A characteristic feature of this photometer is the presence of a reliable laser diode in it, which generates constant laser power in such a way that the aerosol concentration measurements remain stable for a long period of time, and the created air envelope keeps the optics clean at low background levels. In addition, the need for maintenance of this photometer is minimized. The 8587A uses an internal reversing valve to measure aerosol concentrations in both flow and counter flow. A special high-speed “purge” mode shortens the purge time when switching from downstream measurements to counterflow measurements.