Atmospheric Humic‐Like Substances (HULIS) Act as Ice Active Entities.

We investigated the ice nucleation activities of humic‐like substances (HULIS), an important component of organic aerosol (OA), derived from atmospheric and biomass burning aerosols, and produced from aqueous‐phase chemical reactions. Respective HULIS can effectively trigger heterogeneous IN under mixed‐phase cloud conditions. HULIS ice active entities (IAE) were aggregates in size between 0.02 and 0.10 μm. At −20°C, the IAE numbers per unit HULIS mass varied from 213 to 8.7 × 104 mg−1. Such results were different than those detected in aquatic humic substances (HS) from previous studies, implying using HS as surrogates may not robustly estimate the IAE concentrations in the real atmosphere. Combining the abundance of atmospheric HULIS with the present results suggests that HULIS could be an important IAE contributor in the atmosphere where other ice nucleating particle species, such as dust and biological particles, are either low in concentration or absent. Plain Language Summary: Humic‐like substances (HULIS), a ubiquitous component of atmospheric organic aerosol, were found to nucleate ice under mixed‐phase cloud conditions. HULIS ice active entities (IAE) were shown to be aggregates in size between 0.02 and 0.10 μm. The concentration of IAE per unit mass of HULIS varied from 213 to 8.7 × 104 mg−1 at −20°C. Such results were different than those detected in aquatic humic substances done by previous studies. Combining the reported concentrations of HULIS in the atmosphere with the present results suggests that HULIS could be important ice nucleating particle contributor in the atmosphere. Key Points: Humic‐like substances (HULIS) originating from atmospheric and biomass burning aerosols, and produced from aqueous‐phase reactions, can trigger immersion freezingThe abundance of ice active entities (IAE) per unit HULIS mass varied from 213 to 8.7 × 104 mg−1 at −20°CHULIS could be an important IAE contributor in the atmosphere where other ice nucleating particle species are either low in concentration or absent [ABSTRACT FROM AUTHOR]