1. The mass of suspended particulate aerosols per mass or volume of air. 2. A measure of the amount of mass of particulates per unit time that activities in a region generate in the atmosphere. If a simple “box model” is used to estimate the associated particulate concentration, the result is proportional to the total emission rate and is inversely proportional to the wind speed and the vertical cross-sectional area of the airflow through a plane perpendicular to the wind.