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 During the field campaign of the Study of Houston Atmospheric Radical Precursors/Surface-Induced Oxidation of Organics in the Troposphere (SHARP/SOOT) in Houston, Texas, a suite of aerosol instruments was deployed to directly measure a comprehensive set of aerosol properties, including the particle size distribution, effective density, hygroscopicity, and light extinction and scattering coefficients. Those aerosol properties are employed to quantify the mixing state and composition of ambient particles and to gain a better understanding of the formation and transformation of fine particulate matter in this region. During the measurement period, aerosols are often internally mixed, with one peak in the effective density distribution at 1.55 ± 0.07 g cm−3, consistent with a population composed largely of sulfates and organics. Episodically, a second mode below 1.0 g cm−3 is identified in the effective density distributions, reflecting the presence of freshly emitted black carbon (BC) particles. The measured effective density demonstrates a clear diurnal cycle associated with primary emissions from transportation and photochemical aging, with a minimum during the morning rush hour, increasing from 1.4 to 1.5 g cm−3 on average over 5 h, and remaining nearly constant throughout the afternoon. The average BC concentration derived from light-absorption measurements is 0.31 ± 0.22 µg m−3, and the average measured particle single scattering albedo is 0.94 ± 0.04. When elevated BC concentrations are observed, typically during the morning rush hours, single scattering albedo decreases, with a smallest measured value of about 0.7. Aerosol hygroscopicity measurements indicate that larger particles (e.g., 400 nm) are more hygroscopic than smaller particles (e.g., 100 nm). The measurements also reveal discernable meteorological impacts on the aerosol properties. After a frontal passage, the average particle effective density decreases, the average BC concentration increases, and the aerosol size distribution is dominated by new particle formation.
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 Houston is the fourth largest city in the United States in one of the most rapidly expanding regions in the country [U.S. Census Bureau, Population Division, 2012], which has a history of exceeding the Environmental Protection Agency's (EPA) National Ambient Air Quality Standards (NAAQS) [Lei et al., 2004; Zhang et al., 2004a]. Between 2000 and 2005, this region exceeded the 8 h O3 standard of 75 parts per billion (ppb) on 219 occasions [Cowling et al., 2007]. Several atmospheric field campaigns, including the Texas Air Quality Study (TexAQS) I (2000) and II (2005–2006), the Gulf of Mexico Atmospheric Composition and Climate Study in 2006, and the TexAQS-II Radical and Aerosol Measurement Project TexAQS-II Radical and Aerosol Measurement Project (TRAMP), have been conducted to assess emissions of volatile organic compounds (VOCs) and nitrogen oxides and ozone formation in southeast Texas [Daum et al., 2003; Olaguer et al., 2009; Parrish et al., 2009; Lefer et al., 2010]; photochemical oxidation of VOCs in the presence of NOx contributes to ozone formation [Nesbitt et al., 2000; Bond et al., 2002; Daum et al., 2003]. The overarching goals of the previous field campaigns have been focused to identify cost-effective solutions to the ozone problem for this region. For example, considerable attention has been placed on the rapid formation and transport of ozone [Berkowitz et al., 2004; Jiang and Fast, 2004; Parrish et al., 2009], emissions of VOCs and NOx [Mellqvist et al., 2007], secondary organic aerosols (SOA), and PM2.5 [Russell et al., 2004; Fan et al., 2005, 2006; Massoli et al., 2009]. In particular, rapid ozone formation (> 40 ppb h−1) associated with emissions of highly reactive olefins, such as ethene and propene from refineries and chemical plants, has been identified in the Houston region [Daum et al., 2003; Ryerson et al., 2003; Wert et al., 2003]. Furthermore, during high ozone days, the concentrations of organic aerosols have been frequently observed to exceed 12 µg m−3, which is the current NAAQS annual PM2.5 standard [Dechapanya et al., 2003; Bahreini et al., 2009]. The oxidation products of VOCs emitted from biogenic and anthropogenic sources contribute importantly to secondary organic aerosol formation [Lei et al., 2000a, 2000b; McGivern et al., 2000; Hallquist et al., 2009]. In 2009, the Study of Houston Atmospheric Radical Precursors (SHARP) campaign was conducted to further investigate significant undercounted primary and secondary sources of the radical precursors, including formaldehyde (CH2O) and nitrous acid (HONO), in both heavily industrialized and more typical urban areas of Houston (E. P. Olaguer et al., Overview of the SHARP campaign: Motivation, design, and major outcomes, submitted to Journal of Geophysical Research, 2013).
 In addition to their impacts on air quality and visibility, aerosols exert profound impacts on atmospheric chemical processes [Zhang et al., 1996a; Kroll and Seinfeld, 2008; Hallquist et al., 2009], cloud formation, and regional and global climate [Haywood and Boucher, 2000; Ramanathan et al., 2001; Forster et al., 2007]. For example, aerosols influence the climate directly by reflecting solar radiation [Forster et al., 2007] and indirectly by modifying cloud formation [Fan et al., 2007, 2008], modulating photochemistry [Li et al., 2005; Flynn et al., 2010], and promoting multiphase chemistry [Zhang et al., 1995, 1996b; Zhao et al., 2005; Osthoff et al., 2008; Ziemba et al., 2010; Qiu et al., 2011, 2012]. The overall impact of aerosols is dependent on the size, concentration, and chemical composition of particles. Elevated concentrations of atmospheric aerosols also have an adverse effect on human health [Mokdad et al., 2004; Dominici et al., 2006]. Research on aerosols remains an active field since aerosols correspond to the greatest source of uncertainty in projecting climate changes [Forster et al., 2007; Zhang, 2010].
