Most particulate matter monitoring current measures the total mass of PM below a particular size. As atmospheric particles exhibit a vast range of morphology, the concept of aerodynamic diameter is commonly used to assess particle size a particle with an aerodynamic diameter of 1 um will exhibit the same inertial properties as a sphere with a diameter of 1 m and a density of l g cm irrespective of the particle's actual size, shape, or density. Common abbreviations for particle size classes are PM for particles with an aerodynamic diameter below 10 um and the smaller PM2 s (or PMfine), for particles with a diameter below 2.5 um. PM2 s is therefore a subset of PM10, whereas the fraction between 2.5 and 10 Hm, given by PM10-PM2 , is often referred to as PM Ultra fine particles are frequently (although not coarse universally) defined as those with an aerodynamic diameter below 0.1 m. PM10 and PM2 s correspond approximately to inhalable (thoracic) and respirable (alveolar) particle sizes. Here we focus on measurement of the total particulate mass, which is currently the principal regulatory metric. Techniques for assessing particle composition are briefly introduced.