Zenith scattered light measurement: observations of BrO and OClO

Abstract

Stratospheric BrO and OClO observations have been made for the first time over a tropical station, Pune (18° 31′ N, 73° 55′ E) using a Differential Optical Absorption Spectroscopy (DOAS) technique by measuring zenith sky scattered light spectra in the wavelength range of 346–358 nm by ultraviolet (UV)/visible spectrometer. The Differential Optical Density (DOD) fitting technique is applied for the right selection of a suitable spectral region for the analysis to minimize interference and poorly fitting absorption features, and also to minimize the residual of the fit. Observed DODs of O₃, NO_2, BrO, OClO, O₄, Rayleigh and Ring are well fitted with the calculated DODs and the percentage DODs are found to vary up to 0.5%, 0.8%, 0.15%, 0.13%, 1.5%, 1.2% and 1.3% respectively. Chlorine and bromine species play an important role in the ozone depletion, hence O₃, NO₂, BrO and OClO Slant Column Densities (SCDs) are derived between 76° and 94° Solar Zenith Angles (SZAs). The SCDs of O₃ are found to be decreased in the twilight period (i.e. between 90° and 94° SZA) in the presence of sufficient BrO and OClO. Total Column Densities (TCDs) of O₃, NO₂, BrO and OClO are derived by UV/visible spectrometry, Brewer spectrometry and satellite-based Scanning Imaging Absorption spectrometer for Atmospheric Cartography (SCIAMACHY) for Pune and the higher latitude station Kanpur (26° 28′ N, 80° 24′ E) during the period 1 April–31 June 2008. The day-to-day variations in O₃ and NO₂ TCDs over Pune are found to be more than over Kanpur. BrO TCDs vary between 1.9 × 10¹³ and 4 × 10¹³ molecules cm⁻² over Pune, which are derived by UV/visible spectrometry, while they vary for the high-altitude station Kanpur between 0.5 × 10¹³ and 3.5 × 10¹³ molecules cm⁻² derived by SCIAMACHY. The OClO TCDs are found to have an increasing trend with variations between 2 × 10¹³ and 4.5 × 10¹³ molecules cm⁻² during the above period.

Acknowledgements

The authors are grateful to the anonymous reviewers for their valuable comments and suggestions in making the scientific content of the article more clear. Thanks are also due to Professor P.C.S. Devara, Head, Physical Meteorology and Aerology Division and Professor B.N. Goswami, Director, IITM, Pune, India, for encouragement in carrying out this study. The authors would like to express special thanks to Dr D.B. Jadhav, Dr R.Vijaykumar and Mr P. Murugvel, IITM, Pune, for their contributions to automization of the spectrometer.