Mercury in ambient air: a review on sources, speciation, legislation and measurement methods

Mercury being a neurotoxin poses great threat to the human health as well as environment. It is emitted into the atmosphere through both natural and anthropogenic sources and can travel long distances over time before being scavenged and deposited in the deep ocean beds or mineral soils thus becoming a global concern. It occurs in atmosphere largely in three different forms, viz., gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM) or reactive gaseous mercury (RGM) and particulate bound mercury (PBM). Over > 95% of mercury existing in atmosphere is in the GEM form as it is more stable and has a longer residence time. The knowledge and understanding of the mercury emission pathways, its chemical transformations in the atmosphere and eventual deposition in the reservoirs can prove crucial in its management. Recognizing its health effects and environmental impacts, countries have set up various legislations and rules for its monitoring, control and abatement. Minamata convention is one such international treaty that aims at protecting human health and environment from harmful effects of mercury pollution on the land, ocean and atmosphere. Health effects occurring from ingestion of mercury polluted waters and contaminated fish is a commonly known phenomenon but studies on impacts of inhalation of mercury polluted air are still limited. Various studies on model simulations for estimation of actual mercury levels that can be ascribed to the anthropogenic or natural sources are performed over the last few decades. However, a small number of studies are available on ground-based quantitative measurements of mercury species in ambient air. Also, a few methods have been developed to measure the mercury concentration in the ambient air over the years but even at present date, global consensus on its measurement method has not been achieved. This study reviews the available knowledge on interdisciplinary aspects of the mercury pollution in ambient air including its sources and emissions, its species and their behavior in the atmosphere, existing global legislation and monitoring methods. The study also proposes the way ahead in meeting the international commitments for Asian countries like India through inclusion of a ground measurement network and stricter implementation protocols.