 Aerosols in the atmosphere are broadly categorized as primary and secondary particles according to the origins of their formation. One important primary aerosol type is black carbon (BC), produced from incomplete combustion of fossil fuel and biomass burning [Zhang and Zhang, 2005; Khalizov et al., 2012], while sulfate, ammonia, nitrate, and organics represent the key compounds responsible for formation and growth of secondary particles [Zhang et al., 2004b; Zhang et al., 2012]. Light-absorbing particles such as BC [Horvath, 1993; Bond and Bergstrom, 2006] and brown carbon [Bond, 2001; Andreae and Gelencsér, 2006; Moosmüller et al., 2009] have been identified as a major contributor to global warming [Jacobson, 2001; Bond et al., 2013]. One particular challenge in evaluating the aerosol direct radiative forcing occurs when nonabsorbing aerosols, such as sulfates and organics, and light-absorbing aerosols, such as BC, coexist in the same air mass [Ramanathan and Carmichael, 2008; Ramana et al., 2010]. Recent laboratory experiments [Khalizov et al., 2009a; Xue et al., 2009a, 2009b; Qiu et al., 2012; Khalizov et al., 2013], field observations [Knox et al., 2009; Moffet and Prather, 2009], and modeling calculations [Jacobson, 2000, 2001] show that, when mixed with other nonabsorbing aerosol constituents, such as sulfate and organics, BC is more absorptive and exerts a higher positive direct radiative forcing. Currently, there is a large uncertainty regarding the magnitude of absorption enhancement due to BC coating [Zhang et al., 2008; Subramanian et al., 2010; Cappa et al., 2012]. Also, information on the concentration and the mixing state of BC aerosols in different geographical regions is limited.
 In this paper, we report field measurements of submicron ambient aerosols during late springtime in Houston, Texas. As a subproject under the 2009 SHARP field campaign, the Surface-Induced Oxidation of Organics in the Troposphere (SOOT) led by Texas A&M University was conducted to measure ambient gaseous nitrous acid and aerosol properties and to investigate HONO formation and BC aging using an environmental chamber, with the objective to assess the role of heterogeneous chemistry in the radical budget, VOC oxidation, and O3 formation (Olaguer et al., submitted manuscript, 2013). We emphasize in this work the formation and transformation of submicron aerosols in this region on the basis of a comprehensive set of simultaneously measured ambient aerosol properties.
2 Methodology and Instrumentation
2.1 SHARP/SOOT Campaign
 The SHARP Campaign extended from 15 April through 31 May 2009 (Olaguer et al., submitted manuscript, 2013). The SOOT project consisted of two components, i.e., (1) ambient measurements of gaseous HONO and aerosols and (2) environmental chamber studies of HONO formation and soot aging using experimentally generated BC particles. Our instruments were stationed on the University of Houston (UH) northern Moody Tower (70 m above the ground level), which is an 18-story building located 5 km south of downtown Houston, 5 km southwest of the Ship Channel, and within 8 km of three major highways. This location, shown in Figure 1, allows measurements of a representative sample of inner-city Houston air without the interference of extremely localized vehicular emissions. During the period from 30 April to 7 May, the SOOT project focused on ambient measurements. From 8 to 31 May, a series of environmental chamber studies were conducted. As a result, measurements of ambient particle sizes, effective density, and hygroscopicity were sporadic beyond the week period of 30 April and 7 May.
 The data presented in this work span between 30 April and 7 May 2009, representing the normal springtime synoptic conditions in this region. Figure 1 illustrates the major wind circulation patterns during this period, i.e., the frequent southerly wind from the Gulf of Mexico and the northerly wind after the passage of a cold front.
 A suite of aerosol instruments inside an air-conditioned trailer was deployed to measure particle number size distributions, effective densities, hygroscopic growth factors, and light extinction and scattering. Atmospheric aerosols were sampled through a 6 m long thermally insulated 3/8 inch outer diameter copper tube at 10 liters per minute (lpm) flow rate. Multitube Nafion driers (PD-070-18T-24SS, Perma Pure, Inc.) were used to reduce the relative humidity of the aerosol flow below 10% and to convert particles to an anhydrous state. This procedure was necessary to retrieve the dry-particle diameter and effective density and to establish a closure with the optical and chemical composition measurements. Also, removal of the aerosol water content prevented the variations in the size distributions and optical properties of hydrated particles due to changes in ambient relative humidity [Massoli et al., 2009; Taylor et al., 2011]. Two driers connected in series were used with a 6.5 lpm flow sampled by the aerosol optical instruments. Two additional driers were used with 1.0 and 2.5 lpm flows sampled by the size/density and hygroscopicity instruments, respectively. Evaporative losses of semivolatile species were minimized by placing the driers inside the air-conditioned trailer. Diffusion-controlled particle losses in the driers were taken into account by an inversion algorithm when calculating the particle size distributions. Losses of optically important particles were estimated to be less than 1%.
 Particle size distributions and effective density distributions were measured by a system consisting of a differential mobility analyzer (DMA, model 3081, TSI, Inc.), an aerosol particle mass analyzer (APM, model 3600, Kanomax Inc., Japan), and a condensation particle counter (CPC, model 3760A, TSI, Inc.) [Khalizov et al., 2009b; Pagels et al., 2009; Qiu et al., 2012]. The DMA operated in a recirculating flow configuration, with a sheath flow of 6.5 lpm and a sample flow of 1 lpm, corresponding to a particle mobility diameter range of about 10 to 400 nm. A relatively low sheath-to-sample flow ratio was used to achieve a better detection of small particle concentrations and a shorter residence time in the DMA-APM measurements. The DMA transfer function was broadened but remained symmetrical. The application of the DMA-APM technique in the mass-mobility measurements has been previously described [McMurry et al., 2002; Pagels et al., 2009]. The APM consisted of two cylindrical electrodes that rotated at the same angular speed [Tajima et al., 2011]. Charged particles were introduced axially into the annular space between the electrodes and rotated at the same speed as the electrodes, when particles traveled through the APM. The mass of a particle that passed through the APM was determined by the rotational speed of the cylindrical electrodes and the voltage applied to the inner electrode. The aerosol mass distribution was determined by stepping the voltage at a fixed rotation speed and measuring the concentration of particles passing through APM by CPC. There were 30 points observed between 0.10 and 2.60 g cm−3 for the five distinct particle mobility diameters (46, 81, 151, 240, and 350 nm) approximately every hour. Although the APM allows for direct determination of particle effective density (ρeff) based on the particle mass, substantially higher accuracy (better than 5%) can be achieved by measuring ρeff relative to the polystyrene latex (PSL) particles according to
where VAPM and VAPM, PSL are the peak APM voltages corresponding to the masses of ambient and PSL particles of identical initial mobility diameter, and ρPSL = 1.054 g cm−3 is the material density of polystyrene latex.
