A Change in the Fine-to-Coarse Fraction in a Gobi Desert Town: Evidence for an Emerging Mixed Dust–Pollution Aerosol Pattern

Abstract. Mineral dust from the Gobi Desert is one of the largest natural contributors to the global aerosol burden, yet it is unknown whether urbanisation within the source region is altering the fine-to-coarse aerosol ratio. We analysed 13 years of PM_2.5 and PM₁₀ observations at four sites: Ulaanbaatar (UB) and three sites in the Gobi Desert (Dalanzadgad, DZ; Zamyn-Uud, ZU; Sainshand, SS), to investigate the relationship between winter PM_2.5 and spring r-ratio. Winter PM_2.5 in UB and DZ (112±45 and 80±22 μg m⁻³) substantially exceeded values in ZU and SS (19 ± 5 and <5 μg m⁻³), reflecting anthropogenic dominance, while spring PM₁₀ reached 467–649 μg m⁻³ during dust peaks. In DZ, winter PM_2.5 increased 1.6-fold (65 to 102 μg m⁻³) after 2014 and was significantly correlated with spring r-ratio (r = 0.69, p = 0.05). This association persisted under strong-wind conditions, with the elevated spring r-ratio retained between the pre- and post-2014 periods. These findings indicate an emerging mixed dust–pollution regime in DZ, coexisting with the anthropogenic regime at UB and natural dust at ZU and SS. Fine-particle enrichment in DZ suggests longer atmospheric lifetimes and transport distances and challenges the assumed net cooling effect of Gobi dust in urbanised areas, which warrants investigation through chemical composition measurements.