Recent studies have demonstrated that microplastics (MPs) are ubiquitously present in the atmosphere. They undergo complex weathering and aging processes that may modify their physicochemical properties and consequently ice nucleation ability. However, the role of MPs in atmospheric ice nucleation after atmospheric aging remains poorly understood. Here, we investigated the ice nucleation activity in immersion freezing and deposition nucleation of polystyrene (PS), polyamide (PA), and polymethyl methacrylate (PMMA) MPs before and after aging by 80% H₂SO₄. The median freezing temperature (T₅₀) of 2 g L^-1 pristine MP suspensions was 248.6 ± 1.7 K (PS), 249.5 ± 0.6 K (PA), and 247.6 ± 0.3 K (PMMA), which were higher than that of the water blank (244.3 K). After 6 days of acid aging, T₅₀ increased by 2.0 K (PS), 3.4 K (PA), and 2.6 K (PMMA), indicating enhanced ice nucleation efficiency by immersion freezing. Under cirrus cloud conditions, these three types of pristine MPs initiated deposition ice nucleation at temperatures and relative humidity with respect to ice (RH_ice) above the homogeneous freezing threshold. After aging, the average RH_ice onsets decreased substantially between 210 K- 245 K by 3.8–29.1% for PS, 5.7–13.3% for PA, and 15.5–30.4% for PMMA. This demonstrates a pronounced enhancement in deposition ice nucleation activity after H₂SO₄ aging. Our findings highlight the potential climatic relevance of chemically aged MPs as efficient ice nucleating particles in both mixed-phase and cirrus cloud regimes.
 

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