2 CICATA-IPN, Legaria 694, Col. Irrigación, México 11500 D.F.

Photoacoustic spectroscopy provides a unique method for the on-line determination of gases in low concentrations. Ethylene is an atmospheric air pollution produced by exhaust pipes of automotive vehicles and a lot of industrial burning processes. We present a CO₂ laser photoacoustic system which measures gas traces by means a special resonant cavity containing a sensitive microphone. Normal modes oforganic molecules are excited by the incident CO₂ laser beam, and then decay emitting a pressure wave that is detected by the microphone. The signal is amplified by a lock-in connected to a PC, this allows to obtain data analysis of signal which is a function of ethilene (C₂H₄) concentration. C₂H₄ concentration up to 50 ppbv was found in air samples taken from the Mexico City atmosphere.