Quantifying the importance of stratospheric-tropospheric transport on surface ozone concentrations at high- and low-elevation monitoring sites in the United States

In this study, we quantify the frequency of stratosphere-troposphere exchange (STE) events that result in ozone (O-3) concentration enhancements (i.e., hourly average concentrations >= 50 ppb) observed at 39 high- and low-elevation monitoring sites in the US during the years 2007-2009. We employ a refined forward trajectory-based approach to address the relationship between stratospheric intrusions and enhancements in hourly average O-3 concentrations. The model is applied to high-resolution European Center for Medium-Range Weather Forecasting (ECMWF) analyses to identify specific days when the potential for stratosphere-to-troposphere transport (Sir) exists to affect surface O-3 levels. Our results indicate that SIT down to the surface (STT-S) frequently contributes to enhanced surface O-3 hourly averaged concentrations at sites across the US, with substantial year-to-year variability. The O-3 concentrations associated with the SIT-S events appear to be large enough to enhance the measured O-3 concentrations during specific months of the year. Months with a statistically significant coincidence between enhanced O-3 concentrations and STT-S occur most frequently at the high-elevation sites in the Intermountain West, as well as at the high-elevation sites in the West and East. These sites exhibit a preference for coincidences during the springtime and in some cases, the summer, fall, and late winter. Besides the high-elevation monitoring sites, low-elevation monitoring sites across the entire US experience enhanced O-3 concentrations coincident with SIT-S events. (C) 2012 Elsevier Ltd. All rights reserved.