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

Organic nitrates (RONO) are an important NO sink. In rural environments dominated by biogenic emissions, nocturnal NO-initiated production of RONO is competitive with daytime OH-initiated RONO production. However, in urban areas, OH-initiated production of RONO has been assumed dominant and NO-initiated production considered negligible. We show evidence for nighttime RONO production similar in magnitude to daytime production during three aircraft campaigns in chemically-distinct environments: SEACRS in the rural Southeastern US, FRAPPÉ in the Colorado Front Range, and KORUS-AQ around the megacity of Seoul. During each campaign, morning observations show RONO enhancements at constant, near-background Ox (≡ O + NO), indicating that the RONO are from a non-photochemical source, whereas afternoon observations show a strong correlation between RONO and O resulting from photochemical production. We show there are sufficient precursors for nighttime RONO formation during all three campaigns. This evidence impacts our understanding of the nighttime lifetime and fate of NO.