The impact of nitrogen oxides concentration decreases on ozone trends in the USA

Ozone (O₃) has harmful effects on human health and ecosystems. In the USA, significant reductions of O₃ precursors-nitrogen oxides (NO_x) and volatile organic compounds (VOCs)-have not yielded proportionate decreases in O₃. NO_x is a major precursor of O₃ as well as a quencher of O₃ through NO_x titration, which is especially important during the night and wintertime. In this study, we investigated the potential dual impact of NO_x concentration decreases on recent O₃ trends by season and time of day. We analyzed hourly O₃ and NO_x measurement data between 1994 and 2010 in the continental USA. Nationally, hourly O₃ concentrations decreased by as much as -0.38 ppb/year with a standard error of 0.05 ppb/year during the warm season midday, but increased by as much as +0.30±0.04 ppb/year during the cold season. High O₃ concentrations (≥75th percentile) during the warm season decreased significantly, however, there were notable increases in the cold season as well as warm season nighttime; we found that these increases were largely attributable to NO_x decreases as less O₃ is quenched. These O₃ increases, or "penalties", related to NO_x reductions remained robust at a wide range of O₃ concentrations (5th to 99th percentile), and even after accounting for VOC reductions and meteorological parameters, including temperature, wind speed, and water vapor pressure. In addition, we observed O₃ penalties across rural, suburban, and urban areas. Nonetheless, peak O₃ concentrations (99.9th percentile) were mitigated by NO_x reductions. In addition, there was some suggestive evidence that VOC reductions have been more effective in reducing O₃.