U.S. NOx and O3 now declining very slowly thru 2019. What will 2020 show?
NOx concentrations clearly impacts O3 formation in these areas. The annual fourth highest daily 8-hour maximum O3 concentrations show marked declines from 1995-2014, but little change since then. We can use the observed O3-NOx relationship over the longer time period and the current NOx trends in these urban areas to estimate a timeline for meeting the national O3 standards in these 50 areas. This time ranges from 10 years to almost a century, depending on the assumptions made and the rate of decline in NOx concentrations. This work indicates that meeting the national O3 standard will likely require new programs and technologies to reduce NOx across the U.S.
While our analysis does not include the 2020 O3 season at this time, the 2020 data will be very useful to evaluate the NOx-O3 relationship in these urban areas. For example, based on the historical pattern, a 10% decline in NOx in these urban areas would reduce the median 4th highest MDA8 by about 7 ppb. We will incorporate the 2020 data into our presentation and update our projected timeline for meeting the national O3 standard.