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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°N, 5.316°W) on Sept. 11–15, 1994, episodic enhancement of absorption lines of O4, H2O, O3 and NO2 were observed in coincidence with tropospheric clouds being in the instrumental field of view (1.1° full angle). At a solar zenith angle (SZA) of 88°, absorption enhancements up to roughly a factor of 3 were detected for the tropospheric species O4 and H2O and the tropospheric fractions of the total column of O3 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. |
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