What are aerosols?
Tiny solid or liquid particles can be found suspended in the atmosphere. These are called aerosols. Their size can vary from only a few nanometres to up to 100 microns, about the thickness of a human hair.
On average, several million tons of aerosols are created every day in the world, from multiple natural (volcanic ash, desert dust, sea-spray, etc.) and human (industrial smoke, exhaust, dust from farm fires, etc.). This diversity entails very different properties for each type of aerosols. In the stratosphere, aerosol presence is rare but very resilient; particles can remain at these altitudes for several years. In the low troposphere, aerosol presence is more abundant and much shorter (only a few days). As a result, unlike greenhouse gases, aerosol concentration can vary wildly on a daily or regional basis.
Different types of aerosols
Aerosols can be chemically active; their properties can change while they travel through our atmosphere. They can play a role in the creation and/or destruction of gas molecules (such as ozone) by catalysing chemical reaction on their surface.
What impact do aerosols have on our climate?
By diffusing and absorbing light and by changing clouds’ reflectivity, aerosols have direct, semi-direct and indirect effects on the Earth’s climate.
The direct effect is the parasol effect; it consists in the aerosol particles diffusing and sometimes absorbing solar radiation. Diffusion is most effective on solar wavelengths, particularly for pollution-borne aerosols; it is weakest for thermal infrared. Diffusion usually has a cooling effect, except when it happens in absorbing aerosols located above a highly reflective surface.
Aerosols’ direct effect on climate was first discovered by Benjamin Franklin in 1783. Following a massive volcanic eruption in Iceland, he linked it to a “dry fog” which appeared in France and which he postulated was the cause of the frigid winter in Europe that same year. He concluded that due to the volcanic eruption, a smaller amount of solar energy reached the ground. A similar phenomenon occurred after the Mount Pinatubo eruption in the Philippines in 1991; observations showed a 0.5°C drop in the global average temperature.
Aerosols can also absorb solar radiation to varying degrees; this process has an impact on temperature profiles and through them on cloud formation. This semi-direct effect can make clouds dissipate or change their geographical extension.
Finally, the indirect radiative effect is the result of aerosols interacting with clouds; clouds themselves have a major influence on Earth’s energy balance. Aerosols can act as condensation cores around which clouds will form; in these cases, with equivalent water contents, a cloud which formed in polluted air will hold more droplets than a less polluted cloud. The droplets will be smaller in size, but the cloud in question will be more reflective than one which formed in an aerosol-free air mass. This first indirect effect is a cooling effect. The second indirect effect acts as follows: since the droplets in polluted clouds are smaller, they never reach the critical size at which precipitation occurs; thus, these clouds’ average lifetime is extended. This leads to an increased cloud cover on a global scale. Combined with the evaporation of clouds due to aerosol-induced atmospheric heating at relevant altitudes, this process constitutes the second indirect effect aerosols can have on climate. Depending on the clouds’ altitude (among other things), this process can have either a heating or a cooling effect on our planet’s climate.