Sulfur-rich geothermal emissions elevate acid aerosol levels in metropolitan Taipei

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 150 μg m −3 (105 ppb). 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 H 2S 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 H 2S concentration was 1705 ppb in the geothermal area with low traffic density and the mean concentration was 404.06 ppb, which was higher than WHO guideline and might cause eye irritation. The SO 2 concentrations were relatively low (mean concentration was 3.9 ppb) in this area. It may partially result from the chemical reduction reaction in the geothermal emission, which converted the SO 2 gas into SO 4 2− and H 2S. Consequently, very high sulfate concentrations (mean concentration higher than 25.0 μg m −3) were also observed in the area. The geothermal areas also emitted relatively high levels of aerosol acidity, Cl −, F −, PO 4 3−, and N-containing aerosols. As a result, concentrations of HNO 3, NO 2 −, PO 4 3−, and SO 4 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.