Atmospheric particulates, also called particulate matter (PM), are a complex mixture of extremely small solid particles and liquid droplets suspended in the air. Particles are made up of several components, including acids (i.e., nitrates and sulfates), organic chemicals, metals, and soil or dust. PM sources can be anthropogenic or natural. Natural sources include volcanoes, dust storms, vegetation fires, and sea spray. Anthropogenic sources include coil and oil combustion, fossil-fuel power plants, and other industrial activity. The anthropogenic contribution to total atmospheric PM is estimated to be approximately 10%. Some of these particles are emitted directly into the air (primary emissions), while some are emitted as gases and subsequently form particles in the air (secondary emissions).

PM is categorized by size fractions. As particles are often nonspherical, ...