Westerly‑transported Central Asian biomass burning seasonally pollutes cities in Northwest China

Biomass burning (BB) is strictly prohibited in China, but Northwestern China lies downwind of Central Asia, a region with frequent BB events. Whether transboundary BB worsens local air pollution remains unclear. Hydrolyzed amino acids (HAAs) were used as organic BB tracers in PM_2.5 samples from Urumqi, a typical gateway city in the westerly wind belt of Northwestern China. Our research applied novel amino acid ratio approach (ARA) and compound specific nitrogen stable isotope analysis (SIA based on δ¹⁵N_Gly) to apportion HAAs sources and evaluate their potential for secondary organic aerosol (SOA) formation. Both methods consistently identified BB as the dominant HAA source. However, due to differential atmospheric stability of diagnostic amino acids (e.g., serine t_1/2 ≈ 3 h vs. glycine >2040 h), ARA overestimated BB contributions by 16.0-18.4% relative to SIA during the warm season and dust periods, biasing long‑range transport signals. SIA corrected this bias and revealed clear transboundary influences from Central Asian wildfires. A rough estimate indicates that degradation of HAAs may enhance local SOA formation by 7.1% during the cold season to 31.9% in the dust events, with BB-derived HAAs accounting for 38.2-65.0% of the increase. These findings demonstrate that, despite domestic bans, transboundary BB significantly contributes to hazardous particulate matter and SOA precursors in Urumqi. Integrated control of local residential burning and cross‑border wildfires is therefore critical to mitigate both primary HAAs pollution and the associated secondary organic aerosol risks.