 To determine the size-resolved hygroscopic growth factor (HGF), a hygroscopic tandem-DMA (H-TDMA) was employed. Monodisperse aerosols produced by the first DMA were exposed to elevated humidity in a multitube Nafion drier/humidifier. The changes in the particle mobility diameters were measured by the second DMA [Gasparini et al., 2004; Khalizov et al., 2009b]. The growth factor corresponds to the ratio of the particle sizes measured by the two DMAs,
where Dp and Do are the processed and initially unprocessed particle diameters, respectively.
 The optical properties of ambient aerosols were quantified from direct measurements of light extinction and scattering. Aerosol extinction coefficients at 532 nm, bext, were measured by a cavity ring-down spectrometer (CRDS) [Smith and Atkinson, 2001; Khalizov et al., 2009a; Radney et al., 2009]. The CRDS cavity consisted of two high-reflectivity dielectric mirrors (99.9985% reflectivity, 6 m radius of curvature, Los Gatos Research, Inc., CA) and a stainless steel cell with an aerosol inlet in the center and two outlets near the ends. To prevent contamination, the mirror region was purged with a small flow of filtered dry air. A light pulse (λ = 532 nm) from a Q-switched laser (QG-532-500, CrystaLaser, CA) was injected into the cavity through the front mirror, the light leaking through the rear mirror was detected with a photomultiplier (H6780-02, Hamamatsu), and the signal was digitized by a 100 MHz, 12-bit resolution CompuScope 12100 card (GaGe Applied Technologies, LLC) operated by the LabVIEW software. During measurements, 3000 ring-down traces were averaged at a 50 Hz repetition rate, and the decay time was calculated by nonlinear fitting of the averaged decay data. The detection sensitivity of this instrument was better than 0.5 Mm−1.
 The scattering coefficients at 532 nm, bsca, were derived from measurements by a commercial three-wavelength integrating nephelometer (3563, TSI, Inc.). In the nephelometer, the aerosol flow passed through the measurement volume, where particles were illuminated over an angle of 7° to 170° by a halogen bulb light source. The sample volume was viewed by three photomultiplier tubes (PMTs) through a series of apertures along the axis of the instrument. Light scattered by particles was split into three wavelengths (450, 550, and 700 nm) using color filters in front of the PMT detectors. Following the calibration/correction and measurement protocols suggested by Anderson and Ogren , the detection limit of this instrument was better than 0.2 Mm−1. In order to obtain a closure with the measured extinction coefficient, the scattering coefficient at 532 nm was calculated from a power law fit to the scattering coefficients at the three nephelometer wavelengths [Khalizov et al., 2009a]. During field measurements, the CRDS and nephelometer were automatically zeroed every hour using filtered ambient air.
 Single scattering albedo (SSA) of aerosols is represented as the ratio of scattering (bsca) to extinction (bext) coefficients
 During the observation period, less than 7.5% of the measured SSA values exceeded unity. The absorption coefficient, babs, was calculated from the difference between the extinction and scattering coefficients. This procedure has been previously utilized to obtain aerosol absorption in laboratory experiments and field observations [e.g., Moosmüller et al., 2009, and references therein]. To minimize systematic errors due to temporal variations in the aerosol concentration, extinction and scattering measurements in the optical system were performed nearly simultaneously. The system was calibrated with nonabsorbing ammonium sulfate aerosols, for which the extinction coefficient equals the scattering coefficient. The detection limit for absorption using the difference method was within 0.6 Mm−1, on the basis of laboratory calibrations [Khalizov et al., 2009a; Xue et al., 2009b; Khalizov et al., 2013]. During the field measurements, less than 7% of the derived absorption coefficients were below zero.
 A Sunset organic carbon and elemental carbon aerosol analyzer (Sunset Laboratories, Inc.) was used to determine the mass concentrations of organic carbon (OC) and elemental carbon (EC in the form of optical and thermal EC) according to the modified NIOSH (National Institute of Occupational Safety and Health) protocol [Bauer et al., 2009]. Negative optical EC values may be due to a bias from the loss of native EC at high temperatures, which is less than −0.01 µg m−3 and does not influence the BC concentration. To obtain a closure with the Sunset EC measurements, the mass concentration of BC was also calculated from light-absorption coefficients derived by the difference method, assuming uniform values of the mass absorption cross section (MAC) of 7.5 and 11 m2 g−1 for fresh and aged BC, respectively [Bond and Bergstrom, 2006],
 The absolute error in BC mass concentration due to uncertainties in MAC and babs was estimated to be within 0.21 µg m−1. In addition, MAC was also calculated on the basis of the measured absorption coefficient and Sunset optical and thermal EC concentrations using equation (4).
 Aerosol measurements by DMA-APM, H-TDMA, and the optical instruments were performed simultaneously. Each DMA-APM measurement cycle involved the acquisition of a single size distribution followed by effective density measurements for five different particle diameters; a full cycle took about 1 h to complete. Similarly, H-TDMA measurements involved acquiring a size distribution followed by six size-resolved hygroscopicity scans, each cycle occurring in about 20 min. Optical measurements were carried out continuously with a temporal resolution of 1 min. A schematic of the instrumentation arrangements is provided in Figure 2.
 In addition to the size-average effective density determined by the DMA-APM, the bulk particle density was calculated from the submicron aerosol composition data collected by an aerosol chemical speciation monitor (ACSM, Aerodyne Research, Inc.). The ACSM measured the aerosol total mass loading and chemical composition including ammonium, sulfate, organics, nitrate, and chloride in 30 min cycles. The ACSM overall density ρ was derived from the mass fractions fi and material densities ρi of these components, assuming that ammonium sulfate, ammonium bisulfate, ammonium nitrate, and organics represent the only particle constituents,
where the summation over fi equals 1, and the densities of the ammonium sulfate, ammonium bisulfate, and ammonium nitrate are 1.76, 1.78, and 1.73 g cm−3, respectively. The effective density of organic matter may be dependent on the origin of the air masses, since the UH site is impacted by variable aerosol emission sources. Sensitivity calculations over periods with well-defined air mass origins indicated that the best fit effective densities for marine and land originating organics were 1.34 and 1.45 g cm−3, respectively. These values are within the range of the organic aerosol densities reported previously [Dinar et al., 2006; Malloy et al., 2009]. For simplicity, an average value of 1.40 g cm−3 was adopted in our present analysis.
