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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 16, issue 3 | Copyright
Atmos. Chem. Phys., 16, 1603-1618, 2016
https://doi.org/10.5194/acp-16-1603-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 11 Feb 2016

Research article | 11 Feb 2016

Aqueous-phase mechanism for secondary organic aerosol formation from isoprene: application to the southeast United States and co-benefit of SO2 emission controls

E. A. Marais1, D. J. Jacob1,2, J. L. Jimenez3,4, P. Campuzano-Jost3,4, D. A. Day3,4, W. Hu3,4, J. Krechmer3,4, L. Zhu1, P. S. Kim2, C. C. Miller2, J. A. Fisher5, K. Travis1, K. Yu1, T. F. Hanisco6, G. M. Wolfe6,7, H. L. Arkinson8, H. O. T. Pye9, K. D. Froyd3,10, J. Liao3,10, and V. F. McNeill11 E. A. Marais et al.
  • 1School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
  • 2Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA
  • 3Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA
  • 4Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA
  • 5School of Chemistry and School of Earth and Environmental Sciences, University of Wollongong, Wollongong, New South Wales, Australia
  • 6Atmospheric Chemistry and Dynamics Lab, NASA Goddard Space Flight Center, Greenbelt, MD, USA
  • 7Joint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore, MD, USA
  • 8Department of Atmospheric and Oceanic Science, University of Maryland, College Park, MD, USA
  • 9National Exposure Research Laboratory, US EPA, Research Triangle Park, NC, USA
  • 10Chemical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, CO, USA
  • 11Department of Chemical Engineering, Columbia University, New York, NY, USA

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 (γ) for water-soluble isoprene oxidation products, including sensitivity to aerosol acidity and nucleophile concentrations. We apply this mechanism to simulation of aircraft (SEAC4RS) 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 gas-phase 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.

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Isoprene secondary organic aerosol (SOA) is a dominant aerosol component in the southeast US, but models routinely underestimate isoprene SOA with traditional schemes based on chamber studies operated under conditions not representative of isoprene-emitting forests. We develop a new irreversible uptake mechanism to reproduce isoprene SOA yields (3.3 %) and composition, and find a factor of 2 co-benefit of SO2 emission controls on reducing sulfate and organic aerosol in the southeast US.
Isoprene secondary organic aerosol (SOA) is a dominant aerosol component in the southeast US,...
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