Composition Sources And Formation Of Secondary Organic Aerosols From Urban Emissions

Secondary organic aerosols (SOA), known to form in the atmosphere, are a poorly understood but important component of atmospheric fine particles. This study aims to improve the understanding of the composition, source, and formation mechanism of SOA. Ambient particles were measured at urban centers (Mexico City, Mexico; Bakersfield, US), urban pollution-influenced coastal area (San Diego, US), high-elevation (4010 m) site (Altzomoni; 60 km southeast of Mexico City), and onboard the NCAR C130 flight (over Mexico and the coast of the Gulf of Mexico). Ensemble- and single-particle functional group and mass spectral compositions were analyzed using complementary techniques, mainly including Fourier transform infrared spectroscopy (FTIR), high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) coupled with a light scattering module, and scanning transmission X-ray microscopy combined with near-edge X-ray absorption fine structure (STXM-NEXAFS). The organic mass was found to be dominated by alkane, carboxylic acid, hydroxyl, and nonacid carbonyl groups. By applying factor analysis independently to the FTIR- and AMS-measured organic mass, a variety of sources was consistently identified in the urban plumes, with fossil fuel combustion emission accounting for 60- 90% of the organic mass. Volatile organic compounds emitted by the sources underwent fast oxidation. As a result, SOA contributed to 60- 90% of the organic mass, even in regions close to the sources. The SOA components formed from different precursor hydrocarbons were distinguished, with their mass fraction, diurnal cycle, size, and likely formation pathway discussed. The field studies were facilitated by laboratory reaction chamber studies focusing on organonitrate (ON) groups, which are potentially important photochemical products. It was found that ON groups hydrolyze in aerosol water at a rate of 4 day−1 (corresponds to a lifetime of 6 hours) when relative humidity exceeds 20%, which could explain the lower concentration of ON groups in aerosol particles than model prediction. Overall, the combined field and laboratory studies demonstrate that SOA formation is a dynamic and multivariate process; more work is needed to characterize SOA for quantitative and predictive understanding of the impacts of aerosols.