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Estimate of OH trends over one decade in North American cities

Zhu, Qindan and Laughner, Joshua L. and Cohen, Ronald C. (2022) Estimate of OH trends over one decade in North American cities. Proceedings of the National Academy of Sciences of the United States of America, 119 (16). Art. No. e2117399119. ISSN 0027-8424. PMCID PMC9169711. doi:10.1073/pnas.2117399119.

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The hydroxyl radical (OH) is the most important oxidant on global and local scales in the troposphere. Urban OH controls the removal rate of primary pollutants and triggers the production of ozone. Interannual trends of OH in urban areas are not well documented or understood due to the short lifetime and high spatial heterogeneity of OH. We utilize machine learning with observational inputs emphasizing satellite remote sensing observations to predict surface OH in 49 North American cities from 2005 to 2014. We observe changes in the summertime OH over one decade, with wide variation among different cities. In 2014, compared to the summertime OH in 2005, 3 cities show a significant increase of OH, whereas, in 27 cities, OH decreases in 2014. The year-to-year variation of OH is mapped to the decline of the NO₂ column. We conclude that these cities in this analysis are either in the NOₓ-limited regime or at the transition from a NOₓ suppressed regime to a NOₓ-limited regime. The result emphasizes that, in the future, controlling NOₓ emissions will be most effective in regulating the ozone pollution in these cities.

Item Type:Article
Related URLs:
URLURL TypeDescription CentralArticle
Zhu, Qindan0000-0003-2173-4014
Laughner, Joshua L.0000-0002-8599-4555
Cohen, Ronald C.0000-0001-6617-7691
Additional Information:© 2022 the Author(s). Published by PNAS. This open access article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND). Edited by Mark Thiemens, University of California San Diego, La Jolla, CA; received September 21, 2021; accepted February 18, 2022. We acknowledge support from NASA Grant 80NSSC19K0945. We acknowledge high-performance computing support from the Savio computational cluster resource provided by the Berkeley Research Computing program at the University of California, Berkeley (supported by the University of California, Berkeley Chancellor, Vice Chancellor for Research, and Chief Information Officer). We acknowledge use of the WRF-Chem preprocessor tools (boundary conditions using MOZART global model output, MOZBC) provided by the Atmospheric Chemistry Observations and Modeling laboratory of National Center for Atmospheric Research. Author contributions: Q.Z., J.L.L., and R.C.C. designed research; Q.Z. performed research; Q.Z. and J.L.L. contributed new reagents/analytic tools; Q.Z. analyzed data; and Q.Z. wrote the paper. The authors declare no competing interest. This article is a PNAS Direct Submission. Data Availability: Zip data have been deposited in “Supporting data for ‘Estimate of OH trends over one Decade in North American cities”’ ( The analysis code is available at Zenodo, (37). This article contains supporting information online at
Funding AgencyGrant Number
University of California, BerkeleyUNSPECIFIED
Subject Keywords:hydroxyl radical; ozone control; NOx emission
Issue or Number:16
PubMed Central ID:PMC9169711
Record Number:CaltechAUTHORS:20220413-921120100
Persistent URL:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:114277
Deposited By: Tony Diaz
Deposited On:13 Apr 2022 17:39
Last Modified:07 Jun 2022 23:49

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