 Trace gases were sampled through perfluoropolymer tubing. The VOC mixing ratios were obtained with a proton transfer reaction mass spectrometer (PTR-MS, Ionicon Analytik, Austria). SO2, CO, NOx, and O3 mixing ratios were measured with the modified Thermo 43, 48, 42, and 49, respectively. A detailed description of the measurements of trace gases has been provided in this special issue (Olaguer et al., submitted manuscript, 2013).
3 Results and Discussion
3.1 Meteorological Conditions
 Air quality in the Houston region is also dependent on the local meteorological conditions [Lei et al., 2004]. The meteorological parameters from 30 April through 7 May 2009 are presented in Figure 3. This region is typically humid because of its proximity to the Gulf of Mexico and the frequent influx of marine air. There is a frontal passage at 17 h (inferred from the change in the wind direction) on 3 May 2009, which causes precipitation and brings in northerly winds and colder air. The wind is 88% southerly flow (ESE-SW, referred as to the southerly flow condition) and 12% northerly flow (WNW-ENE, referred as to the northerly flow condition) during 30 April and 7 May, with an average wind speed of 5 m s−1 (Figure 3a). The average temperature is 25°C (77°F), and the average relative humidity is about 75% during this period (Figure 3b). A northerly flow is characteristic of a relatively polluted continental air mass, while a southerly flow is associated with a relatively clean marine air mass (J. P. Pinto et al., Intercomparison of field measurements of nitrous acid (HONO) during the SHARP Campaign, submitted to Journal of Geophysical Research, 2013). There is little occurrence of wind direction reversal in the late afternoon and nighttime hours during this period (Figure 1), which otherwise circulates aged pollutant plumes characterized by elevated O3 and NOx back to the Houston region (Pinto et al., submitted manuscript, 2013). Figure 3 shows a period with a wind speed of less than 5 m s−1 after the frontal passage, indicating the stagnant conditions that are favorable for pollutant accumulation [Lei et al., 2004].
3.2 Trace Gases
 Ozone, VOCs (i.e., isoprene, toluene, and benzene), CO, NOx, and SO2 measured between 30 April and 7 May are depicted in Figure 4. There are overall lower concentrations of gaseous pollutants during the southerly flow condition than those after the frontal passage. This is particularly noticeable in the O3 concentration, which exceeds the EPA 8 h O3 standard of 0.075 ppm on 4 May but is well below the standard on the southerly wind days with an average ozone concentration of 25.82 ± 3.80 ppb. On 4 May, the concentrations of SO2, toluene, and benzene rise noticeably after the frontal passage. The isoprene concentration is relatively independent of the synoptic pattern, but there exists a strong diurnal variation, indicating the biogenic production [Li et al., 2007]. The concentration of NOx is comparable to measurements by other groups during the same period [Wong et al., 2012] and during previous field campaigns in Houston [Stutz et al., 2010]. A peak NOx concentration of about 25 ppb occurs on 3 May during the transitory period of the two distinct wind patterns, likely attributable to local emissions. It is evident from Figure 4 that the increases in CO and NOx occur during the morning and evening rush hours throughout the measurement period, along with concurrent decreases in O3 by NO titration, which are indicative of traffic emissions.
3.3 Aerosol Size Distributions
 Figure 5 shows the aerosol size distributions observed between 30 April and 7 May. The normalized plot is obtained by dividing each observation (consisting of approximately 150 points between 20 and 400 nm) by the maximum concentration in each observation (Figure 5b). The normalized plot illustrates the most prevalent diameter, displaying the evolution of the peak particle mobility diameter during a day. For example, the morning rush hour emissions are distinguished by an increase in the concentrations of 50–100 nm aerosols [Ban-Weiss et al., 2010]. The aerosol growth is also discernable, because the highest concentration of particles is located between 50 and 100 nm in the morning and then between 150 and 250 nm in the afternoon. During a typical day, high concentrations of small particles (50–100 nm) are observed near sunrise and sunset, likely due to the morning and evening traffic rushes and consistent with the correlated increases in CO and NOx emitted by traffic and O3 decreases by NO titration in the trace gas measurements (Figure 4). The greatest concentration of larger particles (> 150 nm) occurs in the afternoon, likely reflecting the growth of small particles emitted or freshly formed in the morning.
 On several occasions, a high concentration of less than 30 nm particles is observed, which may be explained by new particle formation [Zhang et al., 2012]. Although our instrumentation does not observe particles below 10 nm, the observed changes in the aerosol properties and concentrations suggest the occurrence of new particle formation. New particle formation is particularly evident by the highest particle number concentrations and growth from 10 nm and above after the frontal passage (Figure 5): 10–50 nm size particles account for 53% of the total aerosol concentration. New particle formation is likely explainable by elevated gaseous sulfur and organic compound levels (Figure 4), which enhance the nucleation and growth rates of nanoparticles [Zhang et al., 2009]. In particular, sulfur dioxide can be oxidized to sulfuric acid to enhance new particle formation [Wooldridge et al., 1995; McMurry et al., 2005; Zhao et al., 2009]. Also, a lower preexisting aerosol concentration may enhance the condensation of atmospheric gases onto nanoparticles (< 3 nm) and reduce scavenging losses of freshly nucleated particles [Wang et al., 2010a, 2010b]. On 4 May, the particle size distribution clearly exhibits the “banana” growth curve, characteristic of new particle formation [Yue et al., 2011]. New particle formation on this day occurs at 9–10 h, when the concentration of 20–40 nm particles reaches 4 × 104 cm−3. The particle surface area is about 5 µm2 cm−3, during this episode of new particle formation, indicating a lower condensation sink for condensable gases and freshly nucleated particles. The SO2 concentration is about 0.2–0.3 ppb between 9 and 10 h and increases to over 3 ppb in the late afternoon. Also, as shown in the normalized plot of Figure 5b, throughout the morning and afternoon of 4 May, newly nucleated particles grow to 75 nm in diameter, and the growth rate of the fine particles is about 10 nm h−1. On 4 May, the temperature, relative humidity, and wind speed are noticeably low in the morning hours (Figure 3), in the range of 17–23°C, 55–75%, and 4–5 m s−1 during 8–12 h, respectively. Hence, during this northerly flow period, the environmental conditions are favorable for new particle formation, since the temperature, wind speed, and relative humidity are low, the existing aerosol surface area is low, but the anthropogenic sulfate and organic gases (i.e., SO2 and toluene) are elevated. New particle formation also occurs during the frontal passage in the afternoon hours on 3 May.
