Observation of pollution plume capping by a tropopause fold

Airborne lidar measurements reveal a case in which a layer of high-ozone ai r extruding from a tropopause fold appears to cap a pollution plume and for ce it to spread out in the lower troposphere. The morphology of the high-oz one layer resembles a three-dimensional model of tropopause fold evolution that produces a low-altitude potential vorticity tube. This is a mechanism that can complete the irreversible transfer of air from the stratosphere, a nd can also affect pollution levels at the surface if the capping layer rea ches the top of the boundary layer.