosol-sampling time interval of about 3 min. For analyses using the DC-8 data set, all data were averaged for the mist chamber sampling time interval of about 1 min.

3. Study Area and Emission Data

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Measurements
  5. 3. Study Area and Emission Data
  6. 4. Approach
  7. 5. Results and Discussion
  8. 6. Summary
  9. Acknowledgments
  10. References
  11. Supporting Information

3.1. Study Area

[12] Figure 1 shows the flight tracks of the P-3B aircraft during TRACE-P. Five-day kinematic back trajectories of air masses sampled from the P-3B were calculated using the method described by T. A. Yamanouchi et al. (Arctic Study of Tropospheric Aerosol and Radiation (ASTAR2000) campaign: An overview and first results, submitted to Bulletin of the American Meteorological Society, 2003), with meteorological data provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) on a 1° × 1° latitude-longitude grid. For the present study, we defined the study area as 30°N–42°N and 124°E–140°E and trajectories for air masses sampled in this area are shown in Figure 2. A latitudinal boundary of 30°N was chosen because back trajectories of air masses sampled south of 30°N (17°N–30°N) passed over Southeast Asia, where hot spots due to biomass burning were frequently observed by the Advanced Very High Resolution Radiometer (AVHRR) satellite instrument during the TRACE-P period [Heald et al., 2003]. In fact, air masses likely influenced by biomass burning (high CH3Cl and K+ concentrations) were occasionally observed at these latitudes in the free troposphere. A longitudinal boundary of 140°E was chosen, because a volcano on Miyakejima Island (34°N, 139°E) was quite active during the TRACE-P period, and a huge amount of SO2 was likely emitted from it. Furthermore, air masses which could have been influenced by the Miyakejima Island volcano were discarded based on back trajectory analysis. As seen in Figure 2, almost all of the air masses sampled in the study area were transported from over the continent at latitudes between 30°N and 60°N by predominant westerly flow [Fuelberg et al., 2003]. These air masses are expected to have been influenced by anthropogenic emissions over the continent, and we therefore denoted air masses sampled in the study area as “continental air masses.” They were sampled during five flights (P-3B flight 13, 14, 16, 18, and 19) made between March 17 and April 2.

image

Figure 1. Flight tracks of the NASA/P-3B during TRACE-P. Locations where “continental air masses” and “Chinese air masses” were sampled are also shown. “Continental air masses” are those sampled in the study area defined as 30°N–42°N and 124°E–140°E (excluding air masses that could have been influenced by a volcano on Miyakejima Island). “Chinese air masses” are a subset of “continental air masses” and are those passed over China at altitudes below the 800-hPa level (boundary layer) within 5 days prior to measurement without passing over South Korea or Japan at these low altitudes. Locations where air masses in which ΔCO > 30 ppbv were sampled (signature of influences from anthropogenic emissions) are also shown.

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image

Figure 2. Five-day backward trajectories for air masses sampled by the P-3B aircraft in the study area (30°N–42°N and 124°E–140°E). Color of the trajectories shows atmospheric pressure along the trajectories.

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[13] Emission ratios of CO, CO2, NOx, and SOx (described in the next section) in Japan and the Republic of Korea (South Korea) are different from those in China. We thus further distinguished “Chinese air masses” from “continental air masses” using back trajectories. “Chinese air masses” are defined as those that passed over China at altitudes below the 800-hPa level (boundary layer) within 5 days prior to the measurements. We excluded air masses of Chinese origin that passed over South Korea and/or Japan at altitudes below 800 hPa. Locations where “Chinese air masses” were sampled are shown in Figure 1. They are a subgroup of “continental air masses” and will be used to confirm the results of the influences from China derived using “continental air masses.”

3.2. Emission Data

[14] A comprehensive inventory of air pollutant emissions in Asia was developed for the year 2000 [Streets et al., 2003]. For this estimate, all major contributing sources, such as power plants (coal and oil), transportation (oil), industrial sources (coal and oil), domestic sources (coal and biofuel), and biomass burning were taken into account. Because emissions of various species were calculated from the common model of activity rate (e.g., fossil fuel energy use rate), these estimates are considered to be internally consistent. In general, other emission estimates were made in less consistent ways, as different investigators use different models for different species. The consistency of emission estimates is important in the present study, because we estimated the transport efficiency using emission ratio information, as shown in equations (1) and (2).

[15] In this study, emission data for CO, CO2, NOx, and SO2 were used. Figure 3 shows the emissions of these species in the East Asian countries in the year 2000. High-emission regions are generally located in the coastal area of the northeastern part of China, South Korea, and Japan. Annual emissions from China were estimated to be 4.13 tera-mol (Tmol) for CO, 86.7 Tmol for CO2, 0.247 Tmol for NOx, and 0.318 Tmol for SO2 with a 95% confidence level of ±156%, ±16%, ±23%, and ±13%, respectively [Streets et al., 2003]. Uncertainty is greatest for CO because emission factors are highly dependent on the efficiency of the combustion process. In fact, a comparison of model calculations using this emission inventory with observations suggests that CO estimates for China are low [Palmer et al., 2003; Carmichael et al., 2003]. It is noted that although annual average emission rates are used in this study, emissions in March (TRACE-P period) are close to the annual averages [Streets et al., 2003].

image

Figure 3. Distribution of anthropogenic emissions of (a) CO, (b) CO2, (c) NOx, and (d) SO2 for the year 2000 at 1° × 1° resolution [Streets et al., 2003]. To distinguish high-emission areas, only areas in which emissions are higher than certain criteria (CO > 3 Gmol/yr, CO2 > 70 Gmol/yr, NOx > 0.2 Gmol/yr, and SO2 > 0.15 Gmol/yr) are plotted. Note that emissions greater than the highest values given in color-scales are also shown using the highest color-code (red).

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[16] In Figures 4a–4c the correlation between the emission rates of CO, CO2, NOx, and SO2 in the northeastern part of China within the region of 25°N–50°N and 100°E–130°E (except for South Korea) is shown using gridded emission data with a spatial resolution of 1° × 1°. This region was chosen because most of the air masses sampled in the study area (30°N–42°N and 124°E–140°E) were transported through this region. As seen in these figures, emission rates assigned to each grid point show positive correlations with CO emissions. A correlation coefficient is slightly better when CO2 is used instead of using CO as a reference gas; r2 value for NOx-CO2 and SOx-CO2 is 0.96 and 0.82, respectively. These good correlations are unexpected because the emission ratios ECO/ECO2, ENOx/ECO, and ESO2/ECO in China are actually quite different for different emission sources. For example, coal-fired power plants are the largest source for SO2 and NOx in China, though CO emission from this source is negligible [Streets et al., 2003]. It is likely that large sources of CO (such as transportation) are colocated with these power plants and a relatively good correlation has resulted. Our method to estimate the transport efficiency defined by equations (1) and (2) requires the same emission ratio be used for all air masses influenced by local emissions at different locations. Further refinements of the emission database should indicate whether the assumption can be justified.

image

Figure 4. Scatter plot between anthropogenic emissions (1° × 1° gridded data) in the region of 25°N–50°N and 100°E–130°E, except for South Korea (30°N–38°N and 126°E–130°E). This region covers mainly industrial regions in the northeastern part of China. Ratios of total emission amounts in this region (Table 2) are also shown by solid lines. These ratios were used as ECO/ECO2, ENOx/ECO, and ESO2/ECO in the present analyses.