 Figure 6 shows two averaged aerosol size distributions, calculated by averaging all the 18 aerosol size distributions corresponding to southerly and northerly wind conditions. For the southerly flow condition, there exist two distinct peaks: the peak occurring between 100 and 200 nm has an average concentration of 10,000 particles cm−3, and the secondary maximum near 30 nm has an average concentration of 5,000 particles cm−3, likely representing primary aerosols emitted directly with subsequent growth and new particle formation, respectively. After the frontal passage, the total concentration approximately quadruples. While aerosols between 175 and 400 nm are comparable in concentration to that of the southerly flow, particles of less than 100 nm in diameter are significantly more prevalent after the frontal passage. There is one major peak in this distribution occurring near 40 nm with a concentration of nearly 40,000 particles cm−3, suggesting that new particle formation represents the dominant process.
 Figure 7 depicts the averaged diurnal cycle of the aerosol diameter and number concentration between 30 April and 7 May. The averaged particle size increases during the morning hours, decreases during the afternoon hours, and remains nearly constant during the night. The highest particle concentration occurs prior to sunrise, decreases during the daytime, and increases after 16 h, likely to be jointly attributable to emissions and the variation of the planetary boundary layer height that regulates the vertical mixing.
3.4 Particle Effective Density
 Atmospheric aerosols can be mixed externally or internally. Since individual particle constituents have distinct effective densities, the chemical makeup of particles can be assessed on the basis of the measured particle density distributions. Although the material density of EC, the major component of BC, is relatively high (1.7 to 1.9 g cm−3) [Bond and Bergstrom, 2006], the effective density of freshly emitted BC particles can be as low as 0.1–0.6 g cm−3 because of their open, fractal morphology [Zhang et al., 2008; Pagels et al., 2009]. The observations of lower density peaks with the values below unity provide an indicator of fresh or partially aged BC. Figure 8 demonstrates the effective density distributions of 151 nm particles measured on two different days (i.e., 2 and 4 May). On 2 May, the density distribution is dominated by a single peak, likely signifying an internal mixture. Several features are noticeable in the shown peak density distribution; the effective density distribution peak ranges from 1.3 to 2.1 g cm3 with a maximum peak value of near 1.6 g cm3, which may indicate the presence of sea salt (material density of 2.17 g cm3), ammonium sulfate (material density of 1.76 g cm3), ammonium sulfate internally mixed with organic materials (~1.65 g cm3), or mostly organics (~1.5 g cm3) [Kostenidou et al., 2007]. On 4 May, the density distribution is bimodal, likely indicating an external mixture. There exists a prominent low-density peak at 0.55 g cm−3, which corresponds to the presence of freshly emitted or partially aged BC. The high-density peak at 1.4 g cm−3 reveals more organics but less ammonium sulfate.
 Figure 9 presents the temporal trends of the effective density distributions for five particle sizes, i.e., 46, 81, 151, 240, and 350 nm. The temporal resolution of our measurements is sufficient to capture the diurnal density changes and is more detailed than previously published [McMurry et al., 2002; Geller et al., 2006; Malloy et al., 2009]. While the overall trends in the effective densities of the five particle sizes are similar, some variances between different particle sizes are apparent. For all particle sizes, the lowest effective densities occur in the early morning, and the largest effective densities are observed in the afternoon. The magnitude of the effective density changes varies between the particle sizes, showing a decreasing trend for larger particles.
 Figure 10 shows the temporal trend of the measured effective density peak maximum of the five particle sizes. The effective density distributions exhibit a single peak between 1.40 and 1.70 g cm−3, consistent with particles dominated by ammonium sulfate (1.77 g cm−3) internally mixed with a variable fraction of organic materials (1.2–1.6 g cm−3) [Dinar et al., 2006; Malloy et al., 2009]. In some of the observations, the effective densities show a second, lower density constituent that is attributable to the presence of fresh or partially aged BC. Particles of 151 nm diameter have the largest percentage of two effective density components (32%) followed by 240 nm particles (7%), indicating fresh BC mostly in the 150–240 nm size range. This observation is in agreement with the previously reported ranges for the effective densities and size distributions of soot emitted from diesel engines [Park et al., 2003; Burtscher, 2005; Ban-Weiss et al., 2010]. For the other three sizes (i.e., 46, 81, and 350 nm), the frequency of observation of bimodal effective density distributions is less than 3%, indicating that smaller (i.e., 46 and 81 nm) and larger particles (350 nm) are less likely to contain freshly emitted BC. It should be noted that the absence of lower density particles does not necessarily imply the absence of BC but merely indicates that BC particles are heavily aged and internally mixed. Few effective density distributions exhibit the peaks between 1.70–1.80 and 1.00–1.40 g cm−3, which correspond to externally mixed sulfate and organics, respectively. Turpin and Lim  suggested that SOA from the oxidation of aromatics and alkanes emitted in the Houston Ship Channel has a density between 1.20 and 1.40 g cm−3, whereas Kostenidou et al.  found the SOA density to be in the range 1.40–1.65 g cm−3.
