Changes in ozone photochemical regime in Fresno, California from 1994 to 2018 deduced from changes in the weekend effect.

Significant progress has been made in reducing emissions of air pollutants in the San Joaquin Valley in California. Nevertheless, from May to October, the valley still experiences numerous exceedances of the ozone health standard. As the standards are tightened, it is becoming harder to design policies to attain them. To better understand historical emissions reductions in the context of necessary future control efforts, we analyze 25 years of hourly measurements of ozone and nitrogen oxides concentrations for the hottest one third of days in Fresno using multiple linear regression analysis. We then analyze the changing dynamics of the weekend effect over the years in order to evaluate the growing importance of day-to-day carryover on ozone concentrations. A simplified model of the day-of-week pattern of ozone concentrations is used to explore the impact of same-day and previous-day concentrations. In addition to ozone, Ox (O3 + NO2) is used to distinguish reductions of atmospheric oxidants from short-duration exchanges between O3 and NO2. The analysis shows that there has been a significant increase in the importance of day-to-day carryover on ozone levels, and that consequently the ozone weekend effect in Fresno has changed over the last 25 years. In the 1990s, lower NOx on the weekend led to increased ozone on Saturdays and Sundays but levels of Ox remained constant. In the 2010s, lower weekend NOx led to reduced ozone on Saturdays, Sundays and Mondays showing that reductions in primary pollutants are sufficient to yield immediate decreases in secondary pollutants. Overall, the photochemical regime in the atmosphere has evolved such that carryover and regional pollution will be increasingly important in determining local ozone concentrations. Policies will therefore need to pay greater attention to regional emissions as local reductions may not be sufficient to meet the health standard.