2002 Annual

Thursday, 17 January 2002: 4:30 PM
Relationship between mean wind direction and daily ozone trends at LaPorte, Texas
Lisa S. Darby, NOAA/ETL, Boulder, CO; and R. M. Banta, R. D. Marchbanks, C. J. Senff, and E. J. Williams
LaPorte, TX is situated southeast of the city of Houston, TX, within 10 km of the shoreline of Galveston Bay and the Ship Channel. The Ship Channel is a waterway connecting the Port of Houston to Galveston Bay. It is a highly industrialized region that is the source of an abundance of pollutants, including ozone precursors.

Ozone concentration patterns across the city of Houston and surrounding regions are highly sensitive to wind direction, particularly the presence or absence of a wind shift associated with the bay breeze or gulf breeze (thermally driven circulations resulting from the temperature differences between the land and the bay or gulf). These patterns include spatial distribution, i.e., there may be a strong gradient of ozone measurements across the region, and changes in time, with ozone increases of 40 ppbv, or more, occurring within an hour.

During the Texas Air Quality Study in the summer of 2000 (TexAQS 2000), the general aviation airport in Laporte, TX was heavily instrumented with meteorological and chemical sensors, including a Doppler lidar that measured radial wind speed and a differential absorption lidar that detected ozone. These two lidars operated simultaneously, side-by-side, during TexAQS 2000.

In this study, we investigate 3 days with different daytime wind flow regimes at LaPorte, as determined from Doppler lidar vertical profiles of the horizontal wind: 1) offshore flow all day; 2) onshore flow all day; and 3) transition from offshore to onshore flow. The wind profiles, derived from Doppler lidar conical scans, show the winds from near the surface to 4 km above ground level (AGL), with greatest detail in the lowest 400 m. The lowest 400 m of data were used to categorize the days by wind direction. High-resolution surface measurements of wind speed and direction followed the same daily trends as the lidar measurements. Additionally, temperature, relative humidity, and pressure measurements gave insight into some of the changes occurring at Laporte, such as indicating marine air mass intrusions. We will show that differences in the high-resolution in-situ ozone time series coincide with differences in the mean wind pattern.

Profiles of ozone concentrations from the ozone detecting lidar indicate changes in ozone with both height and time from 30 m to 3000 m AGL, with a time resolution of approximately 30 minutes. The profile measurements are used to address the following questions: 1) How does the mean wind direction affect the profiles of ozone? 2) Is the presence or absence of the residual layer of ozone in any way related to the mean wind direction of the previous day?, and 3) What happens to the ozone concentrations above the surface after the bay breeze or gulf breeze becomes established?

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