The role of surfactants in Köhler theory reconsidered

[摘要] Atmospheric aerosol particles typically consist of inorganic saltsand organic material. The inorganic compounds as well as theirhygroscopic properties are well defined, but the effect of organiccompounds on cloud droplet activation is still poorlycharacterized. The focus of the present study is the organiccompounds that are surface active i.e. tend to concentrate ondroplet surface and decrease the surface tension. Gibbsiansurface thermodynamics was used to find out how partitioningbetween droplet surface and the bulk of the droplet affects the surface tensionand the surfactant bulk concentration in droplets large enough to act as cloudcondensation nuclei. Sodium dodecyl sulfate (SDS) was usedtogether with sodium chloride to investigate the effectof surfactant partitioning on the Raoult effect (soluteeffect).While accounting for the surface to bulk partitioning is known to lead tolowered bulk surfactant concentration and thereby to increased surface tensioncompared to a case in which the partitioning is neglected, the presentresults show that the partitioning also alters the Raoult effect,and that the change is large enough to further increase the criticalsupersaturation and hence decrease cloud droplet activation. The fractionof surfactant partitioned to droplet surface increases with decreasing dropletsize, which suggests that surfactants might enhance the activationof larger particles relatively more thus leading to less denseclouds. Cis-pinonic acid-ammonium sulfate aqueous solutions werestudied in order to study the partitioning with compounds found in theatmosphereand to find out the combined effects ofdissolution and partitioning behavior. The results show thatthe partitioning consideration presented in this paper alters the shape of theKöhler curve when compared to calculations in which thepartitioning is neglected either completely or in the Raoulteffect. In addition, critical supersaturation was measured for SDSparticles with dry radii of 25-60nm using a static parallelplate Cloud Condensation Nucleus Counter. The experimentallydetermined critical supersaturations agree very well withtheoretical calculations taking the surface to bulk partitioningfully into account and are much higher than those calculated neglectingthe partitioning.

[学科分类] 大气科学