AbstractIn situ campaigns focused on aerosol–cloud interactions were performed to describe the size-dependent activation of aerosols of various origins during variable meteorological conditions. Low cloud episodes, coded as fog, freezing fog, or rain with fog, were compared with nonphenomenon episodes. From the difference in aerosols measured behind the whole air inlet and PM_2.5 inlet, the activated fraction (AF; a share of activated particles from all those available) was calculated. For fog, the AF was stable, resulting in a small variability in the activated size. During freezing fog, a higher variability in supersaturation was deduced from larger variability in the AF and smaller effective radii of cloud droplets. The AF during rain with fog showed a connection to the air mass origin, less effective activation, and smaller cloud droplets. The analysis of the relationship between meteorological conditions and activations suggested that the different hydrometeors were connected with different air masses. No effect of photochemistry was found; in contrast, some dependence on relative humidity, temperature, wind speed, and liquid water content (LWC) was described. With increasing humidity, smaller particles were able to activate. For lower RH, the importance of supersaturation fluctuations increased, moving to a fluctuation-influenced regime. The strongest connection was found between activation and LWC; for the LWC below 0.10 g m⁻³, a strong decrease in activated particle size was found with increasing LWC, due to turbulence, number of particles, and availability of condensable water. From 0.10 g m⁻³ LWC and higher, the LWC and the connected supersaturation could be the main factors influencing the activation.