Amazon basin smoke plumes in the central andes

Although fire ignition is caused by human activity, fire distribution intensity also depends on the severity of drought conditions that may take place due to ENSO warming events3 and anomaly warming conditions in the north tropical Atlantic Ocean23. Historically, it has been found that the number and intensity of fire events correlate with prolonged droughts23,24,25. During severe droughts in the Amazon Basin, smoke plumes may propagate through vast regions of South America, moving over the banks of the Andean Mountains, as previously detected from satellite observations in 201025 (Fig. 1a). To detail this condition and to investigate where measurable impacts of fire events on ice cover do exist, we compiled all fire events occurring in the Amazon Basin from 2000 to 2016 (fire database was obtained from the Brazilian Institute of Space Research - INPE26). From this, we were able to identify major source regions of smoke plumes and to calculate the air mass forward trajectories using the HYSPLIT/NOAA model27 (Hybrid Single Particle Lagrangian Integrated Trajectory model) from high biomass burning regions. The model output shows the average pattern of aerosol transport from 2000–2016 (Fig. 1b) and clearly depicts that the Central Andes, mainly Bolivia and Peru, is a potential region for biomass burning smoke plume impacts as a result of the predominant east-to-west air circulation at the tropical-equatorial region of the Amazon Basin due to the Intertropical Convergence Zone (ITCZ)28. Such an impact can occur due to aerosol BC deposition on the glacier (among other biomass burning byproducts) and heat transport.

Figure 1 Smoke plume transport from the Amazon Basin to the Central Andes. (a) Formation of a mega-smoke plume resulting from the union of several biomass burning emissions (red dots represent the locations of fire events; clouds appear bright white and smoke appears in shades of grey). The images are daily MODIS true color band composition (1-red, 4-green, 3-blue) images from the Aqua satellite captured on 23 August 2010 (NASA Worldview application, link to the image: https://worldview.earthdata.nasa.gov). On that day, 148,946 fire events were registered in South America. (b) Trajectory density showing the average pattern of air mass trajectories during the biomass burning season from 2000–2016. Air mass trajectories at 6-hour time steps were analyzed each September (peak of the fire season in South America) from 2000 to 2016, resulting in a total of 2.040 trajectories. Trajectories were obtained using the HYSPLIT/NOAA model (https://www.ready.noaa.gov/HYSPLIT.php). Trajectory density (the number of trajectories over a 1.5 km radius) was calculated using ArcGIS® 10.2 software. Light blue circles represent glaciers located in South America (data from the Randolph Glacier Inventory database65), and black stars (regions I, II, III, IV, V, VI, and VII) indicate the origins of the trajectories located within regions with a higher density of fire events. Full size image

Among the glaciers located in the Central Andes, along the plume displacement, we chose the Zongo Glacier, located in Cordillera Real (−16.279°S; −68.142°W), Bolivia, as a study area (Supplementary Material, Fig. 3). Our choice of the Zongo Glacier was based on the availability of longer and continuous meteorological, surface mass balance and melting water discharge data for the glacier provided by the GLACIOCLIM program (https://glacioclim.osug.fr/) and the GREAT ICE program29.

One important point to observe is that any impacts of biomass burning on Andean ice can only be postulated if the convective rising of Amazon plumes can at least reach the altitudes of the mountain snowline. Over the Amazon Basin, previous LiDAR (Light Detection and Ranging) observations indicated that aerosol plumes during fire events can reach approximately 3.0–5.0 km in altitude30. Although Zongo Glacier extends from 4.9–6.0 km in altitude (the upper limit being beyond the smoke plumes), airflow striking the Andes Cordillera can be subjected to orographic effects that could lead the elevation of air masses to the altitude of the glacier. This mechanism was observed in South-Southeast Asia, where PM10 aerosols from biomass burning haze reached the upper troposphere due to orographic lifting over the Malaysian mountains31.

