Chemistry and deposition in the Model of Atmospheric composition at Global and Regional scales using Inversion Techniques for Trace gas Emissions (MAGRITTE v1.1) – Part 1: Chemical mechanism

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).

A new model mechanism for atmospheric oxidation of isoprene: global effects on oxidants, nitrogen oxides, organic products, and secondary organic aerosol

“…RCIM's higher first-generation nitrate yields and faster tertiary nitrate hydrolysis relative to the GEOS-Chem v11-02c mechanism lead to reduced ozone formation from the ISOPOO + NO pathway. In the southeastern United States, where past mechanisms have tended to overestimate surface ozone (Murazaki and Hess, 2006;Yu et al, 2007Yu et al, , 2010Lin et al, 2008;Fiore et al, 2009;Rasmussen et al, 2012;Travis et al, 2016), RCIM results in a 1.5 % decrease in annual mean boundary layer ozone relative to the GEOS-Chem v11-02c mechanism. Reduced NO x transport in RCIM and reduced MPAN formation under low-NO conditions also results in higher sustained ozone over the Amazon (+3 ppbv) and lower ozone in the remote Southern Hemisphere (−1.5 ppbv) than the GEOS-Chem v11-02c mechanism.…”

Importance of isomerization reactions for OH radical regeneration from the photo-oxidation of isoprene investigated in the atmospheric simulation chamber SAPHIR

“…The concentrations of OH, HO 2 , and RO 2 radicals were measured with the laser-induced fluorescence (LIF) instrument permanently in use at the SAPHIR chamber and described previously (Holland et al, 2003;Fuchs et al, 2011). Several studies have been recently published showing the presence of an interference in OH radical detection with the LIF for ambient measurements in some environments (Mao et al, 2012;Novelli et al, 2014;Rickly and Stevens, 2018). The interference depends on the chemical conditions of the sampled air and on the geometry of the different instruments.…”

Comprehensive isoprene and terpene gas-phase chemistry improves simulated surface ozone in the southeastern US

“…In the southeastern US, there are particularly large emissions of biogenic hydrocarbons like isoprene and terpenes, which motivates updating the formation and fate of isopreneand terpene-derived organic nitrates in order to assess if more complex and current chemistry reduces model biases in surface ozone. The recent significant improvements in our understanding of isoprene oxidation chemistry (Wennberg et al, 2018, and references therein) have motivated many models to update their isoprene chemistry including GEOS-Chem (Fisher et al, 2016;Bates and Jacob, 2019), GFDL AM3 (the atmopsheric component of the Geophysical Fluid Dynamics Laboratory Coupled Model; Li et al, 2018), MAGRITTEv1.1 (Model of Atmospheric composition at Global and Regional scales using Inversion Techniques for Trace gas Emissions; Müller et al, 2019), WRF-Chem (Weather Research and Forecasting model coupled with Chemistry; Zare et al, 2018), CESM2 (Community Earth System Model;Emmons et al, 2020), and CMAQ (Community Multiscale Air Quality; Pye et al, 2013).…”

“…In general terpene oxidation has been ignored or heavily reduced in chemical schemes in many regional and global models used in the past despite recent field campaigns suggesting the importance of terpene chemistry (Xu et al, 2015;Zhang et al, 2018). Many mechanisms represent monoterpenes as a single tracer (e.g., Emmons et al, 2020;Li et al, 2018;Müller et al, 2019). Monoterpenes were expanded to include two surrogate compounds first in WRF-Chem (Browne et al, 2014;Zare et al, 2018) and then in GEOS-Chem (Fisher et al, 2016).…”

“…4.3). For isoprene, the chemical mechanism updates are of similar complexity to Müller et al (2019) and Bates and Jacob (2019) and more complex than Travis et al (2016) and Li et al (2018). For terpenes, the chemistry updates are significantly more complex than any other reduced scheme currently available (Browne et al, 2014;Fisher et al, 2016;Zare et al, 2018).…”