 Figure 11 depicts the diurnal cycle of the averaged effective density of the five particle sizes between 30 April and 7 May. For all particle sizes, the effective densities vary between 1.45 and 1.65 g cm−3. The effective densities increase from the morning to the afternoon (i.e., 7 to 17 h), likely due to the increase in the particle-phase sulfate and oxidized organic components. There are two minima in the averaged effective densities at 7 and 19–20 h. Increased BC emissions from transportation are likely responsible for the two minima, since the times correspond closely to the morning and evening rush hours and are consistent with those trends in the trace gases (i.e., elevated NOx and CO and decreased O3 shown in Figure 4). The effective densities remain nearly constant throughout the nighttime. Figure 11 also reveals that particles of 46 nm (350 nm) exhibit the largest (smallest) diurnal variability.
 The size-averaged effective density from our APM measurements is compared with the particle density derived from the ACSM in Figure 12. Both the APM and ACSM density data show a similar temporal and diurnal trends. The average effective density is higher during the southerly flow (1.58 ± 0.04 g cm−3) than after the frontal passage (1.47 ± 0.09 g cm−3). The elevated BC level observed after the frontal passage (Figure 9) may be responsible for a reduced particle effective density on 4 May (Figure 12a). After the frontal passage, the effective density range is reduced and the diurnal cycle is less apparent. Figure 12b shows that between 30 April and 7 May the averaged effective density derived from APM and ACSM are 1.55 ± 0.07 and 1.56 ± 0.08 g cm−3, respectively.
 When particle composition is dominated by organics and ammonium sulfate, the measured particle density can be used to infer the mass fractions of the two components. As shown in the ACSM measured chemical compositions (Figure 12c), the two-component assumption holds true for most times in the Houston area, since chloride (1.21% ± 1.08) and nitrate (4.25% ± 2.06) represents minor constituents in submicron aerosols [Russell et al., 2004; Bates et al., 2008]. Figures 12c and 12d compare the aerosol mass fractions derived from the APM effective density using equation (5) and from the ACSM measurements. During the southerly flow, the dominance in the aerosol mass fraction oscillates between sulfate and organics. The peaks of organics occur in the midafternoon, indicating a photochemically driven aerosol formation mechanism. Bates et al.  suggested that sulfate constitutes 51%–61% of the particle mass with the land originating air masses and 20% when influenced by marine air in the Houston area. The mass fractions are 64 ± 16% for sulfate and 36 ± 16% for organics after the frontal passage and 46 ± 11 for sulfate and 54 ± 11 for organics under southerly winds. The high sulfate mass fraction during and after the frontal passage can be explained by the low wind speed, which is favorable for stagnation. Overall, there is a good agreement between the APM and ACSM time-averaged mass fractions for both organics (50 ± 13% and 54 ± 19%, respectively) and sulfate (49 ± 13% and 46 ± 19%, respectively). On 4 May, the derived mass fraction exceeds unity because the two-component assumption becomes invalid. This may be explainable by the presence of high concentrations of freshly emitted BC shown in Figure 9 (also to be discussed later).
 From the measured size-resolved effective density and number size distributions, the mass concentration of particles between 10 and 400 nm (PM0.4) is calculated
 Figure 13 depicts the calculated PM0.4 along with the BC mass concentration and fraction derived from the optical measurements (to be discussed below). The average PM0.4 is 9.4 ± 5.1 µg m−3; the lowest daily average of 4.1 µg m−3 occurs on 30 April, and the highest daily average value of 13.3 µg m−3 is observed on 3 May. Note that although the particle number concentration maximizes after the frontal passage on 4 May (Figure 5), the total PM0.4 is relatively low, since the nucleation mode particles contribute negligibly to the total particle mass. Furthermore, a lower concentration of preexisting particles on 4 May favors new particle formation [Zhang et al., 2012]. Our calculation is in agreement with previous submicron mass concentrations measurements of 6.5 to 20.8 µg m−3 in this region [Bates et al., 2008].
3.5 Aerosol Optical Properties
 The temporal trends of the aerosol extinction, scattering, absorption, and single scattering albedo are illustrated in Figure 14. The average extinction and scattering coefficients are 49.7 ± 23.9 and 46.9 ± 22.7 Mm−1, indicating that scattering dominates the attenuation process. The scattering coefficient is also calculated using the Mie theory on the basis of the measured particle size distributions and components, and the predicted values are in agreement with the measurements (Figure 14a). Since SSA is closely correlated with the BC concentration, SSA exhibits a lesser day-to-day variation than the scattering coefficient, because of little variation in daily vehicle emissions. After the frontal passage on 4 May, the SSA variation is significantly enhanced, ranging from 0.70 to 1.00, and the average SSA decreases to 0.89 ± 0.06. The lowest SSA is observed to be around 0.7 on 4 May. The temporal trend of the Ångstrom exponent derived from wavelength dependence of the scattering coefficient and the aerosol surface area is also illustrated in Figure 14b. The high values of the Ångstrom exponent indicate that there is little presence of dust during the measurements.
 The diurnal averaged optical coefficients are presented in Figure 15a. The scattering, absorption, and extinction coefficients all increase during the daytime, attributable to the greater concentration of larger particles during the daytime (Figure 5). The average SSA is 0.94 ± 0.04 and exhibits a clear diurnal cycle (Figure 15b). SSA is the lowest in the morning hours (between 6 and 7 h), typically with a value between 0.8 and 0.9 and remains above 0.9 during the rest of the day. Occasionally, it is evident that there exists a secondary decreased SSA near 21 h, likely due to an increase of BC emissions from the evening rush hours.
 The measured hygroscopic growth factors range from 0.9 to 2.0 for six different particle sizes (i.e., 15, 20, 50, 100, 200, and 400 nm). Similar to the effective density measurements shown in Figure 9, the weighted average for each hygroscopic distribution is calculated. Three representative particle sizes (20, 100, and 400 nm) of the six observed particle sizes are shown in Figure 16a, along with the average hygroscopicity of all six particle sizes. Figure 16b displays the frequency of the hygroscopic growth factor by 0.10 intervals for the three selected particle sizes. Figure 17 shows the hygroscopic growth factor distributions averaged over the northerly and southerly winds. Since the H-TDMA system operates in multiple configurations during the field campaign, the ambient hygroscopicity measurements are sporadic to resolve an appropriate temporal trend.
