Secondary Organic Aerosol Formation During Continental to Oceanic Transition in South China Coast: Effects of Photostationary State, Aqueous‐Phase Processes, and Heatwaves

Despite the overall decreasing trend of particulate matter (PM) pollution across urban China, PM pollution persists in certain areas, especially in fast‐developing southeastern coastal regions. The characteristics and evolution of PM during the more polluted autumn and winter seasons have been widely studied, but studies focusing on spring and summer, when continental to oceanic air mass transition affects coastal regions, remain limited. Herein, we use a suite of online instruments to characterize PM chemical composition, particularly on organic aerosol (OA) in a coastal city in southern China during the spring and summer of 2024. Results show that oceanic influences caused substantial deviations from photochemical steady state (PSS) at relatively low NO 2 levels, and OA was mainly comprised of aged secondary organic aerosol (SOA); deviation from PSS at high NO 2 levels, however, facilitated fresh SOA formation with high nitrogen content. High humidity promoted aqueous‐phase formation of highly oxygenated SOA, whereas heatwave events facilitated the photochemical formation of fresher, more volatile, and nitrogen‐rich SOA. Our findings further highlight that heatwave conditions drive a unique nighttime chemistry that sustains high levels of NO 3 radicals contributing to the formation of nitrogen‐rich SOA. This study reveals the interplay among reaction pathways (aqueous vs. gas phases), oxides of nitrogen (both NO x and NO 3 radicals), and air mass origins (continental vs. oceanic) that regulate SOA formation in coastal regions, providing critical insights to improve atmospheric models and inform timely pollution control policies.