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

Previous laboratory measurements suggest that ammonium sulfate crystals (AS, (NH4)2SO4) 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, (NH4)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. Sulfate particles that are partially or fully neutralized by ammonium are an important component of the atmospheric aerosol. The relative humidity in the environment determines whether they are present as aqueous solution droplets or as solid crystals. Depending on the degree of neutralization, three different crystalline solid phases can form: ammonium sulfate, (NH4)2SO4, letovicite, (NH4)3H(SO4)2, and ammonium bisulfate, NH4HSO4. Knowledge of the phase state of the particles (i.e., aqueous or solid) is required because it affects a variety of important atmospheric processes, including light scattering, chemical reactions, and cirrus cloud formation. Cirrus ice crystals can form heterogeneously on solid particles and homogeneously in supercooled aqueous solution droplets. Heterogeneous ice formation by solid (NH4)2SO4 particles has been well studied in the laboratory, while much less is known about the ice nucleation ability of the other two solids involved. We performed experiments in a large coolable cloud chamber at a temperature of about 220 K to derive a parameterization for the heterogeneous ice nucleation ability of particles in the (NH4)2SO4-(NH4)3H(SO4)2-NH4HSO4 system as a function of their composition. This enables a differentiated treatment of this particle class in future model simulations of cirrus cloud formation. The heterogeneous ice nucleation ability of crystalline, ammoniated sulfate particles strongly depends on the degree of neutralizationAmmonium sulfate, letovicite (LET), and their mixtures efficiently form ice by deposition nucleation or pore condensation and freezing at 220 KMixtures of LET and ammonium bisulfate partially deliquesce and nucleate ice less efficiently by immersion freezing at 220 K