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 main component of the Earth’s atmosphere is nitrogen (N2, 78%), followed by oxygen (O2, 21%), argon (Ar, 0.96%) and other trace gases, such as water vapour (H2O), carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4). Other gases such as chloroflurocarbons (CFCs) make up much less than 1%.
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. However, around half of the gases and aerosols are located between 0 and 5.6 km above the Earth’s surface, which is in the lower part of the troposphere. Importantly, most of the Earth’s water vapour is located here too, meaning that nearly all the Earth’s weather happens relatively close to the surface.
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 or human made. Natural sources of aerosols include ash from volcanoes, microscopic spores, sea salt and dust, while man made sources include sulphate and carbon-based aerosol emitted from combustion processes such as domestic cooking and heating, and power plants which burn fossil fuels. There are some sources, such as forest fires, that may be natural or human made.
Aerosol particles emitted directly into the atmosphere are called primary aerosols. Examples include ash from volcanoes, sea salt and mineral dust. Secondary aerosols are not directly emitted, but are formed in the atmosphere by chemical reactions and physical processes. For example, sulphur dioxide gas produced in industrial plants and from transport (cars, trucks, ships) reacts with other gases such as ozone to form sulphate aerosol. Other secondary aerosols include nitrates and organic carbon.
Aerosols can range in size from a few nanometers to tens of micrometers in size*. They usually exist in the atmosphere for a few days to a week meaning that they are generally found close to their sources. This is in stark contrast to greenhouse gases (GHGs) such as CO2 which can have lifetimes of hundreds of years.
*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 (link to AQ section). They can alter incoming solar and outgoing thermal radiation and also influence clouds and rainfall. Most aerosol particles, such as sulphates, reflect incoming solar radiation back out to space and so act to cool the climate. However, aerosols such as black carbon and mineral dust can also absorb sunlight. This can reduce cloud formation by increasing the atmospheric 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. Aerosols can therefore lead to more clouds, but the size of the droplets is smaller and so when aerosol emissions are high the albedo (reflectivity) of the cloud increases. Smaller droplets also take longer to reach the critical sizes for rain formation and are believed to result in longer-lived clouds. Both the cloud albedo and cloud lifetime effects of aerosols lead to a cooling effect by reducing the solar radiation reaching the Earth’s surface and are called the aerosol indirect effects. Over the past 60 years this cooling effect has partly counteracted the warming influence from GHGs. However, demands for improved air quality have led to the reduction of sulphur dioxide emissions from European and North American power stations over the past 15-20 years and there has been a reduction in the cooling influence from aerosols.
Gases: A little can do a lot
The most common gases in the Earth’s atmosphere are nitrogen and oxygen. Oxygen is obviously very important for animal life on Earth, but most of that nitrogen doesn’t do much at all! Amazingly, it is trace gases such as carbon dioxide (CO2), sulphur dioxide (SO2) and ozone (O3), which are present in tiny amounts*, that have the biggest impacts on the climate and air quality.
CO2 is one of the gases we hear most about because it is the most abundant greenhouse gas (GHG) (link to the radiation section). 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 GHG with large human sources. These two gases are not so abundant as CO2, but 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 their short lifetimes, they may have an indirect impact on climate if the compounds they form after a reaction do impact the climate (link to the Earth system section). 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, for example, volcanoes can emit a lot of SO2, wetlands emit CH4, and plants (especially leafy trees) are the largest sources of a volatile organic compound called isoprene. Other gases such as O3 form from chemical reactions in the atmosphere (link to ozone section). 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 in to 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 in to 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 figure below. Air quality is directly affected by the gases and aerosols in the atmosphere. Although many compounds 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, NOx, SO2 and O3 irritate the airways of the lungs, increasing the symptoms of those suffering from lung diseases. Particulate Matter (PM) can be carried deep into the lungs where it can cause inflammation and a worsening of heart and lung diseases. O3 also damages plants and reduces yields in important food crops. In addition, pollutants from these sources can be transported long distances causing poor air quality not only at their source, but many hundreds of kilometres away.
In the past, the main air pollution problem in both developed and rapidly industrialising countries 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, industrial and domestic pollutant sources, together with their impact on air quality, tend to be steady or improving over time. This is due to the impacts of legislation and governmental policies to control air pollution. However, traffic pollution problems (mainly CO, NOx, VOCs and PM) are worsening world-wide. You can find out more about air pollution in the UK here (link to the figure ‘sources of air pollution’) and at https://uk-air.defra.gov.uk/. There is more information about air pollution in Europe 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 for absorbing (and therefore trapping) incoming shortwave, ultraviolet (UV) radiation from the sun. UV radiation damages the DNA of living organisms at Earth’s surface and increases the risk of health problems such as skin cancer. The use of chlorofluorocarbons (CFCs) in aerosol cans and as a refrigerant led to the gradual destruction of ozone in the stratosphere, resulting in the discovery of the Antarctic ‘ozone hole’ in the late 1970s. 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 here: 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 (link to the AQ section). Ozone in the troposphere is formed through photochemical reactions resulting from the action of sunlight on nitrogen dioxide (NO2) which is typically emitted from road vehicles and VOCs, including methane. Ozone has a longer lifetime than the NO2 and VOCs it is formed from. It can therefore be transported long distances and affecting rural areas far from the original emission sites. 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
Gases and aerosols can interact with other parts of the Earth system
Gases and aerosols can directly impact the Earth’s climate through altering the amount of radiation in the atmosphere (link to aerosol section, link to GHG section, link to radiation section). Importantly, gases and aerosols can interact with other parts of the Earth system (e.g. the oceans or the land) to change their impact on the climate. An example of this is the deposition of black carbon aerosol from combustion on to snow. Normally snow has a very low albedo and reflects most radiation back into space. However, black carbon increases the albedo of the snow causing radiation to be absorbed, thus heating and melting the snow. This means that there is less snow to reflect radiation, which then remains in the atmosphere, warming the climate.
Another example looks at the interactions between the atmosphere and the ocean: We tend to think of gases and aerosols as having land-based sources, but many compounds are actually emitted from the ocean too. 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 (link to aerosol section). It is possible that as the ocean acidifies, marine creatures will produce less DMS resulting in less cloud formation, which could in turn, affect the climate.