Spectral infrared transmittance of haze and fog: its measurement and influence on FTIR open-path monitoring

摘要

Abstract: Fourier transform infrared (FTIR) open path monitoring OPM is expected to work as a monitoring system for gaseous emissions and imissions within the open atmosphere. As such it is exposed to the dynamics of the atmosphere. Its methods, originating from laboratory spectroscopy, have to be adapted to the conditions of this specific operational area. Among other effects haze and fog influence the radiation transfer of the atmosphere and hence also FTIR- OPM. Model investigations have been performed to investigate the possible influence of haze and fog on common open path monitoring methods. These methods do not account for radiation emitted and scattered by the atmosphere.It is shown that the effects should not be neglected in certain wavelength bands under haze and fog conditions. As a result a new transmittance measurement method has been developed. It allows to completely cancel out the effects of emission and scattering of the transmitting medium. Furthermore it eliminates the influence of the spectrometer's inner radiation on the measured spectra, hence yielding pure transmittance spectra. From these then absorption coefficients can be derived and gas parameters like temperature and concentration. The method has been applied to determine the spectral transmittance of haze and fog and been compared to reference measurements. !2