Carbonaceous aerosol plays an important role in climate, but its sources and atmospheric processes are least understood in the Tibetan Plateau (TP), a remote yet climatically sensitive region. This study presents the first seasonal cycle of radiocarbon and stable isotope ¹³C of organic and elemental carbon (OC and EC) in the atmosphere of the northeastern TP. Large seasonal variations of EC and OC concentrations were explained by non‐fossil sources. Regardless of the season, fossil contribution to OC was strongly correlated with inverse OC concentrations. This allowed the separating a constant background source and a source responsible for OC variability that was mostly of non‐fossil origin. The ¹³C signature of OC shows that OC was highly atmospherically processed and thus less volatile than OC found near sources or in urban areas. The ¹³C‐depleted secondary sources contributed strongly to more volatile OC, whereas the ¹³C‐enriched less volatile OC suggests the influence of atmospheric aging.

A coal ban policy in northern China during winter 2017 enforced a switch from coal to gas or electricity for residential heating, providing a unique opportunity to study the effect of reduced coal combustion emissions on organic aerosol (OA). This study explores OA composition, sources, and atmospheric transformations in Beijing before and during the coal ban using online aerosol chemical speciation monitor (ACSM) and offline ¹⁴C measurements. Four primary factors (hydrocarbon-like, cooking, biomass burning, coal combustion OA) and one secondary factor (oxygenated OA, OOA) were resolved from ACSM. In response to the coal ban, OA concentrations generally decreased, but coal combustion OA decreased most strongly, consistent with the decreased fossil carbon contributions to OA (67 ± 3% before vs 55 ± 4% during the ban). Concurrently, the OOA fraction increased from 45 to 72%, due to a larger decrease in concentrations of primary OA (POA; 59-88%) compared to OOA (34%), highlighting the enhanced secondary aerosol formation during the coal ban period. This aligns with the ¹⁴C evidence of higher water-soluble carbon in fossil OA (which has mostly secondary sources). During the coal ban period, O_x concentrations doubled and were positively correlated with the OOA fraction, highlighting strong photochemical OA production. The results show that the reduction of POA from stringent clean air actions is partially offset by enhanced secondary OA formation.