1 Cloud Activating Properties of Aerosol Observed during CELTIC

2 Measurements of aerosol size distribution, chemical composition and cloud condensation nuclei (CCN) concentration were performed during the Chemical Emission, Loss, Transformation and Interactions with Canopies (CELTIC) field program at Duke Forest in North Carolina. A kinetic model of the cloud activation of ambient aerosol in the chamber of the CCN instrument was used to perform an aerosol-CCN closure study. This study advances prior investigations by employing a novel fitting algorithm which was used to integrate scanning mobility particle sizer (SMPS) measurements of aerosol number size distribution and aerosol mass spectrometer (AMS) measurements of the mass size distribution for sulfate, nitrate, ammonium and organics into a single, coherent description of the ambient aerosol in the size range critical to aerosol activation (around 100 nm diameter). Three lognormal aerosol size modes each with a unique internally mixed composition were used as input into the kinetic model. For the two smaller size modes, which control CCN number concentration, organic aerosol mass fractions for the defined cases were between 58 % and 77%. This study is also unique in that the water vapor accommodatio