Search

Early growth of atmospheric particles is essential for their survival and ability to participate in cloud formation. Many different atmospheric vapors contribute to the growth, but even the main contributors still remain poorly identified in many environments, such as high-altitude sites. Based on measured organic vapor and sulfuric acid concentrations under ambient conditions, particle growth during new particle formation events was simulated and compared with the measured particle size distribution at Chacaltaya Global Atmosphere Watch station in Bolivia (5240 m a.s.l.) during April and May 2018, as a part of the SALTENA (Southern Hemisphere high-ALTitude Experiment on particle Nucleation and growth) campaign . The simulations showed that the detected vapors were sufficient to explain the observed particle growth, although some discrepancies were found between modelled and measured particle growth rates. This study gives an insight on the key factors affecting the particle growth on the site. Low volatile organic compounds were found to be the main contributor to the particle growth, covering on average 65 % of simulated particle mass in particle with diameter of 40 nm In addition, sulfuric acid had a major contribution to the particle growth, covering at maximum 39 % of simulated particle mass in 40 nm particle during periods when volcanic activity was detected on the area, suggesting that volcanic emissions can greatly enhance the particle growth.

Measurements of methane concentrations were made at the Chacaltaya GAW station (16.3ºS, 68.1ºW, 5240m a.s.l.) in the Andean Cordillera from 2015 to date. During this period two high-precision Picarro-CRDS analyzers were used at the station, regularly calibrated with internationally certified gases (WMO X2004A) via the LSCE primary scale. The site has a privileged location not only due to its altitude but also because air masses arriving from the near Altiplano (3 800 m a.s.l.), the Amazon (so-called low-lands), the Pacific Ocean, and the nearby metropolitan area of La Paz/El Alto (~2 million of inhabitants) can be sampled there. The complex topography of the region represents a challenge for deconvoluting the origin of the air masses and therefore to understanding the sources and/or processes associated with the measurements made at Chacaltaya. Here we show some results based on re-analysis data as well as on high and medium - resolution back trajectories in order to identify the influence of different regions on the station. In addition, satellite products and satellite-derived databases, from TROPOMI and GFED4.1s and WAD2M are used to characterize and interpret daily, seasonal and interannual behavior of the methane concentrations observed in Chacaltaya. The influence of the local atmospheric planetary boundary layer is clearly seen in the measurements, especially in the late morning, but collocated measurements of other atmospheric components such as carbon monoxide or equivalent black carbon have proven that identifying free-tropospheric air masses is not an easy task. The contributions of the largest human conglomerate of the region are also discussed in this context.

Ten years of almost continuous observations at the highest Global Atmosphere Watch Regional station in the world are presented here. The Chacaltaya observatory (5240 m asl, 16.3ºS, 68.1ºW) was set up in December 2011. It is currently the only operational station characterizing optical and chemical properties of climate-relevant aerosol and gases in Bolivia and in a radius of about 1500 kilometers from the station. The observations show a clear influence of the well-marked dry and wet meteorological seasons. In addition, the impact on the Andean mountains of long and mid-range transport of biomass burning products from the lowlands is clearly recorded in different parameters measured at the station. Furthermore, the nearby presence of the largest metropolitan area in the region (~1.8 million inhabitants) is observed almost on a daily basis, and therefore different campaigns were carried out to characterize the area and its influence on our measurements. Specific results from these campaigns are discussed elsewhere. Finally, the topographic complexity represents an important challenge for modeling efforts in order to understand sources and sinks (and associated processes) of the observed parameters, requiring not only high spatial resolution and the correct choice of model options, but a novel way of interpreting these results. The decade of collaboration of an international consortium made it possible to keep the station running successfully. The challenge is now to preserve its functioning for the coming decades in a region with historically few high-quality observations while disrupting environmental and socio-economic changes take place.

