Relationship between the aerosol number distribution and the cloud condensation nuclei supersaturation spectrum

Abstract

Though Cloud Condensation Nuclei (CCN) are a subset of atmospheric aerosol, relatively little is known of what links the two. A recently developed instrument called the CCN Remover, which directly relates the CCN supersaturation spectrum to the aerosol number distribution, is described. Instrumental errors are quantified and laboratory tests used to verify the instrument's accuracy are also presented. We made measurements with the CCN Remover in the Aerosol Characterization Experiment 2 (ACE 2) and the INdian Ocean EXperiment (INDOEX). These two multinational field campaigns shared the objective of investigating aerosol particles' ability to modulate cloud albedo by activating as CCN. In both instances, we found that aerosol particles were not activating with the characteristics of pure ammonium sulfate, which is generally regarded as the major component of the majority of aerosol particles which act as CCN. Either a substantial fraction of the aerosol was not participating in the activation process or the presence of a hydrophobic surface film inhibited water vapor transport. Measurements of the aerosol's chemical composition and hygroscopic growth factors are used to examine these possibilities. Anthropogenic activity is modifying the properties of natural aerosol particles in a way which could affect their ability to act as CCN. We discuss evidence for aerosol particles coated with sulfuric acid in an Arctic air mass in support of this claim. In some instances, the connection between aerosol and CCN can be inferred directly from the aerosol number distribution. Clouds segregate aerosol into two populations---those that act as CCN and those that do not, and when the cloud evaporates, the aerosol number distribution bears the signature of the cloud through which it has cycled---a minimum in the aerosol number distribution. The diameter at which this minimum occurs can be related to the maximum supersaturation in the cloud, and the number of particles larger than the minimum is the population of particles that acted as CCN. Over 1,000 bimodal aerosol number distributions from five widely separated locations have been analyzed for maximum supersaturation and cloud droplet (or CCN) et (or CCN) concentrations.