Abstract: Apparatus for monitoring the mass concentration of atmospheric pollutants above and below a predetermined size. A receptacle has an air inlet and outlet. Air is continuously drawn through the inlet and thence through the receptacle and the outlet. A first collector in the receptacle collects pollutants above the predetermined size. This first collector is movable through a first station and collects pollutants above the predetermined size from the air entering the receptacle through the inlet on successive areas thereof. A second collector in the receptacle collects pollutants below the predetermined size. This second collector is movable through a second station and collects pollutants below the predetermined size from the air which has passed the first station on successive areas thereof. A holder is provided for a first source of radiation which passes through the area of the first collector at the first station.