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[17] Total emissions in the region of 25°N–50°N and 100°E–130°E, except for emissions from South Korea, are given in Table 2. They correspond to about 70% of the total emissions from China. In Table 2, ratios of these total amounts are also given and are shown using solid lines in Figures 4a–4c. We used these values as ECO/ECO2, ENOx/ECO, and ESO2/ECO ratios as representative of the emissions in the region in the following analyses. The values are not very sensitive to small change in the locations of the boundary because the boundary of the chosen region is far away from the locations of highest emissions.

Table 2. Emission and Emission Ratio of CO, CO2, NOx, and SO2
SpeciesAreaAmount, tera-mol/yrRatio to CO, mol/mol
  • a

    Northeastern China defined in this study is the region of 25°N–50°N and 100°E–130°E except for South Korea.

COnortheastern Chinaa3.081
 South Korea0.1011
 Japan0.2431
 
CO2northeastern China63.720.7
 South Korea9.3592.7
 Japan27.3112
 
NOxnortheastern China0.1620.0527
 South Korea0.02870.285
 Japan0.04780.197
 
SO2northeastern China0.2120.0690
 South Korea0.01290.128
 Japan0.01250.0515

[18] It is noted that emission ratios of CO, CO2, NOx, and SO2 in South Korea and Japan are very different from those in China (Table 2): ENOx/ECO is larger, while ECO/ECO2 is smaller. The value of ESO2/ECO in South Korea is also larger than that in China. Consequently, influences from South Korean and Japanese emissions are expected to be different from those from Chinese emissions that we estimate in this study. These influences are examined in the following sections and uncertainties in the transport efficiency estimates are discussed in section 5.7.

4. Approach

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Measurements
  5. 3. Study Area and Emission Data
  6. 4. Approach
  7. 5. Results and Discussion
  8. 6. Summary
  9. Acknowledgments
  10. References
  11. Supporting Information

4.1. Estimation of Transport Efficiency

[19] As described in section 1, the transport efficiency of anthropogenically emitted reactive nitrogen and sulfur compounds, defined in equations (1) and (2), was estimated in the present study. For this purpose, background levels of CO, CO2, NOy, and SOx were determined for individual sampled air masses, instead of using common values for all air masses. The definition of the “background value” of an air mass in this study is the value, which an air mass would have had if it had not been influenced by anthropogenic sources in the East Asian countries.

[20] To describe our approach to the transport efficiency estimation, an example of the CO-CO2 relationship is shown in Figure 5a. An air mass can be identified as a point in the diagram by the measured CO and CO2 concentrations. We made two simple assumptions in our analyses. First, CO and CO2 values in background air masses have a linear relationship. Second, the emission ratio, ECO/ECO2, is constant so that the ratio of increases between CO and CO2 (ΔCO/ΔCO2) is always equal to the emission ratio when an air mass receives anthropogenic emissions. An argument for the validity of the first assumption is described in the next section. The second assumption is supported by the relatively good correlation between emissions of CO and CO2 shown in Figure 4a. In addition, we assumed the loss of CO and CO2 is negligible. Because air masses were sampled in our study area only 0 to 5 days after leaving the continent, this assumption is also reasonable.

image

Figure 5. Schematic diagrams showing the procedure to derive background values (C) from observations (C′ or C″).

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image

Figure 5. (continued)

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[21] In Figure 5a, A and B are background air masses, and a line connecting them shows the background linear relationship. Let us assume an air mass C is produced by the mixing of A and B. Air mass C can also be considered as a background air mass, because it is not affected by anthropogenic emissions. Now let us assume another case in which both air masses A and B receive (different amounts of) emissions (denoted A′ and B′). The slopes between A and A′ and between B and B′ are equal to the emission ratio, ECO/ECO2 from the second assumption. Consider that air mass C′ is produced by the mixing of A′ and B′ with the same ratio of mixing as when C is produced from A and B. The difference between C and C′ can be considered as the influence of anthropogenic emissions. Simple geometrical arguments show that the slope between C and C′ is equal to the slope for AA′, thus equal to the emission ratio ECO/ECO2. Consequently, once C′ is obtained from observations, a unique background value C can be calculated for this particular air mass as the interceptions between the background linear relationship (assumption 1) and a line thorough C′ with ECO/ECO2 as its slope (assumption 2). Values of ΔCO and ΔCO2 can then be calculated from the difference between values in air masses C and C′. Note that C′ can be considered either as resulting from mixing between A′ and B′ or as resulting from directly adding anthropogenic emissions to the background air mass C. Because of the linearity of the mixing process, a unique background value can be determined for an air mass that has experienced multiple mixing and multiple injection processes.

[22] The method to calculate the transport efficiency of NOy is now described using Figure 5b where a measured data point is denoted as C′. We assume again that NOy and CO values in background air masses have a linear relationship and that a ratio of enhancement following emissions can be expressed by an emission ratio, ENOx/ECO. Because we know the ΔCO (and, therefore, the background value of CO) value for this air mass from the CO-CO2 correlation (Figure 5a), we can calculate a background value of NOy (NOy value in air mass C). The expected NOy value due to inputs of anthropogenic emissions (NOy value in air mass expected-C′ in Figure 5b) can then be calculated using ENOx/ECO. The measured NOy would have been at this expected NOy value if NOy were to behave as a passive tracer. The observed NOy value (NOy value in air mass C′), however, can be lower than the value in air mass expected-C′, because of the loss of NOy. The ΔNOy value is defined by the difference between the observed value (C′) and background value (C). The transport efficiency of NOy, ε(NOy), is defined as in equation (1), which is obtained from the ratio of ΔNOy to the expected increase in NOy (the difference between C and expected-C′). Note that the NOy value in an observed air mass can be even lower than the “background relationship line” in the NOy-CO diagram, as shown as air mass C′′ in Figure 5b. Even in this case, the ΔNOy value is still positive when the background value of this air mass (C) is lower than the observed value C′′.

[23] In the same way, the transport efficiency of SOx, ε(SOx), can be calculated. Note that if CO2 is used as a reference gas to calculate the transport efficiency defined by equations (1) and (2) instead of using CO, the same result is obtained. This is because ΔCO and ΔCO2 are interconnected as ΔCO/ΔCO2 = ECO/ECO2.

[24] In our approach we assume the background relationship does not change, even when a loss of NOy or SOx takes place and a decrease in NOy or SOx (the difference between expected-C′ and C′) is attributed to a loss within anthropogenically injected NOy or SOx. In reality, however, NOy or SOx molecules, belonging to background air masses are also lost simultaneously when a loss process takes place. Consequently, the transport efficiency estimated by this approach is considered a lower limit. In the case of ε(NOy) estimation, lightning NO production and aircraft emissions can cause an overestimation and these effects will be addressed in section 5.7.

[25] An advantage of this method is that it can be used under the condition of multiple mixing, multiple injection, and multiple loss processes. Although we cannot tell when NOy or SOx is lost, we can calculate the fraction of net loss. The first key to this method is that instead of using a single value of CO or CO2 as the background, we use a linear relationship between CO and CO2 in background air masses, which covers certain ranges of CO and CO2, to derive ΔCO. The other key is the use of emission ratios to estimate background values. In the next section, a determination of the background relationships is described.

