Glaciers in the tropical Andes have been rapidly losing mass since the 1970s.
In addition to the documented increase in temperature, increases in light-absorbing particles deposited on glaciers could be contributing to the
observed glacier loss. Here we report on measurements of light-absorbing
particles sampled from glaciers during three surveys in the Cordillera Blanca
Mountains in Peru. During three research expeditions in the dry seasons
(May–August) of 2011, 2012 and 2013, 240 snow samples were
collected from 15 mountain peaks over altitudes ranging from 4800 to
nearly 6800 m. Several mountains were sampled each of the 3 years and
some mountains were sampled multiple times during the same year. Collected
snow samples were melted and filtered in the field then later analyzed using
the Light Absorption Heating Method (LAHM), a new technique that measures the
ability of particles on filters to absorb visible light. LAHM results have
been calibrated using filters with known amounts of fullerene soot, a common
industrial surrogate for black carbon (BC). As sample filters often contain
dust in addition to BC, results are presented in terms of effective black
carbon (eBC). During the 2013 survey, snow samples were collected and kept
frozen for analysis with a Single Particle Soot Photometer (SP2). Calculated
eBC mass from the LAHM analysis and the SP2 refractory black carbon (rBC)
results were well correlated (
A substantial portion of the population of South America lives on the west coast, where the water supply is supported by the runoff from glacier melt (Barnett et al., 2005). When a region with glaciers is experiencing warming, the water availability for the region will change even if precipitation patterns remain constant. Initially, runoff from glaciers will increase as melting increases, but eventually the glacier area will be sufficiently reduced so that the increased melting rate will be offset by the decreased glacier area available for melting and increased water loss due to evaporation, sublimation, or absorption into ground surfaces. Thus, over time the typical water discharge increases as warming starts, reaches a peak, then reduces until a new steady state is reached, presumably after the climate has stopped changing or after the glaciers have completely melted (Baraer et al., 2012). Post warming water flow rates are expected to be significantly below pre-warming flow rates. If glacier melt becomes insignificant, surface water flow rates are then driven more by precipitation, and as a result, dry season water flows can be substantially reduced while wet season flows may not be as significantly impacted. Baraer et al. (2012) estimated the evolution of water flow rates for the Cordillera Blanca region. Their results show that most valleys are past their highest levels of runoff, implying diminishing water supplies into the future if melting persists. The effect of increased BC and dust on this glacial “life cycle” is to accelerate the process. Increased BC and dust will lead to faster glacier recession, though it is unclear what the effect on runoff would be during the transition phase.
In our warming climate, glaciers are melting at a fast rate and tropical
glaciers are being substantially impacted. Glaciers in Peru account for more
than 70 % of the world's tropical glacial area (Kaser, 1999). Rabatel et
al. (2013) showed that glaciers in the tropical Andes have receded in area
approximately 30 % since the 1970s and that areal loss rates have
increased substantially in the first decade of the new millennium. Vuille et
al. (2008) showed that surface air temperature has increased by
0.10
In addition to air temperature changes, dust and BC deposited on snow could potentially be a major factor leading to glacial mass loss. BC and dust reduce snow's albedo by absorbing solar radiation, which leads to increased melt and sublimation rates (Warren and Wiscombe, 1980). It has recently been shown by Painter et al. (2013) that BC likely led to the end of the little ice age in the European Alps in the mid-1800s. Dust on snow in Colorado has led to substantially increased melt rates (Painter et al., 2012) and has decreased total runoff (Painter et al., 2010). The effects of BC on snow in the Northern Hemisphere has been subject to intensive study, however, despite the fact that glaciers are a critical water source for large populations in South America, no studies to date have measured BC and dust in this region.
