Cloud condensation nuclei concentrations in Southern Arizona: Instrumentation and early observations

During an ongoing study to measure cloud condensation nuclei (CCN) concentrations, a new automated thermal diffusion CCN counter was operated during the months January through June 1994 near the summit of Mt. Lemmon in the Catalina Mountains at about 2700 m elevation. The instrument records data continuously at 5-minute intervals 24 hours a day. The status of the instrument and the data are monitored remotely by telephone modem. For the 123-day, continuous study the average CCN concentration was found to range between a few tens and a few hundred nuclei per cm³ at supersaturations between 0.3 and 0.7%. Diurnal variations were observed, as well as other possible influences, such as wind speed, wind direction and mixing depth. At a constant supersaturation of 0.35% the CCN concentration is typically on the order of 50 to 150 nuclei per cm³. A description of the instrumentation and early observations are presented and discussed.