Increasing springtime ozone mixing ratios in the free troposphere over western North America

• Published in: Nature (London) Vol. 463; no. 7279; pp. 344 - 348
• Main Authors: Cooper, O. R, Parrish, D. D, Stohl, A, Trainer, M, Nédélec, P, Thouret, V, Cammas, J. P, Oltmans, S. J, Johnson, B. J, Tarasick, D, Leblanc, T, McDermid, I. S, Jaffe, D, Gao, R, Stith, J, Ryerson, T, Aikin, K, Campos, T, Weinheimer, A, Avery, M. A
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 21.01.2010; Nature Publishing Group
• Subjects: 704/106/35; 704/106/35/824; Accuracy; Air Pollutants - analysis; Air Pollutants - chemistry; Air pollution; Air quality; Air quality standards; Americas; Anthropogenic factors; Applied sciences; Asia; Asian; Atmosphere - chemistry; Atmospheric and Oceanic Physics; Atmospheric ozone; Atmospheric pollution; Ecosystem; Emission measurements; Environmental aspects; Exact sciences and technology; Greenhouse Effect; Greenhouse gases; Health aspects; History, 20th Century; History, 21st Century; Humanities and Social Sciences; Hydroxyl radicals; International trade; letter; Meteorology; Methods; Mixing ratios; multidisciplinary; North America; Outdoor air quality; Oxidizing agents; Ozone; Ozone - analysis; Ozone - chemical synthesis; Ozone - chemistry; Physics; Pollutants physicochemistry study: properties, effects, reactions, transport and distribution; Pollution; Precursors; Sample Size; Science; Science (multidisciplinary); Seasons; Troposphere; US West Coast; Asia; United States; North America; America; Canada; USA; Photochemical oxidants; Troposphere; Ozone; Air quality; Air mass; Spring(season); Long range pollutant transport; Trend analysis; Observation data; Air pollution; Time-series analysis; Mixing ratio; Concentration distribution
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature08708

In the lowermost layer of the atmosphere—the troposphere—ozone is an important source of the hydroxyl radical, an oxidant that breaks down most pollutants and some greenhouse gases. High concentrations of tropospheric ozone are toxic, however, and have a detrimental effect on human health and ecosystem productivity. Moreover, tropospheric ozone itself acts as an effective greenhouse gas. Much of the present tropospheric ozone burden is a consequence of anthropogenic emissions of ozone precursors resulting in widespread increases in ozone concentrations since the late 1800s. At present, east Asia has the fastest-growing ozone precursor emissions. Much of the springtime east Asian pollution is exported eastwards towards western North America. Despite evidence that the exported Asian pollution produces ozone, no previous study has found a significant increase in free tropospheric ozone concentrations above the western USA since measurements began in the late 1970s. Here we compile springtime ozone measurements from many different platforms across western North America. We show a strong increase in springtime ozone mixing ratios during 1995–2008 and we have some additional evidence that a similar rate of increase in ozone mixing ratio has occurred since 1984. We find that the rate of increase in ozone mixing ratio is greatest when measurements are more heavily influenced by direct transport from Asia. Our result agrees with previous modelling studies, which indicate that global ozone concentrations should be increasing during the early part of the twenty-first century as a result of increasing precursor emissions, especially at northern mid-latitudes, with western North America being particularly sensitive to rising Asian emissions. We suggest that the observed increase in springtime background ozone mixing ratio may hinder the USA’s compliance with its ozone air quality standard.