The Cordillera Blanca range is located in north central Peru. The mountain
range is home to 17 peaks over 6000 m and hundreds over 5000 m. The bulk of
precipitation comes from the Amazon basin in the wet season, which generally
runs from October until May. During the dry season, the atmospheric flows
generally come from the west and include more dry air. The dry season in the
Cordillera Blanca is not completely dry as it is possible for small storms to
pass through the range typically on a weekly basis. These dry season storms
can produce snow from a few centimeters to a few tens of centimeters which
can sometimes obscure the expected variability of light-absorbing particles
by depth. Schauwecker et al. (2014) estimated that for the Cordillera Blanca
region, there has been a modest temperature increase as well as a modest
(60 mm decade
Sources of BC and dust in the Cordillera Blanca are numerous. Obvious anthropogenic sources of BC include industry and transportation (especially from diesel-fueled vehicles). Industrial sources are mostly centered in and around Huaraz, the largest city (population: 100 000, altitude: 3052 m altitude) abutting the Cordillera Blanca. Regional emission inventories for BC are not available, and emission inventories developed for global models are generally based on national emission statistics and do not contain regional specifics. There is substantial agriculture and BC from agricultural burning in the Cordillera Blanca region. Dust from land clearing and livestock grazing could impact the Cordillera Blanca cryosphere as well. Another potential source of BC is forest clearing and biomass burning in the Amazon basin. Amazonian burning has been estimated to account for approximately 50 % of the carbonaceous aerosols in the amazon region (Lamarque et al., 2010; Kaiser et al., 2012). Hydroelectric power is one of the most common forms of energy production, and coal burning is not significant in South America. Increases in dust can be caused by agriculture, construction, mining, and increased traffic on dirt roads.
The measurements described in this publication were collected by scientists
and volunteers participating in American Climber Science Program (ACSP)
expeditions (
Here, we report on 3 years of measurements of light-absorbing particles sampled on glaciers in the Cordillera Blanca. Section 2 describes the field campaigns and the sample collection methods. Section 3 describes a newly developed technique for the quantification of light-absorbing particles collected on filters. Section 4 presents the results from the 3 years of measurements, and the final section discusses the main conclusions of the project.
The ACSP has conducted three research expeditions during the dry season (austral winter) to the Cordillera Blanca region of Peru beginning in 2011. The sampling of snow and ice for light-absorbing particles has been a primary research project during the three expeditions. During the 3 expeditions, 48, 100, and 90 samples were collected respectively. Samples were taken from glacial surfaces from a minimum altitude of 4800 m and then at regular altitude intervals to the mountain top. Several mountains were sampled during all three expeditions, and some mountains have been sampled more than once during a single expedition. On one occasion multiple samples were taken at different depths by collecting ice from the walls of a crevasse; these data are used to identify long-term trends in particle loadings.
When working in remote and demanding conditions it is important to use simple techniques that provide robust scientific results. Several techniques for snow particle sampling were considered before a filtering technique was chosen. The technique for sampling particles in snow is similar to the technique used by the University of Washington's “Soot in Arctic Snow” group (as described in Doherty et al., 2010).
During research expeditions, volunteer scientists collect snow samples from predetermined locations on each mountain. Samples are collected from approximately 100 m above the snow line to the summit at 200 to 500 vertical meter intervals, the exact interval depends on the height of the mountain and the complexity of the terrain. Volunteers are instructed to avoid areas near exposed rock as well as areas that may be near avalanche paths or could be affected by debris from avalanches. Because mountain climbing is a physically demanding and potentially dangerous activity, the ultimate decisions on how high and where to collect samples is left to the discretion of the climbing team with safety being the highest priority. Global Positioning System waypoints are taken and, if possible, samples are collected from the same location as on previous climbs. Snow samples were collected by scooping snow into 4-liter ziplock plastic bags. Bags are labeled on the outside with a permanent marker after the bag has been sealed. Typically, ACSP volunteers collected samples with hands that had been “contaminated” with local snow. This is done by washing hands with snow so that any contaminants on the hands are similar to those in the snow. Since snow is collected on the descent usually well after sunrise, the snow is soft enough that it is easy to fill a bag contacting the snow only minimally. For each sample approximately 1 kg of snow is collected at each site from both the surface (defined as the top 2.5 cm) and the subsurface (deeper than 2.5 cm). The idea was that the surface sample should give an indication of any dry deposition that has occurred since the last snow storm as well as any accumulation of contaminants on the surface due to melting and sublimation while the subsurface samples should contain any contaminants that came with the most recent snowstorm either as ice nuclei or as having been scavenged by falling snow. In reality, it was commonly found that fresh snowfall during the dry season amounted to less than 10 cm. The result was that the subsurface sample often contained snow that had been subject to dry deposition and surface accumulation before the most recent snowfall. Once samples were collected and labeled, the snow samples are packed together into backpacks and returned to base camp for processing.
