Aerosol size distribution variability over the Southern Ocean: implications for cloud droplet number concentrations
Authors: Schmale Julia , Nenes Athanasios , Thurnherr Iris , Henning Silvia , Tatzelt Christian , Baccarini Andrea , Gysel-Beer Martin 
Published: March 2024

The Southern Ocean is a key component of the climate system, where clouds especially matter. Therefore, it is important to correctly simulate clouds in climate models. Even though there has been substantial improvement, climate models still struggle in their representation of cloud microphysical properties.In this study, based on data from the Antarctic Circumnavigation Expedition in 2026/17, we explore environmental factors, such as stable water isotopes in atmospheric water vapor, cyclones and boundary layer stability, that influence the abundance of aerosols and their size distribution, the most important variables for particles to act as cloud condensation nuclei (CCN), along a latitudinal gradient from 35&#176;S to 75&#176;S. Moreover, we use a cloud parcel model to estimate the cloud droplet number concentration and cloud maximum supersaturation (SS) based on the particles&#8217; size distribution, hygroscopicity and measured updraft velocities.Based on the latitudinal gradient of observed CCN, which features a distinct minimum around 60&#176;S, and the carbon monoxide mixing ratios, which reach background levels south of 60&#176;S indicating absence of anthropogenic influence, we compare aerosol properties north and south of this latitude. The northern aerosol population features two distinct Aitken modes, a nucleation mode and a mode with a Hoppel minimum around 60 nm. The presence of cyclones reduces the particle number concentrations over all diameters. We also observe a stronger Aitken mode presence in unstable boundary layer conditions, where downward mixing of freshly formed particles in the outflow of clouds in the free troposphere can occur. The southern population features only three modes, a nucleation mode and two distinct bimodal distributions with Hoppel minima around 70 nm. Only in stable boundary layer conditions an Aitken mode emerges in the 75th percentile that is larger in particle number than the accumulation mode, pointing towards a potential source of condensable vapors from the ocean surface that grow the Aitken mode, leading to observably higher kappa values. The Aitken mode is further associated with air masses with relatively less depletion in d18O, pointing towards a marine source further north.The cloud droplet number concentration simulations feature the same latitudinal pattern as the measured CCN with the &#8220;dip&#8221; around 60&#176;S. This is consistent with droplet observations from satellites. Interestingly, the simulated cloud maximum SS tends to increase with latitude, from roughly 0.27% at 40&#176;S towards 0.43% at 75&#176;S. To estimate the sensitivity of clouds towards available aerosol particles, we form the ratio of the particle number concentration larger than the observed Hoppel minimum over the simulated cloud droplet number concentrations. We find that clouds north and south of 60&#176;S experience elevated sensitivity (ratio < 1) to aerosol concentrations in 23 % and 27 % of the time, respectively. This demonstrates that the Southern Ocean cloud regime is indeed sensitive to aerosol number and size distributions, which in turn are influenced by synoptic features (e.g., cyclones) and marine boundary layer stability. On the other hand, frequent occurrence of low SS, demonstrates that cloud formation is also often updraft limited.