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Atmospheric Chemistry and Physics









The chemical link between isoprene and formaldehyde (HCHO) is a strong, nonlinear function of NOx (i.e., NO + NO2). This relationship is a linchpin for top-down isoprene emission inventory verification from orbital HCHO column observations. It is also a benchmark for overall photochemical mechanism performance with regard to VOC oxidation. Using a comprehensive suite of airborne in situ observations over the southeast US, we quantify HCHO production across the urban–rural spectrum. Analysis of isoprene and its major first-generation oxidation products allows us to define both a "prompt" yield of HCHO (molecules of HCHO produced per molecule of freshly emitted isoprene) and the background HCHO mixing ratio (from oxidation of longer-lived hydrocarbons). Over the range of observed NOx values (roughly 0.1–2 ppbv), the prompt yield increases by a factor of 3 (from 0.3 to 0.9 ppbv ppbv−1), while background HCHO increases by a factor of 2 (from 1.6 to 3.3 ppbv). We apply the same method to evaluate the performance of both a global chemical transport model (AM3) and a measurement-constrained 0-D steady-state box model. Both models reproduce the NOx dependence of the prompt HCHO yield, illustrating that models with updated isoprene oxidation mechanisms can adequately capture the link between HCHO and recent isoprene emissions. On the other hand, both models underestimate background HCHO mixing ratios, suggesting missing HCHO precursors, inadequate representation of later-generation isoprene degradation and/or underestimated hydroxyl radical concentrations. Detailed process rates from the box model simulation demonstrate a 3-fold increase in HCHO production across the range of observed NOx values, driven by a 100 % increase in OH and a 40 % increase in branching of organic peroxy radical reactions to produce HCHO.


G. M. Wolfe1,2, J. Kaiser3, T. F. Hanisco2, F. N. Keutsch4, J. A. de Gouw5,6, J. B. Gilman5,6, M. Graus5,6,a, C. D. Hatch7, J. Holloway5,6, L. W. Horowitz8, B. H. Lee9, B. M. Lerner5,6, F. Lopez-Hilifiker9,b, J. Mao8,11, M. R. Marvin10, J. Peischl5,6, I. B. Pollack5,6, J. M. Roberts6, T. B. Ryerson6, J. A. Thornton9, P. R. Veres5,6, and C. Warneke5,6

1Joint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore, MD, USA
2Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA
3Department of Chemistry, University of Wisconsin–Madison, Madison, WI, USA
4School of Engineering and Applied Sciences and Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA, USA
5Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, USA
6Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, CO, USA
7Department of Chemistry, Hendrix College, Conway, AR, USA
8NOAA Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA
9Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA
10Department of Chemistry, University of Maryland, College Park, MD, USA
11Program in Atmospheric and Oceanic Sciences, Princeton University, Princeton, NJ, USA
anow at: Institute of Atmospheric and Cryospheric Sciences, Innsbruck University, Innsbruck, Austria
bnow at: Laboratory of Atmospheric Chemistry, Paul Scherrer Institut, 5232 Villigen, Switzerland