In camp, snow samples are melted one at a time by placing the ziplock bags in
warm water (usually around 30
Diagram of the LAHM analysis setup. A light source is aimed at a
45
A new technique has been developed for the analyzing the light absorption
properties of particles collected on filters. The Light Absorption Heating
Method (LAHM) is a cost effective technique that can be used to accurately
quantify the impact of light-absorbing particles on snow. The premise behind
conducting measurements of light-absorbing particles in snow is to estimate
the amount of light energy the particles will absorb leading to increased
melting or sublimation. Because snow is completely absorptive in the thermal
infrared wavelength range, the more critical wavelengths to consider are in
the visible range. Understanding the absorption of solar radiation in the
visible wavelength range by particles in snow is the goal of the research.
The LAHM analysis technique described here approaches the problem directly by
measuring the temperature increase of a particle load on a filter when
visible light is applied. A diagram of the LAHM setup is shown in Fig. 1.
Each filter is suspended on plastic wrap a few centimeters above a pool of
cool water. Water is used because it provided a thermally stable background
compared to a solid surface which could absorb visible radiation. Plastic
wrap does not absorb visible light, and as such it provides a neutral
background and is of low mass per unit area thus having a negligible effect
on measurements. An infrared thermometer (IR thermometer: Omega OS1327D)
which can record data every second is mounted above the filter to measure the
temperature of the filter without touching it. To determine the light
absorption ability of the materials, a laboratory grade light source
(Cole-Parmer Fiber-Lite Fiber Optical Illuminator model 9745-00) with a fiber
optic light pipe to direct the light is mounted close to the filter and
shines light at approximately a 45
Measured temperature profiles for seven filters with a variety of light-absorbing particle loads. Temperature recording started 10 s before the lamp was illuminated, and the lamp was extinguished 30 s later. Temperatures are normalized to the average of first 10 s of data and the temperature usually returned to the start temperature within a minute of the lamp being extinguished.
The LAHM analysis technique was calibrated by sampling a BC standard: water
with a known concentration of BC (Schwarz et al., 2012). The BC standard has
10 % uncertainty. Filters with low and high BC loads were made by
filtering different amounts of BC standard. The BC standard was made with
fullerene soot, a reference material used in the single-particle BC detection
community (Baumgardner et al., 2012). The filters used for calibration
(Millipore mixed cellulose ester filter membrane 0.22 micron) are different
from the filters that have been used in the field (Pallflex Membrane Tissuquartz 0.7 micron). The BC standard was created with BC particles in the
size range generally found in the atmosphere (0.2 to 0.8 microns) which were
not captured well by the Tissuquartz filters. Torres et al. (2014) found that
Tissuquartz filters captured less than 38 % of the BC mass when filtering
rainwater. The mass size distribution for their measurements peaked between
210 and 240 nm, similar to atmospheric BC sizes but smaller than the peak
sizes expected in snow (Schwarz et al., 2012). The Tissuquartz filters are
used in the field because water flow through the filters is much faster
making them much easier to use in the field and the filter material enables
additional types of analysis that are not possible with the Millipore
filters. Temperature profiles recorded for unused filters of both types were
nearly identical (well within the SD of measurements), indicating that the
filter type did not bias the temperature measurement technique significantly.
Three calibration filters at each of four different fullerene soot mass
amounts (5–20
Laboratory measurements of post-filtered samples showed that the Tissuquartz filters did not efficiently collect BC particles smaller than about 0.6 microns. In the atmosphere most BC mass is distributed across particles smaller than this, but BC in snow has been shown to exist, in some samples, at sizes up to a few microns (Schwarz et al., 2012). While it is clear that the Tissuquartz filters may not be collecting all of the BC, it is unclear what portion is being missed. Results of separate mineralogy tests suggest that there were high concentrations of dust which can clog the pores and anecdotal evidence (volunteers noting that it had become very difficult to push the water through the filters) suggests that the effective pore size substantially decreases as dusty samples are being filtered. Given that BC particles in this size range can be missed, the BC on the sample filters is likely an underestimate of the true BC in the snow. Since this under-catch was noticed, tests have been conducted using snow from tropical glaciers that indicate that the missed particles do not account for much absorption. Snow sourced water that had already been passed through a Tissuquartz filter was collected and passed through a 0.22 micron filter. While the Tissuquartz filters in these tests captured high concentrations of particles, the 0.22 micron filters appeared clean after filtering and results from the LAHM technique were below the detection threshold. Of the approximately 20 tests filters, the eBC (effective black carbon) value estimated for the 0.22 micron filter was never more than 20 % of the value determined for the Tissuquartz filter and the average was around 5 %, well within the noise level of the LAHM technique at these levels.
