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

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  • a Earth and Atmospheric Sciences Department, The City College, New York, New York
  • | b Atmospheric Sciences Center, Desert Research institute, Reno, Nevada
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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 (D bar = 6.4 µm), and largest ion concentrations (e.g., SO4 4 = 5.7 mg L−1), while clouds with the largest pH values (5.1) had the smallest N values (189 cm−3), largest D bar values (8.0 µm), and smallest ion concentrations (SO4 4 = 3.9 mg L−1). Nevertheless, all three populations had similar liquid water contents (LWC ≅ 0.070 g m−3). The equation LWC = π/66Dbar 3 Nρ where ρ is the density of water, closely describes the relationship between LWC, D bar 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, D bar and pH values were found in the ten-winter record.

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 (D bar = 6.4 µm), and largest ion concentrations (e.g., SO4 4 = 5.7 mg L−1), while clouds with the largest pH values (5.1) had the smallest N values (189 cm−3), largest D bar values (8.0 µm), and smallest ion concentrations (SO4 4 = 3.9 mg L−1). Nevertheless, all three populations had similar liquid water contents (LWC ≅ 0.070 g m−3). The equation LWC = π/66Dbar 3 Nρ where ρ is the density of water, closely describes the relationship between LWC, D bar 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, D bar and pH values were found in the ten-winter record.

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