Measuring Particulate Emissions from Combustion Sources Since the inception of the Clean Air Act, the petroleum refining industry has been faced with the need to determ ine criteria pollutant emissions from combustion sources. While some of these species, such as NOx, SO2, and CO remain in the vapor phase during and after combustion and are relatively simple to measure, particulate matte r (PM) measurements are much more challenging. This is because while some PM such as fly ash or catalytic cracking catalyst fines is clearly soli d material that is readily collected and measured on a sampling filter, other species that may exist in the vapor phase during combustion can later cond ense into aerosols downstream from the combustion zone. This can occur be fore or after any control devices, depending upon the temper ature and composition of the combustion gases. Consequently, it has been customary to refer to PM as being composed of two PM components, filterable and c ondensable, the relative amounts of each depending on the stack gas comp osition and temperature, control devices in use at the unit, and the method for measuring PM. While measuring filterable PM is relatively straightforward (i.e. PM collected on a filter), condensable PM is a more es oteric quantity and its contribution to total PM emissions is very much dependent upon the choice of the measurement method. The U.S. Envir onmental Protection Agency apparently recognized this issue, and until the interest in measuring and controlling PM 2.5 emissions emerged in the 1990 s, their PM sampling methods were centered on measuring only filterable PM. At the time that these methods were originally instituted, the best available pollution control devices were mainly limited to filterable PM an d could not control the condensable portion of PM emissions. As interest in the health effects associated with PM emissions increased, efforts were cent ered on determining the contribution of the