Particulate matter (PM) is composed of small particles of dust, smoke, and soot that are released or kicked up into the air. PM can also be formed by reactions in the air between chemicals, including some of those that form ozone. Particulate matter smaller than ten microns in diameter, known as "PM-10," is about one-seventh of the diameter of a human hair. Levels of PM pollution vary depending on seasonal activities, rainfall and wind conditions. Here is EPA's description of PM. Here is CARB's description.