Mercury in cloud water collected on Mt. Bamboo in northern Taiwan during the northeast monsoon season

Cloud water was sampled on Mt. Bamboo in northern Taiwan in January-March 2009 for total mercury (Hg) analysis. This was the first cloud water Hg measurement reported from the downwind region of East Asia, which is the major Hg emission source region globally. A total of 11 cloud events were encountered, and 129 and 316 cloud water samples were collected for Hg and major ion analyses, respectively. The mean cloud water pH was 4.14. Cl^- and Na⁺ were the major ions in cloud water. About 2/3 of the samples had molar ratios of Cl^-/Na⁺ close to the seawater ratio of 1.12, demonstrating a significant impact of sea salt aerosols on cloud water chemistry. On the other hand, 90% of the measured SO₄^2- was non-sea-salt-SO₄^2- (nss-SO₄^2-), highlighting the influence of human activities. Concentrations of Hg ranged between 0.5 and 165.5 ng L^-1, with a mean of 9.6 ng L^-1. Elevated Hg concentrations were usually associated with highly acidic samples. Hg concentrations were well correlated with major ion concentrations, especially with NO₃^-, K⁺, nss-SO₄^2-, and NH₄⁺. The results of element ratio calculations, correlation analyses and trajectory analyses indicated that, in addition to sea salt aerosols, coal combustion, industrial activities, and biomass/biofuel burning in the East Asian continent could have contributed to the measured Hg in the cloud water. The contribution of below-cloud scavenging to rainwater Hg was evaluated by comparing the cloud water Hg concentrations to the rainwater Hg concentrations sampled at a nearby mountain site and at an urban site. The mean rainwater Hg values at the mountain site (11.6 ng L^-1) and at the urban site (19.2 ng L^-1) were all higher than the cloud water value, demonstrating the below-cloud scavenging contribution. Nonetheless, the below-cloud scavenging contribution was more significant in the urban area than in the mountain area. About 28% and 91% of the rainwater Hg at the mountain and urban sites could be attributed to the below-cloud scavenging, respectively.