The influence of aerosols on cloud properties remains one of the largest sources of uncertainty in estimates of the anthropogenic component of climate change. Here we report the rate of cloud droplet formation on particles sampled at a site near the California coast that is typically above the marine boundary layer. We observed persistent bimodal diameter spectra which are better explained by kinetic limitations than by differences in equilibrium properties. The slowly-growing mode contained 10-25% of the total cloud condensation nuclei (CCN) and had apparent mass accommodation coefficients (a) 10-30 times smaller than mat measured for ammonium sulfate. Cloud parcel modeling suggests that most of these slowly-growing CCN will not form cloud droplets. The relatively small and narrow size distribution of the low-a droplets suggest that a condensed film is a more likely cause of these limitations than slow dissolution. Copyright 2009 by the American Geophysical Union.