Aerosol Swelling: A Major Source of Error When Estimating the Aerosol First Indirect Effect

The aerosol first indirect effect (AIE) measures the sensitivity/change of cloud droplet size with cloud droplet number concentration (Nd) or cloud condensation nuclei (CCN), while the latter is often linked with aerosol proxy variables such as the more readily measured aerosol optical depth (AOD). Due to the aerosol swelling effect, CCN and AOD may not co-vary proportionally as one is sensitive to the cross-section of particles where the other is to the number concentration. As such, the AIEs derived from the two proxies may not be compatible. The inconsistency is thus investigated in this study using the extensive measurements of aerosol and cloud properties made at four Atmospheric Radiation Measurement sites around the world. We try to identify and quantify the influence of aerosol hygroscopicity and the swelling effect on the aerosol first indirect effect (FIE). These sites have distinct aerosol properties and experience different meteorological conditions. The magnitude of the FIE for aerosol particles with stronger aerosol hygroscopicity is systematically larger than that for aerosol particles with weaker aerosol hygroscopicity. A one-unit enhancement in the aerosol scattering coefficient by the swelling effect leads to a systematic underestimation of the FIE by about 23%. This can result in a significant underestimation of the FIE-related radiative forcing (by several W m^-2 depending on aerosol properties and relative humidity). This likely contributes significantly to the systematic difference between observation-based, especially satellite-based, estimates of the FIE and those simulated by general circulation models. It may also partially explain systematic variations in the FIE with water vapor amount as noted by others.