The APCA-370 is a device for continuous CO2 monitoring in ambient air. It uses cross modulation type non-dispersive infrared (NDIR) absorption method. This method employs one cell and one light source for both sample gas and reference gas what allows precise measurement without generation of zero-drift. Due to precise measurement and long-term stability in continuous monitoring as well as its minimal maintenance requirements, this method has been a preferred method for monitoring the atmospheric pollution.