This Standard sets out a gravimetric procedure for the concurrent determination of the PM10 and PM2.5 fractions of suspended particulate matter in ambient air collected in a dichotomous sampler. The method provides a measure of the mean concentration of PM10 over the sampling period of the two size fractions, viz. the fine fraction (less than approximately 2.5 µm) and the coarse fraction (2.5 µm to 10 µm). A procedure for assessing the performance of dichotomous samplers so that they comply with the sampling requirements of this method, is described in Appendix A. NOTE: Sampling is normally of 24 h duration to average out diurnal variations. Provided that the mass of the filter is determined under carefully controlled laboratory conditions, mean concentrations of 5 µg/m3 (1.67 L/min sample flow rate) or 1 µg/m3 (15 L/min sample flow rate) and greater may be determined, based on a 24 h sampling period.