(17)Providing reference measurement methods is acknowledged to be an important issue. The Commission has already mandated work on the preparation of EN standards for the measurement of polycyclic aromatic hydrocarbons and for the evaluation of the performance of sensor systems for the determination of concentrations of gaseous pollutants and particulate matter (PM 10 and PM 2,5 ) in ambient air with a view to their early development and adoption. In the absence of EN standard methods, the use of international standard reference measurement methods, national standard reference measurement methods or European Committee for Standardization (CEN) technical specifications should be permitted.