4.2. Background Relationships

[26] In Figures 6a–6d, vertical profiles of CO, CO2, NOy, and SOx (=SO2 + nssSO42−) observed from the P-3B aircraft are plotted using potential temperature as the vertical coordinate. For these figures, all data points obtained at west of 155°E are used irrespective of the latitude (17°N–42°N) except for those possibly influenced by a volcano on Miyakejima Island. Minimum values of CO, CO2, and NOy at each potential temperature level change smoothly with potential temperature. The solid line in each figure is a polynomial fit calculated for minimum values at each potential temperature level, and they will be referred to as “minimum value lines.” Considering the fact that all of the air masses sampled in our study area had been transported from the Asian continent, these minimum values suggest the background levels of these species. Note that the minimum value at each potential temperature surface changes little with latitude. When geometric altitude is used instead of potential temperature, the minimum values change with latitude because of air mass motions along isentropic surfaces [Koike et al., 1997]. In the case of SOx, the background level was suggested to be close to zero because very low concentrations were frequently observed.

image

Figure 6. Vertical profiles of CO, CO2, NOy, and SOx (=SO2 + nssSO42−). All of the P-3B data obtained at west of 155°E are used irrespective of the latitude (17°N–42°N) except for those possibly influenced by a volcano on Miyakejima Island. Potential temperature is used as the vertical coordinate. A solid line in each figure is a polynomial fit calculated for minimum values at each potential temperature level and is referred to as “minimum value line” in the text.

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[27] In Figure 7a, a scatter plot between CO and CO2 is shown using “continental air mass” data. In this plot, the color was coded using the altitude at which the air mass was sampled. From Figures 6a and 6b, pairs of CO and CO2 values on the “minimum value lines” were calculated at various potential temperature levels between 280 and 310 K (open squares in Figures 6a and 6b), and they are also plotted in Figure 7a using open squares. As seen in this figure, these “minimum values” are linearly correlated within this CO and CO2 range. Using the argument that these “minimum values” suggest the background levels of CO and CO2, we use this relationship as the “background relationship” between CO and CO2.

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Figure 7. Scatter plot between CO and CO2 in “continental air masses,” which were sampled at 30°N–42°N and 124°E–140°E (P-3B data). Color was coded using (a) sampling altitude, (b) Halon 1211 mixing ratio, (c) OCS mixing ratio, and (d) CH3Br mixing ratio. Open squares show pairs of CO and CO2 values on the “minimum value lines” shown in Figures 6a and 6b. A solid line is a least squares fit to these open squares, and it will be referred to as “background relationship” in the text. The dashed line denotes the emission ratio of CO relative to CO2 (ECO/ECO2) in the northeastern part of China (Table 2).

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[28] As seen in Figure 7a, many data points are distributed around the “background relationship” line. Vertical profiles of CO and CO2 suggest that these air masses have chemical characteristics of a background air mass, which is influenced little by anthropogenic sources over the East Asia. This is further confirmed using a correlation with Halon 1211 (CF2ClBr) and OCS as described below. In Figures 7b and 7c, scatter plots between CO and CO2 are shown and levels of Halon 1211 and OCS are used for color-coding. Halon 1211 is considered to be a unique tracer for Chinese urban emissions [Blake et al., 2001; Blake et al., 2003] because China is currently responsible for about 90% of the world's production following regulation by the Montreal Protocol [Fraser et al., 1999]. It is used in fire extinguishers and its residence time in the atmosphere is 10 to 20 years. OCS has also been found to be a good indicator of anthropogenic emission from China and likely originates from coal burning (J. Green et al., manuscript in preparation, 2003). Coal combustion is a dominant contributor to both CO2 and SO2 emission in China [Streets et al., 2003]. The global average residence time of OCS was estimated to be about 4 years [Chin and Davis, 1995]. As seen in Figures 7b and 7c, both Halon 1211 and OCS values in air masses above the “background relationship” are systematically higher than those in air masses around the “background relationship.” This result indicates that air masses around the “background relationship” were influenced little by anthropogenic emissions over China.

[29] In summary, the “background relationship” between CO and CO2 shown in Figure 7a was established using the minimum values at each potential temperature level observed in our study area. The correlations with both Halon 1211 and OCS confirm the validity of the use of this relationship as the “background relationship.” In this study, one common “background relationship” is defined for the boundary layer air and free tropospheric air. Thus transport efficiency can be calculated even when an air masses in the boundary layer and free troposphere are mixed together. The degree of influence from anthropogenic emissions can be estimated by the value of ΔCO, irrespective of the altitude.

[30] Influences from Japanese and South Korean sources are noted here. In Figure 7d, a scatter plot between CO and CO2 is shown using methyl bromide (CH3Br) for color-coding. During TRACE-P, enhancements of CH3Br were found in air masses when they had been advected over Japanese and South Korean ports, suggesting that it can be used as a unique tracer for influences from these two countries [Blake et al., 2003]. High levels of CH3Br are found in air masses in which CO and CO2 was greater than 200 ppbv and 380 ppmv, respectively as shown in Figure 7d. These air masses are found around the “background relationship” and this is consistent with the fact that emission ratios ECO/ECO2 over Japan and South Korea are systematically lower than that over China and are closer to a slope of “background relationship” (Table 2). These results suggest that Japanese and South Korean sources had only small influences in estimating ΔCO values.

[31] The “background relationship” between NOy and CO was calculated in the same way, using “minimum value lines” shown in Figures 6a and 6c. This relationship is shown as a solid line in a scatter plot between NOy and CO (Figure 8a). Black squares in this figure denote low-ΔCO air masses (ΔCO < 30 ppbv) in which CO and CO2 values are close to the CO-CO2 background relationship line. As seen in this figure, most of the low-ΔCO data correspond to data points around the “background relationship” between NOy and CO. Although clear increases in NOy are seen in some low-ΔCO air masses sampled at low altitudes, they were likely due to influences from Japanese and/or South Korean sources (relatively high CH3Br concentrations as seen in Figure 7d) and a number of data is limited. Consequently, the method to estimate the transport efficiency described in the previous section can be applied to NOy. In the case of SOx, the background level was chosen to be zero, as described above. However, there is a positive correlation between SOx and CO in low-ΔCO air masses, and mixing with background air masses could result in some errors in our estimation (Figure 8b). We note that ΔCO values are estimated more accurately than ΔNOy and ΔSOx values in this study. The accuracy of the background level determination is more important for CO because the relative variability in CO due to anthropogenic emissions is smaller as compared with that in NOy [Parrish et al., 1991; Stohl et al., 2002].

image

Figure 8. Scatter plots between (a) NOy and CO, (b) SOx and CO, (c) ΔNOy and ΔCO, and (d) ΔSOx and ΔCO in “continental air masses” (P-3B data). ΔSOx values are identical to SOx values because a zero background was assumed. Open squares show pairs of NOy and CO or SOx and CO values on the “minimum value lines” shown in Figures 6a, 6c, and 6d. A solid line and dashed line denote the “background relationship” and the emission ratio (ENOx/ECO or ESOx/ECO) in the northeastern part of China (Table 2), respectively. Black squares denote air masses in which ΔCO < 30 ppbv.

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[32] Finally, it is noted that if an emission ratio ECO/ECO2 is similar to a slope of the “background relationship” between CO and CO2, influences cannot be extract using the present method. As described above, this is the case for emissions from Japan and South Korea. High ECO/ECO2 ratios of Chinese emissions enable us to evaluate their impact.

