The Role of Relative Humidity in Estimating Cloud Condensation Nuclei Number Concentration Through Aerosol Optical Data: Mechanisms and Parameterization Strategies

Abstract The number concentration of cloud condensation nuclei ( N CCN ) is vital for quantifying aerosol‐cloud interactions. Estimating N CCN using aerosol optical properties is essential for obtaining continuous N CCN data. This study highlights the significant impact of relative humidity (RH) on N CCN estimation through aerosol optical data, especially at low supersaturations ( SS ). When RH exceeds a threshold (e.g., 60% at 0.2% SS ), N CCN estimation shifts from underestimation to overestimation, with the overestimation degree increasing with RH. Including RH in the estimation formula can effectively reduce this bias, although the aerosol optical hygroscopicity parameter is found to have a minimal effect on N CCN estimation. Based on these insights, a new parameterization scheme for N CCN estimation is proposed, which can significantly reduce N CCN estimation bias when using wet aerosol optical data at high RH levels (40%–90%).