The importance of stratospheric–tropospheric transport in affecting surface ozone concentrations in the western and northern tier of the United States

Abstract

Stratospheric–tropospheric exchange (STE) processes contribute at both high and low-elevation monitoring sites to background ozone (O3) concentrations. This study addresses the importance of stratospheric intrusions contributing to enhanced hourly average surface O3 concentrations (i.e., ≥50 ppb) at 12 O3 monitoring stations in the western and northern tier of the US for 2006, 2007, and 2008. The Lagrangian Analysis Tool (LAGRANTO) trajectory model identified specific days when stratosphere-to-troposphere transport was optimal to elevate surface O3 levels. The coincidences between the number of days with a daily maximum hourly average O3 concentration ≥ 50 ppb and stratosphere-to-troposphere transport to surface (STT-S > 0) were quantified. The high-elevation site at Yellowstone National Park (NP) in Wyoming exhibited the most coincidences (i.e., more than 19 days a month) during the spring and summer for hourly average O3 concentrations ≥ 50 ppb with STT-S > 0 of the 12 monitoring sites. At this site, the daily maximum hourly springtime average O3 concentrations were usually in the 60–70 ppb range. The maximum daily 8-h average concentrations mostly ranged from 50 to 65 ppb. At many of the lower-elevation sites, there was a preference for O3 enhancements to be coincident with STT-S > 0 during the springtime, although summertime occurrences were sometimes observed. When statistically significant coincidences occurred, the daily maximum hourly average concentrations were mostly in the 50–65 ppb range and the daily maximum 8-h average concentrations were usually in the 50–62 ppb range. For many cases, the coincidences between the enhancements and the STT-S events occurred over a continuous multiday period. Supplementary observations, such as (1) the greater frequency of O3 concentration enhancements occurring during the springtime versus other times of the year, (2) the elevation dependency of the frequency of enhancements, (3) the year-to-year variability, (4) the timing of the hour-by-hour occurrences of the O3 concentration enhancements within and across monitoring sites, and (5) the detailed analyses of O3 enhancement events at specific sites, provide additional support for our modeling and statistical results. Our analysis provides an important step in better understanding the variability of natural background O3 concentrations. The study has provided insight into stratospheric intrusions, with emphasis on the combined role of quasi-isentropic large-scale advection and mesoscale boundary layer turbulence for stratospheric air influencing enhanced surface O3.