This Special Issue collects results on atmospheric mercury measurements and modelling applications obtained during the EU-funded Project "Global Mercury Observation System (GMOS)" within the 7th framework program. It summarizes original research work about sources, chemistry, transport, and deposition of atmospheric Hg carried out between 2010 and 2015. During the GMOS, a coordinated global observational network for atmospheric Hg was established. As one result the spatial and temporal coverage of atmospheric mercury measurements at remote locations around the globe has been significantly expanded, in particular in the Southern Hemisphere. To date, there are more than 40 monitoring sites participating in the GMOS network involving over 50 research institutions worldwide.

In combination with global mercury deposition measurements carried out at a substantial number of these sites, this unprecedented data base has supported the development and validation of both regional and global atmospheric Hg models.

Research work summarized in this Special Issue has strong ties to the UNEP Global Partnership on Atmospheric Mercury Transport and Fate Research (UNEP F&T), the Task Force on Hemispheric Transport of Air Pollutants (TF HTAP) of the UNECE-LRTAP convention, and GEO Task HE-02: Tracking Pollutants related to Hg and POPs as well as the Arctic Monitoring and Assessment Program (AMAP).

The manuscripts within this Special Issue have been selected under three main topic cover issues concerning (a) aircraft Hg measurements focusing on vertical profiles of Hg concentrations over Europe discussing influences of industrial emission sources on Hg levels; (b) atmospheric mercury measurements at ground-based monitoring sites globally distributed at high altitude and at sea level locations in the Northern and Southern hemispheres, as well as in the equatorial area highlighting seasonal, annual, and spatial changes of Hg levels and total wet deposition fluxes; and (c) regional and global scale modelling applications and results highlighting the atmospheric mercury pattern and process, deposition, and source apportionment.