Contrasting Inland-Coastal Aerosol Mixing States: An Entropy-Based Metric for CCN Activity

Simplified assumptions of aerosol hygroscopic mixing states in modeling studies often introduce substantial uncertainties in estimating cloud condensation nuclei (CCN) concentrations and their climatic impacts. This study systematically investigates the contrasting relationships between mixing states and CCN activity by comparing ambient measurements from inland and coastal sites. We show distinct seasonal variations of the particles mixing state. In winter, externally mixed particles dominated both sites, with comparable mixing state indices (χ) of 0.38±0.12 and 0.39±0.09 respectively for coastal air masses and inland air. However, summer measurements showed pronounced differences: photochemical processes promoted significantly higher internal mixing in coastal aerosols (χ=0.69±0.19), whereas inland χ values only increased moderately to 0.47±0.12.​ A universal logarithmic correlation was identified between the critical diameter (Dcri) characterizing CCN activity and χ (Dcri = -32.15ln(χ)+84.71, Pearson r = -0.74), but with distinct decrement rates for coastal vs. inland aerosols. Our further quantitative analysis reveals a 0.1 increase in χ enhanced winter CCN concentrations (NCCN) by 39–65 % under typical cloud supersaturations, whereas this effect diminished to ~9 % in summer. These results underscore that mixing states exert more pronounced control over NCCN in diverse environments. Our work provides critical constraints for parameterizing fine aerosols CCN activity in climate models, thereby reducing uncertainties in aerosol–climate effect estimations.