Invited Presentation: Challenges in aerosol-cloud condensation nuclei closure studies

The prediction of the number concentrations of atmospheric cloud condensation nuclei (CCN) from information on aerosol size distributions and chemical composition is a key element in modeling of aerosol indirect effects, in both present day and future climates. The relative sensitivities of CCN predictions to particle size and assumed overall composition, as well as mixing state, have been addressed in a number of studies. Here we discuss the implications of recent findings with respect to the role of size-dependent composition and its relationship to PM1 and PM2.5 composition. A further variable that has not been comprehensively dealt with to date is the role of temperature on CCN activity. Laboratory and field measurements are generally made at relatively warm temperatures that are not representative of most of the troposphere. Cooling of particles as they are vertically transported can result in changes in phase and solubility, and thus in particle water uptake and activation behavior.