5. Results and Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Measurements
  5. 3. Study Area and Emission Data
  6. 4. Approach
  7. 5. Results and Discussion
  8. 6. Summary
  9. Acknowledgments
  10. References
  11. Supporting Information

5.1. Episodes of High-ΔCO Events

[33] In the statistical analyses described in the following sections, we only used air masses in which ΔCO values were greater than 30 ppbv (i.e., air masses clearly influenced by anthropogenic sources over the northeastern part of China). A threshold value of 30 ppbv was subjectively chosen by assuming a symmetrical distribution of ΔCO values in background air masses around the ΔCO = 0 line (“background relationship” line in Figures 7a–7d). Air masses in which ΔCO > 30 ppbv consisted of 30% of “continental air masses,” and this fraction is higher at 0–2 km as compared with that at 2–7 km. Locations where these air masses were sampled are shown in Figure 1. After this selection, 19–42 and 9–29 data points remain in “continental” and “Chinese” air masses, respectively, in each of the 0–1, 1–2, and 2–4 km altitude ranges, while only 2 and 1 data points remain in these two air masses in the 4–7 km range. Results at 4–7 km likely do not represent the average in the western Pacific region. Uncertainties in the estimation at 4–7 km altitude range are discussed in section 5.5.

5.2. Correlation

[34] In Figure 8a, a scatter plot between CO and NOy is shown using all of the “continental air mass” data. As seen in this figure, enhancements in NOy are positively correlated with those in CO, suggesting that these NOy enhancements were due to anthropogenic emissions. The enhancement was greatest at altitudes below 2 km. A clear positive correlation between NOy and CO, suggesting anthropogenic influences, was also found in the lower and middle troposphere over the western Pacific in February–April 1994, during PEM-West B [Koike et al., 1997].

[35] In Figure 8c, a scatter plot between ΔNOy and ΔCO is shown for “continental air mass” data. The relationship between ΔNOy and ΔCO is similar to that between NOy and CO (Figure 8a) because background values of individual data are not very different. The ΔNOy and ΔCO are positively correlated; however, the slope is lower than the emission ratio, suggesting that anthropogenically emitted nitrogen compounds had been removed from the air masses before they were sampled from the P3-B aircraft. Ratios of increases, ΔNOy/ΔCO, are generally higher at altitudes below 2 km as compared to those at higher altitudes, suggesting removal of NOy during the vertical transport. On the other hand, ratios of increases in air masses in which ΔNOy and ΔCO values are very high (ΔNOy > 6 ppbv and ΔCO > 400 ppbv) are similar to those of other air masses. These air masses are considered to have been influenced by anthropogenic sources more strongly, however the removal rate is similar to those of less-influenced air masses.

[36] It is noted that there are some data which have greater ΔNOy/ΔCO ratios than the image ratio. This is likely due to inhomogeneity in emission ratios and/or influences from Japanese and/or South Korean emissions. Most of these data points are found in air masses in which ΔCO < 30 ppbv and therefore, they were discarded in the following statistical analyses as described in section 5.1.

[37] In Figure 8b, a scatter plot between CO and SOx is shown using “continental air mass” data. A clear correlation was found between the two values, especially at altitudes below 2 km. A positive correlation is also seen in a scatter plot between ΔCO and ΔSOx (Figure 8d). As for NOy, the slope was lower than the emission ratio, ESO2/ECO, suggesting the removal of SOx. Air masses sampled at 2–7 km appear to belong to two groups. In one of the groups, the slope of the increase is similar to that of the 0–2 km data, while very little increase of SOx is found in the other group. These two groups of air masses were sampled during different flights (mostly from flights 13 and 19); however, many of these air masses had been transported from below 800 hPa over the northeastern part of China. Consequently, the observed difference in the transport efficiency was largely due to differences in the transport processes, in which the degree of loss of SO2 and/or nssSO42− was different (more discussion on flight 19 data is given in section 5.4.).

5.3. Vertical Profile of ε(NOy)

[38] Average ΔCO and ΔNOy values in “continental air mass” data were calculated within each altitude range (0–1, 1–2, 2–4, and 4–7 km), and the transport efficiency, ε(NOy), was calculated using these averages (equation image and equation imageequation image). In this calculation, we only used data with ΔCO > 30 ppbv. We took the ratio of the average rather than using the median value of individual ratios because events in which ΔCO and ΔNOy were large should have a greater impact on the overall efficiency of the transport. It has been reported that the majority of transport of pollutants from the Asian continent to the Pacific Ocean is generally the result of episodic events rather than continuous steady flow [Bey et al., 2001; Liu et al., 2003; Jaffe et al., 1999]. We note that the choice of the threshold value of ΔCO (>30 ppbv) is not very sensitive to the results because the averages are used instead of using the medians.

[39] A vertical profile of the average transport efficiency in the “continental air” is shown in Figure 9a (Table 3). In this figure, the transport efficiency calculated using only “Chinese air masses” is also shown. “Chinese air masses” were those that passed over China at altitudes below the 800-hPa level (boundary layer) within 5 days prior to measurement without passing over South Korea or Japan at these low altitudes, as described in section 3.1. We note that when air masses in which ΔCO > 30 ppbv are selected, Chinese air masses in each altitude range consist of 20–70% of the continental air masses. As seen in Figure 9a, a transport efficiency of 20–40% was found at 0–2 km. “Chinese air masses” at 0–2 km were mostly sampled over the Yellow Sea, and they were sampled within 24 hours after they had left the coast of China. The efficiency estimated in this study indicates that 60–80% of NOy had already been removed in the source region before these air masses left the continent, although additional loss could have also occurred during this short transport over the ocean. The efficiency tends to be lower at higher altitudes, and values of about 15% were found at 2–7 km. The lower efficiency in the free troposphere suggests that HNO3 and NO3 were removed by wet deposition during the vertical transport because precipitation is often accompanied by convection and synoptic-scale air mass lifting in association with cold frontal systems, which is known as the warm conveyer belt (WCB). The removal process is examined more quantitatively below.

image

Figure 9. Vertical profiles of the transport efficiency for (a) NOy, defined in equation (1) and (b) SOx (=SO2 + nssSO42−), defined in equation (2). Values of ΔCO, ΔNOy, and ΔSOx were averaged (only using air masses in which ΔCO > 30 ppbv) before taking their ratios. Emission ratios, given in Table 2 were used. Horizontal bars indicate uncertainties in the estimations (50% range).

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Table 3. Transport Efficiency of Reactive Nitrogena
 P-3BDC-8
ε(NOy)ε(NOx)ε(NO3)ε(NOy)
  • a

    Results are for air masses in which ΔCO > 30 ppbv within “continental air masses.” Values in parentheses are for “Chinese air masses.”

