Abstract

Laser photoacoustic spectroscopy is a powerful tool for the monitoring of a large number of gaseous substances at concentration levels varying from the ppb to the percent level. We have developed a fully computer controlled mobile CO₂ laser photoacoustic spectrometer which has been involved in several field studies [1,2]. Our system will be presented including some recent developments. Owing to the strong absorption, CO₂ laser spectroscopy is particularly sensitive to ethene and ammonia, two trace gases that are important for the production of photochemical smog and of acid rain, respectively, and which are both emitted by road traffic and other sources. These compounds are difficult to detect with alternative methods, particularly if a good time resolution and high sensitivity are required. With the widespread introduction of catalytic convenes in cars, the local concentration of traffic-emitted ammonia could have increased substantially in the recent past. We present first results of a field study performed with our system during a five day period in summer 1995 at a freeway tunnel outlet near Zurich. Tunnel air was pumped continuously through the photoacoustic cell. In order to avoid any adverse effects during sampling which could influence the components to be monitored we used neither CO₂ nor H₂O scrubbers but preferred to analyse the PA spectra by taking these two substances into account in the fitting procedure for the spectral analysis. The measurements were done at atmospheric pressure and simultaneous monitoring of CO₂, ethene and ammonia was achieved with a time resolution of 10 minutes by automatic tuning to the appropriate laser transitions. Detection thresholds of a few ppb were obtained for ethene and ammonia. Furthermore, the CO level was monitored independently by a conventional device. The derived concentration profiles could be correlated with the data from an automatic traffic counting system. Rather high gas concentrations were derived, particularly for ethene and ammonia in the range of up to 250 ppb and 400 ppb, respectively. Even at night the ammonia concentrations stayed well above the background level of a few ppb. Based on these data and the calculated air flow through the tunnel, emission factors (mass of an exhausted component per car and km) from road traffic for the four substances could be determined. To our knowledge it is the first time that ammonia emission factors have been derived in Switzerland. An extrapolation with our emission factor of 15 mg ammonia per km and car indicates that traffic contributes approximately 1 % to the total ammonia emission into the air in Switzerland. A similar percentage was estimated for Austria in 1990. Local relative contributions can be much higher and strongly depend on the vehicle composition of the traffic.

© 1996 IEEE