AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
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Modeling the Effects of an Updated Isoprene Oxidation Mechanism on Organic Aerosol, Reactive Nitrogen, and Sulfate Budgets
KELVIN BATES, Eleni Dovrou, Vasquez Krystal, Frank Keutsch, Paul Wennberg, Daniel Jacob, Harvard University
Abstract Number: 467 Working Group: Aerosol Chemistry
Abstract We implement the Reduced Caltech Isoprene Mechanism (RCIM), a recently developed state-of-the-science gas-phase isoprene oxidation mechanism, into the global chemical transport model GEOS-Chem and investigate its global effects of secondary organic aerosol (SOA) formation. We find an SOA yield from isoprene of 13% per carbon, much higher than commonly assumed in models, and likely offset by SOA chemical loss through aging mechanisms not yet included in GEOS-Chem. Global production of isoprene SOA is about one third each from isoprene epoxydiols (IEPOX), organonitrates, and C5 tetrafunctional compounds, with much smaller contributions from glyoxal, methylglyoxal, and hydroxymethyl-methyl-alpha-lactone (HMML). Furthermore, through their interactions with inorganic components of aqueous particles, organic products of gas-phase isoprene oxidation can also play important roles in the budgets of particulate sulfur and nitrogen. Here, we present results from targeted experimental and field observations of two such interactions, along with simulations of their global impacts. First, using synthetic standards of isoprene-derived hydroperoxides (two isomers of C5 hydroxyhydroperoxides, ISOPOOH, as well as hydroxymethyl hydroperoxide), we show that the oxidation of SO2 to sulfate by organic hydroperoxides can proceed at atmospherically relevant rates in aerosol and cloud water due to the compounds' high solubility and reactivity. Second, by comparing GEOS-Chem sensitivity simulations to ambient observations, we constrain the rapid hydrolysis of tertiary organonitrates (τ < 1 s) in the aqueous phase, which converts reactive gas-phase nitrogen oxides into particulate inorganic nitrate. Finally, we perform GEOS-Chem global simulations with RCIM to show that both conversion of SO2 to sulfate by organic hydroperoxides and removal of NOx by isoprene hydroxynitrates play minor roles globally (<5% of total sulfate and nitrate formation), but can be significant (>50%) in areas with high biogenic VOC emissions, where these processes serve to decrease the availability of gas-phase H2SO4 for particle nucleation and NOx for ozone formation.