Evidence of nighttime production of organic nitrates during SEAC4RS, FRAPPÉ, and KORUS-AQ

Organic nitrates (RONO2) are an important NOx sink. In warm, rural environments dominated by biogenic emissions, nocturnal NO3-initiated production of RONO2 is competitive with daytime OH-initiated RONO2 production. However, in urban areas, OH-initiated production of RONO2 has been assumed dominant and NO3-initiated production considered negligible. We show evidence for nighttime RONO2 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 (SEAC4RS) 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 RONO2 enhancements at constant, near-background Ox (≡ O3+NO2) concentrations, indicating that the RONO2 are from a non-photochemical source, whereas afternoon observations show a strong correlation between RONO2 and Ox resulting from photochemical production. We show that there are sufficient precursors for nighttime RONO2 formation during all three campaigns. This evidence impacts our understanding of nighttime NOx chemistry.