Influence of the Neutralization Degree on the Ice Nucleation Ability of Ammoniated Sulfate Particles

Previous laboratory measurements suggest that ammonium sulfate crystals (AS, (NH4)2 SO4) are efficient ice-nucleating particles under cirrus conditions. Sulfate particles not completely neutralized by ammonium are less well studied and include two other solids, ammonium bisulfate (AHS, NH4HSO4 ) and letovicite (LET, (NH 4)3H(SO4)2). In this work, we have obtained the first comprehensive data set for the heterogeneous ice nucleation ability of crystallized particles in the AS–LET–AHS system as a function of their degree of neutralization at a temperature of about 220 K. Quantitative data on nucleation onsets, ice-active fractions, and ice nucleation active surface site densities were derived from expansion cooling experiments in a large cloud chamber and measurements with two continuous flow diffusion chambers. We found a strong dependence of the efficiency and the mode of heterogenous ice nucleation on the degree of neutralization. Ice formation for AS, mixed AS/LET, and LET crystals occurred by the deposition nucleation or pore condensation and freezing mode. The lowest nucleation onset was observed for AS, where 0.1% of the particles became ice-active at an ice saturation ratio of 1.25. This threshold gradually increased to 1.35 for LET, and abruptly further to 1.45 for mixed LET/AHS crystals, which partially deliquesced and induced ice formation via immersion freezing. Pure AHS crystals did not form due to the inhibition of efflorescence. Our data allow for a more sophisticated treatment of ice formation in the AS–LET–AHS system in future model simulations, which have so far only considered the available data for AS alone

The seasonal cycle of ice-nucleating particles linked to the abundance of biogenic aerosol in boreal forests

Ice-nucleating particles (INPs) trigger the formation of cloud ice crystals in the atmosphere. Therefore, they strongly influence cloud microphysical and optical properties and precipitation and the life cycle of clouds. Improving weather forecasting and climate projection requires an appropriate formulation of atmospheric INP concentrations. This remains challenging as the global INP distribution and variability depend on a variety of aerosol types and sources, and neither their short-term variability nor their long-term seasonal cycles are well covered by continuous measurements. Here, we provide the first year-long set of observations with a pronounced INP seasonal cycle in a boreal forest environment. Besides the observed seasonal cycle in INP concentrations with a minimum in wintertime and maxima in early and late summer, we also provide indications for a seasonal variation in the prevalent INP type. We show that the seasonal dependency of INP concentrations and prevalent INP types is most likely driven by the abundance of biogenic aerosol. As current parameterizations do not reproduce this variability, we suggest a new mechanistic description for boreal forest environments which considers the seasonal variation in INP concentrations. For this, we use the ambient air temperature measured close to the ground at 4.2 m height as a proxy for the season, which appears to affect the source strength of biogenic emissions and, thus, the INP abundance over the boreal forest. Furthermore, we provide new INP parameterizations based on the Ice Nucleation Active Surface Site (INAS) approach, which specifically describes the ice nucleation activity of boreal aerosols particles prevalent in different seasons. Our results characterize the boreal forest as an important but variable INP source and provide new perspectives to describe these new findings in atmospheric models.Peer reviewe

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