Evidence of Nighttime Production of Organic Nitrates During SEAC⁴RS, FRAPPÉ, and KORUS‐AQ

Abstract Organic nitrates (RONO 2 ) are an important NO x sink. In warm, rural environments dominated by biogenic emissions, nocturnal NO 3 ‐initiated production of RONO 2 is competitive with daytime OH‐initiated RONO 2 production. However, in urban areas, OH‐initiated production of RONO 2 has been assumed dominant and NO 3 ‐initiated production considered negligible. We show evidence for nighttime RONO 2 production similar in magnitude to daytime production during three aircraft campaigns in chemically distinct summertime environments: Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC 4 RS) in the rural Southeastern United States, Front Range Air Pollution and Photochemistry Experiment (FRAPPÉ) in the Colorado Front Range, and Korea‐United States Air Quality Study (KORUS‐AQ) around the megacity of Seoul. During each campaign, morning observations show RONO 2 enhancements at constant, near‐background O x (≡ O 3 +NO 2 ) concentrations, indicating that the RONO 2 are from a non‐photochemical source, whereas afternoon observations show a strong correlation between RONO 2 and O x resulting from photochemical production. We show that there are sufficient precursors for nighttime RONO 2 formation during all three campaigns. This evidence impacts our understanding of nighttime NO x chemistry.