Contribution of Regional Transport to Ozone Exceedances in Houston and Dallas, Texas

During the TEXAQS and TEXAQS II field experiments, aircraft measured ozone concentrations upwind, across, and downwind of Houston and Dallas, and the transport contributions to Dallas area ozone were quantified using the CAMx photochemical grid model. The transport contribution to ozone exceedances in both urban areas was at least as large as the contribution from local emissions, on average. For example, analysis of a flight over Dallas on a day that had a significant exceedance showed that the background ozone transported into Dallas constituted about 73% of the total ozone concentration. The aircraft measurements show that these two major metropolitan areas can be brought close to exceeding the 8-hour National Ambient Air Quality Standard for ozone solely from the ozone contribution from regional transport and before any contribution from local sources. The CAMx Model-predicted ozone concentrations agreed with the measurements on the importance of regional transport of ozone relative to local ozone formation. These results emphasize the benefits of regional control strategies, suggesting that local controls may not be sufficient to ensure attainment of the 8-hour ozone standard in Houston and Dallas.