Total Gaseous Mercury (TGM) measurements at the Cape Verde Atmospheric Observatory (CVAO)

Atmospheric mercury has been claimed by UNEP to be the second most important global environmental issue after greenhouse gases, and many countries are becoming increasingly concerned about atmospheric mercury pollution. Transported in the atmosphere in the form of gaseous elemental mercury (GEM), which is the less reactive form and accounts for more than 95% of total gaseous mercury (TGM), it has a 0.5 – 2 year residence time in the atmosphere. This lifetime makes it an important global pollutant, recorded as persistent and bio-accumulative toxic, after oxidation processes that culminates in deposition to the surface environment and aquatic food chain, causing several types of damage to human health and ecosystems. Presented here the results from the first two years of total gaseous mercury measurements at the Cape Verde Atmospheric Observatory. These are made as part of the Global Mercury Observation System (GMOS) network, the goal of which is to establish long-term continuous world coverage of mercury measurements. This project contributes high precision data that is representative for a large geographical area in the tropical Atlantic Ocean. Up until now the only existing data in this region have been from short-term ship-based cruise campaigns. The observatory is located at 16.86403º N, 24.86752º W (10 m a.s.l.), approximately 50 m from the coastline, there are no habitation within a 2 km radius and the nearest public road is 1.2 km downwind. The prevailing wind is from the NE from the open ocean bringing air masses from the tropical Atlantic and from the African continent. Measurements made in 2012 and 2013 were broadly consistent with previously published measurements in the region, with typical atmospheric values of between 1.0 and 1.5 ng m ̄ ³. Whilst located in the Northern hemisphere, the low latitude of Cape Verde resulted in observations more similar in concentration to those reported previously in the Southern hemisphere. Gaseous Hg showed little annual variability at Cape Verde, (slight minimum in November and December, and maximum from July to September), but had a strong diurnal cycle with a minimum in the later afternoon. The destruction of Hg during the day was consistent with loss mechanisms by bromine and hydroxyl (OH), which both have maxima during the day. Many synoptic scale events (1-5 days) showed a close relationship between observed mercury and other tracers of long-range pollution transport e.g. CO, or a strong dependence on airmass origin, seen for example during Saharan dust transport.