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Discussion papers
https://doi.org/10.5194/acp-2019-328
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-328
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 15 Apr 2019

Research article | 15 Apr 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

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

Kelvin H. Bates1 and Daniel J. Jacob2 Kelvin H. Bates and Daniel J. Jacob
  • 1Faculty of Arts and Sciences, Harvard University, Cambridge, MA 02138, USA
  • 2School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA

Abstract. Atmospheric oxidation of isoprene, the most abundantly emitted non-methane hydrocarbon, affects the abundances of ozone, 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 each from 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.

Kelvin H. Bates and Daniel J. Jacob
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Kelvin H. Bates and Daniel J. Jacob
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Isoprene Oxidation Model K. H. Bates and P. O. Wennberg https://doi.org/10.22002/D1.247

Kelvin H. Bates and Daniel J. Jacob
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Short summary
Isoprene is a highly reactive chemical released to the atmosphere by plants. Its gas-phase reactions and interactions with chemicals released by human activity have far-reaching atmospheric consequences, contributing to ozone and particulate pollution and prolonging the lifetime of methane, a potent greenhouse gas. We use global simulations with a new isoprene reaction scheme to quantify those effects, and to show how recently discovered aspects of isoprene chemistry play out on a global scale.
Isoprene is a highly reactive chemical released to the atmosphere by plants. Its gas-phase...
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