Atmospheric constituents such as ozone, nitrogen- and sulphur-containing compounds and particulate matter can find their way into soil, marine and fresh water bodies and plant and animal tissue through the naturally occurring processes of dry and wet deposition. Dry deposition is the free fall to Earth directly from the atmosphere of atmospheric trace gases and particulate matter. Wet deposition is the process whereby atmospheric gases mix with suspended water in the atmosphere and are then washed out through rain, snow or fog. Acid rain is an example of wet deposition that damages forests, kills insects, and causes paint to peel, corrosion of steel structures, weathering of stone buildings and statues.
Understanding the spatial distribution and magnitude of total (the sum of wet and dry) deposition of atmospheric pollutants on a global scale is critical to determining the areas, populations, ecosystems and farmlands which are most vulnerable to its negative effects and which would most benefit from measures to control excessive pollutant loads.
The World Meteorological Organization is leveraging the world-wide community of experts on atmospheric deposition modelling and observations to develop global maps of total atmospheric deposition to identify areas most at risk and which would most benefit from measures to control excessive pollutant load. Communities working on ecosystem mapping or food security will participate in developing user-codesigned products. Deposition chemistry observations are performed at a number of sites in the Global Atmosphere Watch (GAW) Programme of WMO.