Influence of springtime weather systems on vertical ozone distributions over three North American sites

The Atmosphere/Ocean Chemistry Experiment (AEROCE) '96 Springtime Intensive was designed to quantify major atmospheric chemical species transported from the United States east coast to the North Atlantic Ocean. During the study ozonesondes were launched almost every day between March 22 and May 3, 1996, from Charlottesville, Virginia; Purdue University, Indiana; and Bermuda. Whenever possible, the Charlottesville sondes were timed to fly behind passing cold fronts into midtropospheric to upper tropospheric bands of dry air. The dry air was hypothesized to contain ozone of stratospheric origin and was detected with color-enhanced satellite water vapor imagery. Soundings were placed in three categories: sondes launched ahead of an approaching cold front, sondes launched behind a passing cold front, and sondes launched into cutoff lows. This stratification explained much of the vertical ozone variation at each site. Tropospheric mean ozone increased with height at all three sites under postfrontal conditions, with relatively little increase with height under prefrontal conditions. Backward trajectories from Charlottesville indicated that the postfrontal air masses originated at high elevations to the northwest, while prefrontal air masses came from relatively lower elevations to the southwest. Transport also explained the lower tropospheric ozone differences between Charlottesville and Bermuda. The enhanced ozone observed in the upper troposphere over Charlottesville compared with the other sites may be linked to upwind conditions more favorable for tropopause folding. However, we believe that selective launching based on the operational use of satellite water vapor imagery allowed us to sample the full range of tropospheric ozone over Charlottesville.