Influence of water-soluble organic carbon on cloud drop number concentration

[1] Studies published to date produce ambiguous results regarding the magnitude and even the sign of the effect of water-soluble organic carbon (WSOC) on cloud drop concentration compared to well-characterized inorganics. We present a systematic investigation of the reasons for these discrepancies by examining the ranges of physicochemical properties of water soluble organics that most influence drop formation. We show that when considered individually, composition parameters such as low solubility, increased molecular weight, and surface tension suppression can lead to significant effects on droplet concentration, compared to the equivalent aerosol size distribution assuming properties of ammonium sulfate solutions. When considered together, these effects tend to counteract one another and produce much smaller changes. In addition, an assessment of the published literature suggests that estimates of composition effects on drop concentration based on equilibrium assumptions can be much larger than similar estimates under nonequilibrium conditions.