Clear sky blue/UV spectrum at sunset
The shows the use of an Ocean Optics USB4000 digital spectrometer to obtain a blue-UV spectrum of a clear blue sky at sunset. Such spectrometers are used extensively for atmospheric measurements - especially for the study of gases in volcanic plumes such as sulphur dioxide and bromium monoxide that have absorption bands in this spectral region.
The blue line spectrum here was obtained in Munich during the evening of the 22 August 2014 using an optical fibre pointed towards the NE at an altitude of about 60°. What is plotted is the ratio of the sky spectrum when the Sun was on the horizon to the spectrum obtained earlier with a Solar altitude of 19°. Such a ratio will remove most (but see below) of the Solar (Fraunhofer) spectral lines and leave what we call the 'telluric' spectrum: the extinction caused by the passage of the Sunlight through the Earth's atmosphere.
This telluric spectrum has been modelled (orange line) using the published X-sections (absorption coefficients) of the molecules O3 (ozone), NO2 (nitrogen dioxide) and O4 (the Collision Induced Absorption produced by the O2.O2 'dimer'). There is also a smooth spectral slope produced by the molecular (Rayleigh) scattering by molecules and the Mie scattering produced by aerosols: this has been modelled over this spectral range using a simple power-law in wavelength. Note that the observed (blue) spectrum has been shifted upwards by 0.1 transmission units for clarity of presentation.
The 'rippled' cut-off below 350nm is caused by the Huggins band of ozone - the gas that protects us from hazardous UV radiation. Ozone also has a much weaker absorption , the Chappuis band, that has some effect at the longest wavelengths in this spectrum. The nitrogen dioxide, in this case probably a pollutant produced by engine exhausts around the city, is apparent as the haze of weak absorption bands mostly between about 400 and 500nm. The O4 only has a small effect here at a wavelength of 476nm.
The two most obvious discrepancies between the observation and the model are marked with the pink rectangles on the plot. On the left, the two strong Fraunhofer lines (H & K) produced in the Solar atmosphere by ionized calcium appear in 'emission' in the ratio spectrum. This is due to Raman scattering on the Earths atmosphere, a process called the 'Ring effect' that has been discussed in another of my posts. The one on the right is produced by a weak absorption band of water between 500 and 510nm that is not included in this model: the strong water bands are further to the red.
A simple model of this kind produces estimates of the column densities of molecular absorbers and scatterers that affect the transfer of light through the atmosphere.
Clear sky blue/UV spectrum at sunset
The shows the use of an Ocean Optics USB4000 digital spectrometer to obtain a blue-UV spectrum of a clear blue sky at sunset. Such spectrometers are used extensively for atmospheric measurements - especially for the study of gases in volcanic plumes such as sulphur dioxide and bromium monoxide that have absorption bands in this spectral region.
The blue line spectrum here was obtained in Munich during the evening of the 22 August 2014 using an optical fibre pointed towards the NE at an altitude of about 60°. What is plotted is the ratio of the sky spectrum when the Sun was on the horizon to the spectrum obtained earlier with a Solar altitude of 19°. Such a ratio will remove most (but see below) of the Solar (Fraunhofer) spectral lines and leave what we call the 'telluric' spectrum: the extinction caused by the passage of the Sunlight through the Earth's atmosphere.
This telluric spectrum has been modelled (orange line) using the published X-sections (absorption coefficients) of the molecules O3 (ozone), NO2 (nitrogen dioxide) and O4 (the Collision Induced Absorption produced by the O2.O2 'dimer'). There is also a smooth spectral slope produced by the molecular (Rayleigh) scattering by molecules and the Mie scattering produced by aerosols: this has been modelled over this spectral range using a simple power-law in wavelength. Note that the observed (blue) spectrum has been shifted upwards by 0.1 transmission units for clarity of presentation.
The 'rippled' cut-off below 350nm is caused by the Huggins band of ozone - the gas that protects us from hazardous UV radiation. Ozone also has a much weaker absorption , the Chappuis band, that has some effect at the longest wavelengths in this spectrum. The nitrogen dioxide, in this case probably a pollutant produced by engine exhausts around the city, is apparent as the haze of weak absorption bands mostly between about 400 and 500nm. The O4 only has a small effect here at a wavelength of 476nm.
The two most obvious discrepancies between the observation and the model are marked with the pink rectangles on the plot. On the left, the two strong Fraunhofer lines (H & K) produced in the Solar atmosphere by ionized calcium appear in 'emission' in the ratio spectrum. This is due to Raman scattering on the Earths atmosphere, a process called the 'Ring effect' that has been discussed in another of my posts. The one on the right is produced by a weak absorption band of water between 500 and 510nm that is not included in this model: the strong water bands are further to the red.
A simple model of this kind produces estimates of the column densities of molecular absorbers and scatterers that affect the transfer of light through the atmosphere.