Effects of the sea breeze and the sea-breeze front on the transport and distribution of air pollutants was a major focus of the Texas Air Quality Study in the Houston vicinity during August and September 2000. In turn, pollutants served as tracers of the flow, and the time-dependent distribution of the pollutants gave insight into the characteristics of the evolution of the meteorological processes. On a few days with moderate offshore flow, including 30 August, the sea breeze was weak and did not start to penetrate inland until late afternoon.

Many aircraft and ground-based instruments were deployed to characterize the meteorology and chemistry of the atmosphere over the Houston-Galveston area. One of these instruments was a downward-looking ozone-profiling differential-absorption lidar (DIAL) operated by NOAA/ETL, mounted in a DC-3 airplane. This lidar mapped out the distribution of ozone and aerosol backscatter in a 2-d curtain, and thus was capable of showing the 3-D distribution of these pollutants by flying back and forth over a region. One region of special interest was the western shore of Galveston Bay and the ship channel to the north, going from the Bay to Houston. These regions contained many refineries and petrochemical plants, which were heavy sources of pollutants of various species that were photochemically active.

The flow for most of the day on 30 Aug was offshore in the Houston-Galveston area, and this carried pollutants offshore over Galveston Bay. The pollutants remained at low levels (and high concentrations) over the water (below 300 m) as determined by airborne ozone lidar flights, because the air in contact with the sea surface was relatively stable. The sea breeze began weakly in the afternoon, and the sea-breeze front only penetrated a few kilometers inland by late afternoon. The stronger offshore flow and the weaker onshore sea-breeze flow behind the front produced a region of strong convergence just inland and parallel to the western shore of Galveston Bay. This was also an optimum condition for high pollution concentrations, because the onshore flow brought pollution that had been released earlier in the day and had drifted out over the bay, back over the sources for a double dose. Although this has often been hypothesized to happen, documented occurrences are rare.

Airborne ozone DIAL measurements revealed how bad the pollution situation became, but it also revealed how strong and deep the convergence was over the linear sea-breeze front along the west shore of the Bay. The DC-3 flew several parallel east-west cross sections across the shore from south to north, thus painting a 3-D picture of the ozone distribution. The pollution formed a spectacular wall of pollution reaching more than 2 km high, with very high concentrations of 200 ppb of ozone occupying much of this volume. Thus the strong updrafts in the sea-breeze convergence zone lofted these excessive pollution concentrations high into the atmosphere.

We present a detailed analysis of this event, emphasizing how the chemistry and airborne DIAL measurements provide insight into the meteorological processes beyond that available from the meteorological instrumentation alone, which included rawinsondes, tethersondes, surface flux sites, surface mesonets, and radar wind profilers.