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

Mineral dust is a major source of ice nucleating particles (INPs) 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, H ₂ SO ₄ , and HNO ₃ , as well as heat treatment. Median freezing temperature ( T ₅₀ ) of Tengger Desert dust increased from 253.6 to 258.5 K as dust mass fraction increased from 1.4 × 10 ⁻⁴  wt % to 1.0 × 10 ⁻¹  wt %. A minor increase in feldspar content (∼ 2 % in surface area) quantified by single particle analysis may contribute to the enhanced ice nucleation active site density ( N _s ). Internal mixing with AS enhanced IMF activity, with the enhancements of T ₅₀ (0.4–4.7 K), N _s , and heterogeneous ice nucleation rate coefficient ( J _het ) exhibiting sigmoidal response to AS concentration from 5 × 10 ⁻⁵ to 5 × 10 ⁻²  M. This sigmoidal response is consistent with a possible contribution from NH4+ adsorption on dust surface. NaCl and acids (H ₂ SO ₄ and HNO ₃ ), as well as heat treatment, had no impact on the overall IMF activity in terms of T ₅₀ , although modest changes in N _s were observed above 258 K. We developed experimentally constrained sensitivity parameterizations of N _s and J _het for Tengger Desert dust with and without aging, which may be used in cloud modeling to study how atmospheric aging modifies the ice nucleation of natural dust.