U.S. Department of Agriculture: Agricultural Research Service, Lincoln, Nebraska


Date of this Version



Published in Micrometeorology in Agricultural Systems (2005) Agronomy Monograph no. 47: 227-245


Particulate matter is the term given to the tiny particles of solid or semi-solid material found in the atmosphere. Particulates in the atmosphere range in size across many orders of magnitude. The expression “particulate size” is based on particle behavior in the earth’s gravitational field. The aerodynamic equivalent diameter refers to a spherical particle of unit density (1 g cm–3) that falls at standard velocity. Size, because it determines atmospheric lifetime and lung deposition, is a very important characteristic of particulates. Particulates ranging in size from <0.1 to 50 μ are called Total Suspended Particulates (TSP). Particulates larger than 50 μ tend to settle out of the air whereas particulate matter 10 μ in diameter and smaller are considered inhalable. This particulate matter is commonly referred to as PM10.