A differential absorption lidar (DIAL) using a Q-switched CO(2) laser and a heterodyne receiver has been developed. The DIAL system is highly automated with computer-controlled laser-line selection and signal processing. The transmitter operates at a pulse-repetition frequency of 20 kHz and has an average output power of 1.8 W. A wideband HgCdTe detector is used together with a high speed adding buffer to detect the return signals. The system has been used in a field experiment to monitor ethylene emission from a petrochemical factory.
Absorption cross sections at CO(2) laser wavelengths have been measured for ethylene and ammonia. The dependence of the cross sections on pressure and temperature have been investigated for pressures and temperatures normally occurring in the atmosphere. The changes in cross section are, e.g., 5% for ethylene at the P(14) 00 degrees 1-10 degrees 0 line and 10% for ammonia at the R(8) 00 degrees 1-10 degrees 0 line for a temperature change of 30 K. A comparison with theoretical calculations is made for ammonia.
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