4–7 km0.18 (0.18)0.0040.0010.14 (0.11)
2–4 km0.13 (0.13)0.0070.0010.13 (0.08)
1–2 km0.21 (0.18)0.0130.0250.26 (0.25)
0–1 km0.38 (0.28)0.0480.160.31 (0.26)

[40] In Figure 10a, vertical profiles of median values of HNO3/NOy and NO3-/NOy in “continental air masses” are shown. These values are also given in Table 4. For this calculation, we only used data with ΔCO > 30 ppbv. As seen in this figure, HNO3 + NO3 accounts for 50% and 30% of NOy at 0–1 and 1–2 km, respectively. Henry's law equilibrium and laboratory studies of HNO3 uptake on ice surfaces indicate that HNO3 is ∼100% fractionated into the condensed phase in both warm and glaciated clouds [Mari et al., 2000, and references therein]. One-dimensional model calculations for tropical continental deep convection suggest that about 80% of HNO3 was scavenged during vertical transport [Mari et al., 2000]. Similarly, very low NO3/NOy values of 0.007 in the free troposphere, as compared with those observed in the boundary layer of 0.1–0.4, indicate that nitrate aerosols were also scavenged quite efficiently during vertical transport. In fact, ammonium nitrate (NH4NO3), which is generally seen in accumulation mode aerosol and which is considered to have been measured by the NOy instrument, is expected to be efficiently scavenged by cloud droplets. Assuming 100% removal of HNO3 and NO3 aerosols in the boundary layer during vertical transport into the free troposphere, the factor of 2 reduction of ε(NOy) estimated in this study can be explained. Formation of HNO3 from NOx during and after vertical transport retains the HNO3/NOy ratios in the free troposphere, while very low NO3/NOy ratios in the free troposphere suggest that the formation of nitrate aerosol was likely limited because an insufficient amount of NH3 was available to neutralize the acidity.

image

Figure 10. (a) Vertical profile of the partitioning within the NOy species (P-3B data). Median values and 67% ranges (horizontal bars) in “continental air masses” (and ΔCO > 30 ppbv) are shown. (b) Vertical profile of nssSO42−/SOx ratio (P-3B data). Median values and 67% ranges (horizontal bars) in “continental air masses” (and ΔCO > 30 ppbv) are shown. Individual data points are also shown.

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Table 4. Partitioning Within NOy and SOx (P-3B)a
 NOySOx
NOx/NOyPAN/NOyHNO3/NOyNO3/NOynssSO42−/SOx
  • a

    Median values for air masses in which ΔCO > 30 ppbv within “continental air masses.” Values in parentheses are 67% ranges.

4–7 km0.0230.530.00660.10
2–4 km0.051 (0.028–0.078)0.46 (0.36–0.64)0.23 (0.12–0.33)0.0078 (0.0015–0.0723)0.49 (0.01–0.87)
1–2 km0.064 (0.036–0.083)0.39 (0.30–0.52)0.17 (0.10–0.23)0.12 (0.00–0.49)0.37 (0.21–0.83)
0–1 km0.12 (0.09–0.17)0.34 (0.25–0.38)0.091 (0.046–0.146)0.42 (0.26–0.74)0.56 (0.45–0.71)

[41] The transport efficiency of NOy during the TRACE-P period was also studied by Miyazaki et al. [2003]. Although a single value was used for the background levels of NOy and CO in their analysis, generally similar results were obtained: 30% and 10–20% in the boundary layer and free troposphere, respectively. Synoptic-scale WCB and convection were found to be important mechanisms for the transport of anthropogenic reactive nitrogen to the free troposphere. The transport efficiency of NOy from North America was studied by Stohl et al. [2002] using North Atlantic Regional Experiment (NARE) 1996 and 1997 data. They used CO and Lagrangian tracers as a reference and found that only 3–5% of NOy emissions reached altitudes above 3 km. D. D. Parrish et al. (Relation of NOy and CO concentrations in the free troposphere: Fraction and composition of NOy transported in polluted air masses lofted from the North American continental boundary layer, submitted to Journal of Geophysical Research, 2003) also studied the relationship between NOy and CO using five field campaigns over the North America (summer season) and the western North Atlantic Ocean (spring and fall) and the NOy transport efficiencies in the free troposphere are estimated to have been 10–30%.

[42] Vertical profiles of the partitioning of reactive nitrogen (Figure 10a and Table 4) show that PAN was the dominant species in “continental air masses” (ΔCO > 30 ppbv). Because of the long lifetime of PAN at low temperatures in the free troposphere, PAN would be transported farther downwind and eventually produce NOx after descending to lower altitudes [e.g., Moxim et al., 1996; Koike et al., 1997]. Consequently, this fraction is considered as a potential source of NOx. Direct transport of NOx, however, was found to be very limited. The NOx/NOy ratio was 0.06–0.12 and 0.02–0.05 at 0–2 km and 2–7 km, respectively (Figure 10a and Table 4). When ε(NOy) values in Table 3 (Figure 9a) are used, the transport efficiency of NOx, ε(NOx), was calculated to be about 1–5% and 0.5% in the boundary layer and free troposphere, respectively. This result means that only 0.5% of anthropogenically emitted NOx remained in the form of NOx when these air masses were transported into the free troposphere. Values of NO3/NOy have large variability at 0–2 km, partly because the NO3 amount in aerosol is generally limited by the availability of NH3. When median values of NO3/NOy are used, the formation efficiency of NO3, ε(NO3), was calculated to be 3–16% and 0.1% in the boundary layer and free troposphere, respectively. This low yield in the free troposphere is due to the efficient scavenging of nitrate aerosol, as described above.

[43] A scatter plot between ε(NOy) and NOx/NOy is shown in Figure 11a. As seen in this figure, these two quantities are positively correlated. Part of the correlation is due to their vertical profiles; however, a correlation is seen even within the boundary layer data. Because the loss of HNO3 and NO3 results in a higher NOx/NOy ratio, a positive correlation cannot be simply explained by this process. When an air mass remained or stagnated within the boundary layer for a longer time period, conversion from NOx to HNO3 by gas-phase reaction with OH and nighttime heterogeneous reaction through N2O5 proceed further, leading to a lower NOx/NOy ratio. Concurrently, the loss of HNO3 and NO3 results in a lower ε(NOy). Furthermore, during vertical transport, we may expect an efficient conversion of NOx to HNO3 due to the relatively large amounts of OH from H2O and wet surface area for heterogeneous reaction. The results presented in this study suggest that the transport efficiency was higher in air masses that spent less time in the boundary layer, though a very limited fraction of emitted NOx can still be brought into the free troposphere.

image

Figure 11. Scatter plot between (a) NOx/NOy and transport efficiency of NOy, and (b) transport efficiencies of NOy and SOx (P-3B data). Air masses in which ΔCO > 30 ppbv within “continental air masses” are used.

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image

Figure 11. (continued)

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5.4. Vertical Profile of ε(SOx)

[44] Vertical profiles of the average transport efficiency of SOx (=SO2 + nssSO42−), ε(SOx), in the “continental air masses” and “Chinese air masses” are shown in Figure 9b (Table 5). As for ε(NOy), averages of ΔSOx and ΔCO in high-ΔCO air masses (ΔCO > 30 ppbv) were calculated (equation image and equation image), and their ratio was divided by the emission ratio given in Table 2. As a result, the transport efficiency at 0–2 km was estimated to be 25–45%. “Chinese air masses” at 0–2 km were mostly sampled over the Yellow Sea, and they were sampled within 24 hours after they had left the coast of China, as described above. The efficiency estimated in this study indicates that 55–75% of SOx had already been removed in the source region before these air masses left the continent. The transport efficiency of 15% at 2–7 km is lower than that at 0–2 km, suggesting that SO2 and nssSO42− were removed by wet deposition during vertical transport. One-dimensional model calculations for tropical continental deep convection suggest that about 30% of SO2 was scavenged during vertical transport [Mari et al., 2000]. Although this value is lower compared to the value of 80% for HNO3 due to the low Henry's Law constant, nssSO42− particles are considered to have been scavenged quite efficiently within the cloud and precipitation leading to lower ε(SOx) values in the free troposphere.

Table 5. Transport Efficiency of SOxa
 P-3BDC-8
ε(SOx)ε(nssSO42−)ε(SOx)
  • a

    Results are for air masses in which ΔCO > 30 ppbv within “continental air masses.” Values in parentheses are for “Chinese air masses.”

