Contributions of inter- and intra-state emissions to ozone over Dallas-Fort Worth, Texas

Simulation of CMAQ with the high-order direct decoupled method (HDDM) for two 2005 episodes was used to assess the impacts of local emissions and regional transport on ozone concentrations in the Dallas-Fort Worth (DFW) region of Texas. The episodes featured east-northeasterly winds conducive to interstate transport of air pollutants. The study revealed that local, intrastate, and neighbouring state emissions of nitrogen oxides (NOx) all contributed significantly to daytime ozone in DFW. Local NOx emissions exerted the strongest impact on local ozone, though the impact was highly variable temporally and spatially within the region. NOx emissions from Texas areas outside DFW contributed on average about 10ppb to daytime DFW ozone. Neighbouring states (Oklahoma, Arkansas, Louisiana, and Mississippi) in total also contributed about 10ppb to DFW ozone. Anthropogenic VOC emissions from outside the DFW region yielded negligible impact on DFW ozone. DFW ozone is shown to respond more nonlinearly to local NOx than to other NOx emission reductions. The CMAQ-HDDM results indicate that for these episodes, a 4ppb reduction in average DFW 8h ozone could be achieved by either a 40% reduction in DFW NOx, a 70% reduction in intrastate NOx, or a 50% reduction in NOx from the four neighbouring states.