Reassessing Seasonal Variations of Gaseous Oxidized Mercury in Urban Beijing

Understanding mercury behaviors in the atmosphere is essential for recognizing global mercury cycling. However, gaseous oxidized mercury (GOM) characteristics remain debated due to limitations in the commonly used KCl-coated denuder method. Here, we reassess GOM characteristics by simultaneously employing both the denuder method and a more reliable dual-channel method with a cation exchange membrane in an integrated system (CSAMS). Based on year-long observations in urban Beijing, we found significantly distinct seasonal patterns of GOM concentrations when using different methods: the denuder method showed GOM peaks in spring and troughs in summer, while the dual-channel method recorded the highest in summer and the lowest in winter. These results indicated significant seasonal variability in method consistency (the ratio of denuder GOM to dual-channel GOM), with values ranging from 7.3% in summer to 18.7% in spring (p-value < 0.05). Using a generalized additive model, we quantified the impacts of different environmental variables on method consistency. GOM, particulate-bound mercury, relative humidity, ozone, and nitric acid were identified as the main factors. Our study suggests temporal and potential spatial biases in GOM measurement introduced by the denuder method. We recommend reassessing the existing understanding of GOM characteristics and atmospheric mercury chemistry in future studies.