Experimental and theoretical studies of cloud condensation nuclei

Abstract

Cloud condensation nuclei (CCN), the subset of atmospheric aerosol that nucleate cloud droplet formation, are a key component in cloud formation, and are an important factor in controlling climatically-relevant cloud properties such as cloud albedo, cloud lifetime, and precipitation rate. A CCN instrument that satisfies the constraints for small aircraft operation — minimum weight, volume, and power consumption, good robustness, and high frequency measurement — was constructed. The measurement technique was based on that of Hudson (1989) because it reportedly offered the ability to make measurements of CCN at all supersaturations simultaneously at high frequency and with good counting statistics. Modelling studies, and laboratory and field measurements, subsequently showed that this technique exhibits poor sensitivity. The CCN instrument was also studied in fixed supersaturation mode, where it is able to accurately measure CCN concentration at a fixed supersaturation, whose value ranges from 0.1 and 2%. The CCN instrument was flown during the 2nd Aerosol Characterization Experiment (ACE-2). The data were reported at a fixed supersaturation of 0.1%. Intercomparison of these measurements with those on two other aircraft shows good agreement. A sublinear relationship between measured CCN concentration and that predicted from aerosol size distribution and chemical composition measurements, Nmeas ~ N0.72pred. In-situ measurements of below-cloud CCN concentration and cloud droplet number concentration are compared. The results are in agreement with model predictions and with previous studies. Cloud droplet concentration is predicted to depend on the CCN spectrum and updraft velocity. Cloud droplet activation has often been assumed to be reasonably described by an equilibrium model. The error in calculated cloud droplet number concentration due to the influence of condensational growth kinetics was shown to be significant for some conditions. Such errors are estimated to lead to overestimates of indirect radiative climate forcing on the order of Wm-2. Accurate interpretation of measured CCN concentration may require consideration of activation kinetics associated with CCN instruments.