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Abstract Cloud droplet number concentration (N_d) is a key microphysical property that is largely controlled by the balance between sources and sinks of aerosols that serve as cloud condensation nuclei (CCN). Despite being a key sink of CCN, the impact of coalescence scavenging on Southern Ocean (SO) cloud is poorly known. We apply a simple source‐and‐sink budget model based on parameterizations to austral summer aircraft observations to test model behavior and examine the relative influence of processes that determineN_din SO stratocumulus clouds. The model predictsN_dwith little bias and a correlation coefficient of ∼0.7 compared with observations. Coalescence scavenging is found to be an important sink of CCN in both liquid and mixed‐phase precipitating stratocumulus and reduces the predictedN_dby as much as 90% depending on the precipitation rate. The free tropospheric aerosol source controlsN_dmore strongly than the surface aerosol source during austral summer.