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Particulate matter describes solids or liquids that are very small and usually found in the air, in which case they also can be referred to as particulate pollution. The size of a particle can range from only two or three molecules to pieces that are clearly visible to the human eye, although still small. There are two types of particulate matter, primary and secondary. Primary particles are created directly from a source and are somehow launched into the air, where they can remain suspended for anywhere from hours to weeks, depending on the size. Secondary particles actually form in the air when different molecules and conditions create chemical reactions.
Most particulate matter is measured in micrometers. The smallest particles, called fine particles, are 2.5 micrometers or smaller. Coarse particles are larger, measuring from 2.5 micrometers to 10 micrometers. Visually, fine particles, if they can be seen at all, will appear like vapor or smoke. The larger coarse particles might be individually visible and can form grainy clouds of the pollutants.
The atmosphere is not the only place where particulate matter can exist. It also can be in liquids such as ocean water and chemical solutions. Particles in a liquid have the possibility of becoming airborne through evaporation or types of agitation that can actually physically throw the particles into the air. One example of this is salt in the ocean that the wind and evaporation can cause to become suspended in the air. Evidence of this can be seen in metal structures near salt water bodies that can suffer from the corrosive effects of air particles carrying the salt onto the land.
There are many sources of particulate matter, some manmade and others natural. One of the largest sources for both fine and coarse particulate pollution is the dust kicked into the air from roads. Other sources include the burning of fossil fuels such as coal, forest fires, volcanic eruptions and emissions from the engines of cars that use gasoline as fuel. The finer the particles, the longer they will stay in the atmosphere, allowing some to travel pretty far before being brought to the ground by rain or gravity.
One of the dangers of particulate matter, especially the fine type, is that it can be inhaled. If the particles, which can be composed of any combination of hundreds of chemicals and elements, are able to get through the defenses of the nose and mouth, then they can find their way into the lungs and possibly even pass into the blood stream. This can cause respiratory problems and potentially contribute to a higher risk for certain types of cancer or heart disease.
Another problem that particulate matter causes is a modification in how the atmosphere absorbs solar radiation. A high enough concentration of particles in the atmosphere can change how heat passes through the atmosphere to the surface and how heat reflected back from the surface travels returns to space. Some of the particles are able to scatter the radiation, causing less heat to reach the ground while also preventing the radiation that does reach the Earth from escaping back into space as it normally would.