Fine particulate matter is airborne microscopic dust and droplets smaller than 2.5 microns (PM2.5) in diameter. The composition of particulate matter depends on its source, the season and atmospheric conditions. Fine particulate matter is comprised primarily of sulfates, nitrates, carbon, organic substances, ground minerals and metals. Particulate matter can travel long distances and is a year-round pollutant common to urban and rural communities. It is a principal component of smog.