title: The impact of nitrogen oxides concentration decreases on ozone trends in the USA.
abstract: Ozone (O3) has harmful effects on human health and ecosystems. In the USA, significant reductions of O3precursors-nitrogen oxides (NOx) and volatile organic compounds (VOCs)-have not yielded proportionate decreases in O3. NOxis a major precursor of O3as well as a quencher of O3through NOxtitration, which is especially important during the night and wintertime. In this study, we investigated the potential dual impact of NOxconcentration decreases on recent O3trends by season and time of day. We analyzed hourly O3and NOxmeasurement data between 1994 and 2010 in the continental USA. Nationally, hourly O3concentrations 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 O3concentrations (≥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 NOxdecreases as less O3is quenched. These O3increases, or "penalties", related to NOxreductions remained robust at a wide range of O3concentrations (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 O3penalties across rural, suburban, and urban areas. Nonetheless, peak O3concentrations (99.9th percentile) were mitigated by NOxreductions. In addition, there was some suggestive evidence that VOC reductions have been more effective in reducing O3.
authors.affiliation: Department of Environmental Health, Harvard School of Public Health, 401 Park Drive, Landmark 4 West (Rm 412J), Boston, MA 02215, USA.
authors.forename: Iny
authors.initials: I
authors.lastname: Jhun
authors.affiliation: Department of Biostatistics, Harvard School of Public Health, 655 Huntington Avenue, Boston, MA 02115, USA.
authors.forename: Brent A
authors.initials: BA
authors.lastname: Coull
authors.affiliation: Department of Environmental Health, Harvard School of Public Health, 401 Park Drive, Landmark 4 West (Rm 412J), Boston, MA 02215, USA.
authors.forename: Antonella
authors.initials: A
authors.lastname: Zanobetti
authors.affiliation: Department of Environmental Health, Harvard School of Public Health, 401 Park Drive, Landmark 4 West (Rm 412J), Boston, MA 02215, USA.
authors.forename: Petros
authors.initials: P
authors.lastname: Koutrakis
keywords: Air pollution; Nitrogen oxides; Ozone; Trends