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Constraining remote oxidation capacity with ATom observations.

Publication ,  Journal Article
Travis, KR; Heald, CL; Allen, HM; Apel, EC; Arnold, SR; Blake, DR; Brune, WH; Chen, X; Commane, R; Crounse, JD; Daube, BC; Diskin, GS; Luo, G ...
Published in: Atmospheric chemistry and physics
July 2020

The global oxidation capacity, defined as the tropospheric mean concentration of the hydroxyl radical (OH), controls the lifetime of reactive trace gases in the atmosphere such as methane and carbon monoxide (CO). Models tend to underestimate the methane lifetime and CO concentrations throughout the troposphere, which is consistent with excessive OH. Approximately half of the oxidation of methane and non-methane volatile organic compounds (VOCs) is thought to occur over the oceans where oxidant chemistry has received little validation due to a lack of observational constraints. We use observations from the first two deployments of the NASA ATom aircraft campaign during July-August 2016 and January-February 2017 to evaluate the oxidation capacity over the remote oceans and its representation by the GEOS-Chem chemical transport model. The model successfully simulates the magnitude and vertical profile of remote OH within the measurement uncertainties. Comparisons against the drivers of OH production (water vapor, ozone, and NO y concentrations, ozone photolysis frequencies) also show minimal bias, with the exception of wintertime NO y . The severe model overestimate of NO y during this period may indicate insufficient wet scavenging and/or missing loss on sea-salt aerosols. Large uncertainties in these processes require further study to improve simulated NO y partitioning and removal in the troposphere, but preliminary tests suggest that their overall impact could marginally reduce the model bias in tropospheric OH. During the ATom-1 deployment, OH reactivity (OHR) below 3 km is significantly enhanced, and this is not captured by the sum of its measured components (cOHRobs) or by the model (cOHRmod). This enhancement could suggest missing reactive VOCs but cannot be explained by a comprehensive simulation of both biotic and abiotic ocean sources of VOCs. Additional sources of VOC reactivity in this region are difficult to reconcile with the full suite of ATom measurement constraints. The model generally reproduces the magnitude and seasonality of cOHRobs but underestimates the contribution of oxygenated VOCs, mainly acetaldehyde, which is severely underestimated throughout the troposphere despite its calculated lifetime of less than a day. Missing model acetaldehyde in previous studies was attributed to measurement uncertainties that have been largely resolved. Observations of peroxyacetic acid (PAA) provide new support for remote levels of acetaldehyde. The underestimate in both model acetaldehyde and PAA is present throughout the year in both hemispheres and peaks during Northern Hemisphere summer. The addition of ocean sources of VOCs in the model increases cOHRmod by 3% to 9% and improves model-measurement agreement for acetaldehyde, particularly in winter, but cannot resolve the model summertime bias. Doing so would require 100 Tg yr-1 of a long-lived unknown precursor throughout the year with significant additional emissions in the Northern Hemisphere summer. Improving the model bias for remote acetaldehyde and PAA is unlikely to fully resolve previously reported model global biases in OH and methane lifetime, suggesting that future work should examine the sources and sinks of OH over land.

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Published In

Atmospheric chemistry and physics

DOI

EISSN

1680-7324

ISSN

1680-7316

Publication Date

July 2020

Volume

20

Issue

13

Start / End Page

7753 / 7781

Related Subject Headings

  • Meteorology & Atmospheric Sciences
  • 3702 Climate change science
  • 3701 Atmospheric sciences
  • 0401 Atmospheric Sciences
  • 0201 Astronomical and Space Sciences
 

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Travis, K. R., Heald, C. L., Allen, H. M., Apel, E. C., Arnold, S. R., Blake, D. R., … Yu, F. (2020). Constraining remote oxidation capacity with ATom observations. Atmospheric Chemistry and Physics, 20(13), 7753–7781. https://doi.org/10.5194/acp-20-7753-2020
Travis, Katherine R., Colette L. Heald, Hannah M. Allen, Eric C. Apel, Stephen R. Arnold, Donald R. Blake, William H. Brune, et al. “Constraining remote oxidation capacity with ATom observations.Atmospheric Chemistry and Physics 20, no. 13 (July 2020): 7753–81. https://doi.org/10.5194/acp-20-7753-2020.
Travis KR, Heald CL, Allen HM, Apel EC, Arnold SR, Blake DR, et al. Constraining remote oxidation capacity with ATom observations. Atmospheric chemistry and physics. 2020 Jul;20(13):7753–81.
Travis, Katherine R., et al. “Constraining remote oxidation capacity with ATom observations.Atmospheric Chemistry and Physics, vol. 20, no. 13, July 2020, pp. 7753–81. Epmc, doi:10.5194/acp-20-7753-2020.
Travis KR, Heald CL, Allen HM, Apel EC, Arnold SR, Blake DR, Brune WH, Chen X, Commane R, Crounse JD, Daube BC, Diskin GS, Elkins JW, Evans MJ, Hall SR, Hintsa EJ, Hornbrook RS, Kasibhatla PS, Kim MJ, Luo G, McKain K, Millet DB, Moore FL, Peischl J, Ryerson TB, Sherwen T, Thames AB, Ullmann K, Wang X, Wennberg PO, Wolfe GM, Yu F. Constraining remote oxidation capacity with ATom observations. Atmospheric chemistry and physics. 2020 Jul;20(13):7753–7781.

Published In

Atmospheric chemistry and physics

DOI

EISSN

1680-7324

ISSN

1680-7316

Publication Date

July 2020

Volume

20

Issue

13

Start / End Page

7753 / 7781

Related Subject Headings

  • Meteorology & Atmospheric Sciences
  • 3702 Climate change science
  • 3701 Atmospheric sciences
  • 0401 Atmospheric Sciences
  • 0201 Astronomical and Space Sciences