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

page2838The chemical composition of PMinline-formula₁₀ and non-overlapping PMinline-formula_2.5 was studied at the summit of Mt. Chacaltaya (5380 m a.s.l., lat. inline-formula−16.346950°, long. inline-formula−68.128250°) providing a unique long-term record spanning from December 2011 to March 2020. The chemical composition of aerosol at the Chacaltaya Global Atmosphere Watch (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 minima occurring during the wet season (December to March) and maxima occurring during the dry and transition seasons (April to November). Ions, total carbon (EC inline-formula+ OC), and saccharide interquartile ranges for concentrations are 558–1785, 384–1120, and 4.3–25.5 ng minline-formula⁻³ for bulk PMinline-formula₁₀ and 917–2308, 519–1175, and 3.9–24.1 ng minline-formula⁻³ for PMinline-formula_2.5, respectively, with most of the aerosol seemingly present in the PMinline-formula_2.5 fraction. Such concentrations are overall lower compared to other high-altitude stations around the globe but higher than Amazonian remote sites (except for OC). For PMinline-formula₁₀, 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, NOinline-formula $M12inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimgee54bb0fff66afdafaf51bed1fde360d acp-24-2837-2024-ie00001.svg9pt16ptacp-24-2837-2024-ie00001.png , NHinline-formula $M13inlinescrollmathml{}_{\mathrm{normal\; 4}}^{+}$ 8pt15ptsvg-formulamathimgf1ca5762abf079d28af10bf21d382d4c acp-24-2837-2024-ie00002.svg8pt15ptacp-24-2837-2024-ie00002.png , glucose, and Cinline-formula₂Oinline-formula $M15inlinescrollmathml{}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 2}-}$ 13pt17ptsvg-formulamathimg9def59c1763723bf85d4c029a1ebd14e acp-24-2837-2024-ie00003.svg13pt17ptacp-24-2837-2024-ie00003.png for the nearby urban and rural areas; OC, EC, NOinline-formula $M16inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimg06954914259a113e7faaa0d01a8ee756 acp-24-2837-2024-ie00004.svg9pt16ptacp-24-2837-2024-ie00004.png , Kinline-formula⁺, acetate, formate, levoglucosan, and some Finline-formula⁻ and Brinline-formula⁻ for biomass burning; MeSOinline-formula $M20inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimg9a17d6ab4c67d7f6701d29ccd0703b2e acp-24-2837-2024-ie00005.svg9pt16ptacp-24-2837-2024-ie00005.png , Nainline-formula⁺, Mginline-formula²⁺, Kinline-formula⁺, and Cainline-formula²⁺ for aged marine emissions from the Pacific Ocean; arabitol, mannitol, and glucose for biogenic emissions; Nainline-formula⁺, Cainline-formula²⁺, Mginline-formula²⁺, and Kinline-formula⁺ for soil dust; and SOinline-formula $M29inlinescrollmathml{}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 2}-}$ 13pt17ptsvg-formulamathimg29af680a2c2c3e13b3242191be5b1002 acp-24-2837-2024-ie00006.svg13pt17ptacp-24-2837-2024-ie00006.png , Finline-formula⁻, and some Clinline-formula⁻ for volcanism. Regional biomass burning practices influence the soluble fraction of the aerosol between June and November. 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 inline-formula $M32inlinescrollmathml\mathrm{normal\; OC}/\mathrm{normal\; EC}$ 40pt14ptsvg-formulamathimgde7a86866b2e8a9da956d9c46cef780c acp-24-2837-2024-ie00007.svg40pt14ptacp-24-2837-2024-ie00007.png mass ratio being practically constant all year round (10.5 inline-formula± 5.7, IQR 8.1–13.3). Peruvian volcanism has dominated the SOinline-formula $M34inlinescrollmathml{}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 2}-}$ 13pt17ptsvg-formulamathimg23ee1f08807182dc6cd06a1b188c59a7 acp-24-2837-2024-ie00008.svg13pt17ptacp-24-2837-2024-ie00008.png concentration since 2014, though it presents strong temporal variability due to the intermittence of the sources and seasonal changes in 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.