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Abstract: Several studies have demonstrated that millions of people globally are potentially exposed to volcanic gases. Hydrogen sulfide is a typical gas in volcanic and geothermal areas. The gas is toxic at high concentrations that predominantly affects the nervous, cardiovascular, and respiratory systems. The WHO air quality guideline for hydrogen sulfide is 150μgm−3 (105ppb). The northwest part of Taipei is surrounded by sulfur-rich geothermal and hot springs. Active fumaroles and bubbling springs around the geothermal area emit acidic gases. In combination with automobile emissions, the pollution of acid aerosols is characteristic of the metropolis. This study considered sulfur-rich geothermal, suburban and downtown locations of this metropolis to evaluate geothermally emitted acid aerosol and H2S pollution. Acid aerosols were collected using a honeycomb denuder filter pack sampling system (HDS), and then analyzed by ion chromatography (IC). Results indicated that long-term geothermal emissions, automobile emissions and photochemical reactions have led to significant variations in air pollution among regions of metropolitan Taipei. The highest H2S concentration was 1705ppb in the geothermal area with low traffic density and the mean concentration was 404.06ppb, which was higher than WHO guideline and might cause eye irritation. The SO2 concentrations were relatively low (mean concentration was 3.9ppb) in this area. It may partially result from the chemical reduction reaction in the geothermal emission, which converted the SO2 gas into SO4 2− and H2S. Consequently, very high sulfate concentrations (mean concentration higher than 25.0μgm−3) were also observed in the area. The geothermal areas also emitted relatively high levels of aerosol acidity, Cl−, F−, PO4 3−, and N-containing aerosols. As a result, concentrations of HNO3, NO2 −, PO4 3−, and SO4 2− in metropolitan Taipei are significantly higher than those in other urban areas considered in other studies. These results provide evidence that geothermal emissions represent a significant source of acid aerosols in metropolitan Taipei. Therefore, in this metropolitan area, the combined health effects of air pollutants from automobiles and geothermal emissions should be particular concern. [Copyright &y& Elsevier]