The Aethalometer® is an instrument that uses optical analysis to determine the mass concentration of Black Carbon particles collected from an air stream passing through a filter. These particles are directly emitted to the air during all combustion, but are primarly associated with coal or diesel smoke. They adversely affect public health; contribute to local and global climate change; and reduce visibility. Aethalometers are used by air-quality monitoring programs; public-health protection agencies; research laboratories; and community groups. The Aethalometer technology is available on different chassis and platforms: Rack Mount, Portable, Modular, High Range, and Transmissometer. The Aethalometer is also offered in two configurations of light analysis: 2-Wavelength (infra-red 880 nm for Black Carbon; ultraviolet 370 nm for aromatic organic compounds) and 7-Wavelength (7 sources from 370 nm to 950 nm). The latest model AE33, 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, the new 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.