Introduction – what are aerosols?

Aerosols are small particles in the atmosphere of both natural and human-made origin. Aerosols can affect the climate by directly interacting with solar radiation (energy in the form of sunlight) (Box 1) and by playing a role in cloud formation and precipitation. How a given aerosol will influence the climate depends on the material it is composed of, its colour, size, and where it is in the atmosphere (i.e., vertical location and whether it is over land or ocean)¹.  

Natural aerosols include dust, wildfire smoke, volcanic emissions (ash and sulphates) and sea spray². Human-made aerosols result from industrial activity and the burning of fossil fuels and wood. Two types of aerosols have a notable influence on climate: black carbon and sulphate aerosols. Black carbon is produced when coal, gas and wood are burned. Aerosols containing black carbon tend to be dark.  Sulphate aerosols are emitted during volcanic eruptions (Box 2) and the burning of fossil fuels and industrial processes such as smelting. Aerosols comprised of sulphates tend to be light in colour. Darker-coloured aerosols absorb radiation, resulting in atmospheric warming; lighter-coloured aerosols reflect radiation, resulting in atmospheric cooling. Darker-coloured aerosols can also cause additional warming when they settle on snow and ice because they make the surface darker, causing more radiation to be absorbed on these otherwise reflective surfaces. 

The length of time that aerosols stay in the atmosphere is known as the residence time. Unlike greenhouse gases, which typically reside in the atmosphere for decades³, the residence time of aerosols is generally between 2 to 30 days, with shorter residence times in rainy conditions and longer residence times in dry conditions. Volcanic aerosols are the exception; they are typically emitted high in the atmosphere and, consequently, have much longer residence times (Box 2). Since most aerosols have short residence times, concentrations of these particles are highest around the sources of emissions. As such, the climate impacts of aerosols are typically confined to local or regional scales. 

The reflection and absorption of radiation by aerosols is affected by the colour of the surface of the aerosol, the aerosol’s chemical composition, and the presence and type of clouds⁵. The presence of aerosols, and the corresponding warming and cooling of the atmosphere, can change where clouds form or whether they form at all. Aerosols also alter cloud properties and affect precipitation.  

Aerosols act as “seeds” for cloud formation. It is difficult for water to condense into droplets on its own, but when a seed particle is present, moisture can condense onto the particle allowing clouds to form. When the concentration of aerosols increases, there are more small cloud droplets for the same amount of water. Smaller cloud droplets make clouds brighter and more reflective, causing enhanced atmospheric cooling^{5, 6}.  

Aerosols affect precipitation by influencing the absorption and reflection of radiation, and through cloud formation. The influence of aerosols on precipitation is an active area of research, with the general scientific consensus being that aerosols tend to decrease global precipitation⁷. Overall, aerosols in the atmosphere decrease the amount of radiation reaching the Earth’s surface, which lowers the energy available for evaporation of surface moisture and water, thus, reducing precipitation⁷. As noted, when there are more aerosols, cloud droplets tend to be smaller^5,7,9. Smaller droplets require a longer time to reach a size where they fall as precipitation. In addition, smaller droplets evaporate more easily, resulting in smaller clouds with shorter lifetimes than larger clouds. For these reasons, the amount of precipitation from clouds is reduced when there are more aerosols⁵.  

While more aerosols generally lead to a reduction in precipitation, it is important to note that aerosols vary in their radiative properties and in their ability to serve as cloud seeds⁷. As such, there are situations where aerosols can increase precipitation, especially at the regional scale^7,10.

Air-polluting aerosols harm human and environmental health

Overall, aerosols have a cooling effect on the climate since they predominantly reflect sunlight and increase cloud reflectivity. Since the industrial era, high concentrations of aerosols from human-caused emissions of air pollutants have masked some of the warming from greenhouse gases. Efforts to reduce emissions of air pollutants to protect human and ecosystem health have reduced the concentrations of aerosols in the atmosphere¹¹. As a result, some amount of recent global warming can be attributed to the reduction of aerosols^11,12. Since human emissions of aerosols cause smog and acid rain, which have detrimental effects on environmental and human health, it is important to continue to reduce aerosol emissions alongside efforts to reduce greenhouse gas emissions.