Drop size-dependent chemical composition in clouds and fogs

Cloud drop composition can be size-dependent. Size-dependent drop composition is important because it impacts the fate of some atmospheric species. Two new cloud water collectors were developed to provide improved resolution of drop size-dependent composition. Both collectors are active, multi-stage, rectangular jet cascade impactors designed to separate drops into three or more fractions across the expected drop size spectrum. The FROSTY collector (FROSTY) is a three stage impactor for super-cooled drops in mixed-phase clouds. The Colorado State University 5-Stage collector (CSU 5-Stage) is a five stage impactor for use in warm clouds and fogs. While field data are limited, the new collectors' performance was evaluated against existing cloud water collectors and other equipment. The chemical composition comparison between FROSTY and the Caltech Heated Rod Cloud water Collector was consistent, although the sampled mass evaluations suggest neither collector may consistently sample the ambient drop population. Field performance evaluations of the CSU 5-Stage, size-fractionating Caltech Active Strand Cloud water Collector (sf-CASCC), and Caltech Active Strand Cloud water Collector #2 suggest they all work reasonably as intended, although some open questions remain regarding the overall sampling efficiency of the Caltech collectors, particularly in low liquid water content (<0.1 g m-3) radiation fogs. Both new collectors' results suggest that there is more variation in size-dependent drop composition than previously measureable with our existing equipment. As the CSU 5-Stage can consistently separate the very largest drops from the very smallest, it can discern size-dependent drop variation in the principal inorganic species---sulfate, nitrate, and ammonium---not otherwise observed in orographic clouds sampled using the sf-CASCC at Whiteface Mountain, NY (July 1998). The CSU 5-Stage data also indicate additional variation in radiation fogs than observed using the sf-CASCC (Davis, CA (January 1999)). The CSU 5-Stage data are useful for interpreting other observations, such as the in-fog removal rate of species via deposition. Data from the new cloud water collectors can be used for model validation and should provide improved insights into both chemical and physical processes in-cloud.