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

摘要

Isoprene emitted by vegetation is an important precursor of secondary organicaerosol (SOA), but the mechanism and yields are uncertain. Aerosol isprevailingly aqueous under the humid conditions typical of isoprene-emittingregions. Here we develop an aqueous-phase mechanism for isoprene SOAformation coupled to a detailed gas-phase isoprene oxidation scheme. Themechanism is based on aerosol reactive uptake coefficients () forwater-soluble isoprene oxidation products, including sensitivity to aerosolacidity and nucleophile concentrations. We apply this mechanism to simulationof aircraft (SEACRS) and ground-based (SOAS) observations over thesoutheast US in summer 2013 using the GEOS-Chem chemical transport model.Emissions of nitrogen oxides (NO  ≡  NO + NO) overthe southeast US are such that the peroxy radicals produced from isopreneoxidation (ISOPO) react significantly with both NO (high-NOpathway) and HO (low-NO pathway), leading to different suites ofisoprene SOA precursors. We find a mean SOA mass yield of 3.3 % fromisoprene oxidation, consistent with the observed relationship of total fineorganic aerosol (OA) and formaldehyde (a product of isoprene oxidation).Isoprene SOA production is mainly contributed by two immediate gas-phaseprecursors, isoprene epoxydiols (IEPOX, 58 % of isoprene SOA) from thelow-NO pathway and glyoxal (28 %) from both low- and high-NOpathways. This speciation is consistent with observations of IEPOX SOA fromSOAS and SEACRS. Observations show a strong relationship between IEPOXSOA and sulfate aerosol that we explain as due to the effect of sulfate onaerosol acidity and volume. Isoprene SOA concentrations increase as NOemissions decrease (favoring the low-NO pathway for isopreneoxidation), but decrease more strongly as SO emissions decrease (due tothe effect of sulfate on aerosol acidity and volume). The US EnvironmentalProtection Agency (EPA) projects 2013–2025 decreases in anthropogenicemissions of 34 % for NO (leading to a 7 % increase in isopreneSOA) and 48 % for SO (35 % decrease in isoprene SOA). ReducingSO emissions decreases sulfate and isoprene SOA by a similar magnitude,representing a factor of 2 co-benefit for PM from SO emissioncontrols.