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

Organic nitrates (RONO₂) are an important NO_x 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 O_x (≡ O₃+NO₂) concentrations, indicating that the RONO₂ are from a non-photochemical source, whereas afternoon observations show a strong correlation between RONO₂ and O_x 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 NO_x chemistry.