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.

Cloud seeding is considered a practical but unproved method to enhance precipitation or suppress hail, due to insufficient knowledge of ice formation and evolution after seeding clouds with ice nucleating particles. This study investigates the size effects on the immersion freezing of aerosol produced from commercial silver iodide (AgI) containing flares at mixed‐phase cloud temperatures from 243 to 267 K. Flare‐generated aerosol exhibited comparable ice nucleation ability (INA) to pure AgI particles in the size range of 200 and 400 nm. Non‐AgI impurities reduced the INA of flare‐generated particles ≤90 nm, which is lower than pure AgI particles ≤40 nm. The critical mass ice‐active site density of the generated aerosols (critical‐n_m) was derived, indicating the minimum mass of AgI particles required for efficient ice nucleation. The new parameterization to predict critical‐n_m can serve as a reference to optimize the effectiveness of cloud‐seeding materials for practical use.

Ice crystals occurring in mixed-phase clouds play a vital role in global precipitation and energy balance because of the unstable equilibrium between coexistent liquid droplets and ice crystals, which affects cloud lifetime and radiative properties, as well as precipitation formation. Satellite observations proved that immersion freezing, i.e., ice formation on particles immersed within aqueous droplets, is the dominant ice nucleation (IN) pathway in mixed-phase clouds. However, the impact of anthropogenic emissions on atmospheric IN in the urban environment remains ambiguous. In this study, we present in situ observations of ambient ice-nucleating particle number concentration (N_INP) measured at mixed-phase cloud conditions (−30 ^∘C, relative humidity with respect to liquid water RH_w= 104 %) and the physicochemical properties of ambient aerosol, including chemical composition and size distribution, at an urban site in Beijing during the traditional Chinese Spring Festival. The impact of multiple aerosol sources such as firework emissions, local traffic emissions, mineral dust, and urban secondary aerosols on N_INP is investigated. The results show that N_INP during the dust event reaches up to 160 # L⁻¹ (where “#” represents number of particles), with an activation fraction (AF) of 0.0036 % ± 0.0011 %. During the rest of the observation, N_INP is on the order of 10⁻¹ to 10 # L⁻¹, with an average AF between 0.0001 % and 0.0002 %. No obvious dependence of N_INP on the number concentration of particles larger than 500 nm (N₅₀₀) or black carbon (BC) mass concentration (m_BC) is found throughout the field observation. The results indicate a substantial N_INP increase during the dust event, although the observation took place at an urban site with high background aerosol concentration. Meanwhile, the presence of atmospheric BC from firework and traffic emissions, along with urban aerosols formed via secondary transformation during heavily polluted periods, does not influence the observed INP concentration. Our study corroborates previous laboratory and field findings that anthropogenic BC emission has a negligible effect on N_INP and that N_INP is unaffected by heavy pollution in the urban environment under mixed-phase cloud conditions.

A drop-freeze array (PeKing University Ice Nucleation Array, PKU-INA) was developed based on the cold-stage method to investigate heterogeneous ice nucleation properties of atmospheric particles in the immersion freezing mode from −30 to 0 °C. The instrumental details as well as characterization and performance evaluation are described in this paper. A careful temperature calibration protocol was developed in our work. The uncertainties in the reported temperatures were found to be less than 0.4 °C at various cooling rates after calibration. We also measured the ice nucleation activities of droplets containing different mass concentrations of illite NX, and the results obtained in our work show good agreement with those reported previously using other instruments with similar principles. Overall, we show that our newly developed PKU-INA is a robust and reliable instrument for investigation of heterogeneous ice nucleation in the immersion freezing mode.

The ice‐nucleating particles (INPs) modulate the microphysics and radiative properties of clouds. However, less is known concerning their abundance and sources in the most pristine and climatic sensitive regions, such as the Tibetan Plateau (TP). Here, to our best knowledge, we conduct the first investigation on INPs in rainwater collected in the TP region under mixed‐phase cloud conditions. The value of INP concentrations varies from 0.002 to 0.675 L⁻¹ air over the temperature range from −7.1 to −27.5°C. This is within the INP spectra derived from precipitation under worldwide geophysical conditions and is also comparable with INP concentrations in the Arctic regions. The heat‐sensitive INPs account for 57% ± 30% of the observed INPs at −20°C and become increasingly important at higher temperatures, indicating biological particles as major contributors to INPs at temperatures above −20°C over the TP, especially on the day with additional input of biogenic materials carried by dust particles. Chemical analysis demonstrates that the rainwater components are mixture of dust particles, marine aerosol, and anthropogenic pollutants. Combined with the backward trajectory analysis, we show that dust particles transported from the surrounding deserts and originated from ground surface of TP may contribute to the heat‐resistant INPs at temperatures below −20°C.

Biomass burning aerosol (BBA) contributes significantly to the global aerosol burden, yet its chemical nature and ice nucleation activities (INAs) are unconstrained due to the heterogeneity in biomass sources and complex evolution of atmospheric aging processes. This study comprehensively investigates the chemical composition and INA of BBA generated through laboratory-controlled burns with different biomasses and burning conditions. Both freshly emitted and photochemically aged BBA produced from different processes exhibit distinct and reproducible chemical compositions. However, the INA of BBA shows substantial variability at mixed-phase cloud temperatures and cannot be predicted by the chemical variability of the enriched carbonaceous materials. This indicates the negligible role of carbonaceous materials in determining the INA of BBA. Using laboratory data, we further evaluate the impact of BBA on atmospheric ice nucleation using particulate matter mass concentration and particle equivalent spherical radius. The estimated ice nucleating particle (INP) concentrations contributed by laboratory-produced BBA are lower than those observed during BBA pollution in field studies. This discrepancy is likely attributed to co-lofted mineral particles during real-world biomass burning, such as ash or soil particles, rather than carbonaceous-rich particles from combustion. We encourage further research to quantify the contribution of mineral particles to the INP concentrations of BBA.