Abundance, variability and sources of Ice-Nucleating Particles in the Beijing area

Ice-nucleating particles (INPs) are microscopic aerosol particles that serve as the seeds for the formation of ice crystals in clouds and by that also the initiation of precipitation processes. These processes exert fundamental influences on cloud properties and the climate system. Large uncertainties in the formulation of aerosol cloud interactions in current climate models underscore the urgent need for improved understanding of INPs. Beijing’s proximity to major Asian dust sources and the strong anthropogenic emissions in the surrounding area make it a unique natural laboratory to investigate INPs under complex atmospheric conditions.

Long-term and high-resolution INP observations in such anthropogenically influenced environments have been scarce, leaving a critical knowledge gap. This thesis used an innovative multi-method approach, combining controlled laboratory experiments using advanced facilities, long-term field observations at mountain, suburban, and urban sites in Beijing, and also an intensive campaign during the 2022 Winter Olympics emission control period with detailed aerosol and INP measurements. The use of multiple instruments with cross-validation significantly enhanced the robustness and reliability of the results. ... mehr

The results reveal that Asian dust was the most efficient INP source that influences North China, and laboratory experiments further demonstrate that Asian dust has an ice-nucleating active site (INAS) density which decreases with temperature at a steeper rate than that represented by existing global dust parameterization, indicating unique ice-nucleating properties of Asian dust. According to the long-term INP offline observations, the INP concentrations exhibited pronounced seasonal and spatial variations, with values peaking in spring and dropping to a minimum in winter, and with the mountain site registering the highest and most variable concentrations versus lower, more stable levels at the urban and suburban sites.

Intensive online observations with CFDC and PINE revealed a clear correlation between INP concentrations and pollution levels, with INP numbers being approximately one order of magnitude higher during extremely polluted episodes compared to clean periods. However, discrepancies were observed between online and offline INP measurement techniques. Long-term offline observations using filter sampling showed that INP concentrations were lowest during winter when large-scale emissions are typically higher, suggesting that anthropogenic aerosols may not be efficient INPs. These findings highlight the need for further research to understand the discrepancies between different INP measurement techniques. Despite the differences, the nucleating efficiency (\(n_s\)) was consistently found to be low.Therefore, a new parameterization using surface area larger than \SI{500}{\micro\metre} ($SA_{500}$)was chosen to predict the INPs over the anthropogenically influenced area.