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.
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Rapid hydrolysis of tertiary isoprene nitrate efficiently removes NO x from the atmosphere
The formation of a suite of isoprene-derived hydroxy nitrate (IHN) isomers during the OH-initiated oxidation of isoprene affects both the concentration and distribution of nitrogen oxide free radicals (NOx). Experiments performed in an atmospheric simulation chamber suggest that the lifetime of the most abundant isomer, 1,2-IHN, is shortened significantly by a water-mediated process (leading to nitric acid formation), while the lifetime of a similar isomer, 4,3-IHN, is not. Consistent with these chamber studies, NMR kinetic experiments constrain the 1,2-IHN hydrolysis lifetime to less than 10 s in deuterium oxide (D2O) at 298 K, whereas the 4,3-IHN isomer has been observed to hydrolyze much less efficiently. These laboratory findings are used to interpret observations of the IHN isomer distribution in ambient air. The IHN isomer ratio (1,2-IHN to 4,3-IHN) in a high NOxenvironment decreases rapidly in the afternoon, which is not explained using known gas-phase chemistry. When simulated with an observationally constrained model, we find that an additional loss process for the 1,2-IHN isomer with a time constant of about 6 h best explains our atmospheric measurements. Using estimates for 1,2-IHN Henry’s law constant and atmospheric liquid water volume, we show that condensed-phase hydrolysis of 1,2-IHN can account for this loss process. Simulations from a global chemistry transport model show that the hydrolysis of 1,2-IHN accounts for a substantial fraction of NOxlost (and HNO3produced), resulting in large impacts on oxidant formation, especially over forested regions.
more » « less- NSF-PAR ID:
- 10205325
- Publisher / Repository:
- Proceedings of the National Academy of Sciences
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 117
- Issue:
- 52
- ISSN:
- 0027-8424
- Page Range / eLocation ID:
- p. 33011-33016
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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