[CF-metadata] atmosphere_optical_thickness at 550 nm

[Jonathan Gregory]

Dear Heinke

> atmosphere_optical_thickness_due_to_black_carbon_ambient_aerosol
> 
> This is one parameter in the Hammoz model but at 550 nm. Where is the
> right place
> for 550 nm?

I would suggest a scalar coordinate variable of radiation_wavelength.

Cheers

Jonathan