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Isoprene is the largest single VOC emission to the atmosphere. Although itis primarily oxidized photochemically during daylight hours, late-dayemissions that remain in the atmosphere at sunset undergo oxidation byNO3 in regionally polluted areas with large NOx levels. A recentaircraft study examined isoprene and its nocturnal oxidants in a series ofnight flights across the Northeast US, a region with large emissions ofboth isoprene and NOx. Substantial amounts of isoprene that wereobserved after dark were strongly anticorrelated with measured NO3 andwere the most important factor determining the lifetime of this radical. Theproducts of photochemical oxidation of isoprene, methyl vinyl ketone andmethacrolein, were more uniformly distributed, and served as tracers for thepresence of isoprene at sunset, prior to its oxidation by NO3.A determination of the mass of isoprene oxidized in darkness showed it tobe a large fraction (>20%) of emitted isoprene. Organic nitratesproduced from the NO3+isoprene reaction, though not directly measured, were estimated to accountfor 2–9% of total reactive nitrogen. The mass of isoprene oxidized byNO3 was comparable to and correlated with the organic aerosol loadingfor flights with relatively low organic aerosol background. The contributionof nocturnal isoprene oxidation to secondary organic aerosol was determinedin the range 1–17%, and isoprene SOA mass derived from NO3 wascalculated to exceed that due to OH by approximately 50%

Nocturnal isoprene oxidation over the Northeast United States in summer and its impact on reactive nitrogen partitioning and secondary organic aerosol