Lack of Vertical Co-Location Between Aerosols and Clouds Biases the Satellite-Based Quantification of the Twomey Effect

Uncertainties in aerosol–cloud interactions (ACI) remain significant. Recent studies report an anti-Twomey phenomenon, where cloud droplet effective radius (R_e) exhibits a positive correlation with aerosol optical depth (AOD), contrary to the classical Twomey effect. Using long-term satellite and reanalysis data over East Asia, we found a strong positive R_e-AOD correlation. Further analysis reveals that AOD, as a column-integrated quantity, lacks aerosol vertical information and cannot fully represent cloud condensation nuclei (CCN), potentially misleading the interpretation of the Twomey effect. Consequently, we analyzed aerosol vertical profiles and introduced a metric, Elevated Aerosol Ratio (EA_ratio), defined as the ratio of aerosol concentration integrals above and below clouds. Higher EA_ratio values correlated with a stronger anti-Twomey phenomenon, while lower values aligned with the classical Twomey effect. Notably, the anti-Twomey phenomenon primarily appeared under low liquid water path (LWP) conditions: as LWP increased, both the anti-Twomey phenomenon and the influence of EA_ratio diminished. Moreover, the marine anti-Twomey phenomenon is linked to pollution. Elevated aerosol layers in heavily polluted nearshore areas—driven by anthropogenic emissions and atmospheric dispersion—distort the R_e-AOD relationship. Additionally, while coarse aerosols suppress marine anti-Twomey phenomenon, the EA_ratio-based analysis remains valid when excluding their influence as much as possible. Our findings demonstrate that the observed anti-Twomey phenomenon is primarily a statistical artifact arising from the lack of vertical co-location between aerosols and clouds. This mismatch biases the satellite-based quantification of the Twomey effect when column-integrated AOD is used as a proxy for CCN.