| 30 Apr 2026
Ice nucleating properties of α-pinene- and limonene-derived secondary organic aerosol under cirrus conditions
Abstract. The contribution of biogenic secondary organic aerosol (BSOA) to cirrus cloud formation remains unresolved, contributing to uncertainty in aerosol-cloud interactions in global climate models. Laboratory studies report highly variable ice nucleating efficiencies for BSOA, suggesting that these particles may act as either homogeneous or moderately effective heterogeneous ice nuclei. Here, we investigate the deposition ice nucleating properties of α-pinene- and limonene-derived BSOA, including both self-nucleated particles and BSOA coatings on ammonium sulfate and ammonium bisulfate seed particles. Deposition ice nucleation relevant to cirrus clouds (−45 °C, −40 °C, −35 °C; 1.0 ≤ Sice ≤ 1.6) was measured using the SPectrometer for Ice Nucleation (SPIN). Bulk physicochemical properties relevant to ice nucleation were characterized using aerosol mass spectrometry (AMS) and volatility distributions. Pre-cooling was applied to modulate phase state as inferred from glass transition temperature (Tg).
BSOA ice nucleating properties were strongly precursor dependent (p < 0.001). Tg was an unreliable predictor of freezing behavior, correctly anticipating freezing mode for only two of eleven particle combinations. Limonene-derived BSOA nucleated ice almost exclusively via heterogeneous freezing, with Sice onsets as low as 1.27±0.07 at -39.8±0.3 °C. α-pinene-derived BSOA predominantly nucleated ice homogeneously. BSOA coatings on ammonium bisulfate shifted freezing from homogeneous to heterogeneous, while the role of acid-catalyzed multiphase chemistry in ice nucleation remained inconclusive due to experimental limitations. These results demonstrate that cirrus-relevant BSOA parameterizations must explicitly account for precursor specific chemistry and broad classifications of BSOA ice nucleating abilities are inappropriate.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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