4–7 km0.15 (0.14)0.0150.06 (0.06)
2–4 km0.18 (0.16)0.0870.14 (0.06)
1–2 km0.27 (0.24)0.100.38 (0.38)
0–1 km0.45 (0.29)0.250.43 (0.42)

[45] A vertical profile of median values of nssSO42−/SOx ratio is shown in Figure 10b (and Table 4). At 0–1 km altitude, nssSO42−/SOx ratios of about 0.6 were observed during each of the four fights, suggesting that the conversion from SO2 to SO42− had taken place quite efficiently near the source region. Ratios of nssSO42−/SOx of about 0.3–0.7 were obtained from the ground-based measurements at Cheju Island in the East China Sea [Carmichael et al., 1997] and Taean station in Korea facing the Yellow Sea [Kim et al., 2001], which also suggest rapid conversion within the boundary layer. On the other hand, at 2–4 km, nssSO42−/SOx ratios differ significantly between the flights. In air masses sampled over Japan Sea during flight 19, nssSO42−/SOx ratios were quite low (Figure 10b), while the ε(SOx) values were higher than those of the other flights. During this flight, the CO, SO2, and NOy values reached 300–400 ppbv, 5–9 ppbv, and 2–4 ppbv, respectively, at 3 km, suggesting a significant impact from anthropogenic emissions. Because ε(SOx) values at 2–4 km during this flight were close to those at 0–1 km, the observed low nssSO42−/SOx ratios might have been due to a limited formation of nssSO42− particles, rather than the removal of nssSO42− particles. When a median value of nssSO42−/SOx is used with the transport efficiency of SOx, a formation efficiency of sulfate aerosol is estimated to be 10–25% and 2–9% at 0–2 km and 4–7 km, respectively. The efficiency in the free troposphere was higher than that of nitrate aerosol in the same altitude rage (ε(NO3) = 0.1%) likely because sulfate aerosols can be formed more easily in the free troposphere after removal during vertical transport.

[46] A scatter plot of the transport efficiencies for NOy and SOx is shown in Figure 11b. As seen in this figure, they are positively correlated, even within each altitude range. This result suggests that loss processes of NOy and SOx proceeded simultaneously.

5.5. Comparison With ε(NOy) and ε(SOx) Values Derived From DC-8 Data

[47] In this study, the P-3B aircraft data were used exclusively. However, because measurements of NOx, PAN, HNO3, SO2, and SO42− (particles with diameters smaller than 2.7 μm) were also made onboard the NASA/DC-8 during TRACE-P, we performed the same analyses using the DC-8 data set obtained in the study area defined for the present study (30°N–42°N and 124°E–140°E). For NOy values, we summed the DC-8 observations of NOx, PAN, and HNO3. In HNO3 measurements, particulate nitrate of diameters smaller than 1–2 μm is considered to have been measured with gas phase HNO3. When we plot a CO-CO2 scatter plot using Halon 1211 and OCS as markers of Chinese urban emissions, very similar results to those shown in Figures 7a–7c are obtained (not shown). The “background relationship” derived from the P-3B also fits the DC-8 data set well. This is partly because the same research groups made measurements onboard the two aircraft for CO, CO2, and species obtained from whole air sampling. In fact, very good agreement between the two sets of measurements was found during the P-3B/DC-8 intercomparison flights [Eisele et al., 2003].

[48] Vertical profiles of ε(NOy) and ε(SOx) values derived from DC-8 “continental air” and “Chinese air” are shown in Figures 9a and 9b, together with the P-3B results. DC-8 “Chinese air masses” were selected using the same definition as that for P-3B data using air mass trajectories. These results are also summarized in Tables 3 and 5. As seen in the figure, most of the results obtained from measurements from the two aircraft agreed to within 50%. When only the transport efficiencies of SO2 are compared, a similar degree of agreement was found at 0–2 km, although the P-3B values in the boundary layer and free troposphere were found to be lower and higher, respectively. Disagreement by a factor of 3 was found between the P-3B and DC-8 SO2 measurements during the P-3B/DC-8 intercomparison flights [Eisele et al., 2003]. However, the agreement in the estimates indicates that the results obtained in this study are robust.

[49] As described in section 5.1, only two samples of air masses, in which ΔCO > 30 ppbv, were used for P-3B data analyses at 4–7 km (continental air masses), and therefore they do not represent the average in the western Pacific region. A number of data with ΔCO > 30 ppbv is small at this altitude range, primarily due to a limited number of uplifting events. When a criterion of ΔCO > 5 ppbv is used instead, 7 data (10% of the total number of data at 4–7 km) remain and ε(NOy) and ε(SOx) of 22 and 20% are obtained, suggesting a range of uncertainties in our estimations. Measurements at altitudes above 4 km were made more frequently by the DC-8. Although data above 7 km were also used for the DC-8 statistics, the agreement with the DC-8 estimates (within 25% and 85% for NOy and SOx, respectively) derived using a greater number of data (32 data) supports the validity of the results obtained from the P-3B data at 4–7 km (Tables 3 and 5).

5.6. Export Flux and Budget

[50] The transport efficiencies ε(NOy) and ε(SOx) defined in equations (1) and (2) were determined for individual air masses. The value of ε(NOy) was derived from the number density of observed NOy in one air mass divided by the expected NOy number density in that air mass assuming that there is no loss after anthropogenic input. Consequently, the efficiency can be 100% in individual air masses. In the next step, we estimated a time-averaged eastward flux of NOy and SOx across the 130°E meridional plane at 30°N–40°N during the TRACE-P period. Because of the limited air mass sampling frequency, only fractional contributions from various altitudes are considered. To estimate these quantities, we first examined an eastward flux of CO. The export flux fraction of CO is determined in this study as follows.

  • equation image

where Fair is the eastward flux of air molecules at 130°E meridional plane at 30°N–40°N derived using ECMWF data. Here γ is the occurrence frequency of the events in which ΔCO > 30 ppbv were observed from the P-3B aircraft. For equation image, averaged values in the continental air masses (ΔCO > 30 ppbv) were used. These three quantities were determined as a function of altitude and calculations were made below 7 km, where P-3B data are available. Note that the occurrence frequency of pollution transport events (events in which ΔCO > 30 ppbv) is highly dependent on where and when P-3B aircraft measurements were made, although only the relative frequency among the various altitude ranges was used for our estimation. Consequently, the flux estimated here should be considered as a case study for air masses which we sampled during TRACE-P. Using equation (3), it was found that 30% of CO export took place through the boundary layer (0–2 km) and 70% was transported through the free troposphere (2–7 km). Although the size of the ΔCO value was similar at altitudes below 4 km, higher wind speeds resulted in a greater flux in the free troposphere. The CO flux within each 1-km layer was suggested to be greatest in the 2–4-km altitude range. This result is consistent with the GEOS-CHEM model for the PEM-West B period (February–March 1994) shown by Bey et al. [2001]. Liu et al. [2003] also showed similar results for the TRACE-P period. When we attempted to estimate the absolute value of the averaged eastward flux of CO instead of the fractional contributions, values of 1–3 × 10−9 mol cm−2 s−1 were obtained at altitudes below 4 km. This is again in reasonable agreement with the results obtained by Liu et al. [2003].

[51] The export flux fraction of NOy is determined in the similar way.

