Analysis of sunlight absorption spectra related to atmospheric trace gases in the tropics

ABSTRACT

Zenith sky-scattered light intensity spectra of wavelength ranges of 325–500 nm have been recorded with UV-visible spectrometer over tropical station Pune (18° 31′ N, 73° 55′ E). Zenith scattered light spectra in the spectral range of 346–358 nm are analysed to find out differential optical depth (DOD) for the period 15–18 November 2010. In DOD spectra, depths are noticed at relevant wavelength due to the absorption by atmospheric gases such as NO₂ (nitrogen dioxide), O₃ (ozone), BrO (bromine monoxide), and OClO (chlorine dioxide). These DOD spectra are analysed by a matrix inversion technique to calculate individual DOD spectrum of the gases. The observed and calculated DODs are found to be in a good agreement. The coefficient of determination (R²) between observed and calculated DODs of NO₂, O₃, BrO, OClO, O₄ (oxygen dimer), and Ring effect are observed to be 0.55, 0.77, 0.73, 0.75, 0.82, and 0.91, respectively. Filling-in of solar Fraunhofer lines in the observed zenith scattered sunlight is known as ‘Ring effect’. The slant column densities of the above gases are found to be increased due to increasing absorption path length with solar zenith angles. The vertical column densities (VCDs) of O₃ and NO₂ derived using ground-based spectrometer are compared with the Ozone Monitoring Instrument (OMI) on board Aura satellite during the period 1 March–31 December 2010. The day-to-day variations are found to be similar; however, the percentage differences in VCDs of O₃ between ground-based spectrometer and satellite-based OMI are observed to be varying from 1% to 15%, while for NO₂, they vary from 1% to 10%. Also, the seasonal mean values of VCDs of O₃ and NO₂ are discussed. The O₃ mean values in the rainy season are found to be higher than that of in the summer and winter seasons from both ground- and satellite-based measurement. Whereas, the NO₂ mean values in the winter season are found to be higher than that of in the summer and rainy seasons from both the measurement techniques. The VCDs of O₃ are observed to be lowest in winter season due to the loss of ozone within NO₂ and O₃ reaction active during the winter season.

Acknowledgements

We are grateful to Dr. M. Rajeevan, Director, Indian Institute of Tropical Meteorology (IITM), Pune. We also thank the reviewers and editors for their valuable comments on this article.

Disclosure statement

No potential conflict of interest was reported by the authors.