Mercury is a toxic metal released to our environment through mankind's activities (e.g., coal burning) and natural processes. Once it gets into the sea or other aqueous environments it can be transformed into toxic forms, which can then get into the food chain and can cause damage to the central nervous systems of animals. Mercury pollution is a grave human health concern and so it is important to understand its different global sources. Recent work has shown that continuously degassing volcanoes represent a significant source of mercury to the atmosphere compared to other sources on the global scale. Recent measurements at active volcanic craters have also shown that volcanic mercury is primarily emitted as a gas. This gas is not very soluble (e.g., in rainwater) and so would not be very efficiently removed from the atmosphere meaning that it could be transported very long distances from its source making volcanic mercury a global environmental hazard. However reactive bromine compounds have also been observed in volcanic plumes downwind from the point of emission. In the Polar Regions and at the Dead Sea reactive bromine compounds are thought to lead to the rapid transformation of gaseous mercury (present in these environments at low levels due to remote sources) into more soluble form that are then rapidly deposited to the local environment. If similar chemistry occurs in volcanic plumes then they will represent a significant mercury hazard to local ecosystems as this chemistry would not only lead to the rapid deposition of low levels of atmospheric mercury from remote sources but also the very significantly enhanced levels present in volcanic plumes. We want to understand the behaviour of mercury in volcanic plumes and the balance between local deposition and global dispersion. We will make measurements of the levels and types of mercury found in the persistent non-explosive volcanic plume from Masaya volcano in Nicaragua at the crater rim and at two locations downwind of the volcano where the plume is known to ground regularly. We will also measure reactive bromine levels and other constituents of the volcanic plume at these locations and will put out a network of samplers to measure the deposition flux of mercury to the local downwind environment from the volcano. These measurements will allow us to test our hypothesis that in volcanic plumes, gaseous mercury will be rapidly transformed to more reactive forms that are then deposited to the local environment. These measurements will then be used to adapt existing computer models of the atmospheric chemistry occurring in volcanic plumes to include mercury chemistry. This will allow us to further our understanding of volcanic mercury processing as well as to build up a picture of the fate of the mercury emitted from active volcanoes and specifically to understand whether volcanic mercury emissions pose a local or global hazard.