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

Past work has shown that traffic patterns in the US and resulting NO_X emissions vary by day of week, with NO_X 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 NO_X emissions perturbations in different urban areas. Specifically, areas with lower NO_X but higher ozone on the weekends (the weekend effect) can be characterized as NO_X -saturated and areas with both lower NO_X and ozone on weekends (the weekday effect) can be characterized as NO_X-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 NO_X 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 NO_X -saturated (i.e., VOC-limited) at the beginning of the analysis period but transitioned to mixed chemical regimes or NO_X-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 NO_X-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 NO_X -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 NO_X-saturated conditions than the observations but do a good job of capturing year-to-year changes in weekend-weekday ozone patterns. [ABSTRACT FROM AUTHOR]