Immersion freezing of surface dust particles from Tengger Desert in China: effects of mineral composition and aging treatments on ice nucleating activity

Abstract. Mineral dust is a major source of ice nucleating particles (INPs) for mixed-phase clouds and thereby influences cloud properties, precipitation, and radiative forcing. However, the ice nucleation properties of natural dust and their response to atmospheric aging remain poorly constrained. Here, we investigated immersion freezing (IMF) of surface-collected dust from Tengger Desert, China, and examined the effects of internal mixing with ammonium sulfate (AS), NaCl, H2SO4, and HNO3, as well as heat treatment. Median freezing temperature (T50) of Tengger Desert dust increased from 251.6 K to 256.7 K as dust mass fraction increased from 1.4x10-4 wt% to 1.0x10-1 wt%. A minor increase in feldspar content (~2 % in surface area) quantified by single particle analysis enhanced the ice nucleation active site density (Ns) by approximately one order of magnitude. Internal mixing with AS enhanced IMF activity, with the enhancements of T50 (0.4 K – 4.8 K), Ns, and heterogeneous ice nucleation rate coefficient (Jhet) exhibiting sigmoidal response to AS concentration from 5x10-5 M to 5x10-2 M. This enhancement is attributed to NH4+ adsorption on dust surface. NaCl and acids, as well as heat treatment, had no impact on the overall IMF activity in terms of T50, although modest changes in Ns and Jhet were observed above 256 K. We developed the experimentally constrained parameterizations of Ns and Jhet for Tengger Desert dust with and without aging, which can be used in cloud modeling to improve our understanding of how atmospheric aging modifies the ice nucleation of natural dust.