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Published April 14, 2022 | Published + Supplemental Material
Journal Article Open

Direct measurements of ozone response to emissions perturbations in California


A new technique was used to directly measure O₃ response to changes in precursor NOₓ and volatile organic compound (VOC) concentrations in the atmosphere using three identical Teflon smog chambers equipped with UV lights. One chamber served as the baseline measurement for O₃ formation, one chamber added NOₓ, and one chamber added surrogate VOCs (ethylene, m-xylene, n-hexane). Comparing the O₃ formation between chambers over a 3-hour UV cycle provides a direct measurement of O₃ sensitivity to precursor concentrations. Measurements made with this system at Sacramento, California, between April–December 2020 revealed that the atmospheric chemical regime followed a seasonal cycle. O₃ formation was VOC-limited (NOₓ-rich) during the early spring, transitioned to NOₓ-limited during the summer due to increased concentrations of ambient VOCs with high O₃ formation potential, and then returned to VOC-limited (NOₓ-rich) during the fall season as the concentrations of ambient VOCs decreased and NOₓ increased. This seasonal pattern of O₃ sensitivity is consistent with the cycle of biogenic emissions in California. The direct chamber O₃ sensitivity measurements matched semi-direct measurements of ratios from the TROPOspheric Monitoring Instrument (TROPOMI) aboard the Sentinel-5 Precursor (Sentinel-5P) satellite. Furthermore, the satellite observations showed that the same seasonal cycle in O₃ sensitivity occurred over most of the entire state of California, with only the urban cores of the very large cities remaining VOC-limited across all seasons. The O₃-nonattainment days (MDA8 O₃ > 70 ppb) have O₃ sensitivity in the NOₓ-limited regime, suggesting that a NOₓ emissions control strategy would be most effective at reducing these peak O₃ concentrations. In contrast, a large portion of the days with MDA8 O₃ concentrations below 55 ppb were in the VOC-limited regime, suggesting that an emissions control strategy focusing on NOₓ reduction would increase O₃ concentrations. This challenging situation suggests that emissions control programs that focus on NOₓ reductions will immediately lower peak O₃ concentrations but slightly increase intermediate O₃ concentrations until NOₓ levels fall far enough to re-enter the NOₓ-limited regime. The spatial pattern of increasing and decreasing O₃ concentrations in response to a NOₓ emissions control strategy should be carefully mapped in order to fully understand the public health implications.

Additional Information

© Author(s) 2022. This work is distributed under the Creative Commons Attribution 4.0 License. Received: 20 Aug 2021 – Discussion started: 20 Sep 2021 – Revised: 11 Mar 2022 – Accepted: 15 Mar 2022 – Published: 14 Apr 2022. The authors thank Michael Miguel, Anthony Esparza, and Aimee Davis of the California Air Resources Board (CARB) for their logistical support surrounding the siting of the chamber experiments. This research has been supported by the California Air Resources Board (grant no. 19RD012), the Coordinating Research Council (grant nos. A-121 and A-121-2), and the University of California Institute of Transportation Studies through funding from the State of California via the Public Transportation Account and the Road Repair and Accountability Act of 2017 (SB 1). Author contributions. SW made field measurements and wrote the initial draft of each version of the manuscript. HJL processed TROPOMI data. AR analyzed wildfire vs. no-wildfire periods. SL provided project management. TK constructed the initial version of the chambers. JHS hosted initial measurements and helped revise the manuscript. MJK designed the experiment, directed the data analysis, coded the chamber model, and revised the manuscript. Data availability. Daily chamber measurement data including O₃ concentration, O₃ sensitivity, and NOₓ concentration can be accessed through https://datadryad.org/stash/share/ktJh3AxAs0K7y8Iku8-VL3v7ZuGwBGQodYhRT-wHZ04 (Wu et al., 2022). The supplement related to this article is available online at: https://doi.org/10.5194/acp-22-4929-2022-supplement. The contact author has declared that neither they nor their co-authors have any competing interests. This document is disseminated in the interest of information exchange and does not necessarily reflect the official views or policies of the State of California, the California Air Resources Board, or the Coordinating Research Council. This paper was edited by Andreas Hofzumahaus and reviewed by William Stockwell, David Parrish, and two anonymous referees.

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Published - acp-22-4929-2022.pdf

Supplemental Material - acp-22-4929-2022-supplement.pdf


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August 22, 2023
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