16AEROSOL Ice Nucleation Activity of Humic-like Substances (HULIS) and Its Evolution in Photochemical Aging Process

Humic-like substances (HULIS) is a ubiquitous component of atmospheric organic aerosol (OA), which plays important roles in global radiative forcing, heterogeneous atmospheric reactions, and public health. HULIS can effectively trigger heterogeneous IN under mixed-phase cloud conditions. HULIS derived from atmospheric and biomass burning aerosols, and produced from aqueous-phase chemical reactions, can effectively trigger heterogeneous IN under mixed-phase cloud conditions. At −20 °C, the ice active entities (IAE) numbers per unit HULIS mass (n_m) varied from 213 to 8.7 × 10⁴ mg⁻¹. 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.

The ice nucleation activity (INA) of HULIS will be modified due to photochemical aging process. The n_m of HULIS declined more than an order of magnitude after being exposed to ultraviolet (UV) radiation for 16 hours. The medium freezing temperatures of HULIS decreased from −16.8 to −20.2 °C due to photochemical aging. The formation of low-molecular-weight organic acids partially explained the decreased INA. Compared with fresh HULIS, more than 10 low-molecular-weight organic acids were detected in the photochemically aged samples. For these compounds, their concentrations, which was represented by the signal intensities detected by high-resolution mass spectrometry, clearly negatively correlated to n_m at given temperature. In addition, the decreased average molecular weight of HULIS can be another factor in declining the INA. The ~9% decrease in the average molecular weight might inhibit the formation of aggregates or micelles, leading to limited surfaces to initiate ice, and therefore decreased the INA of HULIS.