The LAHM technique does not discriminate between BC and dust. Therefore, the values derived from the LAHM technique should be treated as eBC, meaning that the visible light absorptive capacity of the particles on the sample filter are equivalent to the given amount of BC (Grenfell et al., 2011).
The increase in temperature of the calibration filters after 10 s of
exposure to light was used to derive a fit equation to predict the mass of BC
on the calibration filters. The
Data from 2011 were collected with a slightly different filtering setup than subsequent expeditions. The change was made because the type of filter housing used in 2011 occasionally came into contact with the surface of the filter leading to the removal of some of the particles when the filter housing was opened. We estimate that this may have reduced the filter loading by up to 25 %. This estimate is based on the approximate area of the filter that appeared to be scraped clean as well as the fact that the filter holder had obvious particle loading that corresponded to the cleaned area. While it is not possible to determine the exact amount of material lost, the eBC values estimated for the affected filters were multiplied by 1.25.
Temperature profiles measured for a calibration filter with fullerene soot (lower curve), and a sample filter with dust and black carbon (upper curve). The curves are nearly identical until 20 s (10 s of heating) after which they diverge. The continued more rapid increase in temperature of the dust – black carbon mixture is thought to be due to the dust.
During the 2013 expedition, 12 unmelted snow samples were collected in acid
washed glass vials and returned to the US for analysis with a Single Particle
Soot Photometer (SP2) instrument (see Schwarz et al., 2012). These samples
were collected from two mountains: one in a region suspected to be highly
polluted due to its proximity to Huaraz and another mountain in a more remote
region. Additional snow samples were collected for filtering from the same
locations for comparison. Results from the LAHM technique are compared to SP2
refractory black carbon (rBC; Baumgardner et al., 2012) mass concentration
measurements in Fig. 5. Note that while there is some variability between the
techniques, the results are reasonably correlated (
Plot showing the relationship between eBC as determined from the
LAHM analysis and refractory black carbon as determined by the SP2 instrument
(
The eBC values determined by the LAHM technique for all measurements
collected during the three expeditions are shown in Fig. 6 plotted by
altitude. Values of eBC range from the quite low: 2.0 ng-eBC g-H2O
As stated in the field measurements section, it was rare to collect samples where the surface sample contained long term dry deposition and surface accumulation and the subsurface sample contained only pristine snow. Often, the snow from the previous storm had compacted enough that the subsurface sample contained snow that had been previously exposed as a surface. This led to the eBC values for the surface and subsurface samples to not be consistently related. During the 3 expeditions there were 92 paired measurements where a surface sample and a subsurface sample were collected. Of those samples, in 56 cases, the surface sample contained higher eBC values than the subsurface samples while for the remaining 36 samples the opposite was true. The average ratio (averaged in log-space) was 0.96 suggesting that on the average, the surface and subsurface samples were statistically similar. The average difference between the two samples was a factor of 2.17 with a maximum difference of a factor of 12. Based on the fact that it does snow from time to time in the dry season, and that samples are never collected near the zone of ablation, the differences are thought to be due to short term changes in the snow pack rather than annual variability. Figure 7 shows a probability distribution function of the ratio between the surface measurement to the subsurface measurement for the 92 pairs of measurements.
complete data set of eBC values determined from the LAHM analysis plotted versus altitude.
Probability distribution function of the ratio of the eBC value
measured for surface samples divided by the eBC value measured for
subsurface samples presented.
Google map image of the Cordillera Blanca mountain range. The five
regions as well as the city of Huaraz are indicated on the map by black
arrows. Average eBC values as determined from the LAHM analysis are shown in
the plot for each of the 3 years. The thin lines indicate
Figure 8 shows a map of the Cordillera Blanca region and the eBC values in
snow determined from filter samples using the LAHM analysis technique for
filter samples collected in different areas during each of the three
expeditions. For the three expeditions, the data are grouped into five
different valleys or mountains that were sampled each of the 3 years.
