Estimation of foreign versus domestic contributions to Taiwan's air pollution

Based on the analysis of observational data obtained over a period of 17 years, this study developed a novel approach estimating long-term changes in the relative contributions of domestic and foreign sources to air pollution levels over the island of Taiwan. The contribution from foreign sources was calculated using data measured at selected coastal monitoring stations under specific meteorological conditions. The domestic contribution was derived by subtracting the foreign contribution from the overall concentration, which was calculated with island-wide monitoring station data averaged using Thiessen polygon area weighting. The trends of mean CO, NO, NO2 and SO2 concentrations in Taiwan were −2.1%, −3.3%, −1.4% and 0% yr−1; while the background concentrations varied by −1.0%, 5.7%, 3.8% and 6.6% yr−1, respectively. These figures suggest that the efforts of Taiwan in reducing air pollution are largely being negated by foreign contributions. Ozone showed a steady increase of 3.3% yr−1, and a portion of this was associated with a 1.0% yr−1 increase in background values. Local and background PM10 concentrations did not show significant long-term trends, but rather strong inter-annual variations associated with dust storm activity in East Asia. The domestic fractions of NO and NO2 respectively decreased from 90% and 85% in 1994 to less than 60% and 70% in 2010. In contrast, the domestic fraction of SO2 decreased from 82% in 1994 to 27% in 2010. The domestic fraction of CO exhibited no obvious trend due to concurrent decreases in local and background contributions. Background O3 values tended to drop across Taiwan due to the titration effect from domestic NO emissions, and the fraction of domestic O3 titration decreased from 50% in the mid-90s to 25% in 2010.