Smoke in the City: How Often and Where Does Smoke Impact Summertime Ozone in the United States?

Abstract:

The mechanisms and magnitude of the contribution of fires to ozone (O₃) production is poorly understood. In this work we investigate the influence of fire on O₃ abundances over the contiguous United States. Using co-located observations of particulate matter (PM_2.5) and the National Weather Service Hazard Mapping System smoke data, we identify summertime days between 2005 and 2014 that Environmental Protection Agency (EPA) Air Quality System (AQS) O₃ monitors are influenced by smoke. We then compare O₃ mixing ratio distributions for smoke-free and smoke-impacted days for each O₃ monitor. We observe that the mean O₃ abundance measured on smoke-impacted days is higher than on smoke-free days. The magnitude of the effect varies by location with a range of 0 to 37 parts per billion by volume (ppbv). We find that smoke is present on a non-negligible proportion of days when the 8-hour average O₃ mixing ratio exceeds the EPA limit of 75 ppbv in regions and locations with significant O₃ issues, including the Northeast urban corridor, Dallas, Houston, Atlanta, Birmingham, and Kansas City. Most U.S. cities maintain a similar proportion of smoke-impacted exceedence days when they are held against a more stringent limit of 65 ppbv for an 8-hour average. We show that smoke-impacted O₃ mixing ratios are most elevated in U.S. cities with the highest emissions of nitrogen oxides (NO_X).