Optical particle counter, including - a light source, - an integrating sphere, - a light stop, - a photo-detector, - a channel for air inlet, and - a channel for air outlet, wherein the light source is arranged to focus the light beam in the center of the integrating sphere, wherein the light stop is arranged to stop the light beam after exiting the integrating sphere, wherein the channels for air inlet and outlet are configured to allow an aerosol sample to enter the integrating sphere, cross the light beam, and exit, wherein the photo-detector is arranged to collect light scattered by particles suspended in the aerosol sample when hit by the light beam and to convert the scattered light into an electrical signal, wherein the optical particle counter is configured to deduce PM concentration in the air, characterized in that the optical particle counter includes a heater, the heater being arranged to exert a convection force on the aerosol sample entering the integrating sphere and hence supporting its transit through the integrating sphere; and the optical particle counter includes ports (507, 508) allowing the aerosol sample to enter the integrating sphere, wherein the heater is placed in front of one of the ports. - a light source, - an integrating sphere, - a light stop, - a photo-detector, - a channel for air inlet, and - a channel for air outlet, wherein the light source is arranged to focus the light beam in the center of the integrating sphere, wherein the light stop is arranged to stop the light beam after exiting the integrating sphere, wherein the channels for air inlet and outlet are configured to allow an aerosol sample to enter the integrating sphere, cross the light beam, and exit, wherein the photo-detector is arranged to collect light scattered by particles suspended in the aerosol sample when hit by the light beam and to convert the scattered light into an electrical signal, wherein the optical particle counter is configured to deduce PM concentration in the air, characterized in that the optical particle counter includes a heater, the heater being arranged to exert a convection force on the aerosol sample entering the integrating sphere and hence supporting its transit through the integrating sphere; and the optical particle counter includes ports (507, 508) allowing the aerosol sample to enter the integrating sphere, wherein the heater is placed in front of one of the ports. wherein the light source is arranged to focus the light beam in the center of the integrating sphere, wherein the light stop is arranged to stop the light beam after exiting the integrating sphere, wherein the channels for air inlet and outlet are configured to allow an aerosol sample to enter the integrating sphere, cross the light beam, and exit, wherein the photo-detector is arranged to collect light scattered by particles suspended in the aerosol sample when hit by the light beam and to convert the scattered light into an electrical signal, wherein the optical particle counter is configured to deduce PM concentration in the air, the optical particle counter includes a heater, the heater being arranged to exert a convection force on the aerosol sample entering the integrating sphere and hence supporting its transit through the integrating sphere; and the optical particle counter includes ports (507, 508) allowing the aerosol sample to enter the integrating sphere, wherein the heater is placed in front of one of the ports.