Non‐Fossil Origin Explains the Large Seasonal Variation of Highly Processed Organic Aerosol in the Northeastern Tibetan Plateau (3,200 m a.s.l.)

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. Plain Language Summary The climate effects of carbonaceous aerosols (CAs) are highly uncertain and debated. The high‐altitude Tibetan Plateau (TP) is sensitive to climate change, however, sources and atmospheric processes of CAs in the TP are poorly known. This is particularly challenging for organic aerosols, due to the large number of species involved in their formation and transformation processes in the atmosphere. In recent years, dual‐carbon isotope characterization (i.e., radiocarbon 14C and the stable carbon isotope 13C) has become a promising tool to elucidate the sources and formation processes of organic aerosols. With this approach, we found that organic aerosols in the northeastern TP (Qinghai Lake) are highly atmospheric processed, and their seasonal variability is driven by a variable largely non‐fossil source. Findings from this study lead to a better understanding of the sources and formation mechanisms of organic aerosols in different seasons, and thus organic aerosols' effects on climate change. Key Points High seasonality of elemental carbon (EC) and organic carbon (OC) concentrations in the northeastern Tibetan Plateau (TP) was driven by non‐fossil sources In the monsoon season, low EC and OC concentrations reflected local sources, with most evident contributions from fossil sources OC in the northeastern TP was highly atmospherically processed and thus less volatile compared to OC near sources or in urban areas