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Total column density variations of NO2 and O3 by automatic visible spectrometry over Pune, India

Total column density variations of NO2 and O3 by automatic visible spectrometry over Pune, India,G. S. Meena,D. B. Jadhav,C. S. Bhosale

Total column density variations of NO2 and O3 by automatic visible spectrometry over Pune, India   (Citations: 4)
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A single scattering radiative transfer model has been developed to calculate the air mass factors (AMFs) of NO2 and O3 for scattered light observations. The di- rect and scattered intensities reaching the ground have been calculated using this model, which are uti l- ized for computation of AMFs of these species. AMFs have been used to derive the total column densities (TCDs) from slant column densities (SCDs). Daily in- tensity data obtained by spectroscopic observations made at Pune (18° °32''N, 73 ° °51''E) during May 2000- May 2001 are used for the computation of SCDs of NO2 and O3 by differential optical absorption spec- troscopy (DOAS) technique. NO2, O3, H 2O and O4 have characteristic absorption features in the visible range 462-498 nm. These features have been used in the DOAS technique. The percentage differential op- tical depths (DODs) of NO2, O3, H2O and O4 have been computed. The TCD of O 3 has also been derived by other methods. They are in good agreement with Dob- son spectrophotometer o bservations. Here, the daily and seasonal variations in TCDs of NO 2 and O3 at Pune for the above period are discussed. BEGINNING with the pioneering work of Brewer et al. 1 and Noxon2, observations of scattered visible light from the zenith sky have been used to o btain the slant column density (SCD) of stratospheric NO2. Twilight geometry enhances the path of the sunlight in the stratosphere, allowing more sensitivity to stratospheric absorbers. SCDs of trace constituents are deduced from the spectra by diffe rential absorption spectroscopy. The instrume nt used here is an automatic spectrometer operated in spe c- tral band 462-498 nm with resolution (1.3 nm) and pho- tomultiplier tube (PMT) as detector. The spectrometer is set to measure intensity absorptions of O3 and NO2 twice a day during twilight. Spectra are recorded every 2 min during twilight up to 92° solar zenith angle (SZA). These spectra are analysed by differential optical absorption spectroscopy (DOAS) relative to a reference spectrum taken at low SZA. It is possible to retrieve a large nu m- ber of t race species simultaneously in UV -visible region using DOAS technique3-7. Daily/seasonal variations in total column density (TCD) of NO 2 and O3 have been studied using spectroscopic observations by several workers5,6,8-13. The air mass factor (AMF) of the absorbing species is defined as the ratio of the SCD of the absorber to its ve r- tical column density. Calculations of AMFs have been made using single scattering radiative transfer model 14,15. This approach assumes that each photon entering the de- tector is scattered only once before it enters the detector from the zenith direction. The attenuation due to absor p- tion and scattering is calculated along paths through the modelled twilight spherical shell atmosphere for each a l- titude interval, and the intensity -weighted average path through an absorber having a particular alt itude profile is subsequently computed. A visible spectrometer, which observes the zenith sky, provides the simplest method for routine calculation of the SCDs of NO 2, O 3, H 2O and O4. Observations of the sunlight scattered from the zenith sky, can be made in any weather. We use mercury spectra and Fraunhofer lines in the reference spectrum for wavelength calibr a- tion. Atmospheric gases absorb radiation at characteristic wavelength. The efficiency with which a given gas absorbs radiation is expressed by the absorption cross - section, which is a function of wavelength. The absorp- tion cross-sections of NO 2, O 3, H 2O and O4 are shown in Figure 1. At wavelengths where the absorption cross - section is small, we can expect solar radiation to reach the ground with little attenuation, while at wavelengths where the absorption cross-section is large, absor ption occurs fairly high in the atmosphere. The wav elength ranges were chosen to minimize the spectral overlap b e- tween different absorbers, in particular, to avoid spectral interference with the H2O absorption at 448 nm (ref. 16). Differential rather than absolute cross -sections are used in the analysis procedure. The differe ntial cross-sections represent the departures of the cross sections from a mean of zero over the spectral interval of interest 17. The intensity observations taken during morning and evening hours for 75-92° SZA are used to find the abundance of NO2, O 3, H 2O and O4 using Lambert-Beer's law. SCDs are obtained by matrix inversion technique. Details of the observational technique, method of ana lysis and compu- tational procedure adopted for retrieving TCD of NO 2 and O3 are discussed. Comparison of TCDs of O 3 has been made with those obtained by diffe rent methods and daily morning/evening variation in NO 2 and O3 are dis- cussed. Seasonal variation of these species is studied du r- ing the year 2000-01.
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