ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-9-3027-2009 Nocturnal isoprene oxidation over the Northeast United States in summer and its impact on reactive nitrogen partitioning and secondary organic aerosol Brown S. S. 1 deGouw J. A. 1 2 Warneke C. 1 2 Ryerson T. B. 1 Dubé W. P. 1 2 Atlas E. 3 Weber R. J. 4 Peltier R. E. 4 6 Neuman J. A. 1 2 Roberts J. M. 1 Swanson A. 2 5 7 Flocke F. 5 McKeen S. A. 1 2 Brioude J. 1 2 Sommariva R. 1 2 8 Trainer M. 1 Fehsenfeld F. C. 1 2 Ravishankara A. R. 1 NOAA Earth System Research Laboratory, 325 Broadway, Boulder, CO 80305, USA Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, USA RSMAS/MAC, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, USA School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, USA National Center for Atmospheric Research, 1850 Table Mesa Dr., Boulder, CO 80305, USA currently at: Department of Environmental Medicine, NYU School of Medicine, Tuxedo, NY 10987, USA currently at: Chemical Technology Department, Northrup Grumman Space Technology, Redondo Beach, CA 90278, USA currently at: School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK 11 05 2009 9 9 3027 3042 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/9/3027/2009/acp-9-3027-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/9/3027/2009/acp-9-3027-2009.pdf

Isoprene is the largest single VOC emission to the atmosphere. Although it is primarily oxidized photochemically during daylight hours, late-day emissions that remain in the atmosphere at sunset undergo oxidation by NO<sub>3</sub> in regionally polluted areas with large NO<sub>x</sub> levels. A recent aircraft study examined isoprene and its nocturnal oxidants in a series of night flights across the Northeast US, a region with large emissions of both isoprene and NO<sub>x</sub>. Substantial amounts of isoprene that were observed after dark were strongly anticorrelated with measured NO<sub>3</sub> and were the most important factor determining the lifetime of this radical. The products of photochemical oxidation of isoprene, methyl vinyl ketone and methacrolein, were more uniformly distributed, and served as tracers for the presence of isoprene at sunset, prior to its oxidation by NO<sub>3</sub>. A determination of the mass of isoprene oxidized in darkness showed it to be a large fraction (>20%) of emitted isoprene. Organic nitrates produced from the NO<sub>3</sub>+isoprene reaction, though not directly measured, were estimated to account for 2–9% of total reactive nitrogen. The mass of isoprene oxidized by NO<sub>3</sub> was comparable to and correlated with the organic aerosol loading for flights with relatively low organic aerosol background. The contribution of nocturnal isoprene oxidation to secondary organic aerosol was determined in the range 1–17%, and isoprene SOA mass derived from NO<sub>3</sub> was calculated to exceed that due to OH by approximately 50%.

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