Surfactant partitioning in cloud droplet activation: a study of C8, C10, C12 and C14 normal fatty acid sodium salts

Abstract

A B S T R A C T 12 COONa] in the diameter range 33-140 nm at 296 K using a static thermal gradient diffusion cloud condensation nucleus counter. These fatty acid sodium salts are surface active molecules which have all been identified in atmospheric aerosol particles. Experimental critical supersaturations increased systematically with increasing carbon chain length and were in the range 0.96-1.34% for particles with a dry diameter of 40 nm. The experimental data were modelled using Köhler theory modified to account for partitioning of the surface active fatty acid sodium salts between the droplet bulk and surface as well as Köhler theory including surface tension reduction without accounting for surfactant partitioning and Köhler theory using the surface tension of pure water. It was found that Köhler theory using the reduced surface tension with no account for surfactant partitioning underpredicts experimental critical supersaturations significantly, whereas Köhler theory modified to account for surfactant partitioning and Köhler theory using the surface tension of pure water reproduced the experimental data well.

A simple representation of surface active organic aerosol in cloud droplet formation

“…(15) and (16) are exactly the outcome of the detailed model predictions. Notably, the expected consequences for surface active organic CCN properties have been demonstrated experimentally for a number of compounds (Prisle et al, 2008(Prisle et al, , 2010Sorjamaa et al, 2004). For the present purposes of a simplified description, the modest effects of the organic solute on either surface tension or water activity of the activating droplets, as seen in Fig.…”

“…(4) above, for the C8Na-C12Na fatty acid salts and SDS increases in the order of increasing molar mass, M C8Na <M C10Na <M C12Na <M SDS (Prisle et al, 2010(Prisle et al, , 2008, and references therein). The agreement of predictions using rep1 with both experiments and the detailed model increases with the surfactant strength, as expected; the stronger surfactants are relatively more partitioned to the droplet surface, yet the effect on droplet activation of the surface tension reduction attained evidently cannot overcome that of increased water activity due to bulkdepletion of surfactant solute (Prisle et al, 2010).…”

“…2b,3b,4b,and 5b. A number of surface active organics have been demonstrated in previous experimental and theoretical works to act as efficient CCN in single-component particles under atmospherically relevant conditions (Prisle et al, 2008;Sorjamaa et al, 2004). As dry particle surfactant mass fractions increase, critical supersaturations generally increase, and activating droplets become smaller and more concentrated.…”

“…Presence of surface active molecules (surfactants) has been demonstrated in bulk samples of atmospheric cloud and fog waters (Facchini et al, 1999(Facchini et al, , 2000, as well as in aerosol samples (Yassaa et al, 2001;Mochida et al, 2003;Cheng et al, 2004) and aqueous extracts thereof (Oros and Simoneit, 2000;Mochida et al, 2002;Kiss et al, 2005;Dinar et al, 2006;Asa-Awuku et al, 2008) from marine, and rural and urban/polluted continental environments. A number of the identified surface active organic molecules have been observed in laboratory experiments to be CCN active at atmospheric conditions and surface activity can significantly affect the CCN properties of these compounds (Prisle et al, 2010(Prisle et al, , 2008. Comparison to detailed model predictions from thermodynamically consistent relations suggests that surface activity influences cloud droplet activation via both surface partitioning and surface tension effects.…”

“…components (Prisle et al, 2010(Prisle et al, , 2008Sorjamaa et al, 2004). Recent works by Topping (2010) and Raatikainen and Laaksonen (2010) address in particular the first point and present approximate analytical solutions to the partitioning equilibrium that do not require iterative procedures.…”