Sensitivity of Ambient Secondary Organic Aerosol Formation to Perturbations Measured Using Parallel Photochemical Smog Chambers

RASHMI RAJPUT, Shenglun Wu, Benjamin Schulze, Ryan Ward, Qi Zhang, Christopher Cappa, John Seinfeld, Michael Kleeman, University of California, Davis

     Abstract Number: 185
     Working Group: Aerosol Processes and Properties in Changing Environments in the Anthropocene

Abstract
Secondary organic aerosol (SOA) is a major constituent of PM₂.₅, impacting human health, atmospheric visibility, and climate. This study investigates the sensitivity of urban SOA formation to controlled perturbations in volatile organic compounds (VOCs) and nitrogen oxides (NOₓ) at atmospherically-relevant concentrations using mobile chamber experiments at two California locations, Pasadena and Sacramento. SOA produced from photooxidation was measured with a high-resolution Aerosol Mass Spectrometer (HR-AMS) in three parallel chambers filled with ambient air: VOC-perturbed, base-case (control), and NOₓ-perturbed. Chamber air was aged with UV exposure equivalent to a bright summer day in California for three hours. Compared to the base case, SOA production was suppressed in chambers with 8 ppb of NOₓ-addition at both locations, consistent with RO₂ + NO reactions producing more volatile products, including organic nitrates. In Pasadena, 8 ppb of VOC addition increased SOA concentrations by 21% compared to base-case conditions, likely due to enhanced oxidation via RO₂ + HO₂ pathways forming semi-volatile products that absorbed into the existing condensed organic aerosol (OA) matrix. In Sacramento, where initial OA concentrations in the chamber were an order of magnitude lower than Pasadena (because of differences in chamber filling operation), VOC addition reduced SOA production by a factor of two, likely because of reduced condensation owing to reduced OA matrix. Analysis via positive matrix factorization provides further insights into how different OA types evolve with aging and helps explain the sensitivity to VOC/NOₓ perturbations. The measured trends imply that minor reductions in NOₓ emissions could increase SOA production. Moreover, reduction of primary organic aerosol (POA) could reduce SOA yields, a potential strategy to mitigate the impact of increased VOC emissions from increased temperatures. This underscores the dual importance of considering both gas-phase chemistry and phase partitioning when designing control programs to reduce ambient SOA concentrations.