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.

<p><strong>Abstract.</strong> Long-term monitoring 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 (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 Hemisphere, confirming that the gradient observed is mostly driven by local and regional sources, which can be anthropogenic, natural or a combination of both.</p>

Particulate nitrate ( pNO 3 − ) has long been considered a permanent sink for NO x (NO and NO 2 ), removing a gaseous pollutant that is central to air quality and that influences the global self-cleansing capacity of the atmosphere. Evidence is emerging that photolysis of pNO 3 − can recycle HONO and NO x back to the gas phase with potentially important implications for tropospheric ozone and OH budgets; however, there are substantial discrepancies in “renoxification” photolysis rate constants. Using aircraft and ground-based HONO observations in the remote Atlantic troposphere, we show evidence for renoxification occurring on mixed marine aerosols with an efficiency that increases with relative humidity and decreases with the concentration of pNO 3 − , thus largely reconciling the very large discrepancies in renoxification photolysis rate constants found across multiple laboratory and field studies. Active release of HONO from aerosol has important implications for atmospheric oxidants such as OH and O 3 in both polluted and clean environments.

15During the peak period of hurricane activity in the summer of 2010, vertical profiles of 16 ozone using ozonesondes were taken downstream of tropical cyclones in the Western and 17Eastern Atlantic Ocean basin at Barbados and Cape Verde. Measurements are taken for 18 tropical cyclones Danielle, Earl, Fiona, Gaston, Julia and Igor. The measurements show 19an increase in ozone mixing ratios with air originating from the tropical cyclones at 5-10 20 km altitude. We suggest that observed lightning activity associated tropical cyclones and 21the subsequent production of NO X followed by upper level outflow and subsidence ahead 22of the tropical cyclones and aged continental outflow from West Africa thunderstorms 23 produced observed increases in ozone mixing ratios. Hurricane Danielle showed the 24 largest changes in ozone mixing ratio with values increasing from 25 ppb to 70 ppb 25 between 22 and 25 August in the middle troposphere, near 450 hPa; warming and drying 26in the middle and lower troposphere. Measurements of ozone mixing ratios in Cape 27Verde show higher ozone mixing ratios prior to the passage of tropical storm Julia but 28 low ozone mixing ratios and high relative humidity up to 300 hPa when the storm was in 29 close proximity. This is due most likely the vertically transported from the marine 30 boundary layer. 31 32 33 34 3

Abstract. Mercury is a chemical with widespread anthropogenic emissions that is known to be highly toxic to humans, ecosystems and wildlife. Global anthropogenic emissions are around 20 % higher than natural emissions and the amount of mercury released into the atmosphere has increased since the industrial revolution. In 2005 the European Union and the United States adopted measures to reduce mercury use, in part to offset the impacts of increasing emissions in industrialising countries. The changing regional emissions of mercury have impacts on a range of spatial scales. Here we report 4 years (December 2011-December 2015) of total gaseous mercury (TGM) measurements at the Cape Verde Observatory (CVO), a global WMO-GAW station located in the subtropical remote marine boundary layer. Observed total gaseous mercury concentrations were between 1.03 and 1.33 ng m −3 (10th, 90th percentiles), close to expectations based on previous interhemispheric gradient measurements. We observe a decreasing trend in TGM (−0.05 ± 0.04 ng m −3 yr −1 , −4.2 % ± 3.3 % yr −1 ) over the 4 years consistent with the reported decrease of mercury concentrations in North Atlantic surface waters and reductions in anthropogenic emissions. The decrease was more visible in the summer (July-September) than in the winter (December-February), when measurements were impacted by air from the African continent and Sahara/Sahel regions. African air masses were also associated with the highest and most variable TGM concentrations. We suggest that the less pronounced downward trend inclination in African air may be attributed to poorly controlled anthropogenic sources such as artisanal and small-scale gold mining (ASGM) in West Africa.