Precipitation

As discussed above, gas-phase pollutants such as sulfur dioxide can be converted into water-soluble particles in the atmosphere. Many of these particles can then act as nuclei of cloud droplet formation. Increasing the number of CCN in the atmosphere is expected to change the characteristics of clouds. For example, ships' emissions have been observed to cause an increase in the number of droplets in the marine stratus clouds above them. If a constant amount of liquid water is present in the cloud, the average droplet size will be smaller. Higher concentrations of smaller droplets reflect more sunlight, so if pollution-derived particles alter clouds over a large enough region, climate can be affected. Precipitation rates may also decrease, since droplets in these clouds are not likely to grow large enough to precipitate.