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

Ozone (O-3) has harmful effects on human health and ecosystems. In the USA, significant reductions of O-3 precursors-nitrogen oxides (NOx) and volatile organic compounds (VOCs)-have not yielded proportionate decreases in O-3. NOx is a major precursor of O-3 as well as a quencher of O-3 through NOx titration, which is especially important during the night and wintertime. In this study, we investigated the potential dual impact of NOx concentration decreases on recent O-3 trends by season and time of day. We analyzed hourly O-3 and NOx measurement data between 1994 and 2010 in the continental USA. Nationally, hourly O-3 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 O-3 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 NOx decreases as less O-3 is quenched. These O-3 increases, or "penalties", related to NOx reductions remained robust at a wide range of O-3 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 O-3 penalties across rural, suburban, and urban areas. Nonetheless, peak O-3 concentrations (99.9th percentile) were mitigated by NOx reductions. In addition, there was some suggestive evidence that VOC reductions have been more effective in reducing O-3.