  • equation image

The denominator is the expected total flux of NOy molecules that could have crossed the meridional plane, assuming that there is no loss after receiving anthropogenic emissions. The numerator is the observed flux of NOy molecules that actually crossed the meridional plane within each altitude range (continental air masses in which ΔCO > 30 ppbv). The export flux fraction can be considered as a weighted average of the transport efficiency for which air flux and occurrence frequency of pollution transport event are used as a weighting function. Using equation (4), the export flux fraction of NOy at 0–7 km during TRACE-P was estimated to be 18%. About half (8%) of the export flux took place through the boundary layer and the other half (10%) took place through the free troposphere. Similarly, the export flux fraction of SOx at 0–2 km and 2–7 km was estimated to be 10% and 12%, respectively. The fact that export flux fractions were similar between the two altitude ranges for NOy and SOx is in contrast to the case of CO, in which the fraction in the free troposphere was greater as described above. The free tropospheric fraction was less significant for NOy and SOx because loss during the vertical transport resulted in lower ε(NOy) and ε(SOx) values. The flux in the free troposphere is important for long-range transport, due to greater wind speeds and less-frequent removal, while the flux in the boundary layer suggests an impact on neighboring countries, such as Japan.

[52] The budget of NOy and SOx (total export flux versus emissions) over the East Asia region is also examined here. During TRACE-P, predominant westerly flow generally brought air masses influenced by anthropogenic emissions over the East Asia into the western Pacific region. Liu et al. [2003] show using the GEOS-CHEM model that eastward fluxes of Asian CO is strongest at 30–45°N in the boundary layer and at 20–35°N in the lower free troposphere. The former mostly reflects anthropogenic emissions, while the latter includes contributions from both anthropogenic and biomass burning emissions. When we assume that a majority of air masses influenced by anthropogenic emissions is transported to 30–40°N (and sampled from the P-3B aircraft) or an export flux fraction is similar at other latitude ranges, we can use an estimate of export flux fraction as a budget estimate. This assumption may not be very unreasonable, because a total flux of CO at 30–40°N at 130°E meridional plane described above agrees with the total emission of CO at 25°N–50°N and 100°E–130°E (except for emissions from South Korea) within 20%. However considering the limitation of air mass sampling in space and time during TRACE-P, this budget analysis should be considered as a case study as for the flux analysis. The results of the export flux fraction obtained above suggest that of total number of molecules of NOx and SO2 emitted over the northeastern part of China, only 18 and 22% were transported out from the Asian continent into the western Pacific in air masses which we sampled during TRACE-P.

[53] Budget of NOy and SOx in the East Asia region has been estimated in various numerical model studies [e.g., Bey et al., 2001; Chin et al., 2000; Tan et al., 2002]. Usually, the total export flux from a selected domain is calculated from the difference between the total source and total sink. Bey et al. [2001] showed using the GEOS-CHEM model for the PEM-West B period (February–March 1994) that about 70–80% of NOx emitted in Asia was lost within the domain by deposition of HNO3, and the net export flux was 20–30%, although the contribution from biomass burning in the southern part of China was also included in their model calculations. Chin et al. [2000] showed using the GOCART model that the export flux fraction of anthropogenic SOx from eastern Asia was about 16%, with 2/3 of it as SO2. Nearly half of the emitted SO2 is lost by dry-deposition to the surface. Tan et al. [2002] showed using their three-dimensional (3-D) model calculations for the late winter/early spring period that about 50% of the anthropogenic SOx emitted over East Asia was removed from the continental source regions. About 30% is further removed within the neighboring ocean (region west of about 155°E), and 20% is exported out of their model domain. The vast majority of the exported SOx was in the form of sulfate aerosol. These estimates are generally consistent with those obtained in this study.

5.7. Uncertainties

[54] Uncertainties in the estimation of transport efficiency are affected by uncertainties in the emission data, biogenic sources of CO2 [Vay et al., 2003] which is not considered in this study, measurements imperfections, background relationships, the choice of the threshold value of ΔCO (30 ppbv), and loss of NOy and SOx in background air masses. Also, the two assumptions were made in our analyses: first, CO, CO2, NOy, and SOx values in background air masses have a linear relationship, and second, the ratio of enhancements due to anthropogenic emissions is always the same. It is hard to evaluate individual uncertainties in a consistent manner, however we estimated an overall uncertainty in our estimation of transport efficiency to be on the order of 50% (horizontal bars in Figures 9a and 9b).

[55] Some specific sources of uncertainty are described here. First, Palmer et al. [2003] shows using an inverse model analysis that the anthropogenic emissions of CO in China estimated by Streets et al. [2003] is likely to be underestimated by 30%. When CO emissions are increased by 30% in our analyses, the transport efficiencies and the export flux fractions (equation (4)) increase by about 30%.

[56] Second, emissions from Japan and South Korea are examined. Emission ratios of CO, CO2, and NOx in Japan and South Korea are very different from those in China (Table 2), as described in section 3.2. Consequently, when an air mass is influenced by emissions over South Korea or Japan, the ratio of increase, ΔNOy/ΔCO and ΔSOx/ΔCO, will be different from those expected from Chinese emissions. However, as shown in Figures 9a and 9b, results obtained using “continental air masses” generally agreed with those using “Chinese air masses”. As explained in section 3.1, a more strict criterion on the air mass trajectories was applied to select “Chinese air masses,” suggesting the robustness of the current estimates.

[57] Third, in situ sources of reactive nitrogen, lightning NO production and aircraft emissions, are examined. Lightning during the TRACE-P period was limited over India and Southeast Asia to latitudes lower than 30°N [Fuelberg et al., 2003], and their contributions to the budget of reactive nitrogen in the present study area are therefore considered small. Influences from aircraft emissions are also considered to be small at altitudes below 7 km [e.g., Koike et al., 2000]. Mixing of stratospheric air mass can increase ΔNOy values, however results obtained using “Chinese air masses” (originated from below the 800-hPa level) are considered to have been influenced little.

[58] Fourth, transport of NOy and SOx in coarse mode particles is examined. As described in section 2, particulate matter with diameters greater than about 1 μm was not sampled either by aerosol composition measurement (PILS) or the NOy measurement. Because of this size cut, the transport efficiencies estimated in this study are limited for gas phase and fine particles. It has been reported, however, that a significant amount of nitrate was contained in coarse mode aerosols sampled over the western Pacific [e.g., Chen et al., 1997; Kim and Park, 2001]. If nitrate in these particles originated from anthropogenic NOx emission, we have underestimated the total transport amount (gas + fine particles + coarse particles). Although coarse particles may not be very important for long-range transport due to their shorter residence time, they can play an important role in budget of reactive nitrogen. Three-dimensional model calculations for the East Asia region for March 1994 period also suggest that in the presence of mineral dust particles and sea-salt aerosols, a significant amount of HNO3 is partitioned onto these particles because carbonate in the dust particles and chloride in sea-salt aerosols are volatile and easily replaced by nitrate [Song and Carmichael, 2001]. During TRACE-P, coarse mode particles were also collected from the DC-8 [Dibb et al., 2003]. Analyses using these aerosol data indicate that air masses with elevated levels of dust particles (high nssCa2+ levels) are generally mixed with pollutants because these air masses often passed over the northeastern part of China [Jordan et al., 2003]. As a result of an efficient uptake of HNO3 by the dust particles, particulate NO3 constituted 50% of the total NO3 (gas phase HNO3 + particulate NO3) on average in these air masses [Jordan et al., 2003; Dibb et al., 2003]. Because HNO3 constitute 10–20% of NOy (Figure 10a and Table 4), we could have underestimated the total transport efficiency of NOy by 10–20%. Chemical conversion of SO2 to sulfate on mineral dust particles may also take place [Xiao et al., 1997]. Ground based measurements indicate that although most of the sulfate has been found on accumulation mode particles [Chen et al., 1997], significant amounts were found in the coarse mode during heavy dust events [Kim and Park, 2001]. Though no major dust events occurred during TRACE-P, some signatures of uptake of SO2 on dust particles were found from the DC-8 measurements [Jordan et al., 2003]. More studies are needed in order for us to better understand the budget of nitrate and sulfate over East Asia.

