Dependence of daily peak O3 concentrations near Houston, Texas on environmental factors: Wind speed, temperature, and boundary-layer depth

Airborne and surface measurements of ozone (O3) during the Texas Air Quality Study campaigns in 2000 and 2006 (TexAQS 2000 and TexAQS 2006) were used to investigate the relationship between maximum daily O3 and the vector-averaged wind speed <U>, calculated from radar wind profiler data, in the lower atmospheric boundary layer (BL). Both the maximum daily O3 and the peak “add-on” O3 contribution (calculated as the maximum minus the background values) from the Houston area showed a strong correlation (r∼0.7–0.9) with both the reciprocal of this wind speed <U>−1, and the wind speed <U> itself. Data from airborne platforms produced higher correlations in general than surface-measured values. Except for special cases where O3 was measured close to source activity in a location where the BL depth h was suppressed, peak daily ozone concentrations were not strongly correlated with h, and attempts to include 1/h dependence with <U>−1 degraded the correlations—indicating that in general, h was not a strong predictor for maximum daily O3. Inclusion of daily maximum temperature in the regression analysis also failed to improve the correlations significantly. The high correlations for wind speed thus showed that <U> was the meteorological variable most strongly associated with peak daily O3 concentrations. The best-fit regression line of peak daily O3 vs. <U> for the 2000 data lay above the line for 2006 for wind speeds less than 5 m s−1, the difference increasing as the wind speeds weakened. This six-year decrease in O3 concentrations for the weakest-wind, most polluted days suggests that control strategies implemented between 2000 and 2006 may be producing beneficial effects, especially on the most polluted days.