Abstract. Transmission electron microscopy (TEM) was employed to obtain morphology, size, composition, and mixing state of background aerosols with diameter less than 1 μm in the northern Qinghai–Tibet Plateau (QTP) during 15 September to 15 October 2013. Individual aerosol particles mainly contained secondary inorganic aerosols (SIA – sulfate and nitrate) and organics during clean periods (PM_2.5 mass concentration less than 2.5 μg m⁻³). The presence of K–Na–Cl associated with organics and an increase in soot particles suggest that an intense biomass burning event caused the highest PM_2.5 concentrations (> 30 μg m⁻³) during the study. A large number fraction of the fly-ash-containing particles (21.73 %) suggests that coal combustion emissions in the QTP significantly contributed to air pollutants at the medium pollution level (PM_2.5: 10–30 μg m⁻³). We concluded that emissions from biomass burning and from coal combustion both constantly contribute to anthropogenic particles in the QTP atmosphere. Based on size distributions of individual particles at different pollution levels, we found that gas condensation on existing particles is an important chemical process for the formation of SIA with organic coating. TEM observations show that refractory aerosols (e.g., soot, fly ash, and visible organic particles) likely adhere to the surface of SIA particles larger than 200 nm due to coagulation. Organic coating and soot on surface of the aged particles likely influence their hygroscopic and optical properties, respectively, in the QTP. To our knowledge, this study reports the first microscopic analysis of fine particles in the background QTP air.