The importance of stratosphericetropospheric transport in affecting surface ozone concentrations in the western and northern tier of the United States Allen S. Lefohn a, * , Heini Wernli b , Douglas Shadwick c , Sebastian Limbach d , Samuel J. Oltmans e , Melvyn Shapiro f a A.S.L. & Associates, 302 North Last Chance Gulch, Suite 410, Helena, MT 59601, USA b Institute for Atmospheric and Climate Science, ETH Zurich, Universitätstrasse 16, 8092 Zurich, Switzerland c 320 Eastwood Road, Chapel Hill, NC 27514, USA d Institute for Atmospheric Physics, University of Mainz, Becherweg 21, 55099 Mainz, Germany e NOAA Earth System Research Laboratory, Global Monitoring Division, 325 Broadway, Boulder, CO 80305, USA f NCAR, Boulder, CO 80305, USA article info Article history: Received 24 February 2011 Received in revised form 2 June 2011 Accepted 6 June 2011 Keywords: Background ozone Stratosphereetroposphere exchange Stratosphere-to-troposphere transport Surface ozone abstract Stratosphericetropospheric exchange (STE) processes contribute at both high and low-elevation moni- toring sites to background ozone (O 3 ) concentrations. This study addresses the importance of stratospheric intrusions contributing to enhanced hourly average surface O 3 concentrations (i.e., 50 ppb) at 12 O 3 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 O 3 levels. The coincidences between the number of days with a daily maximum hourly average O 3 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 O 3 concentrations  50 ppb with STT-S > 0 of the 12 monitoring sites. At this site, the daily maximum hourly springtime average O 3 concentrations were usually in the 60e70 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 O 3 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 50e65 ppb range and the daily maximum 8-h average concentrations were usually in the 50e62 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 O 3 concentration enhancements occurring during the springtime versus other times of the year, (2) the elevation depen- dency of the frequency of enhancements, (3) the year-to-year variability, (4) the timing of the hour-by- hour occurrences of the O 3 concentration enhancements within and across monitoring sites, and (5) the detailed analyses of O 3 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 O 3 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 O 3 . Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Stratosphere-troposphere exchange (STE) processes contribute to natural background O 3 concentrations at the surface at both high and low-elevation monitoring sites (e.g., Ludwig et al., 1977; Haagenson et al., 1981; Davies and Schuepbach, 1994; Schuepbach et al., 1999; Stohl et al., 2000; Lefohn et al., 2001; Cooper et al., 2005; Hocking et al., 2007; Ordóñez et al., 2007; * Corresponding author. Tel.: þ1 406 443 3389. E-mail addresses: alefohn@asl-associates.com (A.S. Lefohn), heini.wernli@env. ethz.ch (H. Wernli), dougshadwick@nc.rr.com (D. Shadwick), limbach@uni-mainz. de (S. Limbach), Samuel.J.Oltmans@noaa.gov (S.J. Oltmans), mshapiro@ucar.edu (M. Shapiro). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.06.014 Atmospheric Environment 45 (2011) 4845e4857