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Robert I. Sax and James G. Hudson


Measurements of cloud condensation nuclei (CCN) were obtained in the south Florida region as part of NOAA's Florida Area Cumulus Experiment (FACE). During the summer of 1975, CCN measurements were obtained near the bases of cumulus clouds by means of an airborne static diffusion chamber operating at 0.75% supersaturation. Concentrations were highly variable across the peninsula. The change in concentration from low values over the water to very high concentrations over land surfaces was abrupt.

A continuous surface-based CCN monitoring program was carried out during the summer of 1976 with two continuous flow diffusion chambers, operating at 0.4 and 1.0% supersaturation. These confirmed the continental (high CCN) characteristics of the surface aerosol (overall mean 718 cm−3 at 0.49% and 1644 cm−3 at 1.0%). The concentration of CCN was found to vary as a function of wind speed, wind direction, and time of day. Analysis of these data suggested that the concentration of CCN was modulated by short-term localized influences superimposed on the longer term air mass characteristics. Further stratification using low-level trajectories to separate air masses of recent maritime and continental exposures resulted in a mean concentration of 543 and 1291 cm−3 at 0.4 and 1.0% supersaturation, respectively, for the maritime CCN data, and a mean concentration of 1149 and 2049 cm−3 at the respective supersaturations for the continental CCN data.

The picture that emerges is consistent with the view of local production mechanisms (probably photochemical), sometimes superimposed on larger scale advection of aged aerosol.

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