Relative Contribution of Different Source Categories to Ozone Exceedances in the Houston-Galveston-Brazoria Area

Ozone concentrations at a particular location depend on a number of variables such as quantity of emissions, types of source categories, meteorology, and land use. Ozone formation in the Houston-Galveston-Brazoria (HGB) area is associated with emissions from on- and off-road vehicles, power plants and biogenic sources. However, the extensive petrochemical industry that contributes both reactive VOCs and NO_x to the pollutant mix and ozone formation makes it a unique region. In this study, the authors aimed to analyze the relative contribution of different emission source categories to ozone in the HGB area of Texas.

Emission Processing System (EPS3) was used to prepare the emission files for five different source combination cases (Base case, Biogenic, Area + Biogenic, Mobile + Biogenic, Low-level Point + Biogenic). These emission files were used to perform photochemical modeling with Comprehensive Air Quality Model with Extensions (CAMx), and the results were analyzed with Visual Environment for Rich Data Interpretation (VERDI) tool. The daily maximum ozone concentrations and individual contributions of the source categories were analyzed over a 15-day study period between June 1-15, 2012, at three locations (University of Houston-Sugarland, Bayland Park and Conroe).

The results showed that Isoprene emissions in the region, and transport of biogenic VOCs from northeast Houston played a significant role at all three locations, as evidenced by an average contribution of 49.7% ± 12.8%, 43.1% ± 12.0%, and 39.9% ± 9.28% from biogenic sources toward daily maximum ozone concentrations, at Sugarland, Bayland Park and Conroe sites respectively. The relative contribution Mobile + Biogenic source category was the primary factor in this region, indicating the dominance of mobile source NO_X emissions in determining peak ozone exceedances. Conroe site had lower deviations in the diurnal variations of ozone and lower peak concentrations as compared to Sugarland and Bayland park sites, due to the location being distant from the city center and the Houston ship channel. The remoteness of this site is also evidenced by the relatively lower contribution of LPt + Biogenic source category. The overall importance of emission source categories that need to be targeted for regulatory control in the HGB area were observed to be in the following sequence Mobile > Point > Area sources.

Article by Raghava R. Kommalapati, et al, from Prairie View A & M University, Prairie View, USA.

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