[CF-metadata] aerosol standard names (mass and optical depth)

[Jonathan Gregory]

Dear Christiane

> pm10_ambient_aerosol_optical_depth
> 
> Is it a good idea to define the mass for pm10 at a RH given as 
> coordinate variable, and the optical depth at ambient RH?

If that is what you use in practice (mass of pm10 aerosol at a specified
or default RH, but optical depth at ambient RH) then that is what should
be used. It can be described appropriately in the definition of the std names.

Cheers

Jonathan