Smoke transport analysis was conducted to prove that biomass burning emissions can reach the Bolivian glaciers. Herein, we detailed the vertical distribution and aerosol types from two transects from the Amazon Basin to the Central Andean highlands encompassing the region of the Zongo Glacier using remotely sensed LiDAR observations of aerosols from CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) over the 2010 fire season (24 August 2010 and 11 September 2010). On August 24, 2010, the presence of smoke plumes over the Bolivian Amazon was clearly identified in cloud-free regions (Fig. 2a). During this period, numerous fire events were concentrated near the eastern side of the Andean Mountains within the Bolivian and Peruvian portions of the Amazon Basin. Meteorological data recorded at the Zongo Glacier indicated that northeastern and eastern winds were predominant (Fig. 2a). Such wind directions favor smoke plume displacement from the Amazon Basin towards the Andean glaciers. The CALIOP sensor (onboard CALIPSO) recorded an aerosol plume over the Andean Mountains containing the same composition as the aerosol plume located over the Amazon Basin (Fig. 2b). The plume contained mainly dust and smoke pollution. This finding confirms that smoke plumes from Amazonian biomass burning are capable of reaching the top of the Andean Mountains between altitudes of 5.0 and 6.0 km. On September 11, 2010, the plume over the Central Andes contained polluted continental/smoke, dust, and polluted dust. The polluted continental/smoke can be observed from the Amazon Basin to the top of the mountain range. No aerosol plume is observed on the west side of the Andes, whereas the polluted dust and dust observed over the Andean mountains should be caused by local production driven by wind (supplementary material, Fig. 4).

Figure 2 CALIPSO-CALIOP smoke transport analysis. (a) Daily MODIS true color band composition (1-red, 4-green, 3-blue) from the Aqua satellite captured on 08/24/2010 (NASA Worldview application, link to the images: https://worldview.earthdata.nasa.gov/08_24_2010). The Amazon Basin on the east side of the Andean Cordillera is completely covered by smoke from biomass burning. HYSPLIT backward trajectories show that wind masses that reach the Zongo Glacier come from the Amazon Basin (the wind direction at the Zongo Glacier corroborates with the HYSPLIT trajectories). (b) Vertical profile of aerosols over the Andean mountain range during the fire event on 08/24/2010 showing the presence of smoke above the Andean Cordillera. Full size image

Estimating black carbon concentrations at Zongo Glacier

Since the start of the MODIS satellite fire monitoring program in 2002, modeled emissions of BC data available at the global fire emission database (GFEDBv4) noted the 2007 and 2010 fire seasons as most critical for the Amazon Basin (Supplementary Material, Fig. 5a,b). Therefore, to assess the potential maximum impact of BC on the Zongo Glacier, we focused on these episodes. From the above, we modeled continental BC aerosol emissions, atmospheric concentration and deposition from the HYSPLIT/NOAA model. The BC emissions used in our model were from a global fire emission database, which uses a compilation of emission factors retrieved from experiments that include the measurements of BC in the Brazilian Amazon forest during biomass burning events32. From our results, atmospheric BC concentrations values due to Amazonian biomass burning emissions ranged from 0 to 5.0 × 10−3 mg m−3 in the Central Andes (Supplementary Material, Figs 6 and 7), and total BC depositions on the Zongo Glacier were estimated as 0.88 mg m−2 and 1.17 mg m−2 during the fire seasons of 2007 and 2010, respectively (Supplementary Material, Fig. 8). These predictions are in agreement with the measurements of BC at the Chacaltaya station (5 km from Zongo Glacier), where atmospheric concentrations of black carbon ranging from 0.2 to 1.5 × 10−3 mg m−3 were reported (database from 2012–2014)33 and with other global aerosol models that estimate BC deposition values between 0.5–2.0 mg m−2 for the Andes34.

Modeled BC deposition and measured snowfall at the glacier surface were used to calculate BC concentrations in snow at the Zongo Glacier (details are presented in the methods section). Liquid precipitation over the glacier is rare and, therefore, neglected in the model. Our estimated BC concentrations in snow were 41.1 ppb (parts per billion) for September 2007, 73.4 ppb for September 2010, and 29.2 ppb at the end of the fire season in October 2010. The lowest concentration (29.2 ppb of BC) in October 2010 was attributed to the high precipitation over the glacier during that period, which diluted the concentration accompanied by a decrease in fire events in the Amazon Basin. The high BC concentrations estimated for September each year were related to the peak in fire events in South America combined with associated low precipitation rates in the Andes. A comparison of our modeled concentrations with Illimani ice core data shows consistency. The Illimani Glacier is located approximately 55 km southeast of the Zongo Glacier, and due to its geographic proximity, it has a similar influence on the transport of particles from the Amazon Basin (from our model). The BC record from the Illimani ice core displays a strong seasonality, with low values during the wet season and high values during the Amazon fire season (Supplementary Material, Fig. 9), which is consistent with the observed transport of biomass-burning aerosols from the Amazon Basin. BC measurements performed on the Illimani ice core displayed a peak of 58.3 ppb of BC for 2007, which was slightly higher than the concentration of 41.1 ppb that we modeled for the same period for the Zongo Glacier (Supplementary Material, Fig. 9). In fact, our modeled values solely considered biomass burning contributions, whereas emissions from fossil fuels from nearby urban sites likely contributed, to some extent, to the total BC deposition35. Furthermore, post depositional processes must be important and may explain why our BC concentration values in snow were below the measured values in the Illimani core. For investigating major causes of BC interanual variability we examined spatial/temporal correlation between the Illimani BC record and the aerosol index over the South America from 1994 to 2009 (Supplementary Material, Fig. 10). We found significant positive correlation (p < 0.05) between Illimani BC and aerosol index for areas in the Amazon Basin located East of the Illimani Glacier, where extensive fires occur during the fire season. On the other hand, we did not find significant correlation between Illimani BC and aerosol index at La paz/ El alto region. Our results indicate that Amazon Basin is the main source region of the BC deposited at Illimani.

