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Rapid hydrolysis of tertiary isoprene nitrate efficiently removes NOₓ from the atmosphere

Vasquez, Krystal T. and Crounse, John D. and Schulze, Benjamin C. and Bates, Kelvin H. and Teng, Alexander P. and Xu, Lu and Allen, Hannah M. and Wennberg, Paul O. (2020) Rapid hydrolysis of tertiary isoprene nitrate efficiently removes NOₓ from the atmosphere. Proceedings of the National Academy of Sciences of the United States of America, 117 (52). pp. 33011-33016. ISSN 0027-8424. PMCID PMC7777079. doi:10.1073/pnas.2017442117.

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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 NOₓ environment 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 NOₓ lost (and HNO₃ produced), resulting in large impacts on oxidant formation, especially over forested regions.

Item Type:Article
Related URLs:
URLURL TypeDescription Information ItemIHN isomer concentration data Itemchromatograms collected at the field site Itemoutput results of the 1-D atmospheric model Itemupdated isoprene mechanism used in the global chemical transport model ItemGEOS-Chem CentralArticle
Vasquez, Krystal T.0000-0003-4540-4212
Crounse, John D.0000-0001-5443-729X
Schulze, Benjamin C.0000-0002-6405-8872
Bates, Kelvin H.0000-0001-7544-9580
Teng, Alexander P.0000-0002-6434-0501
Xu, Lu0000-0002-0021-9876
Allen, Hannah M.0000-0002-4218-5133
Wennberg, Paul O.0000-0002-6126-3854
Alternate Title:Rapid hydrolysis of tertiary isoprene nitrate efficiently removes NOx from the atmosphere
Additional Information:© 2020 Published under the PNAS license. Edited by Mark Thiemens, University of California San Diego, La Jolla, CA, and approved November 3, 2020 (received for review August 17, 2020). First published December 10, 2020. Development of the GC-CIMS was supported by the NSF Major Research Instrumentation Program under Grant AGS-1428482 and the field and laboratory studies it participated in were supported by additional NSF funding (Grant AGS-1240604). We thank the Caltech campus and affiliated staff for accommodating our 2017 field study. Work performed by K.T.V. and H.M.A. was also supported by NSF through the Graduate Research Fellowship. Author contributions: J.D.C. and P.O.W. designed research; K.T.V., J.D.C., B.C.S., K.H.B., A.P.T., L.X., H.M.A., and P.O.W. performed research; J.D.C. and P.O.W. contributed new reagents/analytic tools; K.T.V., J.D.C., and A.P.T. analyzed data; and K.T.V., J.D.C., and P.O.W. wrote the paper. Data Availability. Atmospheric trace gas measurements and model output data have been deposited in the California Institute of Technology Research Data Repository CaltechDATA. IHN isomer concentration data used here are available online ( along with the chromatograms collected at the field site (, the output results of the 1-D atmospheric model (, and the updated isoprene mechanism used in the global chemical transport model ( GEOS-Chem is available for public use at The authors declare no competing interest. This article is a PNAS Direct Submission. This article contains supporting information online at
Funding AgencyGrant Number
NSF Graduate Research FellowshipUNSPECIFIED
Subject Keywords:atmospheric chemistry; isoprene; hydrolysis; organic nitrates; NOx
Issue or Number:52
PubMed Central ID:PMC7777079
Record Number:CaltechAUTHORS:20201210-160240365
Persistent URL:
Official Citation:Rapid hydrolysis of tertiary isoprene nitrate efficiently removes NOx from the atmosphere. Krystal T. Vasquez, John D. Crounse, Benjamin C. Schulze, Kelvin H. Bates, Alexander P. Teng, Lu Xu, Hannah M. Allen, Paul O. Wennberg. Proceedings of the National Academy of Sciences Dec 2020, 117 (52) 33011-33016; DOI: 10.1073/pnas.2017442117
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:107029
Deposited By: George Porter
Deposited On:11 Dec 2020 16:33
Last Modified:09 Feb 2022 19:51

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