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 ohler equation when calculating critical supersaturation (S c) for real atmospheric samples. Water activity is calculated from osmolality and thus the original ohler equa-tion can be applied avoiding the difficulties with unknown molecular weights, solubili-5 ties, 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 litera-ture values both for electrolytes and a non-electrolyte. Then the applicability of the approach was demonstrated by generating ohler curves from osmolality derived and literature activity data as well as by using the simplified ohler equation. S c values 10 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, CaCl 2 , NaCl and H 2 SO 4 , respectively, while the corresponding errors with the simplified ohler 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 con-15 siderably 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.