LIDAR(LIght Detection And Ranging) technology is very useful for remote sensing of pollutants in the air. To detect rare molecules in the air, it is important to know the accurate absorption coefficients of those molecules. Measurements of Ethylene($C_2H_4$) and Ammonia($NH_3$) absorption coefficients in the IR(10 ㎛) region were done by using a tunable $CO_2$ laser and pyroelectric detectors. With the results of this experiment, the concentrations of Ethylene and Ammonia were detected by using RR(Retro-Reflector) DIAL(DIfferential Absorption Lidar) system with two $CO_2$ lasers.
The cell used for absorption coefficient measurement is made with a Quartz tube 1 m long. The volume of a mixing chamber is about 4 ℓ. $N_2$ gas was used for purzing. Almost the results of this experiment had good agreements with the values of HITRAN database(e.x.: 10P(14) line for Ethylene case, $32.00 atm^{-1}cm^{-1}$ from the database, $30.66 atm^{-1}cm^{-1}$ from the experimental result), but 10R(08) line data for Ammonia did not. Because 10R(08) line was used for on-line, which had large absorption coefficient, this experimental result(not the database value) was used in the field experiment.
For the field experiment, the detecting range between the lasers and a reflector is 345 m. The measured Ethylene and Ammonia densities were 119 ppb and 6.7 ppb respectively, which were resonable results. The limitation of detection, i.e. the standard deviation, of this experiment was about 20 ppb for both molecules. To get more accurate results, more stable laser system is necessary.
