An optical particle counter ( 100 ) includes a light source, an integrating sphere (104), a light stop (105), a photo-detector (106), a channel for air inlet ( 107 ), and a channel for air outlet (108). The light source is arranged to focus the light beam in the center of the integrating sphere (104), The light stop (105) is arranged to stop the light beam after exiting the integrating sphere (104 ). The channels for air inlet (107) and outlet ( 108 ) are configured to allow an aerosol sample to enter the integrating sphere ( 104 ), cross the light beam, and exit.