[CF-metadata] aerosol water

[Jonathan Gregory]

Dear Christiane

> mass_fraction_of_water_in_ambient_aerosol_in_air
> mass_fraction_of_ambient_aerosol_water_in_air
> 
> I would prefer the first because all other aerosol components are called 
> <dry/ambient>_aerosol_in_air.

The first certainly makes sense to me. I wonder whether _in_air is really need
for aerosol species, though, because isn't an aerosol by definition in air?

Best wishes

Jonathan