This standard describes a standard method for determining the PM2,5 mass concentration of suspended particulate matter in ambient air by sampling the particulate matter on filters and weighing them by means of a balance. Measurements are made over a sampling period of about 24 h, and, in line with the Directive, are expressed as µg/m 3 , where the volume of air is the volume at ambient conditions near the inlet at the time of sampling. The range of application of the standard is from 1 µg/m 3 (i.e. the limit of detection of the standard measurement method expressed as its uncertainty) up to 120 µg/m 3 (i.e. the maximum concentration level observed during the field study undertaken by CEN/TC 264/WG 15 to validate the standard). NOTE Although the standard is not validated for concentrations over 120 µg/m 3 , its range of application could well be extended to commonly encountered ambient concentrations up to circa 200 µg/m 3 when using glass or quartz fibre filter. At these high concentrations and particulate mass loadings no filter clogging is to be expected. Also the flow rate can be easily maintained at the nominal setting. The equivalence procedure in Annex A specifies two approaches, depending on whether the candidate method differs slightly or fundamentally from the standard method. In the former case, involving only slight differences from the standard method ("variations on a theme") Annex A provides a restricted procedure to compare only the pertinent differences, instead of a full field test. This part of the annex serves to give practical guidance for determining equivalence for measurement methods commonly used in monitoring networks, and includes examples of common variations to the standard method, such as different filter storing or conditioning procedures and the variation of the standard method for the application as automated filter changer. In the latter case, involving a full set of field tests, the procedure serves to determine equivalence only within the range of conditions under which the field tests are carried out. The equivalence can be shown to hold for conditions prevailing within European countries by carrying out the field test in situations covering a suitable range of relevant ambient parameters (such as concentration and composition of the suspended particulate matter, temperature, and humidity). Although this standard does not explicitly address automatic monitoring methods for the measurement of the PM2,5 mass fraction in ambient air, the equivalence test procedure in Annex A applies both to non-automatic and automatic methods.