Tracing the transboundary transport of atmospheric Particulate Bound Mercury driven by the East Asian monsoon

Taking Beijing-Tianjin-Hebei (BTH) with severe atmospheric mercury (Hg) and PM2.5 pollution as a typical re-gion, this study clarified the characteristics and transboundary transport of atmospheric Particulate Bound Mercury (PBM2.5) affected by the East Asian monsoon. Five sampling sites were conducted in rural, suburban, urban, industrial, and coastal areas of BTH from northwest to southeast along the East Asian monsoon direction. PBM2.5 showed increasing concentrations from northwest to southeast and negative delta 202Hg values, indicating significant contributions from anthropogenic sources. However, the mean Delta 199Hg values of PBM2.5 at the five sites were significantly positive, probably triggered by the photoreduction of Hg(II) during long-range transport driven by the East Asian monsoon. Apart from local anthropogenic emissions as the primary sources, the transboundary transport of PBM2.5, driven by west and northwest air masses originating in Central Asia and Russia, contributed significantly to the PBM2.5 pollution of BTH. Moreover, these air masses reaching BTH would carry elevated PBM2.5 concentrations further transported to the ocean by the East Asian monsoon. In contrast, the southeast air masses transported from the ocean by the East Asian monsoon in summer diluted inland PBM2.5 pollution. This study provides insight into the atmospheric Hg circulation affected by the East Asian monsoon.