[59] Finally, the approach using a single background value is examined, because it has been used in various studies [e.g., Parrish et al., 1991; Stohl et al., 2002; Miyazaki et al., 2003]. When we use background values of CO and NOy of 129 ppbv and 157 pptv, which are the minimum values at 0–2 km in the study area, a transport efficiency of NOy for air masses used for the present study (ΔCO > 30 ppbv in “continental air masses”) at 0–2 km and 2–7 km is estimated to be 25–35% and 20%, respectively. This is well within the uncertainties in our estimate. Because the variability of the CO was large, the choice of a background CO level was not very critical during TRACE-P. However, if we use CO2 as a reference, an error in the estimate will be large. This is because changes in CO2 due to anthropogenic emissions were generally comparable to those caused by background air mass mixing.

6. Summary

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Measurements
  5. 3. Study Area and Emission Data
  6. 4. Approach
  7. 5. Results and Discussion
  8. 6. Summary
  9. Acknowledgments
  10. References
  11. Supporting Information

[60] The transport efficiencies of NOy and SOx (ε(NOy) and ε(SOx)) in the form of gas and fine particles (diameters smaller than 1 μm) were estimated for anthropogenic emissions from the northeastern part of China using NASA P-3B and DC-8 data obtained during the TRACE-P experiment, conducted over the western Pacific between February and April 2001. Transport efficiency was defined as the observed NOy (SOx) number density (over the background level) in one air mass divided by the expected increase in that air mass assuming that there is no loss after receiving anthropogenic emission (equations (1) and (2)). For this purpose, we introduced a new method to estimate the background levels of various species in individual air masses that have experienced multiple injection, mixing, and loss processes. In this method, a linear relationship between CO and CO2 in background air masses was used instead of using a single value of CO or CO2. When CO and CO2 are higher than this background relationship, levels of Halon 1211 and OCS, which are considered to be unique tracers of emissions from China, are also higher, suggesting the validity of our approach. We then estimated an increase of CO, NOy, and SOx in individual air masses (ΔCO, ΔNOy, and ΔSOx), and the transport efficiencies were calculated using CO as a passive tracer.

[61] We selected the study area of 30°N–42°N and 124°E–140°E because air masses sampled in this area had mostly been transported from over the northeastern part of China and had little effects from a volcano on Miyakejima Island. Air masses sampled in the study area were denoted as “continental air masses” in this study. In addition, we defined “Chinese air masses” that passed over China at altitudes below the 800-hPa level (boundary layer) within 5 days prior to measurements without passing over South Korea or Japan at these low altitudes. “Chinese air” is a subset of “continental air.” Emission inventory data showed a good correlation between emissions of CO, CO2, NOx, and SO2 over the northeastern part of China (25°N–50°N and 100°E–130°E, except for South Korea region) where “continental air masses” had generally originated. This result indicates that various emission sources are generally co-located and ensure that the use of single emission ratios in our study, ECO/ECO2, ENOx/ECO, and ESO2/ECO, is reasonable.

[62] Averages of ΔCO, ΔNOy, and ΔSOx values were calculated for air masses in which ΔCO > 30 ppbv in “continental air masses,” and the transport efficiencies were calculated by taking their ratios. Vertical profiles of the transport efficiencies in “continental air masses” generally agreed with those in “Chinese air masses,” within the uncertainties (50%). The results from the P-3B showed that 20–40% and 15% of NOx emitted over the northeastern part of China remained as NOy at 0–2 km (boundary layer) and 2–7 km (free troposphere), respectively. In the free troposphere, PAN was found to be the dominant form, while only 0.5% of emitted NOx remained as NOx. The transport efficiency of SOx was estimated to be 25–45% and 15–20% in the boundary layer and free troposphere, respectively. Median values of SO42−/SOx ratio were 0.4–0.6 both in the boundary layer and free troposphere, though large variability was found in the free troposphere. These estimates were generally consistent with those derived using DC-8 data. The results obtained in this study indicate that more than half of NOy and SOx were lost over the continent and that the vertical transport from the boundary layer to the free troposphere further reduced their amounts by a factor of 2. Because HNO3 + NO3 consisted of about 30–50% of NOy at 0–2 km, a very efficient wet removal of these two compounds during the vertical transport was suggested to be responsible for the observed reduction in NOy. Furthermore, a positive correlation between the transport efficiencies of NOy and SOx indicates that the loss processes of these two species likely proceeded simultaneously. In general, the transport efficiency has been estimated well, however relatively large uncertainties remain in the processes controlling the gas to particle conversion.

[63] The export flux fraction was also estimated for air masses which we sampled during TRACE-P. The export flux fraction was defined as the flux of observed NOy (SOx) molecules in each altitude range divided by the expected total flux (surface to the tropopause) of NOy (SOx) molecules assuming that there is no loss after receiving anthropogenic emission (equation (4)). As a result, export flux fractions of NOy at 0–2 km and 2–7 km were estimated to be 8% and 10%, respectively. The export flux fractions of SOx in these two altitude ranges were estimated to be 10% and 12%, respectively. Flux in the boundary layer and free troposphere was found to have a similar contribution. This result is in contrast with that of CO in which flux in the free troposphere was greater. The difference was likely due to the wet removal of NOy and SOx during the vertical transport into the free troposphere.

[64] Budgets of NOy and SOx are estimated simply using the export flux fraction for air masses sampled during TRACE-P, although the measurements were quite limited in space and time. The estimates of the export flux fraction suggest that the total flux of NOy and SOx from the continent in these cases was 20% of the total emissions over the northeastern part of China.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Measurements
  5. 3. Study Area and Emission Data
  6. 4. Approach
  7. 5. Results and Discussion
  8. 6. Summary
  9. Acknowledgments
  10. References
  11. Supporting Information

[65] We are indebted to all of the TRACE-P participants for their cooperation and support. Special thanks are due to the flight and ground crews of the NASA P3-B and DC-8 aircraft for helping make this effort a success. We thank N. Toriyama, M. Kanada, and H. Jindo at Solar-Terrestrial Environment Laboratory, Nagoya University, Japan, for their technical assistance with the measurements of NO, NO2, and NOy. We also thank B. Liley at National Institute of Water and Atmospheric Research Ltd. (NIWA), New Zealand for calculating the cutoff size for NOy measurements by numerical simulations. The meteorological data were supplied by the European Center for Medium-Range Weather Forecasts (ECMWF). This work was supported in part by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT). The trajectory calculation program used in this paper was developed by Y. Tomikawa and K. Sato at National Institute of Polar Research, Japan.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Measurements
  5. 3. Study Area and Emission Data
  6. 4. Approach
  7. 5. Results and Discussion
  8. 6. Summary
  9. Acknowledgments
  10. References
  11. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Measurements
  5. 3. Study Area and Emission Data
  6. 4. Approach
  7. 5. Results and Discussion
  8. 6. Summary
  9. Acknowledgments
  10. References
  11. Supporting Information

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