What Causes Distinct Absorption Bands in the Sun's Spectrum During Sunset?

[sophiecentaur]

I recently bought a cheap and cheerful hand held spectroscope. Great fun to look at CFL lamps and the modern LEDs. Those LEDs are really good, btw. Also, the 'scope gives a pretty even spread of brightness over the whole visible range. (an equipment check).
I have been looking at the spectrum of the setting Sun on a regular basis. I see distinct absorption bands in the yellow / orange region. Blue is a bit attenuated, perhaps and red is continuous again. I would like to know what the bands are due to. I am looking at the Sun, low over the London conurbation and I suspect it could be something to do with that.
Any ideas?
I must remember to take the scope with me next time I can see a sunset over a clean horizon. Does that justify a short break in Cornwall??

 

[Drakkith]

I see distinct absorption bands in the yellow / orange region.

How many bands can you see? I assume they aren't the sodium lines from the Sun itself?

 

[sophiecentaur]

How many bands can you see? I assume they aren't the sodium lines from the Sun itself?

A question worth asking but the clean solar spectrum is line-free in my scope. I would doubt that the scope resolution would be good enough to spot an absorption line, in any case.
I have been looking at absorption bands from common pollutants and there don't seem to be any obvious candidates. There is much more info on IR absorption, which is what most of the google hits contain.
My observations are very qualitative but I would say that there is a region (say 550 nm to 580nm [Edit Correction- to a bit over 600nm]) where there seem to be two or three ragged bands of dark and the whole of the spectrum there is much dimmer. My eyes and the scope behave themselves fine for the rest of the day. (just been outside and checked again).

 

[Drakkith]

A question worth asking but the clean solar spectrum is line-free in my scope. I would doubt that the scope resolution would be good enough to spot an absorption line, in any case.

To be safe, I'd give your scope a good shake or two to make sure it isn't lying to you. Let it know you mean business and all that.

 

[sophiecentaur]

It would need to be far too intelligent to know just when to play up and when not to! I reckon what I am seeing is genuine but it doesn't seem to fit anything I have read about so far. Perhaps there is a cloud of BS over London?

 

[jim mcnamara]

Brown spectrum( from an M&M coating) matches your description. I think you are definitely on to something. Try your spectroscope over Parliament ! If it matches bilirubin (brown pigment in feces) spectrum you have proof positive.

http://www.trinity.edu/fwalmsle/MandMPictures/BrownSolutionSpectrum.bmp