Global relevance of marine organic aerosol as ice nucleating particles

Ice nucleating particles (INPs) lower the supersaturation required and/or increase the temperature at which supercooled droplets start to freeze. They are therefore of particular interest in mixed-phase temperature regimes, where supercooled liquid droplets can persist for extended periods of time in the absence of INPs. When INPs are introduced to such an environment, the cloud can quickly glaciate following the Wegener-Bergeron-Findeisen process and possibly precipitate out, altering its radiative properties. Despite their potential influence on climate, the ice nucleation ability and importance of different aerosol species is still not well understood and is a field of active research. In this study we used the aerosol-climate model ECHAM-HAM to examine the global relevance of marine organic aerosols (MOA), which have drawn much interest in recent years as a potentially important INP in remote marine regions. We address the uncertainties in emission and ice nucleation activity of MOA with a range of reasonable set-ups and find a wide range of resulting MOA burdens. The relative importance of MOA as an INP compared to dust is investigated and found to depend strongly on the type of ice nucleation parametrisation scheme chosen. Regardless, MOA was not found to affect the microphysical properties of clouds or the radiative balance significantly, due to its relatively weak ice activity and a low sensitivity of cloud ice properties to heterogeneous ice nucleation in our model.