Supersaturation and Critical Size of Cloud Condensation Nuclei in Marine Stratus Clouds.

Observations of marine stratus clouds in clean air off the Californian coast reveal a functional relationship between the number of cloud condensation nuclei (CCN) and supersaturation. Satellite‐derived liquid droplet density estimates the number density of CCN. Combining the estimated supersaturation using Köhler theory, global maps of supersaturation and the critical activation size of CCN are estimated. Here, we show that high supersaturation >0.5% persists over the oceans with a critical CCN size of 25–30 nm, which is smaller than the conventional wisdom of 60 nm. Independent support for such high supersaturation in the marine cloud is obtained from CCN measurements provided by the "Atmospheric Tomography Mission." Higher supersaturation implies smaller activation size for CCN making cloud formation more sensitive to changes in aerosol nucleation. Plain Language Summary: Clouds in Earth's atmosphere are of fundamental importance for the climate by regulating the reflection of sunlight into space and interacting with thermal radiation from Earth. Clouds form when moist air ascends and gets supersaturated with water vapor that condenses on aerosol particles of sufficient sizes, which then grow into cloud droplets. The aerosol number‐density and size spectrum influence the resulting cloud properties, and the supersaturation determines which aerosols can be activated into cloud drops. Here, we show that the supersaturation in marine liquid clouds is significantly higher than in the conventional view. As a consequence, much smaller aerosols can serve as cloud condensation nuclei. This can make cloud formation more sensitive to changes in aerosol properties than previously thought. Such a result should be of general interest and lead to a better understanding of aerosol‐cloud interactions, which presently constitute the largest uncertainty in our understanding of climate. Key Points: On average, supersaturation in marine clouds is significantly higher than the conventional view of 0.2%–0.3%Due to the higher supersaturation, much smaller aerosols get activated into cloud dropletsThe results are essential for better understanding aerosols‐cloud interactions [ABSTRACT FROM AUTHOR]