Effects of highway networks on ambient ozone concentrations-A case study in southern Taiwan

The fraction of days on which ozone concentrations exceeded the ambient air quality standard increased from 2.9% in 2000 to 4.6% in 2004 in southern Taiwan. Mobile sources attributed for 44.7% and 17.6% of the ozone precursors, NO_x and NMHC, respectively, based on the emission inventory of the Taiwan EPA. However, a new second north-south highway was recently constructed in Taiwan. This study evaluates how emissions from the first and second highways affect the ambient air quality, especially ozone concentration by using CMAQ between 14 and 17 December 2002. Two emissions scenarios were examined. The first involved emissions from only the first highway and the second involved emissions from only the second highway. The mean errors of the unpaired peak ozone concentration were generally less than 17%; and those of the paired ozone concentrations were less than 25%. Correspondingly, the results simulated by the model were highly consistent with observed data. Simulation results indicated that most ambient O₃ concentrations increased by more than 2 ppb with hourly increments of up to 6 ppb, in both scenarios. Although the total emissions were the same, the differences among emission locations were responsible for substantial differences in ozone concentrations. The ozone concentrations decreased along the highway as the emissions increased, because of NO titration. However, the ozone concentrations were greater at sites that were further downwind. Therefore, the ozone concentrations changed significantly with the highway network, given the same total emission. The differences in ozone concentrations were caused by the changes in the emission locations of NO_x.