Local circulation and aerosol water-soluble ions - A case study in Taiwan during Mei-yu season

The Mei-yu (plum rain) season is a short but important period when the weat her changes from spring to summer in Taiwan. In this study, size-segregated aerosols were collected alternately at 5 sampling sites in northwestern Ta iwan from June 16 to 24, 1994. For the first time in Taiwan, this study rev ealed the aerosol mass spectra and water-soluble ions in the Mei-yu season. For all samples, a bi-modal aerosol mass spectra was found with modal diam eters at 3.2 and 0.32 mu m, respectively. The aerosol samples were able to be divided into different groups to show their mass and ion spectra accordi ng to the calculated 5-hr backward air trajectory. The utilization of enric hment factors showed that aerosol Cl-, Na+, and Mg2+ for all sizes, and sup er-micron SO42- were related to the sea. Both the scheme of "chlorine loss" (Ohta and Okita, 1990) and a multivariate analysis (Thurston and Spengler, 1985) for categorizing water-soluble ions showed that sea-salts were major contributors in the prevalence of a sea breeze. In contrast, the secondary salts were significant for land breeze and a mix of land-sea breeze. In co nclusion, the influence of local circulation on the distribution of aerosol mass and ionic species was found to be prominent. (C) 1998 Elsevier Scienc e Ltd. All rights reserved.