Size Matters More Than Chemistry for Cloud-Nucleating Ability of Aerosol Particles

Size-resolved cloud condensation nuclei (CCN) spectra measured for various aerosol types at a non-urban site in Germany showed that CCN concentrations are mainly determined by the aerosol number size distribution. Distinct variations of CCN activation with particle chemical composition were observed but played a secondary role. When the temporal variation of chemical effects on CCN activation is neglected, variation in the size distribution alone explains 84 to 96 % of the variation in CCN concentrations. Understanding that particles ’ ability to act as CCN is largely controlled by aerosol size rather than composition greatly facilitates the treatment of aerosol effects on cloud physics in regional and global models. T he response of cloud characteristics and precipitation processes to increas-ing anthropogenic aerosol concentra-tions is one of the largest uncertainties in the current understanding of climate change (1, 2). A major challenge is to determine the ability o