Overview

Atmospheric Particulate Matter (PM) is a term used to describe small particles that are suspended in the air in either solid or liquid form. Some examples of PM, including dust, dirt, smoke, and soot, are large enough that they are easily visible due to their size and color. Other particles are smaller and can only be seen when examined under an electron microscope. These particles vary not only in size, but also in chemical composition and origin. Additionally, such particles can either form organic or inorganic suspensions. Collectively, the suspensions that this PM forms are known as aerosols.

Sources of PM

Organic particulate matter is typically generated from natural sources such as sea spray, volcanoes, dust storms and fires. Inorganic PM produced by human activities is the result of activities such as coal combustion and burning fossil fuels for fueling motor vehicles and factories. Currently, organic particles are the most common type of PM, with dust and sea salt alone accounting for 90% of all aerosols in the atmosphere. Although these sources make up most of the PM, biomass burning contributes substantially to other aerosols that organic and black carbon PM. The remainder of aerosols are composed of inorganic PM resulting from anthropogenic (human) sources. The major source of inorganic PM stems from fossil fuels. Fossil fuel burning releases large quantities of sulfur dioxide into the atmosphere, which react with atmospheric gases including water vapor which produce sulfate aerosols. Other common sources of sulfate aerosols are power plants, motor vehicles, and incinerators.

Sizes of PM

Particulate matter can be categorized into three types: Coarse, fine, and ultrafine. The coarse variety includes all particles that have diameters greater than 2 microns. Fine particles lie within the diametric range of 0.1 to 2 microns, and ultrafine particles are the smallest type, with diameters less than 0.1 microns. The organic PM which makes up most of atmospheric aerosols are usually larger than anthropogenic PM, but many aerosols can have a mixture of both inorganic and organic PM. Moreover, the Southern Hemisphere contains more PM with larger diameters, while the atmosphere over the Northern Hemisphere has greater concentrations of small PM.

This is likely due to the fact that the Northern Hemisphere has more human activity and land area, while the Southern Hemisphere is less populated and is covered more by the ocean. The size of PM is also broken down into two other categories: inhalable, and fine inhalable particles. Inhalable particles (PM10) are any PM with diameters typically less than 10 micrometers, and fine inhalable particles (PM2.5) must have diameters 1/30 the size of that of a human hair.

PM in the United States

Although air pollution is a substantial problem in the United States today, the US has made efforts to improve air quality since the 1970s. Particle pollution and ground-level ozone pollution have decreased over the past few decades, but air pollution is still a pressing environmental and health issue that needs to be addressed further. In 1970, one of the first major American environmental laws was passed, intending to cut air pollution and improve health standards.

The Clean Air Act, as it was named, aimed at supporting regional programs air pollution control programs and provided funding for governmental air pollution research. Since then, the Environmental Protection Agency (EPA) has set standards for PM10 and PM2.5 concentrations, as part of the National Ambient Air Quality Standards. The standards for PM10 air quality have been met every year since 1990, whereas the national standard for PM2.5 air quality has only reached below levels deemed acceptable beginning in 2011. These trends are shown below in graphs provided by the Environmental Protection Agency (EPA) in 2016.

Methods to Reduce Particulate Matter Air Pollution

Now that you have some general knowledge about what particulate matter is and a brief discussion of its status in the US, here are some ways that everybody can lessen PM in the atmosphere, aiding in pollution reduction, as suggested by James Madison University.

1. Stop Smoking
2. Limit the burning of garden waste
3. Lessen the use of wood stoves and fireplaces
4. Combat wildfires and forest fires
5. Reduce diesel emissions

Los Angeles air pollution as seen from the Hollywood Hills (Credit: Wikimedia Commons)

Sources:

Center for Disease Control

Environmental Protection Agency

James Madison University

NASA