In-depth study of the formation processes of single atmospheric particles in the south-eastern margin of the Tibetan Plateau

The unique geographical location of the Tibetan Plateau (TP) plays an important role in regulating global climate change, but the impacts of the chemical components and atmospheric processing on the size distribution and mixing state of individual particles are rarely explored in the south-eastern margin of the TP, which is a transport channel for pollutants from Southeast Asia to the TP during the pre-monsoon season. Thus a single-particle aerosol mass spectrometer (SPAMS) was deployed to investigate how the local emissions of chemical composition interact with the transporting particles and assess the mixing state of different particle types and secondary formation in this study. The TP particles were classified into six distinct types, mainly including the largest fraction of the potassium-rich (K-rich) type in the total particles (30.9 %), followed by the biomass burning (BB) type (18.7 %). Most particle types were mainly transported from the sampling site's surroundings and along the Sino-Myanmar border, but the air mass trajectories from north-eastern India and Myanmar show a greater impact on the number fraction of the BB (31.7 %) and dust (18.2 %) types, respectively. Then, the two episodes with high particle concentrations showed that the differences in the meteorological conditions in the same trajectory clusters could cause significant changes in chemical components, especially the dust and aged elemental carbon (aged EC) types, which changed by a total of 93.6 % and 72.0 %, respectively. Ammonium and dust particles distribute at a relatively larger size ( similar to 600 nm), but the size peak of other types is present at similar to 440 nm. Compared with the abundant sulfate ( 97 HSO 4 - ), the low nitrate ( 62 NO 3 - ) internally mixed in TP particles is mainly due to the fact that nitrate is more volatilized during the transport process. The formation mechanism of secondary speciation demonstrates that the formation capacity of atmospheric oxidation is presumably affected by the convective transmission and the regional transport in the TP. However, the relative humidity (RH) could significantly promote the formation of secondary species, especially 97 HSO 4 - and 18 NH 4 + . This study provides new insights into the particle composition and size, mixing state, and ageing mechanism in high time resolution over the TP region.