Primary pollutants enter the atmosphere from identifiable sources, such as pollen from plants, dust from soil, or carbon monoxide from vehicle emissions. Secondary pollutants form from chemically altered primary pollutants, as in the case of ozone. There are natural sources of both primary and secondary pollutants. Particulates are solid particles or liquids in the atmosphere. Particulates can settle on surfaces.
Particles can provide surfaces on which water condenses.
This can increase fog, clouds, and rain depending upon weather conditions. The sulfur oxides then readily dissolve into existing droplets. Particulates can substantially reduce visibility, which can increase accidents. Particulates will reflect incoming solar radiation. Ozone is a secondary pollutant formed by a chemical reaction between gases in the presence of high temperatures and sunlight.