U.S. NOx and O3 now declining very slowly thru 2019. What will 2020 show?

Abstract:

The U.S. has made steady progress in reducing NOx and O₃ concentrations. Nonetheless, there are still more than 100 million people who live in approximately 50 urban areas that exceed the current O₃ standard. We evaluated the long-term trends in observed surface summer NO₂ and O₃ concentrations in these 50 urban areas along with the national NOx emission inventory for 1995-2019. Between 2002 and 2015, there is an excellent relationship between the emission inventory and the observed median NOx concentrations in these 50 urban areas (R²=0.98) and both declined by 50-60%. But starting in 2015 the inventory shows an overly optimistic pattern of reductions, declining by 27% for the 2015-2019 period, whereas the observed median NOx concentrations in these urban centers declined by only 5%.

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 4^th 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.