Atmospheric deposition is the term given to the phenomenon of the transfer of pollutants from the atmosphere to the planet's surface. We are accustomed to hearing how humans pump toxins into our environment, but, when it comes to pollution, what must be kept in mind is that toxins rarely stay in one place when it enters our environment.

For instance, if it is deposited in our rivers, it will eventually end up in our food. In this week's article I will look at the causes and types of atmospheric deposition. I will also look at how the effects of atmospheric deposition are widely felt in different ecosystems.

Pollutants enter our atmosphere as gases or particles from combustion from slash and burn agriculture, vehicular emissions or from industrial processes. This smoke produced contains chemicals like nitrogen, sulphur and even metal compounds. Over time these pollutants are deposited on the earth's surface, in some cases, thousands of kilometres from the source of the pollution. Therefore, atmospheric deposition may be a major source of air pollution.

There are three types of atmospheric deposition; wet deposition, dry deposition and gas absorption. In wet deposition, the pollutant basically mixes with the water vapour to form weak acids. Therefore the pollutant returns to earth as acid rain. With areas that experience a relatively high annual rainfall, wet deposition is the main method of pollutant are transfer to the planet's surface. But rain is not the only vehicle for these pollutants as they can also be found in snow and fog.

In urban areas, wet deposition can cause damage to personal property like houses and vehicles. It can also cause health problems to humans and pets. When this contaminated water enters our waterways, either directly or as run off, it changes the pH level of the water. This change might be slight and not affect species that can tolerate varying degrees of acidity. But to those species that are sensitive to slight changes in acidity, they might become locally extinct. Therefore, besides reducing the biodiversity of a river or stream, it also has the effect of reducing our food supply. This problem is not restricted to aquatic animals, as other animals depend on rivers and streams not only for food but for water. These terrestrial animal species may be what humans hunt for food or they are prey of the animal species hunted for food. Humans also need water in order to survive; acid rain does not discriminate when it comes to where it falls so reservoirs are also affected. This contaminated water can also leach into underwater aquifers, so that our supply of drinking water is compromised.

Acid rain also contains particles of nitrogen, which encourages plant growth. Therefore, when it lands in an aquatic environment, it causes accelerated growth of plants, referred to as eutrophication. This rapid growth uses up the resources available, such as light, nutrients and space and preventing the occupation by other species.

But this environmental problem has to be viewed from an economic standpoint as well. The corrosive action will deteriorate homes and vehicles. We will have to build additional structures like garages to protect the vehicles. These are additional structures that we will have to constantly maintain at a more regular rate due to the corrosive effects of acid rain. Infrastructure like roads, bridges, tunnels will also need more regular maintenance, which means more money being spent. The production of more materials to carry out these repairs also means more energy is needed, which is what caused the pollution in the first place. So it is a never-ending cycle.

Dry deposition refers to how substances in particle form are transferred from the atmosphere to the earth's surface, and is the main method of atmospheric deposition in arid regions like deserts. This method is affected by three main factors. The first factor includes variables like temperature, humidity and wind speed. The second factor deals with the nature of the surface it will be interacting with where it is deposited. Some of the variables to be considered for this factor include the structure, roughness and pH of the surface. Other factors taken into consideration are: if the surface is porous, does it attract or repel water or does it have an electrical charge. These properties lead to the final factor, which is the nature of the material being deposited. In this case the shape, chemical reactivity, solubility, size and whether or not it has an electrical charge is of importance. For instance, if a substance lands on the ground, if the soil is porous like sand when it rains, it will enter the soil, however, if it is non-porous like rock it will be washed or blown away.

Finally, when it comes to gas absorption, the concentration of the pollutants in the air and water determine where they will be deposited. For instance, if the water in a river contains less pollutant, then the pollutant from the air will transfer to the water. However, if the opposite condition occurs, then the pollutant will move from the water to a gaseous state and enter the atmosphere. This method of atmospheric deposition is used by semi-volatile toxins that persist in the environment and usually accumulate in living organisms, thus being transferred along the food chain. Heavy metals like mercury are an example of a pollutant that is transferred by the gaseous absorption method. So when we pump pollutants into our atmosphere, it is often thought that it will stay there or that the effects would only be confined to the atmosphere.

But what we are actually doing is creating a reservoir of pollutants which, when the conditions are right, is deposited on our planet's surface. Therefore atmospheric deposition affects both terrestrial and aquatic ecosystems and all the organisms that occupy them.