Mineral dust dominates the spatial distribution and impacts the climate effects of ice-nucleating particles over the Tibetan Plateau, China
Authors: Wu Zhijun , Chen Jingchuan , Xu Jianzhong , Zhai Lixiang , Hu Min 
Published: February 2023

Mineral dust acts as ice-nucleating particles (INPs) to alter cloud microphysical properties and thus further impact cloud radiative effects (CRE). As the third pole of the world, the Tibetan plateau (TP) is one of the most sensitive regions to climate change along with the Arctic and Antarctic, warming twice as fast as the global average, affecting glacial melt, hydroelectric power, and water supply for billions of people. Several important Asian deserts are distributed around the TP, and large-scale seasonal dust storms lift and sweep this region every spring. However, the effects of these mineral INPs on ice formation and radiation budgets of clouds over the TP are not well understood.In this study, we investigated the immersion mode INP properties of 47 snowmelt samples collected from five glaciers throughout the TP using droplet freezing techniques, identified the dominative factor affecting INP concentration via particle chemistry and morphology analyses and air mass trajectory calculation, and discussed their implications for the cloud microphysical properties by ten-year satellite data (Moderate Resolution Imaging Spectroradiometer, MODIS) and climate reanalysis (ECMWF Reanalysis V5, ERA5). A total of 47 snowmelt samples were collected at five different glaciers over the whole TP in the summer of 2020: Laohugou, LHG (39.4&#176;N, 96.6&#176;E, 4990 m above sea level (masl), 10 samples); Tianshan, TS (43.1&#176;N, 86.8&#176;E, 4070 masl, 12 samples); Dongkemadi, DKMD (33.1&#176;N, 90.1&#176;E, 5708 mas, 10 samples); Ali, AL (32.8&#176;N, 80.9&#176;E, 5967 masl, 10 samples); Zhufeng (Mt. Everest), ZF (35.7&#176;N, 94.2&#176;E, 6525 masl, 5 samples). The average INP concentrations were 1.255, 0.646, 0.257, 0.103, and 0.016 L-1 air at -20&#8451; for LHG, TS, DKMD, AL, and ZF, respectively, showing a spatial distribution with higher INP concentrations in the north and lower INP concentrations in the south. Although the INP concentrations differed by up to three orders of magnitude among these five sampling sites, all 47 snowmelt samples were within the ice nucleation spectra derived from worldwide precipitation samples. Samples collected from ZF presented the lowest INP concentrations, which were comparable with those in the Arctic regions. There was a significant correlation between the INP concentrations and the concentrations of representative chemical components of mineral dust particles (R > 0.75, P value < 0.01). Back trajectory analysis coupled with land cover data also suggested a strong dust source dependence of INPs: the higher the proportion of trajectory from barren and grass sources, the higher the INP concentrations.These results reveal the abundance, distributions, and sources of INPs over the TP, and the INP concentration is controlled by seasonal East Asian dust. Further, the average annual cloud fractions and the cloud top temperatures of the five sampling regions are 42 ~ 65% and -27.0 ~ -17.2&#8451;, respectively, meaning ample opportunities for aerosol-cloud interactions and suitable freezing temperature conditions for mineral INPs. Our work provides a comprehensive and extensive study of INPs over the TP, which could impact ice formation in clouds and may be a key link in understanding the radiation budget in this region.