“…In nonlaser (passive) long-path optical absorption monitoring the effects of atmospheric turbulence can, be eliminated by fast scanning, such as in differential optical absorption spectroscopy, 9 dispersive correlation spectroscopy, 5 and gas-filter correlation spectroscopy.4A example chosen, containing Hg of sufficient vapor pressure-to block out the central part of the resonance line. In DIAL language, in this detection arm the offresonance signal is recorded (actually, two close-lying reference wavelengths are used simultaneously).…”