Inter-pollutant and reactivity-weighted air pollutant emission trading in Texas

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Wang, Linlin

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Emission trading is a market-based approach designed to improve the efficiency and economic viability of emission control programs. Although air pollutant emission trading typically has been confined to trades among single pollutants, inter-pollutant trading (IPT) allows for trades among emissions of different compounds that impact the same air quality end-point, such as ambient ozone concentrations. Designing IPT programs for emissions that lead to elevated ozone concentrations can be challenging because equivalent reductions of different categories of emissions may not always lead to equivalent reductions of ozone. Because emissions of different compounds impact air quality end-points related to ozone concentrations differently, weighting factors or trading ratios (e.g., tons of NOx emissions equivalent to a ton of volatile organic compound (VOC) emissions) must be developed to allow for inter-pollutant trading. This thesis examines two types of IPT of emissions that lead to ozone formation, that have been or may be implemented in Texas. Interpollutant trading of NOx and VOC (ozone precursor) emissions was examined using air quality conditions in Houston and Austin as case studies. Analysis of the Austin case study indicated that inter-pollutant trading ratios were, with few exceptions, constant. The IPT trading ratios were independent of the source category of the emissions and the exact manner in which ozone concentrations were evaluated. In contrast, IPT ratios in Houston were far more variable, due in part to strong spatial and temporal gradients in emissions of ozone precursors. These results suggest that IPT of VOC and NOx emissions will be far more straightforward to implement in Austin than in Houston. Interpollutant trading of various types of VOCs, especially a group of Highly Reactive VOCs (HRVOCs) was also examined, using Houston as a case study. This type of IPT was investigated because a program for trading of HRVOCs is due to be implemented in Houston in 2007. The focus of the analyses was on whether the trading program, as currently designed, could produce undesired localized high concentrations of ozone (“hot spots”). The analyses indicated that the implementation of the trading program would be unlikely to produce ozone hot spots.