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

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. Plain Language Summary Secondary organic aerosols (SOA) are air pollutants formed following the oxidation and often condensation of gas‐phase organic compounds and make up a large fraction of airborne fine particulate matter (PM2.5). Aqueous‐phase reactions within suspended liquid water in the atmosphere are less‐studied among several pathways that produce SOA derived from diverse biogenic and anthropogenic sources. While models and laboratory experiments predict substantial SOA contributions from aqueous‐phase chemistry in many regions, field measurements to quantify this and determine how weather and pollutant concentrations affect aqueous‐phase production in urban areas remain limited. Using ground‐based measurements of aerosols and gases from two sites in New York City, we show evidence for summertime aqueous‐phase SOA production. The production of aqueous SOA was highly sensitive to the combined influence of relative humidity, temperature, and secondary inorganic aerosols. The latter are formed from typically anthropogenic emissions of gas‐phase nitrogen oxides (NOx) and sulfur dioxide (SO2). Aqueous‐phase SOA production showed local variations between sites and its enhancements were strongly dependent on wind patterns, demonstrating the influence of regional environmental conditions. Interestingly, while secondary inorganic aerosol concentrations have decreased considerably, their effect on aqueous‐phase SOA production during warming, humid summers remains important for air quality under a changing climate.