Biological ice nucleation particles in the urban atmosphere of two megacities Beijing and Tianjin in North China

Biological materials are the most active ice nucleating particles (INPs), which can nucleate ice at relatively warm temperatures, affecting cloud properties and regional or even global climate. However, the understanding on the impact of biological INPs in urban areas is quite limited. Beijing is the biggest megacity in North China suffered from severe air pollution. Tianjin is the biggest coastal megacity in North China and influenced by both continental/anthropogenic pollution and marine air masses, especially in summer. In this study, we collected aerosol samples on the urban campuses of Tianjin University (39.11°N, 117.17°E) from 01 to 08 July 2019 and PeKing University (39.99°N, 116.31°E) from 11 to 18 August 2019 with SKC Biosamplers. The concentration of INPs in aerosols has been investigated using the PeKing University Ice Nucleation Array (PKU-INA). The abundance of total bacteria in aerosols was enumerated using the LIVE/DEAD bacterial viability assay and an epifluorescence microscope (DM2500, Leica, Germany). The average concentration of INPs in Beijing (18 ± 23 L^-1) is higher than in Tianjin (8 ± 18 L^-1) at -19 °C. Heat-sensitive INPs inactivated by heat treatment (inactivating ice nucleation protein, 95°C, 15 min) and lysozyme-sensitive INPs (digested by lysozyme) were inferred to biological INPs and bacterial INPs, respectively. The contribution of biological INPs in Beijing (86 ± 14%) was higher than in Tianjin (72 ± 26%), but the proportion of bacterial INPs in Beijing (57 ± 20%) was lower than in Tianjin (64 ± 22%). In addition, we measured the ice nucleation activity of ice nucleating macromolecules (INMs) in filtrate (0.22 µm) and after heat treatment. INMs can be found both in Tianjin and Beijing and the majority of them can be inactivated by heat treatment, indicating most of them were likely proteinaceous materials. Also, we found a significant increase in the concentration of INPs during a rain period with strong wind in Tianjin, which implies rainfall and wind speed may significantly influence the abundance of INPs in this region. Backward air masses trajectories indicated that continental air masses can bring high bacterial INPs in Tianjin and Beijing. Interestingly, the air masses in Tianjin with low bacterial INP concentration were mainly from marine areas. These results imply that biological sources including bacteria may contribute a large fraction of INPs above -19 °C in Tianjin and Beijing in the summer of 2019, and biological INPs potentially play an important role in cloud formation and precipitation in Chinese urban areas.