[1] 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 À3 , consistent with a population composed largely of sulfates and organics. Episodically, a second mode below 1.0 g cm À3 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 À3 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 À3 , 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.

Increased drilling in urban areas overlying shale formations and its potential impact on human health through decreased air quality make it important to estimate the contribution of oil and gas activities to photochemical smog. Flares and compressor engines used in natural gas operations, for example, are large sources not only of NOx but also offormaldehyde, a hazardous air pollutant and powerful ozone precursor We used a neighborhood scale (200 m horizontal resolution) three-dimensional (3D) air dispersion model with an appropriate chemical mechanism to simulate ozone formation in the vicinity ofa hypothetical natural gas processing facility, based on accepted estimates of both regular and nonroutine emissions. The model predicts that, under average midday conditions in June, regular emissions mostly associated with compressor engines may increase ambient ozone in the Barnett Shale by more than 3 ppb beginning at about 2 km downwind of the facility, assuming there are no other major sources of ozone precursors. Flare volumes of 100,000 cubic meters per hour ofnatural gas over a period of 2 hr can also add over 3 ppb to peak 1-hr ozone somewhatfurther (>8 km) downwind, once dilution overcomes ozone titration and inhibition by large flare emissions of NOx. The additional peak ozone from the hypothetical flare can briefly exceed 10 ppb about 16 km downwind. The enhancements of ambient ozone predicted by the model are significant, given that ozone control strategy widths are of the order of a few parts per billion. Degrading the horizontal resolution of the model to 1 km spuriously enhances the simulated ozone increases by reducing the effectiveness of ozone inhibition and titration due to artificial plume dilution.

The Texas Environmental Research Consortium (TERC) funded significant components of the Second Texas Air Quality Study (TexAQS II), including the TexAQS II Radical and Aerosol Measurement Project (TRAMP) and instrumented flights by a Piper Aztec aircraft. These experiments called attention to the role of short-lived radical sources such as formaldehyde (HCHO) and nitrous acid (HONO) in increasing ozone productivity. TRAMP instruments recorded daytime HCHO pulses as large as 32 parts per billion (ppb) originating from upwind industrial activities in the Houston Ship Channel, where in situ surface monitors detected HCHO peaks as large as 52 ppb. Moreover, Ship Channel petrochemical flares were observed to produce plumes of apparent primary HCHO. In one such combustion plume that was depleted of ozone by large emissions of oxides of nitrogen (NOx), the Piper Aztec measured a ratio of HCHO to carbon monoxide (CO) 3 times that of mobile sources. HCHO from uncounted primary sources or ozonolysis of underestimated olefin emissions could significantly increase ozone productivity in Houston beyond previous expectations. Simulations with the CAMx model show that additional emissions of HCHO from industrial flares or mobile sources can increase peak ozone in Houston by up to 30 ppb. Other findings from TexAQS II include significant concentrations of HONO throughout the day, well in excess of current air quality model predictions, with large nocturnal vertical gradients indicating a surface or near-surface source of HONO, and large concentrations of nighttime radicals (approximately30 parts per trillion [ppt] HO2). HONO may be formed heterogeneously on urban canopy or particulate matter surfaces and may be enhanced by organic aerosol of industrial or motor vehicular origin, such as through conversion of nitric acid (HNO3). Additional HONO sources may increase daytime ozone by more than 10 ppb. Improving the representation of primary and secondary HCHO and HONO in air quality models could enhance the simulated effectiveness of control strategies.

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