Particles in the Lower Troposphere over the High Plains of the United States. Part II: Cloud Condensation Nuclei

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Abstract
Analyses are presented of 270 cloud condensation nucleus (CCN) spectra measured in the mixing layer over the High Plains of the United States during the springs and summers of 1975 and 1976. Frequency distributions of the concentrations (N) of CCN at supersaturations (S) of 0.2, 0.5, 1.0 and 1.5% appear to be bimodal, with approximately lognormal modal distributions. At each supersaturation, the mode corresponding to the lower CCN concentrations is tentatively attributed to a “background” source of CCN (possibly gas-to-particle conversion), and the mode corresponding to the higher CCN concentrations is attributed to additional natural and/or anthropogenic sources of CCN. Composite CCN spectra were generated from permissible combinations of the two modal distributions at the various supersaturations, and were fitted to the expression, N = N0(S/S0)k where S0 = 1%. Three representative types of CCN spectra were identified by this process: maritime (N0 = 290 cm−3, k = 0.7); transitional (N0 = 1500 cm−3, k = 2.8); and aged continental (N0 = 2200 cm−3, k = 0.9). The CCN concentrations generally varied with airmass age on a timescale of several days. However, recent involvement in cloud processes was associated with marked decreases in concentrations and changes in spectral type. When the higher concentration mode dominated, the CCN concentrations did not vary greatly in the horizontal or with altitude. When the lower concentration mode dominated, CCN concentrations generally decreased with increasing altitude and showed greater horizontal fluctuations.

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