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 (O3) 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 O3 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 (STT) exists to affect surface O3 levels. Our results indicate that STT down to the surface (STT-S) frequently contributes to enhanced surface O3 hourly averaged concentrations at sites across the US, with substantial year-to-year variability. The O3 concentrations associated with the STT-S events appear to be large enough to enhance the measured O3 concentrations during specific months of the year. Months with a statistically significant coincidence between enhanced O3 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 O3 concentrations coincident with STT-S events.