Calibration and validation methods for DOAS remote sensing systems

This work presents a methodology, based on comprehensive measurements performed in the laboratory using an optical cell with fixed geometrical path (I m), for the calibration and validation of a remote sensing system used in air quality analysis and control. The method was tested on a DOAS (Differential Optical Absorption Spectroscopy) system produced by "Kayser-Italia", but is applicable to other equivalent remote sensors. In the laboratory it was possible to simulate atmospheric depth in the range of 1-500 ppmm, modulating the gas concentration inside an optical cell. To verify the gas calibration curve of the spectrometer, a measurement set of different gas optical paths inside the cell were used. The variation of cell gas concentration was obtained by means of a vacuum pump, starting from an initial filling of the cell using a cylinder mixture of the examined gas with N-2. The accurate value of the gas concentration was inferred from pressure measurement inside the cell. This procedure gives good results for SO2 and NO2 calibration test; for O-3 the calibration procedure is almost the same but there's the necessity of producing the gas during measurement itself. In the case of compounds with relatively high vapour pressures (such as aromatics), which are liquid at standard atmospheric pressure, the above-described measurements were preceded by an evaporation phase at low pressure (about 1-5 mb). This method makes it possible to obtain precise gas calibration curves, based on several points in the complete working span, by means of a single mixture cylinder or pure compound, The results are reliable and repetitive and the method is also applicable in field measurements. Six gases were analysed (SO2, NO2, O-3, Benzene, Toluene, Xylene) and their calibration curves and ACS (Absolute Cross Section) are presented.