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One of the potential application areas for PA systems is the detection of the concentration of ozone, a key substance in the Earth’s atmosphere. Its presence in the atmosphere is crucially important for the quality of our life. Sensitivity in the lower ppb range for our ozone detector based on a quasi-continuous wave (QCW) frequency quadrupled Nd:YAG laser systems is achieved. Wavelength conversion was achieved by focusing the near-infrared light into a nonlinear lithium triborate (LBO) crystal (532 nm). To generate the fourth harmonic (266 nm) the green light was focused to a second non-linear CLBO crystal. The achieved UV wavelength range belongs to the Hartley bands of ozone molecules.

The QCW based system was found to have the potential in practical applications due to its relative simplicity, compactness and automated operation. For the laboratory calibration of this PA system a portable ozone generator (Model 175, Thermo Environment Instruments Inc.) was used. This calibrator was previously checked and calibrated by the accredited Reference Laboratory of the Hungarian Environmental Institute. The minimum detectable concentration is 2.9 ppb.

After laboratory calibration the modulated QCW system was tested at an environmental monitoring station of the Lower Tisza Valley Environmental Inspectorate. This station belongs to the Hungarian Air Quality Monitoring Inspectorate. The PA system was run in parallel with a routinely used ozone measuring instrument (49C Thermo Environmental Instruments Inc.), and showed a reliable operation.

!!!FIGURE CAPTION!!! Red triangles are for the measurements of the QCW PA system and black circles are for the measurements of the reference optical absorption system. Each measurement point is the result of averaging over 30 min. In the insert, the recordings of the systems are shown in detail. The continuous black curve corresponds to the reference instrument and the continuous red curve is the result of the PA system.