Elevated Gaseous Oxidized Mercury Revealed by a Newly Developed Speciated Atmospheric Mercury Monitoring System.

Gaseous oxidized mercury (Hg 2+ ) monitoring is one of the largest challenges in the mercury research field, where existing methods cannot simultaneously satisfy the measurement requirements of both accuracy and time precision, especially in high-particulate environments. Here, we verified that dual-stage cation exchange membrane (CEM) sampler is incapable of gaseous elemental mercury (Hg 0 ) uptake even if particulate matter is trapped on CEM, whereas the Hg 2+ capture efficiency of the sampler is more than 90%. We then developed a Cation Exchange Membrane-Coupled Speciated Atmospheric Mercury Monitoring System (CSAMS) by coupling the dual-stage CEM sampler with the commercial Tekran 2537/1130/1135 system and configuring a new sampling and analysis procedure, so as to improve the monitoring accuracy of Hg 2+ and ensure the simultaneous measurement of Hg 0 , Hg 2+ , and Hg p in 2 h time resolution. We deployed the CSAMS in urban Beijing in September 2021 and observed an unprecedented elevated Hg 2+ during the daytime with an average amplitude of 510 pg m -3 . Using a zero-dimensional box model, the elevated Hg 2+ production rate was attributed to high atmospheric oxidant concentrations, Hg 0 heterogeneous and interfacial oxidation processes on the surface of atmospheric particles, or potential unknown oxidants.