Proxies for atmospheric circulation over the Amazon basin from the aerosol composition in a Nevado Illimani firn core, Bolivia

Current changes in tropical South America due to atmospheric warming, deforestation, and glacier retreat impact moisture and water exchange between the Amazon basin and the Andes. Thus, a deeper understanding of past atmospheric variability is crucial for developing strategies for climate and environmental change scenarios in this region. Within this context, we investigated an 18-year firn core drilled at the Illimani to interpret its aerosol composition (trace elements and major ions) in relation to seasonal processes, particularly atmospheric circulation over the Amazon basin. The resulting 21st-century record showed reduced Cr contamination over the Altiplano in comparison to the late 20th century, which was probably related to reduced emissions from mining activities. Sulfur records suggest the influence of volcanic eruptions in 2006 (Rabaul) and 2014 (Nyamuragira-Nyiragongo). Overall, the aerosol composition was mainly modulated by precipitation variability over the Altiplano at both annual and seasonal timescales. However, Mn was enriched due to strengthened low-level jets in the Amazon basin during the dry season, especially in 2015. This was corroborated by the reanalysis data. Furthermore, Mn, Co, and Fe showed an unprecedented peak in the record during the wet season of 2014, which was consistent with the arrival of a dust plume from Africa over Amazonia. Therefore, the Mn enrichment record can be used as a new proxy for obtaining information about the South American Low-Level Jet, and, when considered together with more elements, might also indicate snow layers that were possibly loaded with aerosols from Africa.

addition, higher temperatures over the Amazon basin are related to events of biomass-48 burning, which increases the concentration of refractive black carbon in Illimani (Osmont 49 et al., 2019). However, aerosols deposited at Illimani mainly reflect conditions over its 50 main source, which is the Altiplano (Figure 1), obfuscating the contribution of biogenic 51 and/or biomass-burning material (Correia et al., 2003;Lindau et al., 2020). 52 Moreover, low-level northeasterly flow over the Amazon basin was found to be re-53 lated to the increased deposition of Amazon-sourced aerosol (more concentrated in ni-  In addition, the Bolivian high, an upper tropospheric anticyclone located over Bolivia, 61 is established during the mature phase of the SAMS, leading to upper tropospheric east-62 knife under a laminar flow HEPA bench inside the cold room. Then, the decontaminated 94 inner part was sampled using a continuous melter system (Osterberg et al., 2006) in an 95 ISO 6 Class clean room, yielding 767 samples (mean sample resolution of 3 cm). The sam-96 ples for elemental analysis were collected into acid-cleaned (Optima HNO 3 ) low-density 97 polyethylene (LDPE) vials and acidified with double-distilled HNO 3 . This procedure was 98 conducted using an ISO 5 laminar flow HEPA bench. Samples were stored for acidifi- Chromatograph ICS-6000 analytical system at the CCI. The method detection limit (MDL, 108 -5-manuscript submitted to JGR: Atmospheres

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The annual signal in ice cores from Illimani is considered to be largely preserved  cate that the area we chose was influenced by a common mode of rainfall variability.

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The reanalysis data set from the European Center for Medium Range Weather Fore-  (Table 2). for all IL2017, IL1999, and Quelccaya were As, Cu, and Cd (Table 2). Interestingly, Cr

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EF showed a reduction in the 21 st century as it was >10 for both IL1999 and Quelccaya, 190 but in IL2017, it was close to unity ( Table 2). The presence of anthropogenic Cr in an 191 ice core from the Alps was attributed to iron, steel, and ferro-alloy production (Van De  (Table 2).

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Bi was not analyzed in IL1999, and in Quelccaya it showed a low EF during the 201 20 th century (Table 2)     In addition, Figure 3a shows a spike for both S EF and SO 2− 4 exc during the wet  lite observations indicate that the SO 2 plume advected toward the Atlantic Ocean due 231 to predominant westerly winds over that latitude, thus not affecting the Illimani region.

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Bi EF showed its most prominent peak for the 2007-08 period, which might indi-  (Table S1). It is estimated that approximately 70% of the worldwide atmospheric

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We observed a common seasonality for the analyzed soluble species during the ma-252 jority of the 1999-2016 period (Figure 6a; Figure S3). This is expected as the extreme  Table S2 shows that PC1e is associated (r > 0.9) 270 with highly enriched elements such as As (mean EF of 69) and Cu (29). To avoid data 271 -12-manuscript submitted to JGR: Atmospheres interpolation, only the detected elements above the MDL in more than 75% of the sam-272 ples (a total of 17 elements, Table S2) were considered for PC analyses.

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The coherence between elemental concentrations was also observed for the EFs. For tends to be closer to soil dust, while during the wet season, regional soil dust aerosols 283 are more efficiently removed than fine, remotely transported aerosols from other sources.

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This is also observed on an interannual timescale.

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We observed that S enrichment was related to increased atmospheric SO 2 over Il-

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The Mn EF, in turn, showed significant correlations at the 95% level with the low-308 level circulation over the Amazon basin during the austral winter (Figure 8), which is 309 related to the SALLJ. In Figure 8, the more intense the SALLJ, the greater the enrich-     Table S1. Correlations between the enrichment factors of the elements emitted by metallurgical smelting and refining processes. All these correlations are significant at the 95% level.
As Bi Cd Cu As 1 Bi 0.5 1 Cd 0.5 0.4 1 Cu 0.6 0.4 0.4 1 Figure S3. Major ions concentration record for the 1999-2016 period. Records are expressed by a 3-sample mean. Blue and red dots denote to samples classified as "wet" and "dry", respectively.