Document Type

Article

Publication Date

2016

Publication Title

Atmospheric Chemistry and Physics

ISSN

1680-7324

Volume

16

Issue

3

DOI

http://dx.doi.org/10.5194/acp-16-1603-2016

Abstract

Isoprene emitted by vegetation is an important precursor of secondary organic aerosol (SOA), but the mechanism and yields are uncertain. Aerosol is prevailingly aqueous under the humid conditions typical of isoprene-emitting regions. Here we develop an aqueous-phase mechanism for isoprene SOA formation coupled to a detailed gas-phase isoprene oxidation scheme. The mechanism is based on aerosol reactive uptake coefficients (gamma) for water-soluble isoprene oxidation products, including sensitivity to aerosol acidity and nucleophile concentrations. We apply this mechanism to simulation of aircraft (SEAC(4)RS) and ground-based (SOAS) observations over the southeast US in summer 2013 using the GEOS-Chem chemical transport model. Emissions of nitrogen oxides (NOx = NO + NO2) over the southeast US are such that the peroxy radicals produced from isoprene oxidation (ISOPO2) react significantly with both NO (high-NOx pathway) and HO2 (low-NOx pathway), leading to different suites of isoprene SOA precursors. We find a mean SOA mass yield of 3.3% from isoprene oxidation, consistent with the observed relationship of total fine organic aerosol (OA) and formaldehyde (a product of isoprene oxidation). Isoprene SOA production is mainly contributed by two immediate gasphase precursors, isoprene epoxydiols (IEPOX, 58% of isoprene SOA) from the low-NOx pathway and glyoxal (28 %) from both low-and high-NOx pathways. This speciation is consistent with observations of IEPOX SOA from SOAS and SEAC4RS. Observations show a strong relationship between IEPOX SOA and sulfate aerosol that we explain as due to the effect of sulfate on aerosol acidity and volume. Isoprene SOA concentrations increase as NOx emissions decrease (favoring the low-NOx pathway for isoprene oxidation), but decrease more strongly as SO2 emissions decrease (due to the effect of sulfate on aerosol acidity and volume). The US Environmental Protection Agency (EPA) projects 2013-2025 decreases in anthropogenic emissions of 34% for NOx (leading to a 7% increase in isoprene SOA) and 48% for SO2 (35% decrease in isoprene SOA). Reducing SO2 emissions decreases sulfate and isoprene SOA by a similar magnitude, representing a factor of 2 co-benefit for PM2.5 from SO2 emission controls.

Comments

AUTHORS

E. A. Marais (School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA) D. J. Jacob (School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA) D. J. Jacob (Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA) J. L. Jimenez (Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA) J. L. Jimenez (Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA) P. Campuzano-Jost (Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA) P. Campuzano-Jost (Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA) D. A. Day (Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA) D. A. Day (Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA) W. Hu (Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA) W. Hu (Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA) J. Krechmer (Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA) J. Krechmer (Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA) L. Zhu (School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA) P. S. Kim (Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA) C. C. Miller (Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA) J. A. Fisher (School of Chemistry and School of Earth and Environmental Sciences, University of Wollongong, Wollongong, New South Wales, Australia) K. Travis (School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA) K. Yu (School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA) T. F. Hanisco (Atmospheric Chemistry and Dynamics Lab, NASA Goddard Space Flight Center, Greenbelt, MD, USA) G. M. Wolfe (Atmospheric Chemistry and Dynamics Lab, NASA Goddard Space Flight Center, Greenbelt, MD, USA) G. M. Wolfe (Joint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore, MD, USA) H. L. Arkinson (Department of Atmospheric and Oceanic Science, University of Maryland, College Park, MD, USA) H. O. T. Pye (National Exposure Research Laboratory, US EPA, Research Triangle Park, NC, USA) K. D. Froyd (Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA) K. D. Froyd (Chemical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, CO, USA) J. Liao (Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA) J. Liao (Chemical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, CO, USA) V. F. McNeill (Department of Chemical Engineering, Columbia University, New York, NY, USA)

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