Summertime Clean-Background Ozone Concentrations Derived from Ozone Precursor Relationships are Lower than Previous Estimates in the Southeast United States

Abstract

Background ozone in this study is defined as the amount of ozone that is not affected by the emissions of ozone precursors in the region of study and is transported from the distant troposphere or the stratosphere. It is one of the factors that must be considered in regional ozone control strategies. Different methods have been applied to define the background ozone level. We develop a new method based on the O₃–CO–HCHO relationships, which can be applied to both observation and modeling data for regions with high isoprene emission ozone, such as the Southeast United States. We make use of the extensive aircraft and surface observations in the Southeast in the summer of 2013. Compared to the diagnostic results using the relationship of O₃–NO_z (total reactive nitrogen excluding nitrogen oxides), zero-emission (model-only), and 5^th percentile methods, the new method is most consistent using observation or model data and the resulting background ozone concentrations are 4–50% lower than the other methods for field campaigns. Using this method, we find that the summertime background ozone at the surface is in the range of 10–15 ppbv in the inland areas of the Southeast, which is lower than that reported in previous studies. This background ozone tends to increase from urban centers to rural regions and from the surface to higher altitude due to changing ozone lifetime driven by anthropogenic emissions and dry deposition to the surface. The better quantification of background ozone using the new method highlights the importance of the contributions by natural emissions to ozone and the necessity to control anthropogenic emissions in ozone nonattainment areas of the Southeast.