In situ measurements on carbon aerosols with photoacoustic spectroscopy

Up to now photoacoustic spectroscopy is the only available approach for an on-line and in-situ measurement of the absorption of radiation by airborne particles. The main disadvantage of the method is the need for a highpower laser in the visible or infrared spectral region with an optical output of 1 W or more as radiation source to achieve a detection limit for particulate carbon in the order of 1 g/m3. For the first time to our knowledge a laser diode was used to test the properties of this laser type as light source for photoacoustic spectroscopy of aerosols. The advantages of a laser diode compared with other high-power lasers in the visible are the emission wavelength of 800 nm, the small size of the laser, and the possibility of an electronic intensity modulation of the light instead of a modulation by a mechanical chopper, which results in a decreased background signal and an extended modulation frequency range.