Particulates

Particulates are classified by aerodynamic diameter, which determines how deeply they penetrate the respiratory system. PM10 includes particles up to 10 micrometers, while PM2.5 (fine particles) are less than 2.5 micrometers and can reach the alveoli. Ultrafine particles (PM0.1) are even smaller and can enter the bloodstream. The composition varies from carbonaceous material to sulfates, nitrates, metals, and organic compounds, depending on source.

Primary particulates are emitted directly from sources such as diesel engines, coal combustion, construction dust, and sea spray. Secondary particulates form in the atmosphere through chemical reactions, notably the oxidation of sulfur dioxide and nitrogen oxides to sulfates and nitrates. Natural sources include wildfires, volcanic eruptions, and pollen. Anthropogenic activities dominate in urban areas, with traffic and industry being the largest contributors.

Exposure to particulates is linked to cardiovascular and respiratory diseases, lung cancer, and premature death. The World Health Organization estimates that air pollution, including particulates, causes about 4.2 million premature deaths annually. Particulates also affect climate by scattering or absorbing sunlight, and they deposit on ecosystems, causing acidification and nutrient imbalances. Children, the elderly, and those with pre-existing conditions are most vulnerable.

Beyond the well-known health effects, particulates have surprising roles. For instance, they can act as cloud condensation nuclei, influencing precipitation patterns. In the 19th century, London's 'pea-soup' fogs were largely particulate pollution, leading to the Clean Air Act of 1956. Recent research has found microplastics in atmospheric particulates, raising new concerns. Additionally, certain desert dust particles can carry pathogens across continents, affecting marine ecosystems and coral reefs.

PM2.5

Fine particulate matter with a diameter of 2.5 micrometers or less.

Aerodynamic diameter

The diameter of a sphere with the same settling velocity as the particle.

Secondary particulates

Particles formed in the atmosphere from gaseous precursors.