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

Abstract

Abstract. Atmospheric oxidation of isoprene, the most abundantly emitted non-methane hydrocarbon, affects the abundances of ozone (O3), the hydroxyl radical (OH), nitrogen oxide radicals (NOx), carbon monoxide (CO), oxygenated and nitrated organic compounds, and secondary organic aerosol (SOA). We analyze these effects in box models and in the global GEOS-Chem chemical transport model using the new reduced Caltech isoprene mechanism (RCIM) condensed from a recently developed explicit isoprene oxidation mechanism. We find many similarities with previous global models of isoprene chemistry along with a number of important differences. Proper accounting of the isomer distribution of peroxy radicals following the addition of OH and O2 to isoprene influences the subsequent distribution of products, decreasing in particular the yield of methacrolein and increasing the capacity of intramolecular hydrogen shifts to promptly regenerate OH. Hydrogen shift reactions throughout the mechanism lead to increased OH recycling, resulting in less depletion of OH under low-NO conditions than in previous mechanisms. Higher organonitrate yields and faster tertiary nitrate hydrolysis lead to more efficient NOx removal by isoprene and conversion to inorganic nitrate. Only 20 % of isoprene-derived organonitrates (excluding peroxyacyl nitrates) are chemically recycled to NOx. The global yield of formaldehyde from isoprene is 22 % per carbon and less sensitive to NO than in previous mechanisms. The global molar yield of glyoxal is 2 %, much lower than in previous mechanisms because of deposition and aerosol uptake of glyoxal precursors. Global production of isoprene SOA is about one-third from each of the following: isoprene epoxydiols (IEPOX), organonitrates, and tetrafunctional compounds. We find a SOA yield from isoprene of 13 % per carbon, much higher than commonly assumed in models and likely offset by SOA chemical loss. We use the results of our simulations to further condense RCIM into a mini Caltech isoprene mechanism (Mini-CIM) for less expensive implementation in atmospheric models, with a total size (108 species, 345 reactions) comparable to currently used mechanisms.

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

“…S3). Several studies showing the global impact of isomerization reactions were performed (Bates and Jacob, 2019;Møller et al, 2019;Müller et al, 2019), all showing, similarly to the results within this study, an enhanced concentration of the Table S6 for more details). The RE is sustained at low levels of NO by the contribution from isomerization reactions, in particular by the aldehyde-H shift and its products.…”

“…However, based on MC-TST calculations incorporating all conformers, we found that thermal decomposition of the tri-HPACYL-I acyl radicals is still sufficiently fast to dominate over O 2 addition; i.e. even when assuming a Boltzmann energy distribution, the predicted rate coefficient of ∼ 2 × 10 8 s −1 at 300 K (see Table 3) is significantly higher than the effective O 2 addition rate for acyl radicals, experimentally measured at ≤∼ 3 × 10 7 s −1 in atmospheric conditions (Sehested et al, 1998;Blitz et al, 2002;Park et al, 2004; Baulch et al, 2005;Atkinson et al, 2006;Carr et al, 2011). One could counter that the presence of -OOH groups might stabilize the acylperoxy radicals formed in the O 2 addition (e.g.…”

“…istry to the formation of tropospheric pollutant ozone (O 3 ). Because high levels of OH radicals were observed in field experiments in mainly forested environments with large concentrations of isoprene (Tan et al, 2001;Ren et al, 2008;Hofzumahaus et al, 2009;Kubistin et al, 2010;Whalley et al, 2011), a large number of investigations over the last decade focused on OH-initiated isoprene chemistry, including laboratory and chamber studies (Crounse et al, 2011;Berndt, 2012;Wolfe et al, 2012;Fuchs et al, 2013;Teng et al, 2017;Berndt et al, 2019), theoretical calculations (Peeters et al, 2009(Peeters et al, , 2014Da Silva et al, 2010;Peeters and Müller, 2010;Wang et al, 2018;Møller et al, 2019), and global model impact (Lelieveld et al, 2008;Taraborrelli et al, 2012;Bates and Jacob, 2019;Møller et al, 2019;Müller et al, 2019). The observed OH levels could only be explained if an OH radical regeneration mechanism exists independently of NO and thus without the formation of O 3 .…”

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).…”

“…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).…”