Why would particles in the atmosphere cause global dimming?
It’s very simple – they reflect sunlight back into space. This fact has been well-known for many decades (it is why volcanoes, which can throw vast numbers of sulphate particles into the upper atmosphere, can have a strong temporary cooling effect on the Earth). What came as a surprise was the so-called indirect effect, whereby particles in the atmosphere change the optical properties of clouds. This happens because the presence of man-made particles in the atmosphere increases the number of sites where water droplets can form. The effect is that up to six times as many water droplets form in a polluted air mass as would naturally, but since the amount of water vapour in the atmosphere does not change, the droplets have to be smaller than they would naturally be. Since the reflectivity of the clouds depends on the surface area of the droplets, these polluted clouds are more reflective than unpolluted ones (since many small droplets have a bigger surface area than fewer big ones). By re
