Brief Guide to Atmospheric Pollutants

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Back in school, you’ll have learned that the air in our atmosphere is composed primarily of nitrogen, at 78%, and oxygen, at 21%, with a number of other trace gases. It’s to these trace gases we’re looking today – more specifically, at how human activity can result in the release of air pollution in to the atmosphere. Here, we examine a number of different chemical compounds that contribute to atmospheric pollution, their specific sources, and their effects.

It’ll come as no surprise to learn that one of the primary sources of atmospheric pollutants is our continued reliance on the burning of fossil fuels for a large proportion of our electrical energy. Carbon dioxide, also produced by natural processes, is the obvious gas produced in this case, produced as a combustion product. However, other pollutants are also produced. Sulfur dioxide is formed as a result of the sulfur impurities in coal and oil, whilst particulate matter and heavy metals can also be released.

Another obvious source of pollutant release is that of vehicle emissions. Pollutants from road transport again include carbon dioxide, but also include carbon monoxide, as well as nitrogen oxides, formed by direct combination of nitrogen and oxygen in combustion engines. The purpose of catalytic converters in cars is to try and remove nitrogen oxides, and carbon monoxide, converting the majority of them into less harmful gases. Cars have also previously been a large contributor to heavy metal pollution, as a consequence of the use of leaded petrol, though this is no longer used in many countries.

The agricultural industry is another that contributes pollutants to the atmosphere. Some of this is a consequence of the use of manure and fertilisers, which can release ammonia, whilst some chemicals used as pesticides can also wind up in the atmosphere – these are known as persistent organic pollutants (POPs). POPs can also be generated in industrial processes; for example, waste incineration can lead to the production of dioxins.

The effects of these different pollutants are varied. Carbon dioxide’s effects have already been extensively documented elsewhere, of course – there’s an excellent NASA site detailing the evidence linking it to anthropogenic global warming here – so we’ll instead focus on the other pollutants detailed.

Carbon monoxide’s effects are also well known – it’s a gas that, in sufficient quantity, can cause toxic effects and death in humans. If we breathe it in, it binds strongly to the haemoglobin in our red blood cells, diminishing the oxygen-carrying capacity of our blood. From the perspective of atmospheric pollution, it’s one of the handful of gases that can react with other atmospheric chemicals to help form ground-level ozone.

Ozone might not immediately spring to mind as a pollutant. After all, it’s present in the higher levels of our atmosphere, and this ozone layer helps shield us from harmful UV radiation. However, ground-level ozone is an entirely different prospect. It is a major component of the smog that occasionally plagues areas of the globe, and can also cause health effects such as irritation, coughing, and chest pains.

Ground-level ozone isn’t directly generated by human activities. However, it can be produced as a result of the reactions of different human pollutants in the atmosphere. Primarily, the reactions of nitrogen oxides with volatile organic compounds, in the presence of sunlight, can produce ozone. These volatile organic compounds (VOCs) can have a range of human sources, but are also produced naturally by vegetation, and other natural processes. VOCs can additionally undergo other reactions with nitrogen oxides to form peroxyacyl nitrates, respiratory and eye irritants present in smog.

Particulate matter in the atmosphere can also be a factor in smog. This matter can be composed of a huge number of chemical entities, and is generally split into three categories: coarse particles, with a diameter between 10 and 2.5 micrometres; fine particles, smaller than 2.5 micrometres; and ultra-fine particles, smaller than 0.1 micrometres. As well as contributing to smog, some of these particles have been linked with human health effects, as the smallest can be breathed deep into the lungs.

Some particulate matter is directly emitted, for example as a result of fossil fuel combustion. Others are generated in the atmosphere from reactions between different atmospheric species. One other atmospheric pollutant that can contribute to the formation of particulate matter is ammonia. Released from manure and fertilisers in agricultural settings, ammonia can react with other pollutants, producing these tiny particles. Ammonia can also have other effects, such as eutrophication. This is when soil or water becomes over-enriched with nitrogen, causing over-promotion of growth, a particular issue in aquatic environments.

A final environmental effect that pollutants can have is the production of acid rain. This is primarily a consequence of sulfur dioxide emissions, though nitrogen oxides can also contribute. They can react with water in the atmosphere, in the case of sulfur dioxide producing sulfurous acid as an intermediate, which can then react further with oxygen to form sulfuric acid. This can cause acidification of aquatic environments, as well as corrosion of some building materials.

It’s clear then, that there are a wide range of atmospheric pollutants – so what are we doing to combat them? A number of environmental agencies worldwide have identified six ‘criteria pollutants’, which are regulated, and measures of which can be used to gauge air quality. These are carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide. There are broad regulations and limits in place to try and reduce the release of these pollutants; however, some countries have failed to meet these emission limits. Just last week, the UK was criticised for failing to meet nitrogen dioxide emissions below required levels.

With that said, progress is certainly being made in some cases. Emissions of lead have been significantly reduced by the removal of tetraethyl lead from petrol, and sulfur dioxide emissions have also been reduced by stricter controls on the sulfur content of fuels. Innovations such as the introduction of catalytic converters into cars have also reduced emissions of nitrogen oxides – however, this has been offset to an extent by increasing numbers of vehicles on the roads.

There are still plenty of challenges to reduce levels of atmospheric pollutants, but hopefully, with greater awareness, we can continue to work on reducing their levels and preventing the associated effects on the environment and our health.

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References & Further Reading