Supersaturation and critical size of cloud condensation nuclei in marine stratus clouds

The critical size of aerosols is the size where they can serve as cloud condensation nuclei and thus get activated into cloud drops. This size depends on the supersaturation of water in a given air mass. The lower the critical size is, the more the cloud formation process is sensitive to changes in the formation of secondary aerosol since they experience large losses as they grow in size. Using observations of marine stratus clouds outside the coast of California, a functional relationship between supersaturation and the number concentration of cloud condensation nuclei has been found (Hudson et al., GRL 37(21), 2010 and Hudson and Noble, J. Atmos. Sci. 71(1), 2014). In this work, we use liquid drop density estimates based on satellite data of optical thickness and liquid water path from the MODIS instrument combined with the above relation to estimate a global map of supersaturation. Applying Köhler's theory to this map, a corresponding map of critical sizes has been made. We find that supersaturation is generally in the range of 0.5-1.0%, which results in a critical size in the range of 25-30 nm, which is lower than the generally used size of about 60 nm. The high supersaturations are supported by observations from the ATom measurement campaign (Brock et al, doi: 10.3334/ORNLDAAC/2111, 2022), at different supersaturations. Furthermore, the “pyrcel” cloud parcel model (Rothenberg and Want, J. Atmos. Sci. 73(3), 2016) is used to investigate the observational relationship between supersaturation and cloud droplets/cloud condensation nuclei. The results hint that cloud formation may be more sensitive to changes in secondary aerosols than previously thought.