Abstract The radiative effect of aerosol on cloud albedo via altering cloud droplet effective radius (re) is a major uncertainty in the Earth’s climate system. Remote sensing studies have reported either negative or positive relationships between re and aerosol number concentration (Na) or other aerosol proxies. However, there are much fewer in situ observational evidences and physical explanation remains elusive for the contrasting Na‐re relationships. Here we quantify the Na‐re relationship by using in situ aircraft measurements, together with a re decomposition method. Our analysis reveals that the cloud‐planetary boundary layer (PBL) coupling plays a pivotal role on the Na‐re relationship. Quantitative re decomposition indicates that the contrasting Na‐re relationships in two cloud‐PBL coupling regimes result from different balances of four distinct aspects. The widely recognized number effect may be outweighed by the joint effects of the remaining three that have been rarely investigated and largely ignored in Na‐re parameterizations.
Observations from the Little Bunker
Last updated: February 24, 2026 10:22 PM
Atmospheric CO₂
429.9 ppm
+17.3
ppm/year
Atmospheric CH₄
1946.47 ppb
+6.49
ppb/year
Global Temperature
+1.48 °C
Peak: +1.73°C
World Population
8.1 billion
+67
million/year