Chemical Heterogeneity across Cloud Droplet Size Spectra in Continental and Marine Air Masses

Abstract

Variations in the chemical composition of cloud droplets of different sizes are predicted in models. Measurements made in natural clouds to verify this prediction are extremely limited, however. During the spring of 1995 and the summers of 1995 and 1996, a size-fractionating version of the California Institute of Technology active-strand cloud water collector was operated on a mountaintop platform in Mount Mitchell State Park, North Carolina (35°44?05"N, 82°17?15"W), to examine differences in drop chemistry between large and small cloud droplets. The size-fractionated measurements also were compared with the chemical composition collected from a passive string?type collector that collects bulk samples. Back-trajectory analysis was used to categorize the source of cloud-forming air masses that arrived at the site as polluted continental, continental, and marine. The differences in cloud drop acidity and chemical constituents were investigated for these different air masses. On average, smaller drops were more enriched in SO42?, NO3?, NH4+, and H+; larger droplets had higher values of Na+, Ca2+, and Mg2+. Samples were collected for which the reverse was true, however. In this study, cloud droplet chemical inhomogeneity between droplet sizes and the effect of airmass origin on variations in the chemical composition were examined. Smaller droplets were found to be more acidic than were larger droplets for both marine and polluted continental air masses. The sodium content was the highest in the larger drops for marine events. The sulfate content in both the larger and smaller droplets was the highest for air masses that were from the polluted continental sector. Slightly higher solute concentrations for the larger droplet size range were found for events caused by orographic lifting; for cloud events influenced by frontal activity, higher solute concentrations were found for the smaller drop size range.