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 PM pollution as a typical region, this study clarified the characteristics and transboundary transport of atmospheric Particulate Bound Mercury (PBM) 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. PBM showed increasing concentrations from northwest to southeast and negative δHg values, indicating significant contributions from anthropogenic sources. However, the mean ΔHg values of PBM 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 PBM, driven by west and northwest air masses originating in Central Asia and Russia, contributed significantly to the PBM pollution of BTH. Moreover, these air masses reaching BTH would carry elevated PBM 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 PBM pollution. This study provides insight into the atmospheric Hg circulation affected by the East Asian monsoon.