Bulk-Interface Partitioning Explains the Enrichment of Organic Compounds in Cloudwater

Abstract

Cloud droplets are known to effectively chemically process water-soluble organic compounds. Field measurements clearly show that concentrations of organic compounds measured in cloudwater can deviate significantly from predictions made with Henry’s law, with high enrichments measured for less water-soluble organic compounds. Several processes are suspected to be the cause of the observed enrichments, but the key process has not yet been elucidated. Here, we use the bulk-interface partitioning approach to predict enrichment coefficients (q) of organic compounds in cloud droplets. A predictive equation is derived as a function of the bulk-interface partition coefficients (K_p) and octanol–water partition coefficients (K_ow). The calculated enrichments are compared to measured q values from different field campaigns. The results show that the predicted values follow the same trend and absolute values as the measurements. Highly water-soluble compounds have small enrichments, with values around 1, while less soluble compounds have very high enrichments of up to >10³. A sensitivity study is performed for the range of K_ow values obtained from different models, and for the range of measurements for different measurement conditions. The results of the sensitivity study show that the q measurements and predictions lie within the same range, thus showing that bulk-interface partitioning can be a good predictor for organic enrichments in cloudwater.

This study presents a new approach describing the deviations of organic concentrations in cloudwater from Henry’s law using bulk-interface partitioning. This approach provides a simple but accurate estimation of the enrichment of various organic compounds in cloud droplets.