Atmospheric Deposition

Atmospheric Deposition

Atmospheric deposition is an important process that removes gases and particles from the atmosphere. However, it also is a major environmental issue in several parts of the world due to concerns over acidification and eutrophication of natural ecosystems, bioaccumulation of toxic substances and metals, impacts on biodiversity, human health, and global climate change. Increased concentrations of pollutants in the atmosphere due to human activities results in more atmospheric deposition of pollutants, with negative effects on human health, crop yields, and land and marine ecosystems. 

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. 

Effects of atmospheric deposition on ecosystems

Eutrophic lake

High concentrations of atmospheric pollutants can result in atmospheric deposition of these compounds exceeding the critical level. This leads to negative effects on the main ecosystem services provided by terrestrial plants, such as biodiversity conservation, production of food and forest-based products, carbon sequestration and water regulation.

For instance, enrichment of nutrients in water bodies causes excessive growth of plants and algae, a phenomenon known as eutrophication, and can lead to algal blooms and may result in oxygen deprivation of the water body with detrimental effects for the plants and animals. Food chain dynamics may also be disrupted.

Effects of atmospheric deposition on vegetation and crops

Ozone damage alone accounts for annual losses of 85 million tonnes of wheat grain worldwide and economic losses in the US$ 10-20 billion range due to its impact on staple food crops such as wheat, soybean and corn, according to WMO’s 2018 Reactive Gases Bulletin. Ozone-related damage causes economic losses in the range of US$ 10-20 billion due to its effect on staple food crops such as wheat, soybean and corn. Furthermore, ozone uptake by plants reduce forest biomass and alter the  species composition of grasslands and semi-natural vegetation.

Effects of atmospheric deposition on human health

From a human health perspective, the single most damaging pollutant is particulate matter (PM). The toxicity of PM, including what chemical constituents are responsible for triggering health effects, is poorly understood. There is a strong relationship between particle size and health effects – i.e. the smaller the particles, the deeper they penetrate into the respiratory system, with PM in size equal or smaller than 2.5 µm being of primary concern.

How is atmospheric deposition estimated?

 

 

Monitoring site in Bondville, Illinois, United States, operated by US National Atmospheric Deposition Program since 1979. Bondville is the flagship station of the monitoring effort in the United States.

Estimating total deposition of atmospheric pollutants cannot be done in the same way as other atmospheric variables, such as  temperature, humidity or wind. Estimation of atmospheric deposition involves, on the one hand, field measurements of atmospheric pollutant concentrations both in ambient air or dissolved in water and, on the other hand, modelled estimates of deposition velocities. 

 

 

 

 

These have  then to be combined using mathematical and statistical techniques known as Measurement-Model Fusion (MMF) resulting in maps of deposition for different atmospheric pollutants. Due to the observational and computational infrastructure and resources required, MMF is only carried out in a limited number of countries in North America and Europe. The resulting deposition maps are only regional in scope and for a limited number of  pollutants. 

Developing a global, detailed and comprehensive tool for estimating the spatial extent and magnitude of atmospheric deposition is the aim of the MMF Global Total Atmospheric Deposition (MMF-GTAD) project, one of the core projects of the Global Atmosphere Watch Programme. The project, in its initial stages, will define the parameters and best practices for such a tool based on the experience of those countries that have already developed MMF products.