The photoacoustic soot spectrometer (PASS) measures light absorption by aerosol particles. As the particles pass through a laser beam, the absorbed energy heats the particles and in turn the surrounding air, which sets off a pressure wave that can be detected by a microphone.
The PASS instruments can also simultaneously measure the scattered laser-light at three wavelengths and therefore provide a direct measure of the single-scattering albedo.
The PASS is part of the aerosol observing system (AOS).