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 (PM2.5 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 PM2.5 concentrations (> 30 µg m−3 ) during the study.