Measurements of Submicron Aerosols in Houston, Texas

During the field campaign of the Study of Houston Atmospheric Radical Precursors/Surface–induced Oxidation of Organics in the Troposphere (SHARP/SOOT) in Houston, TX, 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 (PM) in this region. In this presentation, we will present an updated characterization of the submicron aerosols in the region. Emphasis will be placed on the characteristics of black carbon containing particles with the expectation of obtaining a greater understanding of their composition and concentration and on the discussion of a nucleation event that was observed during the campaign. 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 comprised 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 hours, 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 (SSA) 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 discernible 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.