This study documents case studies of stratosphere-troposphere exchange (STE) events over the continental and coastal United States that led to the exceedence of Environmental Protection Agency (EPA) standards for ground-level ozone. Harmful air quality indices (AQI) were recorded at several surface sites in the presented cases and multi-platform observations were used to determine that the source of these chemical species was the stratosphere, a natural source of ozone. NASA A-Train observations qualitatively show the evolution of these STE events from inception in the upper troposphere and lower stratosphere (UTLS) to dissipation in the mid-to-lower troposphere.

To confirm the location of the originating jet streak or upper-level front, and to identify the underlying dynamical mechanisms of the event, North American Regional Reanalysis (NARR) temperature and wind fields are used. Observations of water vapor and ozone from A-Train satellites, in conjunction with NARR derived potential vorticity (PV), highlight the dynamic vertical coupling of dry, ozone-rich, high PV air that manifests along baroclinic zones accompanying jet / front systems. In the vicinity of the jet streak, two large secondary circulations, one thermally direct (frontogenesis) and one thermally indirect (frontolysis), form due to the two main forcing functions: temperature and vorticity advection. When these two closed circulations are in phase, there is subsidence near the cyclonic part of entrance and anticyclonic part of the exit of the jet streak. Air parcels are transported across the tropopause resulting in the rapid vertical transport of dry, ozone-rich air from the stratosphere to the lower atmosphere as viewed by NARR and A-Train observations in these cases of STE.

Studies in the past diagnosed the vertical motions associated with the formation of secondary ageostrophic circulations mentioned above by examining: the solutions to the quasi-geostrophic (QG) omega equation and the dynamical forcing terms of the QG form of the Sawyer-Eliassen equation. In this study, we utilize QG diagnostic variables like Q-vector calculated from NARR and the proportional relationship of its divergence to omega to complement potential vorticity and geostrophic wind fields. These parameters capture vertical transport between the stratosphere and lower troposphere.

Multiple surface site measurements from polluted areas compare well temporally and spatially with the satellite and reanalysis observations. When stratospheric air extended downward close to the boundary layer and was mixed down to the surface, it contributed to high levels of surface ozone. The analysis demonstrates the direct influence of the stratosphere on local surface air quality (AQ).