LASER PHOTOACOUSTIC SPECTROMETRY FOR TRACE GAS MONITORING

Spectroscopic methods offer important features with respect to atmosph eric trace gas monitoring. Some relevant techniques are briefly discus sed. Emphasis is put on various aspects of photoacoustic spectrometry. Its basic principles and important experimental characteristics are o utlined. Depending on molecular absorption cross-sections, a detection limit in the low ppbv concentration range is typically achieved witho ut the use of any preconcentration device. In multi-component mixtures this limit mag be increased owing to absorption interferences. Two ph otoacoustic (PA) systems have been developed and some specific applica tions are presented. A CO-laser PA system operating in the 5-6.5 mu m wavelength range was applied to the analysis of motor vehicle exhausts . Apart from nitrogen oxides, CO2 and water vapour, ten different vola tile organic compounds (VOCs) including o-, m- and p-xglene were selec tively detected at parts per million by volume (ppmv) concentrations. As a second instrument, an automated CO2 laser PA system was implement ed in a trailer for is situ measurements. It was applied to the detect ion of stack gases at a pharmaceutical plant and to air monitoring in urban and rural environments. As an example, ethene, ammonia and ozone were detected at ppbv concentrations in rural air. Finally, new aspec ts are discussed, in particular the introduction of CO2 isotopes as th e laser medium, the design of PA Stark cells and the development of a continuously tunable high-pressure CO2 laser and of a widely tunable m id-infrared laser based on frequency mixing in a non-linear crystal. A lthough PA spectrometry already represents a valuable alternative to m ore established spectroscopic and non-spectroscopic schemes, these nov el developments should further enhance its potential and versatility,