04 Sep 2023
 | 04 Sep 2023
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Disappearing day-of-week ozone patterns in US nonattainment areas

Heather Simon, Christian Hogrefe, Andrew Whitehill, Kristen M. Foley, Jennifer Liljegren, Norm Possiel, Benjamin Wells, Barron H. Henderson, Lukas C. Valin, Gail Tonnesen, K. Wyat Appel, and Shannon Koplitz

Abstract. Past work has shown that traffic patterns in the US and resulting NOX emissions vary by day of week, with NOX emissions typically higher on weekdays than weekends. This pattern of emissions leads to different levels of ozone on weekends versus weekdays and can be leveraged to understand how local ozone formation changes in response to NOX emissions perturbations in different urban areas. Specifically, areas with lower NOX but higher ozone on the weekends (the weekend effect) can be characterized as NOX -saturated and areas with both lower NOX and ozone on weekends (the weekday effect) can be characterized as NOX-limited. In this analysis we assess ozone weekend-weekday differences across US nonattainment areas using 18 years of observed and modeled data from 2002–2019 using two metrics: mean ozone and percentage of days > 70 ppb. In addition, we quantify the modeled and observed trends in these weekend-weekday differences across this period of substantial NOX emissions reductions in the US. The model assessment is carried out using EPA’s Air QUAlity TimE Series Project (EQUATES) CMAQ dataset. We identify 3 types of ozone trends occuring across the US: disappearing weekend effect, disappearing weekday effect, and no trend. The disappearing weekend effect occurs in a subset of large urban areas that were NOX -saturated (i.e., VOC-limited) at the beginning of the analysis period but transitioned to mixed chemical regimes or NOX-limited conditions by the end of the analysis period. Nine areas have disappearing weekend effect trends in both datasets and with both metrics indicating strong agreement that they are shifting to more NOX-limited conditions: Milwaukee, Houston, Phoenix, Denver, Northern Wasatch Front, Southern Wasatch Front, Las Vegas, Los Angeles – San Bernardino County, Los Angeles – South Coast, and San Diego. The disappearing weekday effect was identified for multiple rural and agricultural areas of California which were NOX -limited for the entire analysis period but appear to become less influenced by local day of week emission patterns in more recent years. Finally, we discuss a variety of reasons why there are no statistically significant trends in certain areas including complex impacts of heterogeneous source mixes and stochastic impacts of meteorology. Overall, this assessment finds that the EQUATES modeling simulations indicate more NOX-saturated conditions than the observations but do a good job of capturing year-to-year changes in weekend-weekday ozone patterns.

Heather Simon et al.

Status: open (until 20 Oct 2023)

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  • RC1: 'Comment on egusphere-2023-1974', Anonymous Referee #1, 25 Sep 2023 reply

Heather Simon et al.


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Short summary
We assess observed and modeled ozone weekend-weekday differences in the US from 2002–2019. A subset of urban areas that were NOX-saturated at the beginning of the period transitioned to NOX-limited conditions. Multiple rural areas of California were NOX-limited for the entire period but become less influenced by local day-of-week emission patterns in more recent years. The model produces more NOX-saturated conditions than the observations but captures trends in weekend-weekday ozone patterns.