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 EPA projects 2013-2025 decreases in anthropogenic emissions of 34% for NOx (leading to 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.
Highlights
Isoprene emitted by vegetation is a major source of secondary organic aerosol (SOA) (Carlton et al, 2009, and references therein) with effects on human health, visibility, and climate
It may be directly emitted by combustion as primary organic aerosol (POA), or produced within the atmosphere as SOA by oxidation of volatile organic compounds (VOCs)
The dominance of gas-phase loss over aerosol uptake for both isoprene epoxydiols (IEPOX) and glyoxal implies that isoprene SOA formation is highly sensitive to their reactive uptake coefficients γ and to the aqueous aerosol mass concentration
Summary
Isoprene emitted by vegetation is a major source of secondary organic aerosol (SOA) (Carlton et al, 2009, and references therein) with effects on human health, visibility, and climate. Standard isoprene SOA mechanisms used in atmospheric models assume reversible partitioning onto pre-existing organic aerosol, fitting the dry chamber data (Odum et al, 1996) This may not be appropriate for actual atmospheric conditions where aqueous-phase chemistry with irreversible reactive uptake of water-soluble gases is likely the dominant mechanism (Ervens et al, 2011; Carlton and Turpin, 2013). We present a mechanism for irreversible aqueousphase isoprene SOA formation integrated within a detailed chemical mechanism for isoprene gas-phase oxidation, linking isoprene SOA formation to gas-phase chemistry and avoiding more generic volatility-based parameterizations that assume dry organic aerosol (Odum et al, 1996; Donahue et al, 2006) We use this mechanism in the GEOS-Chem CTM to simulate observations from the SOAS (surface) and SEAC4RS (aircraft) field campaigns over the southeast US in summer 2013, with focus on isoprene SOA components and on the relationship between OA and formaldehyde (HCHO). SEAC4RS measurements were made from the NASA DC-8 aircraft with extensive boundary-layer coverage across the southeast (Toon and the SEAC4RS science team, 2016; SEAC4RS Archive, 2015)
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