Crystallization, deliquescence, and ice nucleation ability of ammoniated sulphate particles in the cirrus cloud temperature range

Cirrus are high-level clouds composed uniquely of ice crystals. To correctly estimate their radiative contribution to the Earth’s energy budget, it is necessary to know their optical properties,  which in turn depend on the formation mechanism. Cirrus clouds can form either by homogeneous freezing of supercooled aqueous solution droplets or by heterogeneous freezing with the contribution of an ice nucleating particle (INP). Therefore, it is fundamental to understand which aerosol particles are present in the upper troposphere and contribute to initiate heterogeneous ice nucleation.

Sulphate particles are among the most abundant aerosol types in the upper troposphere, and their degree of neutralization with ammonia significantly varies with geographical location and altitude. According to the ammonium-to-sulphate ratio (ASR), three pure inorganic salts can form in the H₂SO₄/NH₃/H₂O system: ammonium bisulphate (NH₄HSO₄, ASR = 1), letovicite (NH₄)₃H(SO₄)₂, ASR = 1.5), and ammonium sulphate ((NH₄)₂SO₄, ASR = 2). However, the transport, ageing and processing of atmospheric aerosols are more likely to lead to a variety of mixtures of the different salts than to particles with exact stoichiometry. The ice nucleation ability of aqueous sulphuric acid (ASR=0) and fully neutralized crystalline ammonium sulphate particles has been extensively investigated in the past. The low-temperature phase state and ice nucleation ability of partially neutralized particles, instead, has never been measured before.

In this contribution, we present new AIDA cloud chamber experiments on the crystallization, deliquescence, and ice nucleation ability of partially neutralized particles in the H₂SO₄/NH₃/H₂O system (1<ASR<2) at temperatures between -60 and -40°C. Particles with various ASR were generated i) from bulk solutions with pre-defined composition and ii) from the in situ neutralization of aqueous sulphuric acid aerosol particles. The latter experiments aimed at simulating the gradual neutralization process that acidic solution droplets may experience in the upper troposphere. A comprehensive characterization of the low-temperature phase state of the particles as a function of relative humidity was obtained by combining FTIR spectra, laser light scattering and depolarisation measurements, as well as water uptake experiments in a continuous flow diffusion chamber (CFDC). We measured the ice nucleation ability with expansion cooling experiments in the AIDA cloud simulation chamber and with two CFDCs.

Our results show that in the cirrus cloud temperature range, the phase state and ice nucleation ability of particles in the H₂SO₄/NH₃/H₂O system depend on their degree of neutralization. In particular, we measured an increased ice nucleation ability with increasing degree of neutralization. Quantifying the abundance and neutralization degree of ammoniated sulphate particles in the upper troposphere may thus be critical to correctly represent their direct and indirect effect on climate.