Satellite image of particulate pollution over Beijing, China. Click on image for full size (219 Kb (JPEG)) NASA image courtesy the MODIS Rapid Response Team

Particulates are tiny particles of solid or liquid suspended in the atmosphere. They are also called particulate matter, aerosols, or fine particles. Particulates vary in size from just a few molecules to so large that the air can’t hold them. Coarse particulates have a diameter bigger than 2.5 micrometers. They contain rock and dust from vehicles and factories. Fine particles are even smaller. They include combustion particles and organic and metal compounds.

In general, the smaller and lighter a particle is, the longer it will stay in the air. Larger particles (greater than 10 micrometers in diameter) tend to settle to the ground by gravity in a matter of hours whereas the smallest particles (less than 1 micrometer) can stay in the atmosphere for weeks and are mostly removed by precipitation.

There are both natural and human-made sources of particulates. The biggest natural sources are wind-blown dust, volcanoes, and forest fires. The biggest human sources of particles are combustion sources, like the burning of fossil fuel in cars and power plants, and wind-blown dust from construction sites and other land areas where the water or plants have been removed.

Some particulates, such as dust from roads or soot from wood combustion, are emitted directly into the atmosphere. These are called “primary” particles. Other “secondary” particles are formed in the atmosphere from primary particulates. These include sulfates, formed from sulfur dioxide emissions from power plants and industrial plants, and nitrates, formed from nitrogen oxides emissions from power plants, cars, and other types of combustion sources.

Particulate pollution contains solids or liquid droplets that are so small that they can get deep into a person’s lungs and make them very sick. Small particles cause more problems than big ones, because they can get deep into your lungs, and some may even get into your blood.

Particulate pollution can also affect visibility. Fine particles are the major cause of haze in many parts of the world, including many of treasured natural parks and wilderness areas. There are many natural sources of particles that are not air pollution. One example is a cloud. Every cloud is made up of billions of water droplets, which are particles.