COMPARISON OF DIRECTLY MEASURED CCN WITH CCN MODELED FROM THE NUMBER-SIZE DISTRIBUTION IN THE MARINE BOUNDARY-LAYER DURING ACE-1 AT CAPE GRIM, TASMANIA

Cloud condensation nucleus concentration (CCN) was measured directly a t a supersaturation of 0.5% with a thermal gradient diffusion cloud ch amber at Cape Grim, Tasmania, during the First Aerosol Characterizatio n Experiment (ACE 1) field study in November and December of 1995. Num ber-size distributions N(Dp) from 3 to 800 nm diameter and the hygrosc opic properties of the aerosol in the 30 to 300 nm diameter range (whi ch. contains most of the CCN active at 0.5%) were measured concurrentl y at the same location. This data set provides a basis to compare meas ured and modeled CCN concentrations. A critical particle diameter that would form cloud droplets:at 0.5% supersaturation was derived from th e hygroscopic growth data including consideration of the hydration of the size distribution measurement. This empirically derived diameter i ncorporates the effects of soluble and insoluble mass as well as an ef fective van't Hoff factor and surface tension as described by Kohler t heory for heterogeneous nucleation of cloud droplets. The size distrib utions were integrated for diameters greater than the critical value a nd compared to the directly measured CCN concentrations. The modeled C CN concentration was 95 cm(-3) during baseline sector periods and 128 cm(-3) overall. This was about 20% greater than the directly measured CCN concentration and well correlated (R-2 = 0.7) With measured CCN. T wo thirds of the CCN at 0.5% supersaturation derive from an accumulati on mode (80 nm < Dp < 200 nm); with one sixth from each of the other t wo modes, a larger mode (DP > 200 nm) and the Aitken mode (Dp < 80 Mn) . The data include air masses from continental, anthropogenic, and bio mass burning sources as well as the more dominant marine source.