Speciated atmospheric mercury during haze and non-haze periods in winter at an urban site in Beijing, China: Pollution characteristics, sources, and causes analyses

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Speciated atmospheric mercury (Hg) was measured at an urban site in Beijing, China from February to March 2018. The mean concentrations of gaseous elemental Hg (GEM), gaseous oxidized Hg (GOM), and fine particulate bound mercury (PBM2.5) were 2.81 ± 0.90 ng m−3, 10.7 ± 8.5 pg m−3, and 60.6 ± 43.5 pg m−3, respectively. Reactive Hg (i.e., RM=GOM+PBM2.5) only represented ~2.4% of total atmospheric Hg. The GEM concentration at the monitoring site was 1.7 times higher than the average value in the Northern Hemisphere. In particular, the mean GEM and PBM2.5 concentrations were significantly higher during haze period (hourly average AQI value > 100 for at least consecutive 24 h) (3.65 ± 0.76 ng m−3 and 93.0 ± 37.1 pg m−3) than during non-haze period (2.42 ± 0.73 ng m−3 and 48.5 ± 39.5 pg m−3). One possible explanation is the strong subsidence of air masses during haze period could lead to the accumulation of air pollutants. However, there was no significant difference in GOM concentration between haze and non-haze periods. Although the ozone, relative humidity (RH), and planetary boundary layer (PBL) all followed a clear diurnal cycle, while for RM no distinct diurnal pattern was observed. Additionally, GEM was significantly positively correlated with CO, indicating that they had homogeneous sources; whereas, they were all negatively correlated with wind speed and PBL height. Based on the combination of potential source contribution function and clustering analyses, we found that high atmospheric Hg events during haze days were more likely caused by the domestic sources in the Circum-Bohai-Sea region, while the transboundary transport of the clean air masses originating from Siberia was attributed to the low GEM and PBM2.5 levels during non-haze days.