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We present a new chemical mechanism for Hg(0)/ Hg(I) / Hg(II) atmospheric cycling, including recent laboratory and computational data, and implement it in the GEOS-Chem global atmospheric chemistry model for comparison to observations. Our mechanism includes the oxidation of Hg(0) by Br atoms and OH radicals, with subsequent oxidation of Hg(I) by ozone and radicals, re-speciation of gaseous Hg(II) in aerosols and cloud droplets, and speciated Hg(II) photolysis in the gas and aqueous phases. The tropospheric Hg lifetime against deposition in the model is 5.5 months, consistent with observational constraints. The model reproduces the observed global surface Hg(0) concentrations and Hg(II) wet deposition fluxes. Br and OH make comparable contributions to global net oxidation of Hg(0) to Hg(II). Ozone is the principal Hg(I) oxidant, enabling the efficient oxidation of Hg(0) to Hg(II) by OH. BrHgOH and Hg(OH)2 are the initial Hg(II) products of Hg0 oxidation, re-speciate in aerosols and clouds to organic and inorganic complexes, and volatilize to photostable forms. Reduction of Hg(II) to Hg(0) takes place largely through photolysis of aqueous Hg(II)-organic complexes. 71% of model Hg(II) deposition is to the oceans. Major mechanism uncertainties for atmospheric Hg chemistry modeling include the concentrations of Br atoms, the stability and reactions of Hg(I), and the speciation of Hg(II) in aerosols and clouds with implications for photoreduction. 

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 (NO_x  ≡  NO + NO₂) over the southeast US are such that the peroxy radicals produced from isoprene oxidation (ISOPO₂) react significantly with both NO (high-NO_x pathway) and HO₂ (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 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 NO_x emissions decrease (favoring the low-NO_x 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 NO_x (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 PM_2.5 from SO₂ emission controls.