 A hygroscopic growth factor of unity indicates no change in the particle size after exposure to 90% relative humidity (RH), a value less than 1 indicates a decrease in mobility particle size, and a value greater than 1 indicates an increase in size. Pure ammonium sulfate has a hygroscopic growth factor of 1.70 at 90% RH [Wise et al., 2003]. Organics exhibit lower growth factors, ranging from 1.08 to 1.17 at 90% RH, although there is still a degree of uncertainty [Meier et al., 2009]. While freshly emitted BC particles are often initially hydrophobic, partially aged BC particles may absorb water and restructure upon humidification, resulting in HGF below unity [Khalizov et al., 2009b]. Thus, the hygroscopicity measurements provide additional information on the chemical composition and mixing state of aerosol particles.
 Figures 16 and 17a show that 20 nm particles are less hygroscopic than larger particles. The primary peak between 1.00 and 1.20 accounts for 75% of all 20 nm particles, and the secondary maximum near 1.30 accounts for the remaining 25%. The two distinct peaks of low and high HGF most likely represent the organics (1.00–1.20) and an organic-sulfate mixture (1.20–1.50). The average HGF values of 20 nm particles are not appreciably distinct between northerly (1.18 ± 0.08) and southerly (1.17 ± 0.11) winds, and the HGF peak shifts slightly under the northerly flow condition from 1.14 to 1.16. Particles larger than 100 nm in diameter show multimodal HGFs. For 100 nm particles, the peaks between 1.00 and 1.20 likely represent organics and BC, and the peaks between 1.3 and 1.5 correspond to an internal mixture of organics and sulfate. Under the southerly wind conditions, there is a significant difference in the HGF distributions between 20 and 100 nm particles, but after the frontal passage the 20 and 100 nm particles exhibit similar HGF distributions. Specifically, under the southerly wind condition, there is a major peak between 1.30 and 1.60 and another peak near 1.00–1.20 accounting for 30% of the total hygroscopic distribution. This indicates that more organics are internally mixed with sulfates, and there is less nonhygroscopic material around 100 nm. During northerly winds, the secondary peak (HGFs > 1.3) is reduced to 1.3, and the persistent peak near 1.1 is enhanced. Only 100 nm particles exhibit an overall decrease in the average hygroscopicity (from 1.27 ± 0.17 for southerly to 1.24 ± 0.12 for northerly winds) with the weather change. This may be explained due to an increase in BC for this particle size. The largest particles (400 nm) show the largest hygroscopic growth. For the hygroscopic distribution (Figure 16), the most frequent HGFs occur between 1.30 and 1.70, and 25% of the observation period exhibit an average HGF of 1.60–1.80, which may be associated with ammonium sulfates. Pure organics and BC (HGF = 1.00–1.20) only account for 12% of aerosols measured. A small fraction (7%) of the largest particles exhibits HGFs of less than unity, indicating the presence of partially aged BC. For this particle size, the two synoptic patterns exhibit similar hygroscopic distributions. The average hygroscopicity with the northerly winds is slightly higher (1.40 ± 0.21) than that under the southerly flow (1.36 ± 0.30), and the magnitude of the maxima (HGF = 1.40–1.60) is enhanced under the northerly winds, which may be attributed to the overall increased concentration of ammonium sulfates during this time period.
3.7 Black Carbon Concentration
 The light-absorption coefficients, babs, are employed to derive the BC mass concentration in the ambient air (equation (4)). Two different values of MAC are used in the calculation, depending on the aging of BC particles. The absorption coefficients are derived for a wavelength of 532 nm, and the MAC values of 7.5 m2 g−1 for fresh BC and 11 m2 g−1 for aged BC are assumed [Bond and Bergstrom, 2006]. An appropriate value of MAC is selected using the photochemical age as a proxy for the mixing state of particles of BC [Miyakawa et al., 2008],
where NOx and NOy are the concentrations of combined NO and NO2 and the total reactive oxidized nitrogen, respectively. The air mass is assumed to be photochemically aged if at least 25% of the NOx has been photooxidized. However, this simple analysis does not account for local emission sources and has a tendency to identify BC as aged particles even when the particles are fresh based on the effective density measurements. To correct for such a deficiency, we consider another factor on the basis of the observed effective density and particle size. Since 75% of freshly emitted BC particles in our measurements are mostly within the 150 to 240 nm diameter range and have an effective density below 1 g cm−3, the events corresponding to fresh BC emissions can be easily identified. The consideration of this additional factor ensures that fresh BC emissions can be identified even if BC particles are released into a previously aged air mass.
 The temporal profile of our optically derived BC concentration is presented in Figure 18a, along with the EC concentrations measured by Sunset OCEC field analyzer. The data plotted in Figure 18a are averaged over an hour to enhance the signal-to-noise ratio. Figure 18b displays the MAC derived from the absorption coefficient and thermal and optical EC. The averaged thermal EC MAC is 8.26 m2 g−1, which resembles freshly emitted BC [Bond and Bergstrom, 2006], and the average optical EC MAC is 15.67 m2 g−1. Figure 19 shows the averaged diurnal trends for our optically derived BC, compared with those from EC concentrations measured by the Sunset OCEC field analyzer. The BC concentration ranges between 0.10 and 0.88 µg m−3 throughout a day. The highest concentrations of BC are observed in the early morning (i.e., 7 h), and there exists a second peak at about 22 h, likely attributable to the morning traffic and nighttime traffic or industrial emissions, respectively. It is interesting to note the absence of low-density aerosol components on 3 May (Figures 9 and 10), even though a rise in the overall BC concentration is observed (Figure 18). This is likely caused by a change of the mixing state by the formation of heavily coated BC particles with an effective density comparable to that of other aerosols. While our derived BC concentration is slightly lower than the thermal EC, the two sets of data are in reasonable agreement in terms of the magnitude and temporal trend. On 30 April and 1 May, the two data sets are the least consistent, which may be attributed to the instrumental uncertainties. For instance, charring of OC may cause the thermal EC to be slightly biased toward a higher value when the BC concentration is low [Yang and Yu, 2002; Bauer et al., 2009]. The optical measurements may be biased when the BC concentration is low or an inappropriate value of MAC is selected. Nevertheless, the estimated error in BC mass concentration from the optical measurements remains within 0.2 µg m−3.
