Differentiating between Local and Remote Pollution over Taiwan

ABSTRACT

In this study, an approach has been developed for differentiating between local and remote pollution over Taiwan, based on the homogeneity (variations of the standard deviation) of both AERONET measurements and NASA MERRA aerosol reanalysis (version 2, MERRA-2) over a 15-year period (2002–2017). The analysis of seasonal variations of the standard deviation of aerosol optical depth (AOD) measurements at six AERONET sites and MERRA AOD data in Taiwan showed that in spring, when remote aerosols dominate, the standard deviation is almost three times lower than in autumn, when local aerosols dominate. This finding was supported by MERRA’s AOD over the open ocean: The total AOD data were used to differentiate between local and remote pollution over both Taiwan and the open ocean in the vicinity of Taiwan. Over Taiwan, MERRA’s total AOD showed a primary maximum in spring and a secondary one in autumn. Over the open ocean, where there are no local sources of anthropogenic aerosols, MERRA’s total AOD showed only one maximum in spring and no maximum in autumn. This suggests that in Taiwan, the maximum in autumn is attributable to local air pollution, whereas the pronounced maximum in spring is mainly caused by air pollution from continental Asia. Analyses of the spatial distribution of MERRA’s 15-year monthly mean winds confirmed the above-mentioned results. Furthermore, similar to the total AOD, MERRA’s sulfate AOD peaked in autumn over Taiwan but not over the oceanic area, indicating the contribution of local emissions of anthropogenic aerosols from the industrial sector. The standard deviation of MERRA’s sulfate AOD in spring is two to three times lower than in autumn, which provides additional evidence that in spring, sulfate aerosols from remote sources are predominant, whereas in autumn, sulfate aerosols from local sources dominate.