Dry and Wet Deposition: When Air Pollution Becomes a Land and Water Question

Air pollution is often treated as though its environmental relevance exists only while a substance remains airborne.

That is incomplete.

Pollutants can move from the atmosphere to the Earth's surface.

This process is known as deposition.

Dry deposition

Dry deposition occurs without precipitation.

Particles and gases can be transferred directly to soil, vegetation, buildings and water surfaces.

The rate depends on factors including particle size, chemical properties, wind, surface characteristics and vegetation.

Wet deposition

Wet deposition occurs when pollutants are incorporated into rain, snow, fog or other atmospheric water and transferred to the surface.

This can move contamination considerable distances from its original source.

From air to soil

Once deposited, pollutants may remain near the surface, bind to soil, migrate downward, move in runoff, become resuspended or be taken up biologically.

An air-emissions question can therefore become a soil-management question.

From air to water

Pollutants can reach water directly through deposition, indirectly through contaminated runoff, or through groundwater movement.

Vegetation

Pollutants may deposit onto leaves or enter plant systems depending on contaminant properties.

This does not automatically establish food-chain exposure.

The pathway still requires uptake, persistence, transfer and consumption.

Snow and ice

Deposition can also affect cryosphere systems.

Light-absorbing particles such as black carbon can darken snow and ice surfaces and influence surface energy balance.

This is an example of deposition producing a physical climate effect rather than only chemical contamination.

Monitoring

Stack monitoring does not directly measure deposition.

Ambient-air measurements do not necessarily describe cumulative surface loading.

Where deposition is environmentally important, separate modelling, soil sampling, water sampling and deposition monitoring may be required.

Governing principle

Air pollution does not necessarily remain an air problem.

Deposition can transfer pollutants into soil, water, vegetation, ecosystems and potential food-chain pathways.

That transfer must be demonstrated rather than presumed, but it cannot responsibly be ignored.