Tropopause Folds and Subsequent Mixing of Ozone over the Northwestern United States during the Spring of 2000

To better understand how ozone changes in the troposphere and lower stratosphere, 35 ozonesondes were launched from Pacific Northwest National Laboratory (PNNL) between March and May of 2000 as part of the Tropospheric Ozone Production about the Spring Equinox (TOPSE) program. Ozonesondes were also launched at three other locations and NCAR’s C-130 research aircraft made a series of measurements across North America between Boulder, Colorado and Greenland by other TOPSE participants. The ozonesonde measurements provided evidence of relatively high ozone concentrations in the middle troposphere. These high concentrations were the result of a meteorological process known as a tropopause fold that periodically exchanges a large amount of air between the stratosphere and troposphere. The stratospheric ozone is then mixed with the surrounding tropospheric air over several days. Layers of ozone, with mixing ratios between 90 and 100 ppb, were also observed at relatively low altitudes between 3 and 4 km above the ground during the latter part of the field campaign. These layers may be signatures of the Seattle and/or Portland urban plumes that were subsequently transported by westerly winds across the Cascade Mountains. Preliminary trajectory calculations; however, indicate that strong sinking motions also occurred further upwind over the North Pacific so that the layers may be a combination of both stratospheric ozone and ozone produced near the surface. A coupled meteorological-chemical modeling system will be used to elucidate the interactions of synoptic, mesoscale, and boundary-layer processes associated with ozone measurements made during the field campaign.