Patterns and trends of atmospheric mercury in the GMOS network: Insights based on a decade of measurements

The Global Mercury Observation System (GMOS) network, initially a five-year project (2010-2015) funded by the European Commission, continued as a GEO Flagship program to support the Global Observation System for Mercury (GOS4M). GMOS was envisioned as a coordinated global observing system to monitor atmospheric mercury (Hg) on a global scale, to support and evaluate the effective implementation of the Minamata Convention on Mercury (MCM). Twenty-eight ground-based stations have participated in monitoring activities, following GMOS sampling protocols and related data quality control management. The GMOS network provides representative coverage of all latitudes, from the Northern Hemisphere to the Southern Hemisphere including the Arctic Circle, Antarctica, and the Tropical Zone. This work presents atmospheric Hg data, available as Total Gaseous Mercury (TGM) or Gaseous Elemental Mercury (GEM) concentrations, recorded within the GMOS network from 2011 to 2020. TGM/GEM concentrations were analysed in terms of their variability along latitudinal areas, considering their comparability, temporal trends and patterns. The main results confirmed a clear gradient of TGM/GEM concentrations between the northern (1.58 ± 0.31 ng/m³) and southern (0.97 ± 0.14 ng/m³) hemispheres. Decreasing trends in TGM/GEM levels were found to be strongly significant only for selected remote stations with at least 5 years of data coverage. Seasonality in atmospheric TGM/GEM concentrations was observed to increase with latitude and is greater at inland sites than at coastal sites.