Reduced snow albedo and enhanced energy flux due to black carbon and dust at Zongo Glacier

Very few studies have investigated the impurity content in Andean low-latitude glaciers. Measured dust concentrations ranging from 1 to 9 ppm were reported for the Quelccaya Glacier, Peru (5670 m asl)36. During the wet season, impurity contents ranging from 10 ppm in fresh snow to 100 ppm in 1-week-old snow37 were observed on the Zongo Glacier. Here, we investigated the snow albedo reduction and the consequent enhanced energy flux at the Zongo Glacier due to BC only and BC in the presence of previously reported dust concentrations (10 ppm and 100 ppm of dust). Considering solely the BC concentrations in snow, we performed estimations using the SNICAR snow albedo simulation38, with an albedo reduction ranging from 1.8 to 7.2% (Supplementary Material, Fig. 11). Again, our results are in agreement with radiative transfer calculations, in which BC concentrations in snow on the order of 10–100 ppb correspond to decreases in albedo by 1–8%8,39,40. Considering the albedo reduction due to BC in the presence of 10 ppm and 100 ppm of dust, we estimated reductions ranging from 3.8 to 9.6% and 11.5 to 20.2%, respectively (Supplementary Material, Fig. 11). These results are in agreement with the observations that visible snow albedo during the dry season on the Zongo Glacier can be reduced by 10–20% due to atmospheric aerosols contained in snow37.

The BC contribution to energy flux on the snow/ice surface for the period from 8 September to 31 October 2010 (corresponding to the Amazon fire season) was assessed (Supplementary Material, Fig. 12a). The BC contribution to the energy flux reached a maximum during the peak of the Amazon Basin fire season in mid-September, 10.7 ± 3.5 W m−2. Following this period, the energy flux contribution decreased to values between 4.7 ± 1.6 and 1.3 ± 0.4 W m−2 by the end of October since in this period, BC deposition rates were reduced, while precipitation rates and cloud cover tended to increase (consequently lowering incident solar radiation at the glacier’s surface and diluting the concentration in the snow/ice surface). The sum of the variables radiative balance (R), sensible heat flux (H), latent heat flux (LE) and forcing due to BC corresponds to the total energy flux at the glacier’s surface (Supplementary Material, Fig. 12a). Radiative balance (R) is the sum of the net incoming and outgoing shortwave and longwave radiation. It is the main source of energy for the glacier’s surface and may significantly vary over time (days). Following BC deposition, R presents higher values during September and lower values during October. Most of the time, the sensible heat flux was positive, whereas the latent heat flux remained negative. The sum between the radiative balance and sensible and latent heat fluxes was positive all of the time, even when the latent heat flux was high. The result indicates that although a portion of energy was consumed by sublimation, sufficient energy for generating snow/ice melting conditions during the fire season was present. Based on the above, BC is evidently an effective “melting parameter” since it positively contributes to an increase in the radiative balance due to the albedo effect, thus intensifying melting. The increase in energy flux used to melt snow/ice (QM) due to BC was 4.2 ± 1.4% in September and 3.6 ± 1.2% in October 2010 (Supplementary Material, Fig. 12b).

Enhanced mass loss due to black carbon and dust at Zongo Glacier

A snow/ice surface mass balance model was applied for Zongo Glacier based on a linear regression between QM at the surface and the in situ annual mass balance measured at the glacier between 2005 and 2011. Simulations were made at an elevation range of 5.100–5.200 m a.s.l., which was where the automatic weather station was operated and where the surface mass balance measurements were conducted. The linear regression between the surface mass balance and the energy balance show statistical significance (statistical significances were based upon a Student t test, p < 0.001, n = 6, r² = 0.96). The coefficient of determination (R²) between the measured and modeled surface mass balances was 0.96, indicating the ability of our model to accurately reproduce the observed surface mass balance (Supplementary Material, Fig. 13).

