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

Abstract

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.

Interannual variability of ozone and carbon monoxide at the Whistler high elevation site: 2002–2006

“…Chan and Vet (2010) used a muti-site cluster analysis and also found a significant upward trend in background ozone along the west coast of North America. Similar trends have been identified in free tropospheric measurements through a comprehensive integration of data from multiple years and west coast studies (Cooper et al, 2010). Reasons for these trends are unclear and several possibilities have been suggested including regional continental or ship emissions, biomass burning and trans-Pacific transport (Cooper et al, 2010;Jaffe and Ray 2007;Dalsoren et al, 2010;Jaffe, 2010).…”

Tropospheric Ozone Assessment Report: Assessment of global-scale model performance for global and regional ozone distributions, variability, and trends

“…Factors related to the measurements or their use include: uncertainties in early measurements (e.g., Staehelin et al, 1994;Logan et al, 2012;TOAR-Observations); sampling biases in modelmeasurement comparisons, such as the representativeness of the trends derived from sparse measurements (Cooper et al, 2010;Lin et al, 2015a); and the presence of trends driven by low-frequency climate variability that freerunning CCMs are not expected to reproduce exactly (Lin et al, 2014(Lin et al, , 2015bBarnes et al, 2016;Garcia-Menendez et al, 2017). Factors related to the models include: errors in the trends incorporated in the underlying emission inventories used in the model (Granier et al, 2011;Hassler et al, 2016); limitations of coarse-resolution models in resolving observed baseline conditions (Lin et al, 2017); and weaknesses in the model representation of the processes (chemical, physical, and dynamical) that control the observed trend at a given location.…”

The effects of global change upon United States air quality

“…Furthermore, global emissions of greenhouse gases and other pollutant precursors are projected to increase (IPCC, 2007). Moreover, recent research has provided evidence of increasing long-range transport of O 3 and PM 2.5 precursors from Asia and their influence over the western US (Lelieveld and Dentener, 2009;Wuebbles et al, 2007;Cooper et al, 2010Cooper et al, , 2012Zhang et al, 2010;Ambrose et al, 2011;WMO, 2012;Lin et al, 2012).…”