ON THE INFLUENCE OF TROPOSPHERIC CLOUDS ON ZENITH-SCATTERED-LIGHT MEASUREMENTS OF STRATOSPHERIC SPECIES

Examples for the influence of tropospheric clouds on the ground-based measurement of stratospheric species using the DOAS-technique (Differential Optical Absorption Spectroscopy) are reported. At Camborne/Great Britain (50.216 degrees N, 5.316 degrees W) on Sept. 11 - 15, 1994, episodic enhancement of absorption lines of O-4, H2O, O-3 and NO2 were observed in coincidence with tropospheric clouds being in the instrumental field of view (1.1 degrees full angle). At a solar zenith angle (SZA) of 88 degrees, absorption enhancements up to roughly a factor of 3 were detected for the tropospheric species O-4 and H2O and the tropospheric fractions of the total column of O-3 and NO2. The additional absorptions in the visible spectral range are probably caused by multiple Mie-scattering in tropospheric clouds. For our conditions, a tropospheric light path enhancement (TLPE) of 135+/-40 km can be inferred, being largely independent of SZA. This observation has several important implications for the atmospheric radiative transport, which are briefly discussed.