Tropical tropospheric aerosol sources and chemical composition observed at high-altitude in the Bolivian Andes.

The chemical composition of PM10 and PM2.5 was studied at the summit of Mt. Chacaltaya (5380 masl, lat.-16.346950º, lon. -68.128250º) providing a unique long-term record spanning from December 2011 to March 2020. The chemical composition of aerosol at the Chacaltaya GAW site is representative of the regional background, seasonally affected by biomass burning practices and by nearby anthropogenic emissions from the metropolitan area of La Paz – El Alto. Concentration levels are clearly influenced by seasons with minimum occurring during the wet season (December to March) and maxima occurring during the dry and transition seasons (April to November). Ions, total carbon (EC+OC) and saccharide concentrations range between 558–1785, 384–1120 and 4.3–25.5 ng m^-3 for bulk PM10 and 917–2308, 519–1175 and 3.9–24.1 ng m^-3 for PM_2.5, respectively. Such concentrations are overall lower compared to other high-altitude stations around the globe, but higher than Amazonian remote sites (except for OC). For PM₁₀, there is dominance of insoluble mineral matter (33–56 % of the mass), organic matter (7–34 %) and secondary inorganic aerosol (15–26 %). Chemical composition profiles were identified for different origins: EC, NO₃^-, NH₄⁺, glucose, C₂O₄^-2 for the nearby urban and rural areas; OC, EC, NO₃^-, K⁺, acetate, formiate, levoglucosan, some F^- and Br^- for biomass burning; MeSO₃^-, Na⁺, Mg²⁺, Br^- for aged marine emissions from the Pacific Ocean; arabitol, mannitol, K⁺ for biogenic emissions; Na⁺, Ca²⁺, Mg²⁺ for soil dust, and SO₄^2-, F^-, and some Cl^- for volcanism. Regional biomass-burning practices influence the soluble fraction of the aerosol particularly between July and September. The organic fraction is present all year round and has both anthropogenic (biomass burning and other combustion sources) and natural (primary and secondary biogenic emissions) origins, with the OC/EC mass ratio being practically constant all year round (10.5±38.9). Peruvian volcanism dominates the SO₄^2- concentration since 2014, though it presents a strong temporal variability due to the intermittence of the sources and seasonal changes on the transport patterns. These measurements represent some of the first long-term observations of aerosol chemical composition at a continental high-altitude site in the tropical Southern hemisphere. [ABSTRACT FROM AUTHOR]