A Ten-Winter Record of Cloud-Droplet Physical and Chemical Properties at a Mountaintop Site in Colorado

Abstract

Cloud-droplet spectra and coincident cloud water pH measurements have been made for a portion of ten consecutive winters (1983/84?1992/93) from clouds that enveloped Storm Peak Laboratory in northwestern Colorado; cloud water ion measurements were made for eight of the winters. To determine if the physical and chemical properties are related, the data were stratified into three populations: pH ≤3.6, 3.6 < pH < 4.6, pH ≥4.6. It was found that clouds with the smallest pH values (3.4) had the largest droplet concentrations (N 329 cm?3), smallest mean droplet diameters (Dbar = 6.4 µm), and largest ion concentrations (e.g., SO44 = 5.7 mg L?1), while clouds with the largest pH values (5.1) had the smallest N values (189 cm?3), largest Dbar values (8.0 µm), and smallest ion concentrations (SO44 = 3.9 mg L?1). Nevertheless, all three populations had similar liquid water contents (LWC ? 0.070 g m?3). The equation LWC = π/66Dbar3N? where ? is the density of water, closely describes the relationship between LWC, Dbar and N. The range in pH values could not be completely explained by entrainment, or variations in cloud-base height or in LWC; differences in cloud condensation nucleus composition appear to be a major factor. No significant trends in average winter N, Dbar and pH values were found in the ten-winter record.