What is the atmosphere made of?

The Earth’s atmosphere is made up of gases and aerosols (which are tiny, microscopic particles). You generally can’t see these, but the air around us is a busy, reactive place!

The atmosphere is made up of five main layers. The troposphere is the layer closest to the Earth’s surface and above that is the stratosphere, mesosphere, thermosphere and lastly, the exosphere which extends to around 10,000 km above the Earth’s surface. The main components of the Earth’s atmosphere are nitrogen (N₂, 78%), oxygen (O₂, 21%) and argon (Ar, 0.96%). Other trace gases, such as water vapour (H₂O), carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄) make up much less than 1%.

Although the trace gases and aerosols make up a small percentage of the atmosphere, they can have big impacts. For example, the increased amounts of CO₂, N₂O and CH₄ in the atmosphere relative to pre-industrial times are driving human-induced climate change and emissions from vehicles, industry and power generation can cause poor air quality. Find out more about the composition of the Earth’s atmosphere and its role in the Earth system below.

Aerosols: Tiny with a BIG climate impact

Aerosols are microscopic particles* that can be either natural (e.g sea salt, dust and ash from volcanoes) or human made such as carbon-based aerosol emitted from combustion processes including domestic cooking and heating, and power plants which burn fossil fuels. Sources such as forest fires can be natural or human made. Aerosol particles can be emitted directly into the atmosphere, for example volcanic ash, or they can be formed in the atmosphere by chemical reactions and physical processes. Sulphate, for instance, is formed by the reaction between sulphur dioxide and gases such as ozone. Aerosols generally exist in the atmosphere for a few days to a week, so their impacts are usually local or regional.

*Aerosols are typically a few nanometres to tens of micrometres in size. A nanometre is one billionth of a metre and micrometre is one millionth of a metre. A sheet of paper is 100 000 nanometres (or 1000 micrometres) thick. There are some more examples to help you imagine how small this is here: https://www.nano.gov/nanotech-101/what/nano-size

Aerosols play an important role in regulating the Earth’s climate and are also important for air quality. Aerosols can directly affect the climate by reflecting incoming solar radiation back out to space and so acting to cool the climate.

However, aerosols such as black carbon and mineral dust absorb sunlight. This can reduce cloud formation by increasing the air temperature and reducing the humidity.
Aerosols also form the tiny particles that water vapour (and ice) can condense on to, forming droplets and then clouds. However, clouds formed in this way can be brighter and longer-lived leading to a cooling effect by reducing the solar radiation reaching the Earth’s surface. In this way aerosols indirectly affect the climate.

Over the past 60 years aerosol cooling effects have partly counteracted the warming from greenhouse gases (GHGs). However, demands for improved air quality have led to a reduction in many aerosols and their cooling influence. Nevertheless, aerosols have a wide-ranging impact on the Earth system. An example is the deposition of black carbon aerosol from combustion on to snow, which normally reflects most radiation back into space. When deposited the black carbon absorbs radiation, thus heating and melting the snow. This means that there is less snow to reflect the radiation, which then remains in the atmosphere, warming the climate.

Trace gases can affect the climate directly and indirectly

Amazingly trace gases, such as, carbon dioxide (CO₂), sulphur dioxide (SO₂) and ozone (O₃), which are present in tiny amounts*, can have huge impacts on climate and air quality. CO₂ is one of the gases we hear most about because it is the most abundant greenhouse gas (GHG). CO₂ is not very reactive and after it is first emitted, it stays in the atmosphere for a long time (300 – 1000 years). This means that it can spread evenly around the Earth and is a global problem. Methane (CH₄) and nitrous oxide (N₂O) are also GHGs with large human sources. Although they are not so abundant as CO₂, their greenhouse effect is much stronger. In contrast, gases such as SO₂ and O₃ may only exist in the atmosphere for a few days because they are very reactive. Despite this they may have an indirect impact on climate if the compounds they form after a reaction do impact the climate. For example, marine creatures produce a lot of a gas called di-methyl sulphide (DMS). Like SO₂, DMS reacts in the atmosphere to form sulphate aerosol, which helps to form clouds. It is possible that as the ocean acidifies, due to absorption of CO₂, marine creatures will produce less DMS resulting in less cloud formation, which could in turn, affect the climate.

The burning of fossil fuels for power generation, industry and transport is a major source of gases such as CO₂ and SO₂, while the main sources of CH₄ and N₂O are from agriculture. Some gases also have large natural sources and other gases such as O₃ form from chemical reactions in the atmosphere. You can see the different sources of trace gases and aerosols to the atmosphere.

*In the air very small concentrations of gases are described in parts per million (ppm) and parts per billion (ppb). For example, if you had one litre of pure CO₂ and released it into a room the size of an Olympic swimming pool, you would have one part per million CO₂ in that room. One part per billion would be the equivalent of releasing one millilitre of CO₂ into the room!

Gases and aerosols can cause poor air quality

The main compounds that cause poor air quality around the world are shown in the diagram below. Although many of these have natural sources, human activities can increase levels such that they have a detrimental impact on human health or the environment or the climate, and sometimes all three! For example, NO_x, SO₂, O₃ and Particulate Matter (PM) can cause or exacerbate lung diseases, while O₃ can also damage plants and reduce yields in important food crops. Pollutants can be transported long distances causing poor air quality many hundreds of kilometres away from their source.

In the past, the main air pollution problem was usually high levels of smoke and sulphur dioxide following the combustion of sulphur-containing fossil fuels such as coal. In all except the worst-case situations, legislation and governmental policies have steadied or reduced these types of air pollution. However, traffic pollution problems (mainly CO, NO_x, VOCs and PM) are worsening world-wide. You can find out more about air pollution in the UK in the figure below ‘Sources of air pollution’ and at https://uk-air.defra.gov.uk/, and around the world here: https://atmosphere.copernicus.eu/

Ozone: The good and the bad

Ozone is a very reactive gas that is formed in the atmosphere by chemical reactions. Depending on where the ozone is in the atmosphere, it can have good or bad impacts on humans, plants and animals. In the stratosphere ozone is extremely important to shield the earth from harmful ultraviolet (UV) radiation from the sun. The use of chlorofluorocarbons (CFCs) in aerosol cans and as a refrigerant led to the destruction of ozone in the stratosphere, resulting in the Antarctic ‘ozone hole’. After the introduction of the Montreal Protocol* in 1987, the usage of CFCs has significantly decreased and the ozone layer is gradually recovering. In 2019 the ozone hole was the smallest on record since its discovery, you can find out more at: https://www.nasa.gov/feature/goddard/2019/2019-ozone-hole-is-the-smallest-on-record-since-its-discovery However, in the troposphere (where we live) ozone can cause a number of health problems in both animals and plants. In addition, ozone is also a greenhouse gas!

*There is more information on the Montreal Protocol here: https://www.unep.org/ozonaction/who-we-are/about-montreal-protocol