[CF-metadata] aerosol water

[Jonathan Gregory]

Dear Christiane

> >>mass_fraction_of_water_in_ambient_aerosol_in_air
> >isn't an aerosol by definition in air?
> 
> Aerosol particles can be within hydrometeors (cloud, precip, ice...), so 
> I would like to keep the in_air extension.

OK, I see there could be a distinction between in_air and in_cloudy_air.

Do you need the "ambient"? You draw a distinction with "dry aerosol", but you
wouldn't find "dry aerosol" in air. Doesn't "dry aerosol" mean you have
collected and dried it, and that the water mass fraction is zero?

If aerosol particles are in cloud water or in precipitation, are they still
"ambient" aerosol? Do they still have a meaningful mass fraction of water?

Thanks for continuing the discussion. Cheers

Jonathan