PNNL

Abstract

Ambient aerosol properties were measured in the field campaign of the Study of Houston Atmospheric Radical Precursors and Surface-induced Oxidation of Organics in the Troposphere (SHARP/SOOT) at a site in downtown Houston, TX in April-May 2009. A suite of aerosol instruments were deployed during the field study, including a Scanning Mobility Particle Sizer, an Aerosol Particle Mass Analyzer, a Hygroscopicity Tandem Differential Mobility Analyzer, a Cavity Ring-Down Spectrometer, and an Integrating Nephelometer. A comprehensive set of directly measured aerosol properties, including particle size distributions, effective densities, hygroscopicities, and light extinction and scattering coefficients were used to infer the mixing state and composition of ambient particles and gain a better understanding of formation and transformation of the springtime submicron particulate matter in southeast urban Texas. Throughout most of the campaign, the aerosol particles were often internally mixed, with one peak in the effective density distribution located at 1.55 ± 0.07 g?cm-3, indicating a large concentration of sulfates and organics. Episodically, a second mode below 1 g?cm-3 was identified in the effective density distributions, due to the presence of a small concentration of freshly emitted black carbon (BC). The effective density demonstrated a clear diurnal cycle, being at a minimum during the morning rush hour, increasing by an average of 0.1 g cm-3 (8.4% growth), and then remaining moderately constant throughout afternoon. The average single scattering albedo of aerosols was 0.94 ± 0.04, indicating that scattering was the main extinction process in this urban environment. However, when increased BC concentrations were observed, typically during the morning rush hours, the average single scattering albedo decreased by 0.07 due to increased absorption by BC. The average BC concentration derived from light absorption measurements was 0.31 ± 0.22 µg m 3. Aerosol hygroscopicity measurements indicated that the larger particles (e.g., 400 nm) had higher hygroscopic growth factors (by 0.1 to 0.2) than the smaller particles (e.g., 100 nm), indicating that the larger particles contained more water-soluble materials. The measurements also revealed a noticeable meteorological impact on the aerosol concentrations and properties. During a period of a frontal passage, the average effective density decreased to 1.43 ± 0.08 g cm-3 and the average BC concentration increased to 0.60 ± 0.21 µg m-3, which was approximately twice of the average concentration. Aerosol size distributions shifted significantly to smaller sizes, but there was little variation in aerosol hygroscopicity with the weather patterns.