The Rack Mount Aethalometer Model AE33 measures light absorption by suspended aerosol particles at seven wavelengths, from 370 nm (UV) to 950 nm (IR). Real-time measurements of particle concentrations can be made with a time resolution of two minutes to one hour. Interpretation of the differences in optical response across the wavelength spectrum reveal information regarding aerosol size distribution, physical properties, and assist in identifying the primary components. This instrument is widely used in monitoring studies involving large scale, regional air pollution problems. It incorporates scientific and technical advances designed to offer improved measurement performance, user features, communications and interface, and the ability to perform routine performance tests to verify correct operation. Most importantly, this instrument incorporates the patented DualSpot™ measurement method. This provides two significant advantages: elimination of the changes in response due to ‘aerosol loading’ effects; and a real-time calculation of the ‘loading compensation’ parameter which offers insights into aerosol optical properties, and has been interpreted in models of aerosol origins and aging