Air Pollution Impacts on the Environment

Key messages

• Low levels of air pollution and a high standard of the quality of our air are essential for a good quality of life, to help us protect health and wellbeing, reduce the contributions to, and impacts of, climate change, protect the environment and biodiversity and to help us achieve net zero.

• Air pollution can lead to severe environmental degradation and result in contamination of soils and water, result in imbalances in ecosystems and reduced biodiversity, directly damage plants and animals and contribute to climate change. This can result from short-term air pollution episodes or longer-term chronic exposure to air pollution.

• Further reductions in air pollution requires concerted action across many sectors including national and local government, the private and public sectors, and by individual members of the public. As new evidence emerges, increased awareness and understanding of the key issues around air pollution will promote the change to behaviours required to help reduce emissions and the impacts of air pollution on the environment.

Environmental impacts of air pollution

In the page on Air Pollution and Air Quality we have already seen where air pollution originates from and how it behaves and moves through the environment. Here we will look at some of the direct and indirect impacts of air pollution on our environment.

Links between air pollution and the physical environment – Cleaner Air for Scotland - The Road to a Healthier Future

Deposition

Deposition is the process where air pollutants that have been transported through the atmosphere, and have undergone chemical reactions, are finally deposited on the surface of the environment. This can result in significant contamination of soils and water, cause imbalances in ecosystems and reduce biodiversity and cause direct damage to plants, animals and the built environment. Deposition is especially important where an ecosystem may already be in a sensitive state or subject to stress.

The main air pollutants that can affect habitats and species through deposition are oxides of sulphur (SO_x), oxides of nitrogen (NO_x), and ammonia (NH₃). The air pollutants enter the atmosphere through releases of gases from both natural sources and from anthropogenic activities such as burning of fossil fuels for energy generation, and transport and agricultural production. When these gases are released, they can react in the atmosphere to form acids and nitrogen-rich compounds. These travel through the atmosphere and are then deposited onto, and into, the environment. Depending on the chemical form they are in when they are deposited from the atmosphere, different environmental impacts can be observed.

Deposition of pollutants can be both wet (in water droplets which can form rain, snow and mists) and dry (where the air pollutants have reacted in the atmosphere and are deposited on the surface of the earth as gases or particulate matter). Wet deposition tends to occur over greater distances than dry meaning it can affect ecosystems hundreds or thousands of miles away, whereas dry deposition tends to affect the environment closer to the point of release.

Acid Deposition

SO_x and NO_x can react with water in the atmosphere to form solutions of sulphuric (H₂SO₄) and nitric acids (HNO₃) as secondary pollutants. This is then washed from the atmosphere via wet deposition. This acid rain can cause direct damage to plants, crops, forests, animals and ecosystems and also the built environment where it can weather buildings (made from sandstone and limestone) and lead to the corrosion and weakening of structures.

The deposition of potentially acidifying compounds can lead to the acidification of surface waters and soils, and this can have significant effects on ecosystems and their biodiversity. Soil acidification in agricultural soils is mainly caused by ammonium fertilisers, however deposition can contribute to soil acidification. In soils, acidification affects how micro-organisms break down organic matter and can lead to a reduction in nutrient availability and uptake required for plant growth. Acidified soils may also leach heavy metals which once released can have a direct toxic effect on plants and animals.

Due to effective implementation of legislation to reduce the release of primary SO_x and NO_x into the atmosphere, environmental problems such as acid rain are now far less significant than previously seen; however, many ecosystems are still seriously degraded due to the historic impacts of acidification, and it may be many years still before they fully recover. Releases of NO_x and NH₃ still continue, although at lower levels than before and therefore acidification is still taking place, but on a much smaller scale.

Nitrogen deposition

After nitrogen gases such as NO, NO₂ and NH₃ are released, they can undergo chemical reactions in the atmosphere, be transported, and deposited to the environment. Nitrogen is a nutrient, and when gases, water droplets, or particulates containing it are deposited, it can contribute to imbalances in the natural nitrogen cycle and lead to the eutrophication of ecosystems. Eutrophication is a natural process where an ecosystem becomes increasingly enriched by nutrients. Air pollution can exacerbate and accelerate this process resulting in ecosystem imbalances such as altering the composition of plants so that nitrogen-loving plants outgrow others and reduce overall biodiversity.

The main difference between these two processes is the effects they have on the environment. In contrast to acid deposition, deposition of nitrogen as moisture, gases and particulate matter can disrupt the natural nitrogen cycle that exists within ecosystems. Eutrophication is a natural process where an ecosystem becomes increasingly enriched by nutrients. Air pollution can exacerbate and accelerate this process resulting in ecosystem imbalances such as altering the composition of plants where nitrogen-loving plants outgrow others and reducing overall biodiversity.

In aquatic ecosystems eutrophication accelerates the growth of plant material, leading to the formation of algal blooms which block light, reduce oxygen levels, reduce biodiversity and in some cases can have a direct toxic effect. Phosphorus is the main nutrient which drives aquatic eutrophication, but nitrogen deposition is still an important contributor.