 The frontal passage has a noticeable impact on the concentration of BC. The average concentration from the northerly flow days (0.60 ± 0.21 µg m−3) is significantly higher than the concentration observed on southerly flow days (0.26 ± 0.18 µg m−3). The concentration of BC during the northerly wind remains elevated overnight. The BC concentration range varies between the wind directions as well. The concentration of BC during southerly winds ranges between 0.10 and 0.40 µg m−3 but increases to 0.40 and 0.90 µg m−3 after the frontal passage. As shown in Figure 13, BC accounts for the greatest percentage during the midmorning hours and the lowest after midnight. Overall, BC accounts for 4.5% of particulate matter (PM) on average. After the frontal passage, the relative mass fraction of BC increases to 10%. Our optically derived BC of 0.31 ± 0.22 µg m−3 agrees with the thermal EC of 0.38 ± 0.19 µg m−3, and both are similar to previous measured BC values of 0.37 ± 0.25 µg m−3 during the TexAQS I [Russell et al., 2004] and 0.29 ± 0.21 µg m−3 during TexAQS II [Bates et al., 2008]. However, both values are significantly lower than BC concentrations observed in other large metropolitan areas, such as 1.7 ± 0.9 µg m−3 in Atlanta [Carrico et al., 2003] and 3.4 ± 2.5 µg m−3 [Salcedo et al., 2006] and 2.1 ± 1.8 µg m−3 [Yu et al., 2009] in Mexico City.
 We report the measurements of comprehensive submicron aerosol properties during the springtime in Houston, Texas, from 30 April through 7 May 2009, including the particle size distributions, the effective density for five individual particle sizes, hygroscopicities, and optical properties in order to gain a better understanding of the formation and transformation of fine PM in this region. From those direct aerosol measurements, the particle mass fraction, BC concentration, surface area, mass concentration, and average effective density are determined.
 The average particle effective density is 1.55 ± 0.07 g cm−3, indicating the dominant contributions from sulfates and organics to the particle chemical compositions. The effective density measurements demonstrate a distinctive diurnal cycle. The lowest densities are observed near 7 h, and a second effective density minimum occurs around 19–20 h, both likely corresponding to morning traffic and evening traffic and industrial emissions, respectively. The densities increase from the morning to late afternoon, reflecting photochemical aging of aerosols. The effective density of smallest particles (46 nm) is more variable and increases more noticeably than larger particles (350 nm) during the course of the day. The most frequent effective densities observed are between 1.50 and 1.60 g cm−3, suggesting that the particles are internally mixed. Episodically, an additional mode below 1 g cm−3 is seen in the effective density distributions, likely due to small concentrations of freshly emitted BC.
 The smallest particles (20 nm) exhibit minimal growth when subjected to increased relative humidity and most frequently have a hygroscopic growth factor between 1.00 and 1.20, indicating a small mass fraction of sulfate in this particle size and a large concentration of organics. The midsized submicron particles (100 nm) show the greatest hygroscopic growth factors near 1.10 and 1.50. The largest particles (400 nm) exhibit the highest hygroscopic frequency between 1.30 and 1.70. The hygroscopic distribution for 400 nm particles reveals a higher concentration of sulfates, whereas the 20 and 100 nm hygroscopic distributions indicate more BC or organics than sulfates. Hence, our results reveal that the larger particles are more hygroscopic due to the increasing sulfate contents during atmospheric transformation.
 After the frontal passage, the overall particle effective density lowers to 1.43 ± 0.08 g cm−3, and the concentration of BC increases from 0.26 ± 0.18 to 0.60 ± 0.21 µg m−3, accounting for a higher percentage of the total particulate matter than previously observed in the region. The overall average BC concentration derived from the optical measurements is 0.31 ± 0.22 µg m−3, which is similar to the previous results and the results from the thermal EC measurements of the 2009 SHARP campaign. This BC concentration in Houston, Texas, is lower than observations from other urban cities.
 The meteorology has a noticeable impact on several aerosol properties in this region. Under the conditions of southerly winds from the Gulf Coast characteristic of relatively clean marine air masses, the overall gaseous concentrations of pollutants are lower and the overall particle effective density is higher. The dominant particle mass fraction oscillates between sulfate and organics. Under the condition of the northern winds characteristic of relatively polluted continental air masses, the particle mass fraction becomes sulfate dominant and the average SSA decreases by 0.07. The instantaneous measurements of SSA decrease to 0.69 after the frontal passage, when there is a significant reduction in the effective density for all particle sizes. After a frontal passage, a high concentration (4 × 104 cm−3) of 20–40 nm particles is observed in the morning hours, indicating that the environmental conditions are more favorable for new particle formation. During this northerly flow period, the temperature, wind speed, and relative humidity are low, the existing aerosol surface area is low, but the anthropogenic sulfate and organic gases (i.e., SO2 and toluene) are elevated. It should be pointed out that our present study shows the meteorological impacts on air chemistry for only a single frontal passage, and future studies are clearly needed to address this issue.
 Trace gases were sampled through perfluoropolymer tubing. The VOC mixing ratios were obtained with a proton transfer reaction mass spectrometer (PTR-MS, Ionicon Analytik, Austria). SO2, CO, NOx, and O3 mixing ratios were measured with the modified Thermo 43, 48, 42, and 49, respectively. A detailed description of the measurements of trace gases has been provided in this special issue (Olaguer et al., submitted manuscript, 2013).
 This project was supported by the Houston Advanced Research Center (HACR) and the Robert A. Welch Foundation (A-1417). We were grateful to B. Thomas Jobson of Washington State University for providing the VOC measurements by PTR-MS and Barry Lefer of University of Houston for CO, SO2, and O3 measurements discussed in our analysis.