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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 PM_2.5in urban communities. 

Despite decades of progress in reducing nitrogen oxide (NO_x) emissions, ammonium nitrate (AN) remains the primary inorganic component of particulate matter (PM) in Los Angeles (LA). Using aerosol mass spectrometry over multiple years in LA illustrates the controlling dynamics of AN and their evolution over the past decades. These data suggest that much of the nitric acid (HNO₃) production required to produce AN in LA occurs during the nighttime via heterogeneous hydrolysis of N₂O₅. Further, we show that US Environmental Protection Agency–codified techniques for measuring total PM_2.5fail to quantify the AN component, while low-cost optical sensors demonstrate good agreement. While previous studies suggest that declining NO_xhas reduced AN, we show that HNO₃formation is still substantial and leads to the formation of many tens of micrograms per cubic meter of AN aerosol. Continued focus on reductions in NO_xwill help meet the PM_2.5standards in the LA basin and many other regions.