Atmos. Chem. Phys. Discuss., 9, 225-269, 2009
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Nocturnal isoprene oxidation over the Northeast United States in summer and its impact on reactive nitrogen partitioning and secondary organic aerosol
S. S. Brown1, J. A. deGouw1,2, C. Warneke1,2, T. B. Ryerson1, W. P. Dubé1,2, E. Atlas3, R. J. Weber4, R. E. Peltier4,*, J. A. Neuman1,2, J. M. Roberts1, A. Swanson2,5,**, F. Flocke5, S. A. McKeen1,2, J. Brioude1,2, R. Sommariva1,2,***, M. Trainer1, F. C. Fehsenfeld1,2, and A. R. Ravishankara1
1NOAA Earth System Research Laboratory, 325 Broadway, Boulder, CO 80 305, USA
2Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80 309, USA
3RSMAS/MAC, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33 149, USA
4School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, USA
5National Center for Atmospheric Research, 1850 Table Mesa Dr., Boulder, CO 80305, USA
*now at: Department of Environmental Medicine, NYU School of Medicine, Tuxedo, NY 10987, USA
**now at: Chemical Technology Department, Northrup Grumman Space Technology, Redondo Beach, CA 90278, USA
***now at: School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK

Abstract. 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 NO3 in regionally polluted areas with large NOx 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 NOx. Substantial amounts of isoprene that were observed after dark were strongly anticorrelated with measured NO3 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 NO3. Comparison of a determination of the mass of isoprene oxidized in darkness by NO3 to a calculation of integrated isoprene emissions showed that large amounts (>20%) of emitted isoprene may undergo nocturnal oxidation in this region. Organic nitrates produced from the NO3+isoprene reaction, though not directly measured, were estimated to account for 2–9% of total reactive nitrogen and 7–31% of other long-lived organic nitrates such as PAN. The mass of isoprene oxidized by NO3 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 NO3 was calculated to exceed that due to OH by approximately 50%.

Citation: Brown, S. S., deGouw, J. A., Warneke, C., Ryerson, T. B., Dubé, W. P., Atlas, E., Weber, R. J., Peltier, R. E., Neuman, J. A., Roberts, J. M., Swanson, A., Flocke, F., McKeen, S. A., Brioude, J., Sommariva, R., Trainer, M., Fehsenfeld, F. C., and Ravishankara, A. R.: Nocturnal isoprene oxidation over the Northeast United States in summer and its impact on reactive nitrogen partitioning and secondary organic aerosol, Atmos. Chem. Phys. Discuss., 9, 225-269, doi:10.5194/acpd-9-225-2009, 2009.
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