A Composite Modeling Study of Civil Aircraft Impacts on Ozone and Sulfate over the Taiwan Area

Abstract

Aircraft induced ozone and sulfate perturbations calculated from a Lagrangian plume model were composed into a regional distribution to quantify the impacts of aircraft emissions at the international and domestic flight altitudes of about 11.5 and 6.5 km, respectively, over Taiwan. The evolution of pollutant concentrations within a Lagrangian aircraft plume was first calculated with consideration of gas-phase chemistry and mixing of ambient air. Due to reaction with high concentration of aircraft emitted NO, the O3 concentration in fresh aircraft plumes rapidly decreased by more than 15 ppb during the first few hours after emissions. Then, becauseof mixing with the ambient air that contained higher O3 and lower NOx concentrations, as well as the conversion of NO into NO2, the O3 concentrations in aged plumes increased and became somewhat (up to 2 ppb) higher than in the ambient air as the plumes were transported further downwind. The sulfate concentration within the aircraft plumes, on the other hand, was enhanced due to chemical conversion of the emitted SO2. At both the domestic and international flight altitudes, enhancements of up to 10 and 5 ppt greater than in the background were achieved a couple of days after emissions. In the aged and diluted plumes, however, the production of sulfate can be lower than in the ambient because of the lower OH concentration.

The plume results were converted into regional-scale distributions by including all flight frequencies at each international and domestic flight corridor, as well as the probability distribution of the wind speed at the cruising altitudes to account for the advection effect. These composite results indicated that aircraft emissions led to negative perturbation of O3 concentration at and just downwind of the flight corridors, with maximum O3 reduction of about 1 and 4% at the international and domestic flight altitudes, respectively. The total column (vertically integrated through troposphere) O3, however, was enhanced because the emitted aircraft plumes were transported by various wind speeds and those plumes that stay longer in the domain under low wind speeds had a net production of O3. Since these aged plumes also had much greater volumes, their contributions to column O3 outweighed those from the smaller and more fresh plumes even though the fresh plumes occurred more frequently. Due to the international and domestic aircraft emissions, the tropospheric column O3 can be enhanced by more than 16 and 400 ppt, respectively. The sulfate concentrations at the international and domestic flight altitudes were generally enhanced by the aircraft emissions, and the perturbations could be as large as 0.16 and 0.35 ppt. The column sulfate perturbations were positive due to international flights, with a maximum reaching about 0.055 ppt; whereas the column perturbations from the domestic flights were negative, with a minimum reaching about -2 ppt.

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