2002 Annual

Thursday, 17 January 2002: 2:30 PM
3-D distribution of ozone during the major pollution event of 30 August 2000 during TexAQS 2000
Robert M. Banta, NOAA/ETL, Boulder, CO; and C. J. Senff, L. S. Darby, T. B. Ryerson, M. Trainer, and R. J. Alvarez II
Poster PDF (183.3 kB)
Air pollution in the Houston, Texas area is a product of strong emissions and specialized meteorological conditions. The emissions are from the Houston urban area, power plants, and, to the east and southeast of the city, the many refineries and petrochemical plants along the ship channel and the western shore of Galveston Bay. The specialized meteorology consists of interactions of the local sea-breeze circulations with larger-scale geostrophic or gradient flows. Conditions where the larger-scale flow is offshore and opposes the sea breeze are especially conducive to pollution events, because the sea-breeze front stalls over the sources, and the pollutants remain in the area and accumulate. One especially dramatic example of these processes was 30 August 2000 during the Texas Air Quality 2000 experiment (TexAQS2000).

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.

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, it has been very difficult to obtain documented occurrences.

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. Concurrent airborne air chemistry measurements during midday revealed an industrial source of the pollutants in the ozone-rich air.

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