Measurement of size-fractionated particulate-bound mercury in Beijing and implications on sources and dry deposition of mercury.

Particulate-bound mercury (PBM) plays an important role in the Hg cycling and is with high concentration in urban area. Totally 10-staged (range of 0.01-10μm) size-fractionated PBM was measured in Beijing during December 2016-November 2017. Our study indicated that Beijing suffered from heavy PBM pollution, with an annual average of 297.9 (17.2-1906.6) pgm-3, 22.2-52.5 times higher than that observed in North America. Overall, the concentration of PBM2.5 (PBM2.5 and PBM2.5-10 meant PBM concentration in PM with aerodynamic diameter <2.5μm and in the range of 2.5-10μm, respectively) accounted for 75%-87% of PBM10 in all seasons. The increase of PBM2.5-10 in the polluted days (PM2.5>75μgm-3) indicated the primary influence from natural sources while anthropogenic sources and secondary conversion caused the growth of PBM2.5 concentration. In addition, the generation of secondary PBM during the transport process contributed to the generation of PBM in Beijing, especially when the PM concentrations increase simultaneously. The local emissions and physicochemical conversion were the dominant cause of extremely high PBM pollution event in winter. Enhanced PBM concentrations also lead to severe dry deposition of PBM in Beijing. The estimated annual dry deposition of PBM was 40.6μgm-2yr-1, which was 3.1-203.0 times than observations from other sites in northern hemisphere. PBM2.5 contributed 53% of total dry deposition in all seasons, significantly higher than the other observation sites (28%-36%). In addition, the proportion of PBM2.5 deposition in PBM10 deposition ranged from 47% in summer to 66% in winter, which indicated that both seasonal variations of PBM size distribution characteristics should be taken into consideration. Results from this study revealed the size distribution and dry deposition characteristics of PBM in urban Beijing and provided base for the implementation of Minamata convention on mercury.