Contrasting aerosol mixing states at inland and coastal sites: an entropy-based metric for CCN activity

Simplified assumptions of aerosol mixing states in modeling often introduce substantial uncertainties in estimating cloud condensation nuclei (CCN) concentrations (N_CCN) and their climatic impacts. This study systematically investigates the contrasting relationships between mixing states and CCN activity by combining field measurements of hygroscopicity with the algorithm of entropy at two inland and coastal sites. We show distinct seasonal variations of 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 and inland air. However, measurements in summer showed pronounced differences: aerosols in the coastal atmosphere exhibited a higher degree of internal mixing (χ = 0.69 ± 0.19), whereas inland χ values only increased moderately to 0.47 ± 0.12. Aerosol mixing state is largely influenced by primary emissions and secondary formation process. Externally-mixed particles originate chiefly from anthropogenic emissions in inland or sea salt in coastal. With the aging process, particles become more internally-mixed as the enhanced fraction of more-hygroscopic mode. Both environments show the negative correlations between the critical diameter (D_cri) and χ but with distinct decrement rates for coastal vs. inland aerosols. Specially, D_cri exhibits heightened sensitivity to fluctuations in χ when χ<0.5. This offers a practical approach to estimate D_cri from χ when the particles are not highly aged. Further analysis reveals that N_CCN exhibits heightened sensitivity to fluctuations in χ at low values. These results underscore that mixing states exert different control over N_CCN in diverse environments and provide critical constraints for parameterizing fine aerosols CCN activity in models.