Ice nucleation ability of ammonium sulfate aerosol particles internally mixed with secondary organics

The abundance of aerosol particles and their abil-ity to catalyze ice nucleation are key parameters to correctlyunderstand and describe the aerosol indirect effect on the cli-mate. Cirrus clouds strongly influence the Earth’s radiativebudget, but their effect is highly sensitive to their formationmechanism, which is still poorly understood. Sulfate and or-ganics are among the most abundant aerosol components inthe troposphere and have also been found in cirrus ice crys-tal residuals. Most of the studies on ice nucleation at cirruscloud conditions looked at either purely inorganic or purelyorganic particles. However, particles in the atmosphere aremostly found as internal mixtures, the ice nucleation abilityof which is not yet fully characterized.In this study, we investigated the ice nucleation ability ofinternally mixed particles composed of crystalline ammo-nium sulfate (AS) and secondary organic material (SOM)at temperatures between−50 and−65◦C. The SOM wasgenerated from the ozonolysis ofα-pinene. The experimentswere conducted in a large cloud chamber, which also al-lowed us to simulate various aging processes that the par-ticles may experience during their transport in the atmo-sphere, like cloud cycling and redistribution of the organicmatter. We found that the ice nucleation ability of the mixedAS / SOM particles is strongly dependent on the particle mor-phology. Small organic mass fractions of 5 wt %–8 wt % con-densed on the surface of AS crystals are sufficient to com-pletely suppress the ice nucleation ability of the inorganiccomponent, suggesting that the organic coating is evenly dis-tributed on the surface of the seed particles. In this case, theice nucleation onset increased from a saturation ratio with re-spect to iceSice∼1.30 for the pure AS crystals to≥1.45 forthe SOM-coated AS crystals. However, if such SOM-coatedAS crystals are subjected to the mentioned aging processes,they show an improved ice nucleation ability with the icenucleation onset atSice∼1.35. We suggest that the agingprocesses change the particle morphology. The organic mat-ter might redistribute on the surface to form a partially en-gulfed structure, where the ice-nucleation-active sites of theAS crystals are no longer completely masked by the organiccoating, or the morphology of the organic coating layer mighttransform from a compact to a porous structure.Our results underline the complexity in representing theice nucleation ability of internally mixed particles in cloudmodels. They also demonstrate the need to further investigatethe impact of atmospheric aging and cloud processing on themorphology and related ice nucleation ability of internallymixed particles