The influence of aerosols on cloud properties is an important modulator of the climate system. Traditional Köhler theory predicts the equilibrium concentration of cloud condensation nuclei (CCN); however, it is not known to what extent particles exist in the atmosphere that may be prevented from acting as CCN by kinetic limitations. We measured the rate of cloud droplet formation on atmospheric particles sampled at four sites across the United States during the summer of 2006: Great Smoky Mountain National Park, TN; Bondville, IL; Houston, TX; and the Atmospheric Radiation Measurement Program Southern Great Plains site near Lamont, OK. We express droplet growth rates with the mass accommodation coefficient (α), and report values of a measured in the field normalized to the mean a measured for lab-generated ammonium sulfate (AS) particles (i.e., α′=α/αAS)- Overall, 59% of ambient CCN grew at a rate similar to AS. We report the fraction of CCN that were "low-α′" (α′<10-1, corresponding to α<1.5×10-2). Of the 16 days during which these measurements were made, 8 had relatively few low-α′ CCN (<16%), 6 had moderate low-α′ fractions (27% to 59%), and 2 had large low-α′ fractions (> 82% during at least one ∼30 min period). Day to day variability was greatest in Tennessee and Illinois, and low-α′ particles were most prevalent on days when back trajectories suggested that air was arriving from aloft. The highest fractions of low-α′ CCN in Houston and Illinois occurred around local noon, and decreased later in the day. These results suggest that for some air masses, accurate quantification of CCN concentrations may need to account for kinetic limitations.