Application of osmolality for the determination of water activity and the modelling of cloud formation

Abstract. A simple approach is suggested here to give reliable estimates on the Raoult term of the Köhler equation when calculating critical supersaturation (S_c) for real atmospheric samples. Water activity is calculated from osmolality and thus the original Köhler equation can be applied avoiding the difficulties with unknown molecular weights, solubilities, van't Hoff factors of aerosol constituents and also the interactions in the growing droplet. First, water activity calculated from osmolality data was compared to literature values both for electrolytes and a non-electrolyte. Then the applicability of the approach was demonstrated by generating Köhler curves from osmolality derived and literature activity data as well as by using the simplified Köhler equation. S_c values calculated with the osmolality approach fitted those obtained by using literature water activity data within a relative deviation of less than 0.3%, 0.8%, 1.1% and 3.4% for sucrose, CaCl₂, NaCl and H₂SO₄, respectively, while the corresponding errors with the simplified Köhler equation were 11%, 8.5%, 4.5% and 19% in the dry nucleus size range of 20 nm to 100 nm. Finally, the osmolality method was used to show how considerably S_c is underestimated for organic acids if complete dissociation is assumed.

The method described in this paper can be applied to real atmospheric samples (aerosol extracts, fog water or cloud water) thus improving the reliability of estimates on critical supersaturation and critical droplet diameter in atmospheric modelling.