Humid Summers Promote Urban Aqueous-Phase Production of Oxygenated Organic Aerosol in the Northeastern United States

Abstract

Aqueous-phase uptake and processing of water-soluble organic compounds can promote secondary organic aerosol (SOA) production. We evaluated the contributions of aqueous-phase chemistry to summertime urban SOA at two sites in New York City. The relative role of aqueous-phase processing varied with chemical and environmental conditions, with evident daytime SOA enhancements (e.g., >1 μg/m3) during periods with relative humidities (RH) exceeding 65% and often higher temperatures. Oxygenated organic aerosol (OOA) production was also sensitive to secondary inorganic aerosols, in part through their influence on aerosol liquid water. On average, high-RH periods exhibited a 69% increase in less-oxidized OOA production in Queens, NY. These enhancements coincided with southerly backward trajectories and greater inorganic aerosol concentrations, yet showed substantial intra-city variability between Queens and Manhattan. The observed aqueous-phase SOA production, even with historically low sulfate and nitrate aerosol loadings, highlights both opportunities and challenges for continued reductions in summertime PM2.5 in urban communities.