Mass loss due to BC was estimated for 2010 by considering varying BC concentrations for September and October 2010. Additionally, two other estimations were conducted, taking into account two different conditions. The first was based on constant BC concentrations of 41.1 ppb (representing the BC concentration modeled for September 2007); the second was based on constant BC concentrations of 29.2 ppb (representing the BC concentration modeled to the end of October 2010). For each condition, we also considered the presence of dust (10 ppm and 100 ppm) and different snow grain sizes (300 µm, 500 µm and 1000 µm) (Fig. 3a). Based on these estimations, BC from Amazonian biomass burning has the potential to increase annual melting by 3.3 ± 0.8%, whereas dust alone can increase annual melting by 3.2 ± 0.9%, when dust content is low (10 ppm of dust) and by 10.9 ± 2.6%, when dust content is high (100 ppm of dust) (Fig. 3b). In the combined presence of BC and dust, annual melting can increase from 5.0 ± 1.0% to 11.7 ± 2.3% (Fig. 3b). For comparison, this value is comparable to that for glaciers in Kyrgyzstan (central Asia), which have been documented to have an increase in snow melt rates of 6.3% due to the BC-albedo effect39.

Figure 3 Contribution of aerosol particles in snow to mass loss at Zongo Glacier. (a) Potential annual snow/ice mass loss due to BC and dust for different scenarios. (b) Bar plot showing the average potential contributions of BC and dust to annual mass loss at the Zongo Glacier. Full size image

Comparing biomass burning “forcing” with water discharge at Zongo glacier

A database of in situ interannual water discharge was used for direct comparison with the estimated mass balance. Water discharge behaved similarly to precipitation, displaying a major increase between October and February in 2009/2010 and following radiation flux seasonal variability for the tropics (Fig. 4a,b). A steady decrease in discharge was observed from February to August 2010. During the dry season (May-July), net radiation was slightly lower, and almost no precipitation was observed on the glacier (Fig. 4a,b). The dry period was marked by western/northwestern winds (Fig. 4c), which prevent humidity flow from the Amazon Basin to the eastern Andes41,42. A synoptic analysis indicated that days with easterly winds during the dry period were uncommon, occurring approximately 5% of the time41; consequently, low humidity and western winds favored sublimation at the glacier’s surface, consuming much of the available energy and resulting in low melting41,43. From August onward, the predominant wind direction changed from western/northwestern to eastern/northeastern (Fig. 4c). This change is associated with the weakening and southward migration of the subtropical jet that occurs due to the southward migration of the ITCZ in conjunction with the establishment of the Bolivian high42. This change in the predominant wind direction results in the prevalence of a weak easterly wind in the middle and upper troposphere within the Central Andes Altiplano. Such a change occurs during the fire season and favors smoke transport to the glacier (Fig. 4c,d). One may observe from Fig. 4a that a secondary peak in water discharge does exist from the beginning of August to the end of September, occurring outside the rainy season, comprising approximately 9% of the total annual discharge. This secondary peak in water discharge began in August and extended until the end of September, coinciding with the fire season of the Amazon Basin (Fig. 4a,d) and following the increase in aerosol emissions from biomass burning (Fig. 4f,g). For September 2010, water discharge corresponded to 4.5% of the annual discharge, which was approximately the same magnitude as expected for glacier runoff due to the impact of BC and mineral dust for the same period (3.3 ± 0.8% for black carbon only; 5.0 ± 1.0% for black carbon in the presence of 10 ppm of dust). Net radiation flux and temperature were maintained nearly the seasonal variability over this period (Fig. 4b,e). Additionally, precipitation was low over the glacier, indicating that the increase in water discharge most likely resulted from an increase in snow/ice melting at the glacier’s surface.

Figure 4 Hydrological and meteorological conditions of Zongo Glacier during the 2010 fire season. (a) Daily water discharge at Zongo Glacier during the 2009/2010 hydrological year and precipitation over the glacier (AWS data). (b) Daily fire outbreaks within the Amazon Basin (the INPE fire database). (c) The wind direction (AWS data). (d) Incident solar radiation (AWS data). (e) Air temperature (red) and humidity (blue) of the Zongo Glacier (AWS data). (f) Monthly aerosol index map for September 2010. The green line represents the geographical limit of the Amazon Basin, the blue dot represents the location of the Zongo Glacier, and the pink dashed line represents the hot spot regions for fire events used for the analysis of AI time series. (g) Water discharge of Zongo Glacier and the AI within the Amazon Basin during the peak of the 2010 fire season. Full size image