Long-term monitoring of data of ambient mercury (Hg) on a global scale to assess its emission, transport, atmospheric chemistry, and deposition processes is vital to understanding the impact of Hg pollution on the environment. The Global Mercury Observation System (GMOS) project was funded by the European Commission (http://www.gmos.eu) and started in November 2010 with the overall goal to develop a coordinated global observing system to monitor Hg on a global scale, including a large network of ground-based monitoring stations, ad hoc periodic oceanographic cruises and measurement flights in the lower and upper troposphere as well as in the lower stratosphere. To date, more than 40 ground-based monitoring sites constitute the global network covering many regions where little to no observational data were available before GMOS. This work presents atmospheric Hg concentrations recorded worldwide in the framework of the GMOS project (2010–2015), analyzing Hg measurement results in terms of temporal trends, seasonality and comparability within the network. Major findings highlighted in this paper include a clear gradient of Hg concentrations between the Northern and Southern hemispheres, confirming that the gradient observed is mostly driven by local and regional sources, which can be anthropogenic, natural or a combination of both.

The atmospheric deposition of mercury (Hg) occurs via several mechanisms, including dry and wet scavenging by precipitation events. In an effort to understand the atmospheric cycling and seasonal depositional characteristics of Hg, wet deposition samples were collected for approximately 5 years at 17 selected GMOS monitoring sites located in the Northern and Southern hemispheres in the framework of the Global Mercury Observation System (GMOS) project. Total mercury (THg) exhibited annual and seasonal patterns in Hg wet deposition samples. Interannual differences in total wet deposition are mostly linked with precipitation volume, with the greatest deposition flux occurring in the wettest years. This data set provides a new insight into baseline concentrations of THg concentrations in precipitation worldwide, particularly in regions such as the Southern Hemisphere and tropical areas where wet deposition as well as atmospheric Hg species were not investigated before, opening the way for future and additional simultaneous measurements across the GMOS network as well as new findings in future modeling studies.

Research has focused on the effect of sulphate and nitrate atmospheric deposition on river water chemistry in the Baikal region. During long-term monitoring it was ascertained that anthropogenic SO₂ and NO_x in the plumes emitted from regional coal Power Plants (PPs) cause extra nitrate and sulphate deposition in the Pereemnaya River basin that is located 100–150 km away from the emission sources. As a result, average concentrations of sulphate and nitrate in the water of the Pereemnaya River are 2–3 times higher than those in Lake Baikal, whilst total mineralisation of the river water is approximately 3 times lower. Because of the acidity of atmospheric deposition in the Pereemnaya river basin, pH value of the snow cover and precipitation is about 4.7. That makes the river water pH significantly lower (=6.8) than that in the lake and other tributaries (=7.8). It is recommended to strengthen monitoring of the natural environment in the southern basin of Lake Baikal and to apply measures to reduce pollutant emissions from PPs.