Influence of tropospheric clouds on ground-based measurements of stratospheric trace gases at Tropical station, Pune

The effects of tropospheric clouds on the measurements of the stratospheric trace species like NO2, O3, H2O and O4 using differential optical absorption spectroscopy have been discussed. For the present study, the spectroscopic observations were carried out in the spectral region 462–498 nm at Pune (18°32′N, 73°51′E) on cloudy days from 4 May to 29 November 2000. Differential slant column density (SCDdiff) of NO2, O3, H2O and differential optical depth of O4 are derived to make the comparison between clear and cloudy sky conditions. Also, daily vertical column densities (VCDs) of O3 and NO2 are computed. Additional optical path lengths (enhanced/reduced path lengths in cloudy sky condition compared to clear sky condition) are derived at 90° solar zenith angle (SZA) for cloudy days using O3 densities. The enhancements/reductions in the path lengths inside the clouds affect the ground-based measurements of the stratospheric gases. The study reveals that the enhancement in the densities of above gases are found inside the optically thick clouds like Cb, Ac whereas reductions in the densities are noticed in optically thin clouds like Ci, As in comparison with clear sky case. Langley plots of SCDdiff of O3 also support the above facts. Enhancement in optical path length during Indian southwest summer monsoon season (June–September 2000) and pre monsoon (May 2000) appears to be due to multiple reflections between layered clouds. Whereas, enhancement in optical path lengths during post-monsoon (October–November 2000) season appears to be linked with photon diffusion, multiple Mie-scattering and multiple reflections due to isolated patches of optically thick clouds. The reductions in optical path lengths due to optically thin clouds are noticed during all the above seasons. Prominent day-to-day variability in the VCDs of NO2 and O3 is observed.