Cloud chemistry varies with drop size

Abstract

Samples of fog and cloud drops were collected by drop size from several locations around the United States. Sites were selected to represent a variety of cloud types (coastal stratus clouds, frontal clouds, and radiation fog) as well as both clean and polluted environments. A size‐fractionating version of the Caltech Active Strand Cloudwater Collector (sf‐CASCC) and a two‐stage Eidgenössische Technische Hochschule (ETH) cloud impactor were each used to simultaneously collect small and large cloud drop fractions. The sf‐CASCC is designed to collect drops >23 μm in diameter in the large fraction and drops between 4 and 23 μm in the small fraction, while the ETH impactor is typically configured to collect drops >10 μm in diameter and drops between 3 and 10 μm in diameter in its two size fractions. Concentrations of sulfate and ammonium were typically observed to be enriched in small cloud drops. A similar pattern was observed for nitrate at several locations. Coarse mode aerosol species were often observed to be enriched in large cloud drops. These findings suggest that the size‐dependent chemical signature of the precursor aerosol particle size distribution is at least partially maintained in the cloud drop size spectrum. The observed size dependence of the cloud drop composition can enhance rates of aqueous phase sulfate production relative to expectations based on average cloud drop composition, bias efficiencies with which individual chemical species are incorporated into precipitation within clouds, and affect patterns of chemical deposition to plant canopies or within the respiratory tract.