Abstract
Abstract. A new chemical mechanism for the oxidation of biogenic volatile organic compounds (BVOCs) is presented and implemented in the Model of Atmospheric composition at Global and Regional scales using Inversion Techniques for Trace gas Emissions (MAGRITTE v1.1). With a total of 105 organic species and over 265 gas-phase reactions, 69 photodissociations, and 7 heterogeneous reactions, the mechanism treats the chemical degradation of isoprene – its main focus – as well as acetaldehyde, acetone, methylbutenol, and the family of monoterpenes. Regarding isoprene, the mechanism incorporates a state-of-the-art representation of its oxidation scheme accounting for all major advances put forward in recent theoretical and laboratory studies. The recycling of OH radicals in isoprene oxidation through the isomerization of Z-δ-hydroxyperoxy radicals is found to enhance OH concentrations by up to 40 % over western Amazonia in the boundary layer and by 10 %–15 % over the southeastern US and Siberia in July. The model and its chemical mechanism are evaluated against the suite of chemical measurements from the SEAC4RS (Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys) airborne campaign, demonstrating a good overall agreement for major isoprene oxidation products, although the aerosol hydrolysis of tertiary and non-tertiary nitrates remain poorly constrained. The comparisons for methylnitrate indicate a very low nitrate yield (<3×10-4) in the CH3O2+NO reaction. The oxidation of isoprene, acetone, and acetaldehyde by OH is shown to be a substantial source of enols and keto-enols, primarily through the photolysis of multifunctional carbonyls generated in their oxidation schemes. Oxidation of those enols by OH radicals constitutes a sizable source of carboxylic acids estimated at 9 Tg (HC(O)OH) yr−1 and 11 Tg(CH3C(O)OH) yr−1 or ∼20 % of their global identified source. The ozonolysis of alkenes is found to be a smaller source of HC(O)OH (6 Tg HC(O)OH yr−1) than previously estimated, due to several factors including the strong deposition sink of hydroxymethyl hydroperoxide (HMHP).
Highlights
The terrestrial biosphere is, by far, the largest source of nonmethane volatile organic compounds (NMVOCs) into the global atmosphere (Guenther et al, 2012)
We have presented a new biogenic volatile organic compounds (BVOCs) oxidation mechanism for use in large-scale tropospheric chemistry-transport models
Its main focus is on isoprene, owing to its high chemical complexity and very large share of global BVOC emissions: of the 105 organic chemical species included in the mechanism, 97 compounds (74 stable compounds and 23 radicals) are involved in the chemical degradation of isoprene alone
Summary
The terrestrial biosphere is, by far, the largest source of nonmethane volatile organic compounds (NMVOCs) into the global atmosphere (Guenther et al, 2012). We present a semiexplicit mechanism of intermediate complexity incorporating the major advances reported above It covers the oxidation of isoprene, monoterpenes, methanol, acetone, acetaldehyde, ethanol, and 2-methyl-3-butene-2-ol (abbreviated as methylbutenol or MBO). The mechanism incorporates important new mechanistic developments related to, e.g. the revisited role of hydroperoxycarbonyl photolysis (Liu et al, 2017, 2018) and the fate of enols and keto-enols produced from such processes Due to these developments, the oxidation of isoprene, as well as of other compounds (e.g. acetone and acetaldehyde), by OH entails a previously unsuspected source of formic and acetic acid, Geosci. Simulations with the MAGRITTE model and the updated chemical mechanism are presented in Sect. 4, including an evaluation against airborne measurements over the eastern United States (Sect. 4.3) and a presentation of the global sources of carboxylic acids (Sect. 4.4) and glyoxal (Sect. 4.5) resulting from the implementation of the chemical mechanism
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