Improved Estimation of Trends in U.S. Ozone Concentrations Adjusted for Interannual Variability in Meteorological Conditions

Daily maximum 8-hour average (MDA8) ozone (O₃) concentrations are well-known to be influenced by local meteorological conditions, which vary across both daily and seasonal temporal scales. Previous studies have adjusted long-term trends in O₃ concentrations for meteorological effects using various statistical and mathematical methods in order to get a better estimate of the long-term changes in O₃ concentrations due to changes in precursor emissions such as nitrogen oxides (NO_X) and volatile organic compounds (VOCs). In this work, the authors present improvements to the current method used by the United States Environmental Protection Agency (US EPA) to adjust O₃ trends for meteorological influences by making refinements to the input data sources and by allowing the underlying statistical model to vary locally using a variable selection procedure. The current method is also expanded by using a quantile regression model to adjust trends in the 90^th and 98^th percentiles of the distribution of MDA8 O₃ concentrations, allowing for a better understanding of the effects of local meteorology on peak O₃ levels in addition to seasonal average concentrations. The revised method is used to adjust trends in the May to September mean, 90^th percentile, and 98^th percentile MDA8 O₃ concentrations at over 700 monitoring sites in the U.S. for years 2000 to 2016. The utilization of variable selection and quantile regression allow for a more in-depth understanding of how weather conditions affect O₃ levels in the U.S. This represents a fundamental advancement in our ability to understand how interannual variability in weather conditions in the U.S. may impact attainment of the O₃ National Ambient Air Quality Standards (NAAQS).