Abstract

A computer-controlled, mobile CO₂ laser photo-acoustic system has been developed and applied to the in situ monitoring of atmospheric trace gases in urban and rural environments. A line-tunable sealed-off CO₂ laser with ¹²C¹⁶O₂ and ¹³C¹⁶O₂ isotopes is used as radiation source. The special design of the resonant photoacoustic cell permits measurements on flowing air. Emphasis is put on high sensitivity, selectivity, and temporal resolution of the detection. For this purpose, the laser is repetitively tuned to preselected laser transitions chosen on the basis of the characteristic absorption by the molecules to be detected and potential absorption interferences. The minimum detectable absorption coefficient amounts to 10⁻⁸ cm⁻¹, which for many components corresponds to parts in 10⁻⁹ concentrations. Field studies concern measurements on industrial exhausts in multicomponent mixtures and on ambient air. In a recent measurement campaign we successfully montitored in situ five different types of compounds in rural air. The concentrations of water vapor, CO₂′ ammonia (NH₃), ozone (O₃), and ethene (C₂H₄) were recorded simultaneously during a 6-day period with a temporal resolution of 10 min. Further results and new developments that demonstrate the great potential of this technique will be discussed.

© 1992 Optical Society of America