Examples of algal blooms and scums – SEPA

Ammonia concentration

Ammonia (NH₃) is a highly reactive and soluble alkaline gas resulting from the breakdown and volatilisation of urea. NH₃ is one of the main sources of the formation of secondary PM, which can have both environmental and health impacts. The main sources of ammonia emissions are from agriculture, in particular livestock manure, urine, and the spreading of slurries, sludges and digestates and fertiliser application. As a result of these activities, local concentrations of NH₃ may be high where animal waste materials are produced in large quantities in intensive farming installations such production of cattle (dairy and beef), poultry and pigs.

NH₃ emissions can lead to the previously described effects resulting from nitrogen deposition and acidification, and it is one of the main contributors to nitrogen pollution from the atmosphere. However, it can also have direct effects on the environment and exert a direct toxic effect on some plants and habitats (certain types of habitats and species of plant are very susceptible to exposure to NH₃, even at relatively low concentrations). This can result in reduced biodiversity and damage to the environment, especially where it happens in conjunction with the other forms of atmospheric deposition.

Ammonia sources and impacts – UK Government, Department for Food and Rural Affairs (Defra)

Emissions of NH₃ from agricultural activities such as spreading materials to land are a major contributor to air pollution. When these materials (such as manures and slurries) are exposed to air during collection, storage and spreading, the NH₃ enters the atmosphere (especially when conditions are warm and windy). Depending on how the NH₃ then moves through the atmosphere it can exert the environmental effects previously discussed (at local, regional and transboundary scales).

Another significant environmental problem from agricultural spreading is the run-off of material into the water environment. Agricultural materials are extremely polluting due to their high organic content. Where this material enters the water environment it can contribute to eutrophication and has a direct effect by reducing light and increasing the suspended solids content. The organic matter is also broken down by micro-organisms resulting in the water becoming severely depleted of oxygen. This can lead to the death of plant, fish and invertebrate life in the watercourse.

To reduce emissions to air and water from the spreading of agricultural materials care must be taken in how and when materials are spread. New controls requiring precision/low emission spreading systems should ensure that agricultural materials are not exposed to air to the same extent, resulting in lower NH₃ emissions and an appropriate and even application across the land. Weather conditions are important for the release of ammonia from applied manure into the atmosphere. Materials should not be applied when land is waterlogged, or when heavy rain is forecast, and material application should also be even and not beyond the receiving capacity of the soils.

Precision spreading of slurry – SEPA

Ground-level ozone

Ground-level ozone (also called tropospheric ozone) is a highly reactive secondary pollutant created by chemical reactions between pre-cursor (primary) pollutants such as NO_x, non-methane volatile organic compounds (NMVOCs) and methane (CH₄). It is formed when these pre-cursor pollutants, emitted from sources such as transport and energy generation react in the atmosphere through complex chemical reactions in the presence of sunlight. Due to the time taken for these reactions to happen in the atmosphere and meteorology at the time, ground-level zone is often formed large distances downwind from the source of release of pre-cursor pollutants, to rural areas.

Ground-level ozone pollution tends to occur seasonally when weather conditions are most suitable and it can reach peak concentrations when the weather is hot, sunny, with little wind/mixing and can affect both urban and rural environments. Annual mean concentrations of ground-level ozone are generally lower in urban areas.

In the natural environment ground-level ozone can significantly affect vegetation and ecosystems. It affects vegetation by reducing levels of photosynthesis, slowing growth and development, increasing risk to disease, pests, severe weather and other stressors and causing physical damage to the surface of leaves. As plants and vegetation are impacted there are then consequential impacts on ecosystems such as reduced biodiversity, reduced crop yield, changes to flora and fauna, reduced quality of habitats and contributions to increase the impact of other ecosystem problems such as acidification and eutrophication. Ground-level ozone also contributes to the formation of photochemical smogs and can have significant health impacts.

Particulate matter (PM)

Particulate matter (PM) in the atmosphere arises from a wide variety of sources, both natural and manmade. Both the size and chemical composition of particles can vary widely in relation to the nature of the source and how the particle has moved through the atmosphere.

Because of the wide variety of sources, types and chemical compositions involved in PM emissions, it can exert a wide range of environmental and health effects. These include where PM (as dust) can cover plant surfaces and form a physical barrier to photosynthesis, be deposited on a local or transboundary scale (which has the effects previously described), and cause the scattering of light and the formation of hazes and smog.

PM as black carbon and soot can also contribute to climate change. Black carbon is a short-lived climate pollutant which when in the atmosphere absorbs heat and light leading to atmospheric warming and also can affect meteorology (such as the formation of clouds and weather patterns). Due to the nature of the material, it can circulate globally and as a result when it is deposited on snow and ice it reduces the reflective ability leading to localised heating and melting of the surrounding areas and again resulting in an overall warming effect.    

Exposure to PM and other air pollutants has been linked to a variety of conditions which may impact on our health. Further information on the impacts of air pollution on health is available from Public Health Scotland (PHS).