Regions 1 and 2 lie on the north end of the range and consist of the
mountains in the Santa Cruz and Paron valleys (region 1, 70 km from Huaraz),
and Llanganuco valley (region 2, 54 km from Huaraz). These regions are
characterized by higher precipitation and lower nearby population densities.
The mountains around Ishinca valley (region 3, 21 km from Huaraz),
Vallunaraju mountain (region 4, 14 km from Huaraz), and the mountains around
the Quilcayhuanca valley (region 5, 24 km from Huaraz) are all clustered
near Huaraz, the largest population center in the region. The eBC values
shown in Fig. 8 are determined by taking an average of all measurements of
surface and subsurface snow in each of the regions. As the relationship
between the surface and subsurface values was highly variable (as shown in
Fig. 7), the averaged values are thought to better represent the typical
conditions for the top 10 cm of snow. Standard deviations of the
measurements in each region are shown in Fig. 8. Data from all 3 years
show a distinct trend, with the southern regions having 2–3 times more light-absorbing particles compared to the northern regions. The SP2 measurements
discussed in the calibration section were collected in 2013 in region 2 and
4. The region 2 SP2 measurements averaged 0.65 ng g
LAHM determined eBC values averaged by altitude bins plotted with
Figure 9 shows the mean value and standard deviation for data collected from all 3 years separated by altitude. Data are binned into two groups: region 1 and 2 in one group and 3–5 in the second group for all 3 years combined. Note the relatively linear decrease in eBC with altitude for both regions. Thin lines show the standard deviation calculated for the more numerous measurements. In general, eBC concentration levels decrease with altitude, though it should be noted that there are few measurements at altitudes above 6000 m and those are mostly in region 2.
During the 2012 expedition, samples were collected from the wall of a newly
opened crevasse on Vallunaraju mountain in region 4. Vallunaraju is the
nearest glaciated mountain to Huaraz and has some of the highest eBC
measurements using the LAHM technique as well as the highest concentration
observed by the SP2 (75 ng-rBC g-H2O
eBC from snow samples collected from the walls of a crevasse on Vallunaraju (region 4) the nearest high mountain to Huaraz. A decreasing trend with depth is noted. These lowest samples were visually the cleanest snow samples from the crevasse walls, and were likely to have accumulated during the wet season. The points at 4, 6, and 8.5 m were thin noticeably dark layers.
It is well documented that tropical glaciers are melting rapidly with concurrent effects on critical water supplies. Numerous factors could be contributing to low latitude glacier mass loss, including larger concentrations of light-absorbing particles on glacier surfaces. Because melting glacial water is an important natural resource in the region for agriculture, hydroelectric power, and drinking water, an increased understanding of the pressures on this resource could aid regional planners in adapting to future regional climate changes. This paper presents the results of 3 years of measurements of light-absorbing particles on the glaciers in the Cordillera Blanca in Peru. Samples were collected by volunteers participating in American Climber Science Program expeditions. Snow samples collected from glacier surfaces were melted, and the light-absorbing particles collected on filters.
A new technique (called the LAHM technique herein) to quantify the amount of
visible radiation the light-absorbing particles can absorb has been
developed and calibrated using filters with a known amount of black carbon.
These “effective black carbon” estimates from the LAHM technique were found to be well correlated (
In the Cordillera Blanca, the concentration of light-absorbing particles was
highest near Huaraz, the largest city in the region, while samples from more
remote regions contained substantially lower amounts of absorptive material.
The measured levels of light-absorbing particles in the snow near Huaraz
absorb a similar amount of visible radiation as snow that contains between
20 to 80 ng g
This work would not have been possible without the help of the American Climber Science Program volunteers for the 2011, 2012, and 2013 expeditions. The authors specifically wish to thank Frank Nederhand for initiating the program that would lead to the ACSP and Chris Benway at La Cima Logistics in Huaraz, Peru for helping to organize the expeditions. Thanks to Darrel Baumgardner for helping to facilitate the SP2 measurements as well as for helping to bring this work to the attention of the Pollution and its Impacts on the South American Cryosphere (PISAC) Initiative. Thanks to Karl Froyd and Jin Liao for their helpful comments and analysis with the PALMS instrument. Thanks also to Huascaran National Park, Jesus Gomez park director, for helping ensure the long term success of this research. Additional thanks go to the many generous donors who have made the ACSP expeditions possible. Also, the American Alpine Club whose early assistance was invaluable. Edited by: A. Klein