The aqueous film formed on aerosol surfaces enhances the absorption of urban locally emitted free amino acids during the dust event

Free amino acids (FAAs) are essential bioavailable organic nitrogen compounds, yet their atmospheric concentrations, distributions, sources, and secondary formation processes during dust events remain poorly understood. Here, we investigated FAA concentrations, molecular distributions, and δ 15 N signatures in PM 2.5 collected from four cities in the Beijing-Tianjin-Hebei air pollution transport channel during spring 2018 to investigate the impact of a dust event on the concentrations, sources and formation mechanism of FAAs in PM 2.5 . During the dust event, total FAA (TFAA) concentrations across all sites were substantially higher than those environments not influenced by the dust events and exhibited a strong linear correlation with PM 10 ( r = 0.6, p < 0.01), indicating that dust events significantly enhance FAA levels. We observed regional variability in the average TFAA concentration during the dust event. The concentrations in Beijing, Tianjin, Shijiazhuang and Taiyuan increased by factors of 1.05, 1.36, 1.55 and 1.63, respectively. Among these four sites, Taiyuan exhibited the most pronounced increase, with its TFAA concentration rising from 0.24 ± 0.07 μg m -3 during the non-dust period to 0.39 ± 0.003 μg m -3 during the dust event ( p < 0.05). These regional differences underscore the influence of local sources and atmospheric quality parameters. The consistent FAA composition profiles and δ 15 N values of TFAA and glycine between dust and non-dust periods across all sites further supported that local sources, rather than long-range transport from the Gobi Desert, was the major contributor to atmospheric FAA pool even during the dust event. In Taiyuan, temporal variations of aerosol liquid water (ALW) were highly correlated with total and subgrouped FAAs (r > 0.8, p < 0.01), identifying ALW as a key factor controlling FAA concentrations. It suggested that even under the low relative humidity conditions of dust events, hygroscopic aerosol growth can facilitate the partitioning of FAAs into the particle phase, leading to higher observed FAA concentrations. Our findings provide new insights into the impact of dust events on FAAs in polluted urban environments and provide a basis for assessing the ecological effects of atmospheric dust transport. • Dust events enhance FAA concentrations in PM 2.5 in downwind urban areas. • During the dust event, elevated ALW levels may facilitate the adsorption of locally emitted FAAs onto aqueous particle surfaces. • Low ALW levels and a weak atmospheric oxidant capacity suppress the extensive degradation of proteinaceous material during long-range transport from dust source regions.