Aqueous-phase mechanism for secondary organic aerosol formation from isoprene: application to the Southeast United States and co-benefit of SO ₂ emission controls

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 (SEAC ⁴RS) and ground-based (SOAS) observations over the Southeast US in summer 2013 using the GEOS-Chem chemical transport model. Emissions of nitrogen oxides (NO _x ≡ NO + NO ₂) over the Southeast US are such that the peroxy radicals produced from isoprene oxidation (ISOPO ₂) react significantly with both NO (high-NO _x pathway) and HO ₂ (low-NO _x 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-NO _x pathway and glyoxal (28%) from both low- and high-NO _x pathways. This speciation is consistent with observations of IEPOX SOA from SOAS and SEAC ⁴RS. 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 NO _x emissions decrease (favoring the low-NO _x pathway for isoprene oxidation), but decrease more strongly as SO ₂ 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 NO _x (leading to 7% increase in isoprene SOA) and 48% for SO ₂ (35% decrease in isoprene SOA). Reducing SO ₂ emissions decreases sulfate and isoprene SOA by a similar magnitude, representing a factor of 2 co-benefit for PM _2.5 from SO ₂ emission controls.