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
