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. Principal component analysis also shows that the highest TGM concentrations observed are a result of forest fire smoke near the monitoring station. Back trajectory analysis highlights the importance of long-range transport, indicating that unseasonably high TGM concentrations are generally associated with air from the southeast and west, while unseasonably low TGM concentrations are a result of arctic air moving over the monitoring station. In general, TGM concentration appears to be driven by diel and seasonal trends superimposed over a combination of long-range transport and regional surface-air flux of gaseous mercury. OPEN ACCESSAtmosphere 2013, 4 473

Speciation studies are necessary to understand the biogeochemical cycling of arsenic in aquatic systems. The species of arsenic present, their behaviour and toxicity will change depending on the biotic and abiotic conditions in the water. In groundwater, arsenic is predominantly present as arsenite (As III ) and arsenate (As V ). The major arsenic species in freshwater are As III and As V and minor amounts of MMA , DMA and methylated As III have also be detected. In seawater, the arsenic speciation differs in the surface and deep zone. In addition to the above species, uncharacterized arsenic species may constitute a significant portion of the total arsenic present in some water and the identification of these compounds is necessary to fully understand the arsenic biogeochemistry in water.A knowledge of the speciation of As in natural water is important because the bioavailability and the physiological and toxicologial effects of As depend on its chemical forms. Arsenic occurs in a variety of chemical forms in water, and this is a result of many chemical and biological transformations in the aquatic environment. The mechanism of transport and biological availability of arsenic depends on the chemical species present.The main arsenic compounds detected in natural waters are arsenite [As 111 ],

Arsenic is the twentieth most abundant element in the earth's crust; it occurs naturally in the environment in both inorganic and organic forms. Arsenic is also released to the environment by anthropogenic activities such as pesticide use, wood preservation, mining, and smelting. Human exposure to arsenic by the general population occurs primarily from drinking water and food. In areas of endemic arsenic poisoning such as Bangladesh, India, Inner Mongolia, and Taiwan, the main exposure is through drinking water where inorganic arsenic levels can reach concentrations in the hundreds or thousands of micrograms per liter. The arsenic is released from natural mineral deposits into the groundwater in endemic areas. Groundwater is the primary drinking water source in these areas. Arsenic is present in a variety of inorganic and organic chemical forms in water. This is a result of chemical and biological transformations in the aquatic environment. The specific arsenic compound present determines its toxicity, biogeochemical behavior, and environmental fate.