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Constraining remote oxidation capacity with ATom observations

Travis, Katherine R. and Heald, Colette L. and Allen, Hannah M. and Apel, Eric C. and Arnold, Stephen R. and Blake, Donald R. and Brune, William H. and Chen, Xin and Commane, Róisín and Crounse, John D. and Daube, Bruce C. and Diskin, Glenn S. and Elkins, James W. and Evans, Mathew J. and Hall, Samuel R. and Hintsa, Eric J. and Hornbrook, Rebecca S. and Kasibhatla, Prasad S. and Kim, Michelle J. and Luo, Gan and McKain, Kathryn and Millet, Dylan B. and Moore, Fred L. and Peischl, Jeffrey and Ryerson, Thomas B. and Sherwen, Tomás and Thames, Alexander B. and Ullmann, Kirk and Wang, Xuan and Wennberg, Paul O. and Wolfe, Glenn M. and Yu, Fangqun (2020) Constraining remote oxidation capacity with ATom observations. Atmospheric Chemistry and Physics, 20 (13). pp. 7753-7781. ISSN 1680-7324. doi:10.5194/acp-20-7753-2020.

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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 N_Oy 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 (cOHR_(obs)) or by the model (cOHR_(mod)). 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 cOHR_(obs) 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 cOHR_(mod) 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.

Item Type:Article
Related URLs:
URLURL TypeDescription Information
Travis, Katherine R.0000-0003-1628-0353
Heald, Colette L.0000-0003-2894-5738
Allen, Hannah M.0000-0002-4218-5133
Apel, Eric C.0000-0001-9421-818X
Brune, William H.0000-0002-1609-4051
Chen, Xin0000-0002-0952-0008
Commane, Róisín0000-0003-1373-1550
Crounse, John D.0000-0001-5443-729X
Diskin, Glenn S.0000-0002-3617-0269
Elkins, James W.0000-0003-4701-3100
Evans, Mathew J.0000-0003-4775-032X
Hall, Samuel R.0000-0002-2060-7112
Hintsa, Eric J.0000-0002-5289-630X
Hornbrook, Rebecca S.0000-0002-6304-6554
Kasibhatla, Prasad S.0000-0003-3562-3737
Kim, Michelle J.0000-0002-4922-4334
Luo, Gan0000-0002-9588-7008
McKain, Kathryn0000-0002-8323-5758
Millet, Dylan B.0000-0003-3076-125X
Moore, Fred L.0000-0002-8461-2535
Peischl, Jeffrey0000-0002-9320-7101
Ryerson, Thomas B.0000-0003-2800-7581
Sherwen, Tomás0000-0002-3006-3876
Ullmann, Kirk0000-0002-4724-9634
Wang, Xuan0000-0002-8532-5773
Wennberg, Paul O.0000-0002-6126-3854
Wolfe, Glenn M.0000-0001-6586-4043
Yu, Fangqun0000-0001-8862-4835
Additional Information:© Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Received: 11 Oct 2019 – Discussion started: 03 Jan 2020 – Revised: 12 Apr 2020 – Accepted: 03 May 2020 – Published: 03 Jul 2020. We are grateful for helpful conversations and advice from Andrea Molod, Rachel Silvern, Eloïse Marais, Sarah Safieddine, Martin Brüggemann, Christian George, and James Crawford. We acknowledge Tom Hanisco and Jason St. Clair for the use of their formaldehyde observations from ATom and Barbara Barletta and Simone Meinardi for their contribution to the UCI WAS measurements. This research has been supported by the National Science Foundation (grant no. AGS-1564495), the National Center for Atmospheric Research (grant no. 1852977), the National Oceanic and Atmospheric Administration (grant no. NA18OAR4310110), and the National Aeronautics and Space Administration (grant no. NNX14AP89G, grant no. IAT NNH15AB12I, grant no. NNX17AG35G, grant no. NNX15AG61A, grant no. NNX15AG71A). Author contributions. CLH and KRT designed the study and wrote the paper with input from the co-authors. KRT modified the code, performed the simulations, and led the analysis. HMA, ECA, DRB, WHB, RC, JDC, BCD, GSD, JWE, SRH, EJH, SRH, MJK, KM, FLM, JP, TBR, ABT, KU, POW, and GMW provided ATom measurements used in the analysis. XW provided the model code for the sensitivity runs including acid displacement of chloride on coarse-mode sea-salt aerosols. TS, ME, and PSK provided the model code for the photolysis of particulate nitrate. GL and FY were responsible for the code for the revised treatment of wet scavenging in the model. DBM and XC provided the methanol seawater concentration and assisted in the ocean budget analysis. SRA provided the biogenic ocean isoprene emissions. Data availability. The ATom-1 and ATom-2 data (Wofsy et al., 2018) are available here: The supplement related to this article is available online at: The authors declare that they have no conflict of interest. Review statement. This paper was edited by Yafang Cheng and reviewed by three anonymous referees.
Funding AgencyGrant Number
National Center for Atmospheric Research (NCAR)1852977
National Oceanic and Atmospheric Administration (NOAA)NA18OAR4310110
Issue or Number:13
Record Number:CaltechAUTHORS:20200723-122440518
Persistent URL:
Official Citation:Travis, K. R., Heald, C. L., Allen, H. M., Apel, E. C., Arnold, S. R., Blake, D. R., Brune, W. H., Chen, X., Commane, R., Crounse, J. D., Daube, B. C., Diskin, G. S., Elkins, J. W., Evans, M. J., Hall, S. R., Hintsa, E. J., Hornbrook, R. S., Kasibhatla, P. S., Kim, M. J., Luo, G., McKain, K., Millet, D. B., Moore, F. L., Peischl, J., Ryerson, T. B., Sherwen, T., Thames, A. B., Ullmann, K., Wang, X., Wennberg, P. O., Wolfe, G. M., and Yu, F.: Constraining remote oxidation capacity with ATom observations, Atmos. Chem. Phys., 20, 7753–7781,, 2020.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:104537
Deposited By: George Porter
Deposited On:24 Jul 2020 14:10
Last Modified:16 Nov 2021 18:32

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