Mixing state and sources of submicron regional background aerosols in the northern Qinghai–Tibet Plateau and the influence of biomass burning

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.