The other is a sensing beam that measures the intensity of the same light that crosses sample spots where the aerosol is continuously accumulating. The Aethalometer also measures signals at the sensors when the light source is turned off for the reference and sensing beams. These values are subtracted from the sensors’ levels when the light source is turned on in order to correct the measurements from dark currents. Thus, the attenuation at each wavelength can be calculated based on the intensity difference of the reference and sensing beams between light source on and off [Cheng et al., 2006].