10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Secondary Organic Aerosol and Organochloride Formation from Alkanes
DONGYU S. WANG, Lea Hildebrandt Ruiz, University of Texas at Austin
Abstract Number: 1425 Working Group: Aerosol Chemistry
Abstract Alkanes account for nearly 40% of anthropogenic volatile organic compounds emitted each year. Enhanced alkane mixing ratios have been reported within unconventional natural gas production regions. The air quality impacts of anthropogenic alkanes remain highly uncertain due to scarce ambient and laboratory data on the precursor abundance and oxidation product composition, especially for larger alkanes.
Environmental chamber experiments were carried out to investigate the photo-oxidation of linear, branched, and cyclic C8-12 alkanes. Nitrogen oxides (NOx) were added to simulate polluted environments. Chlorine radical was added to simulate early morning oxidation chemistry and effects of atmospheric aging. A High-Resolution Time-of-Flight Chemical Ionization Mass Spectrometer (CIMS) was used to characterize the gas-phase chemical composition. A Filtered Inlet for Gas and AEROsol (FIGAERO) was coupled to the CIMS to collect aerosol and to analyze its molecular composition via thermal desorption. The thermal desorption signal of individual species, when viewed as a function of the desorption temperature, can be used to assess the relative volatility of particulate compounds. As an extension of this 1-D thermogram representation, a 2-D thermogram framework was developed to visualize the particle-phase chemical composition and volatility distribution simultaneously using unit-mass-resolution FIGAERO-CIMS data. A clear correlation between molecular mass and volatility of organic aerosol components was observed using the 2-D thermogram. An Aerosol Chemical Speciation Monitor (ACSM) was used to quantify the SOA mass. The observed SOA yields were much higher than SOA yields previously reported for OH-initiated oxidation of alkanes under high NOx conditions. A comparison of OH-alkane and chlorine-alkane 2-D thermograms shows that oligomers and highly oxidized organic compounds contributed to the high SOA yield observed for chlorine-initiated oxidation of alkanes. Both FIGAERO-CIMS and ACSM measurements show trace amounts of alkane-derived organochloride reaction products, and a potential formation pathway is proposed. The chamber relative humidity was varied between 0 to 80% to explore its effect on SOA formation. Alkane-derived organochloride formation was suppressed under humid conditions, consistent with the proposed reaction mechanism. Overall, the results suggest that long-chain anthropogenic alkanes could have greater contribution to aerosol loading over its atmospheric lifetime than previously thought.