Kinetic Limitations Affect Cloud Condensation Nuclei Activity Measurements Under Low Supersaturation

The commercial continuous-flow streamwise thermal-gradient cloud condensation nuclei (CCN) counter is widely used in aerosol CCN activity measurements which are critical for investigating aerosol-cloud interactions. In CCN measurements, a critical threshold is needed for distinguishing interstitial aerosols and activated droplets, and a default threshold of 0.75 mu m was set in CCN counter. Theoretically, interstitial aerosols could also grow larger than 0.75 mu m in CCN counter at low supersaturations, thus theoretical thresholds derived from the Ko''hler theory were suggested. Here, we report that droplet growth in the CCN counter is kinetically limited and CCN-active droplets does not reach their theoretical diameters under low supersaturations. Neglecting the kinetic growth limitation in CCN identification would count less CCN and underestimate the aerosol hygroscopicity parameter kappa $\kappa $ by up to 50% for supersaturations lower than 0.1%. We recommend that thresholds considering kinetic limitations should be used as new criteria for CCN identification under low supersaturations.