The SALTENA Experiment: Comprehensive Observations of Aerosol Sources, Formation, and Processes in the South American Andes

Abstract

This paper presents an introduction to the Southern hemisphere high altitude experiment on particle nucleation and growth (SALTENA). This field campaign took place between December 2017 and June 2018 (wet to dry season) at Chacaltaya (CHC), a GAW (Global Atmosphere Watch) station located at 5240 m a.s.l. in the Bolivian Andes. Concurrent measurements were conducted at two additional sites in El Alto (4000 m a.s.l.) and La Paz (3600 m a.s.l.). The overall goal of the campaign was to identify the sources, understand the formation mechanisms and transport, and characterize the properties of aerosol at these stations. State-of-the-art instruments were brought to the station complementing the ongoing permanent GAW measurements, to allow a comprehensive description of the chemical species of anthropogenic and biogenic origin impacting the station and contributing to new particle formation. In this overview we first provide an assessment of the complex meteorology, air mass origin, and boundary layer – free troposphere interactions during the campaign using a 6-month high-resolution WRF (Weather Research and Forecasting) simulation coupled with FLEXPART (FLEXible PARTicle dispersion model). We then show some of the research highlights from the campaign, including i) chemical transformation processes of anthropogenic pollution while the air masses are transported to the CHC station from the metropolitan area of La Paz/El Alto, ii) volcanic emissions as an important source of atmospheric sulfur compounds in the region, iii) the characterization of the compounds involved in new particle formation, and iv) the identification of long-range transported compounds from the Pacific or the Amazon basin. We conclude the article with a presentation of future research foci. The SALTENA dataset highlights the importance of comprehensive observations in strategic high-altitude locations, especially the undersampled Southern Hemisphere.

Evidence for Interhemispheric Mercury Exchange in the Pacific Ocean Upper Troposphere

“…The Chacaltaya (CHC) regional GAW station (16.35023°S, 68.13143°W) lies at an altitude of 5,240 m asl. in tropical Bolivia (Andrade et al., 2015; Bianchi et al., 2022) (see Figure 3a, Figure S1 in Supporting Information ), more than 1,000 m above but only ∼16 km away from the major urban agglomeration of La Paz/El Alto. CHC samples mostly free tropospheric air, but can be influenced by the planetary boundary layer of the surrounding Altiplano, mainly during daytime (Aliaga et al., 2021; Andrade et al., 2015; Chauvigné et al., 2019).…”

Measurement Report: Long-range transport and fate of DMS-oxidation products in the free troposphere derived from observations at the high-altitude research station Chacaltaya (5240 m a.s.l.) in the Bolivian Andes

“…To identify the specific source regions of chemical species measured at CHC, the SRR from FLEXPART can be combined with the in-situ measurements of such species via an elastic net regression (Pedregosa et al 2011). The same 265 method was used to identify likely sources of black carbon measured at CHC (Bianchi et al, 2021). In contrast to Bianchi et al (2021), however, we included also the lateral boundaries of the FLEXPART domain in the footprint analysis to visualize the long-range transport into the domain through these boundaries.…”

“…The same 265 method was used to identify likely sources of black carbon measured at CHC (Bianchi et al, 2021). In contrast to Bianchi et al (2021), however, we included also the lateral boundaries of the FLEXPART domain in the footprint analysis to visualize the long-range transport into the domain through these boundaries.…”

“…The variety of sources and air masses influencing air composition at CHC (e.g. Bianchi et al (2021);Rose et al (2015); Wiedensohler et al (2018); Achá et al (2018)) required a careful distinction of air masses impacted by local emissions and long-range transport, which we present in the results, using strict thresholds for typical markers of local urban emissions in agreement with the FLEXible PARTicle dispersion model (FLEXPART). We test this distinction by analysing the observations of different instruments in the different air masses.…”

“…Possible sources for such condensible vapors in the south-west to the north-west of the station are volcanic outgassing on the Altiplano (a large semi-arid plateau in the Andes), the close-by metropolitan area La Paz / El Alto, or the 75 km distant southern shores of Lake Titicaca during the day, while the Pacific coast is 330 km away from the station. The intensive measurement campaign Southern hemisphere high altitude experiment on particle nucleation and growth (SALTENA) (Bianchi et al, 2021), lasting from the end of December 2017 to the beginning of June 2018 was planned to improve our understanding of the chemistry behind the observed new particle formation. With the Amazon basin to the east and the open Pacific to the west, the air mass composition reaching CHC in the wet and dry seasons are likely very different, as the prevalent wind direction shifts from the lowlands in the east during the wet-season to the west during dry-season.…”