Sensitivity of Surface Ozone Simulation in Houston, TX: Ozone Exceedance Formation, Fine Grid Resolution, and Future Mobile Emissions Projection

The Houston metropolitan area is classified as a nonattainment area for ozone (O3) by the United States Environmental Protection Agency. In order to obtain a better understanding of the O3 budget in Houston, a WRF-SMOKE-CMAQ air quality modeling system was used to investigate the sensitivity of surface O3 simulation due to precursor emissions, meteorology, and grid resolution. The simulation period was September 2013, coincident with NASA’s DISCOVER-AQ campaign. A WRF run initialized with high-resolution NOAA GOES satellite sea surface temperature data reduced the model wind speed and slowed the dilution rate. However, it still failed to reproduce the exceptionally high surface O3 on 25th September 2013, possibly attributed to episodic flare emissions, dry sunny postfrontal stagnated conditions, and land-bay/sea breeze transitions. Adjusting wind fields to represent the morning weak westerly and afternoon converging zone significantly improved the predictions of magnitude, timing, and location of the O3 hotspots. We compared two simulations with meteorology and emissions prepared for grid spacing of 1 km and 4 km, respectively. The peak NOx emissions at an industrial and urban site differed by a factor of 20 for the 1 km and 8 for the 4 km grid, but simulated NOx concentrations changed only by a factor of 1.2 in both cases. Hence, due to the interplay of the atmospheric processes, we cannot expect a similar level of reduction of gas-phase air pollutants as the reduction of emissions. Future projections of trucking emissions and evaluation of various scenarios with varying levels of emission control suggested that to maintain the current air quality conditions, applying highest emission reductions to at least 50% of the freight traffic is needed.