ESTIMATION OF THE AMOUNT OF TROPOSPHERIC OZONE IN A CLOUDY SKY BY GROUND-BASED FOURIER-TRANSFORM INFRARED-EMISSION SPECTROSCOPY

The problem of retrieving minor concentrations of constituents by grou nd-based Fourier-transform infrared emission spectroscopy is addressed by means of the concept of differential optical emission spectroscopy in analogy to the concept of differential optical absorption spectros copy. Using the prominent nu(3) ozone feature at 1043 cm(-1), we show that the strength of the spectral signature depends not only on the am ount of ozone but also on the atmospheric thermal structure. This depe ndence can be described by a rather accurate approximation, which was used to construct a simple diagram to estimate the amount of column oz one between the instrument site and a cloud deck as well as to determi ne the detection limit. The detection limit is shown to depend on clou d base height. For a given thermal lapse rate it was found that the lo wer the detection limit, the higher the cloud base altitude. However, as shown in a case study with variable cloud base height, the concept fails for semitransparent clouds. Multiple scattering of the emitted r adiation within the clouds yielded a path enhancement that simulated a n enhanced amount of constituent. The path enhancement was estimated t o be 2.4-4 km at 1000 cm(-1) for low-level clouds, equivalent to an en hancement factor of 6-21. The multiple scattering effect has considera ble consequences for ground-based as well as for nadir satellite retri eval techniques in cloudy skies. (C) 1998 Optical Society of America.