NEW DEVELOPMENTS IN CO2-LASER PHOTOACOUSTIC MONITORING OF TRACE GASES

Air pollution control requires sensitive and selective, preferentially multi-component, detection schemes. Present air pollution studies foc us on the monitoring also of minor but important constituents as well as of their reactions. After a brief presentation of relevant air moni toring schemes we focus on a thorough discussion of the newest develop ments and applications of a mobile CO2-laser photoacoustic (PA) system for in situ monitoring of gaseous components in ambient air. The inst rument is computer-controlled and can be operated unattended for many hours. We performed PA measurements in rural air with this mobile (CO2 )-C-12 laser system that is installed in a trailer. During extended ti me periods we recorded for the first time and simultaneously the conce ntrations of the following ambient trace gases: water vapor (H2O abs.) , carbon dioxide (CO2), ammonia (NH3), ozone (O3) and ethene (C2H4). W e obtained the concentrations of these air components with an iterativ e least square analysis on the basis of the repetitively measured PA s ignals on nine laser transitions. A temporal resolution of approx. 10 min was achieved. Although the sealed-off C-12-O-16(2) laser emits on ca 60 transitions in the wavelength range between 9 and 11 mum, well w ithin the atmospheric window from 8 to 14 mum, we have to take absorpt ion interferences mainly by CO2 and H2O vapor into account. Hence, we have evaluated other CO2 isotopes as laser media that could be applied to air pollution monitoring in a mobile system. The introduction of a n isotopic CO2 laser reduces the interferences with ambient CO2 and pr ovides at the same time an additional set of laser wavelengths for spe ctroscopic purposes. Natural CO2 exhibits drastically lower absorption cross sections at the ca 60 laser transitions of the selected C-13-O- 16(2) isotopic laser compared to the C-12-O-16(2) laser. We present ca libration data of important air compounds on C-13-O-16(2) laser transi tions.