Formation of atmospheric particulate mercury in the Tokyo metropolitan area

Abstract

The sources and formation of atmospheric particulate mercury, Hg(p), in the Tokyo metropolitan area were investigated. Nine municipal solid waste (MSW) incinerators are located within 10 km of the study site. Airborne particles >10 μm in diameter were collected on a quartz fiber filter, and Hg(p) was measured by AAS following thermal desorption and gold trap amalgamation. Total gaseous mercury (TGM) was also determined using a continuous analyzer employing gold trap amalgamation and AAS. An average Hg(p) level of 0.098±0.051 ng m −3 (3.5±1.4% for total Hg=Hg(p)+TGM) was observed during the period from April 2000–March 2001, and Hg(p) levels tended to decrease during the summer. We investigated the relationship between concentrations of Hg(p) and Pb, a marker element for particles from MSW incineration. The propriety of using Pb as the marker element was verified based on the lead isotope ratios and the relationship between Pb, Cd and Zn concentrations. The results showed that Hg(p) concentration was correlated positively with Pb concentration and negatively with air temperature. On the other hand, the results of chemical leaching treatment for airborne particles indicated that most of the Hg in the particles might exist in the elemental form, Hg 0. This suggests that some of the Hg 0 emitted from MSW incinerators was adsorbed onto MSW incinerator particles in the atmosphere due to an abrupt decrease in temperature after emission, depending on air temperature. Thus, it is likely that the Hg(p) level in the Tokyo metropolitan area is closely related to the gaseous Hg 0 emissions from MSW incinerators. In addition, from the thermodynamic analysis, it is inferred that the formation of Hg(p) is governed by the physical adsorption equilibrium of Hg 0 between gas and particle phases.