The EPA web site maintains excellent access to this history of proposals and changes. Particulates are mixtures of solid and liquid droplets of material that vary in size and origin. Particulates of this size are referred to as PM-10 particles. Measured and reported by air pollution monitors or samplers , PM-10 particles in urban environments are often composed of soot, acid condensates, and sulfate and nitrate particles. High temperature combustion of fossil fuels in motor vehicles is the primary source of nitrogen oxides in outdoor air.
Nitrogen oxides are relatively insoluble in water.
Ozone, may form in the lower atmosphere as a result of nitrogen dioxide pollution. Sulfur Dioxide Sulfur dioxide is a very irritating, colorless, soluble gas with a strong, pungent odor and taste. When sulfur dioxide comes into contact with water, it forms sulfuric acid, which irritates the eyes, mucus membranes, and skin. Sulfur dioxide is detectable by the nose at 0.5 ppm. The primary source of sulfur dioxide in outdoor air is the combustion of sulfur-containing fuels such as those used in some power plants.