Amazonian deforestation has accelerated during the last decade, threatening an ecosystem where almost one third of the regional rainfall is transpired by the local rainforest. Due to the precipitation recycling, the southwestern Amazon, including the Amazon-Andes transition region, is particularly sensitive to forest loss. This study evaluates the impacts of Amazonian deforestation in the hydro-climatic connectivity between the Amazon and the eastern tropical Andes during the austral summer (December-January-February) in terms of hydrological and energetic balances. Using 10-year high-resolution simulations (2001–2011) with the Weather Research and Forecasting Model, we analyze control and deforestation scenario simulations. Regionally, deforestation leads to a reduction in the surface net radiation, evaporation, moisture convergence and precipitation (~ 20%) over the entire Amazon basin. In addition, during this season, deforestation increases the atmospheric subsidence over the southern Amazon and weakens the regional Hadley cell. Atmospheric stability increases over the western Amazon and the tropical Andes inhibiting convection in these areas. Consequently, major deforestation impacts are observed over the hydro-climate of the Amazon-Andes transition region. At local scale, nighttime precipitation decreases in Bolivian valleys (~ 20–30%) due to a strong reduction in the humidity transport from the Amazon plains toward Andes linked to the South American low-level jet. Over these valleys, a weakening of the daytime upslope winds is caused by local deforestation, which reduces the turbulent fluxes at lowlands. These alterations in rainfall and atmospheric circulation could impact the rich Andean ecosystems and its tropical glaciers.

High-quality data of atmospheric mercury (Hg) is rare for South America, especially for its tropical part. In consequence, mercury dynamics are still highly uncertain in this region, a significant deficiency, as South America appears to play a major role in the global budget of this toxic pollutant. To address this issue, we performed nearly two years (July 2014–February 2016) of continuous high resolution total gaseous mercury (TGM) measurements at the Chacaltaya (CHC) mountain site in the Bolivian Andes, which is subject to a diverse mix of air masses coming predominantly from the Altiplano and the Amazon rainforest. For the first eleven months of measurements, we obtained a mean TGM concentration of 0.89±0.01 ng m−3, in good agreement with the sparse amount of data available from the continent. For the remaining nine months, we obtained a significantly higher TGM concentration of 1.34±0.01 ng m−3, a difference which we tentatively attribute to the strong El Niño event of 2015–2016. Based on HYSPLIT back-trajectories and clustering techniques, we show that lower mean TGM concentrations were linked to either westerly Altiplanic air masses or those originating from the lowlands to the south-east of CHC. Elevated TGM concentrations were related to northerly air masses of Amazonian or southerly air masses of Altiplanic origin, the former possibly linked to artisanal and small scale gold mining (ASGM), while the latter might be explained by volcanic activity. We observed a marked seasonal pattern, with low TGM concentrations in the dry season (austral winter), rising concentrations during biomass burning (BB) season, and highest concentrations at the beginning of the wet season (austral summer). With the help of simultaneously sampled equivalent black carbon (eBC) and carbon monoxide (CO) data, we use the clearly BB influenced signal during BB season (August to October) to derive a mean TGM/CO emission ratio of (2.3±0.6)·10−7 ppbvTGM ppbvCO−1, which could be used to constrain South American BB emissions. Through the link with in-situ measured CO2 and remotely sensed solar-induced fluorescence (SIF) as proxies for vegetation activity, we detect signs of a vegetation sink effect in Amazonian air masses and derive a best guess TGM/CO2 uptake ratio of 0.058±0.017 (ng m−3)TGM ppmCO2−1. Finally, significantly higher Hg concentrations in western Altiplanic air masses during the wet season as compared to the dry season point towards the modulation of atmospheric Hg by the Eastern Pacific Ocean.

In July and August 2003 a field campaign was conducted to explore the diurnal circulation of the Bolivian Altiplano. Vertical soundings by remote-controlled aircraft yielded profiles of temperature, pressure, and humidity at six passes and in a valley. Pilot balloon observations provided wind profiles. Two permanent stations collected additional data. Typically, inflow toward the Altiplano commences a few hours after sunrise at about the time when the stable nocturnal layer near the ground is transformed by the solar heating into an almost neutrally stratified convective boundary layer. The depth of the inflow layer is comparable to but normally less than that of this boundary layer. There are indications of return flow aloft. The inflow continues at least until sunset. Moisture is imported at the passes leading to the Yungas in the east. Strong upvalley flows were found in the valley of the Rio de La Paz, which connects the wide canyon of La Paz with the tropical lowlands to the east. Inflow was absent at one of the passes despite favorable synoptic conditions. Cases of synoptically forced flows are presented as well where the diurnal signal is difficult to separate. A simple flow scheme is presented that fits the observations reasonably well.

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 5,240 m MSL in the Bolivian Andes. Concurrent measurements were conducted at two additional sites in El Alto (4,000 m MSL) and La Paz (3,600 m MSL). 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, airmass origin, and boundary layer–free troposphere interactions during the campaign using a 6-month high-resolution Weather Research and Forecasting (WRF) simulation coupled with Flexible Particle dispersion model (FLEXPART). 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.