PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
May 2014
389
S
now
A
lbedo
At visible wavelengths, ice is nearly non-absorbing,
which means that spectral albedo is controlled by the
absorbing particles mixed in with the glacier ice. It
also means that the albedo of snow over dark surfaces
can be influenced by the dark surface below. During
the 2012 ACSP expedition, samples were collected in
a crevasse to identify changes in contaminant levels
over time. The crevasse wall showed intermittent
dark layers which were often separated by a meter
or more of cleaner ice (Figure 4). Even if these dark
layers were optically perfectly absorbing, they would
likely have little influence on the spectral albedo.
Black carbon and dust in snow and ice cause the
greatest reductions in albedo when the effective ice
particle size is large (Warren and Wiscombe, 1980).
Gardner and Shart (2010) show calculations of snow
pack albedo using different assumptions of particle
size and impurity levels. The uncertainty due to lack
of knowledge of effective particle size is substantial.
This issue is mitigated somewhat for tropical
glaciers. In the tropics, the temperature does not
vary as significantly as in mid-latitudes. With more
predictable temperatures, the evolution of ice particle size is more
easily predictable. To test this, snow pits have been excavated
during ACSP expeditions. Observations showed that typical
particle sizes were similar in different years at the same location
and that particle sizes were uniform (2-5 mm) with respect to
depth (down to 1.2 meters below the surface).
C
onclusions
While remote sensing detection of black carbon in annual snow
packs is complex, a number of complicating issues are mitigated
when considering tropical glacial ice measurements versus snow
measurement and using extensive field data collection. Glaciers
are thick, which means that sub-glacier absorbing features will
not play a role in albedo. Due to consistent temperature in the
tropics, the effective ice particle sizes that make up glacial ice can
be more easily predicted. Low level clouds such as diamond dust
or ice fog which can lead to uncertainties in the arctic regions are
far less likely to be issues above tropical glaciers.
Field measurements such as those conducted by the American
Climber Science Program can help to validate remote sensing
measurements. The Cordillera Blanca mountain range contains
approximately 500 square kilometers of glacier area, down 20-30%
since1970 (Rabatel et al., 2013). Light absorbing particles may
have played an important role in the reported glacial loss. Figure
5 shows the relative concentrations of light absorbing particles
based on ACSP measurements. These results indicate that there
are substantially more light absorbing particles including black
carbon on the glaciers near Huaraz, the largest nearby city. ACSP
measurements on glaciers further from Huaraz showed that most
of the light absorbing particles in glacial ice were dust particles.
This would suggest that remote sensing studies should initially
consider glaciers in reasonable proximity to large population
centers, as the signal might be significantly stronger and easier
to detect.
The ACSP plans to expand its measurements of glaciers to
different regions in the coming years. Regions which are likely
highly impacted by pollution from cities as well as from biomass
burning, such as in the Amazon basin, will be targeted. This
research will be conducted in collaboration with the Pollution and
its Impacts on the South American Cryosphere (PISAC) Initiative
), an international collaborative group of
scientists whose goal is to investigate key sources and impacts
of black carbon and co-pollutant emissions in the Andean and
Patagonian regions.
A
cknowledgements
We would like to thank the American Alpine Club for their
support of the ACSP expeditions. We would also like to thank
Ellen Lapham for her unwavering support of ACSP Peru
expeditions and thanks to all of the volunteers who have made
these expeditions a success. The authors also thank Leslee
Schmitt for editing the manuscript.
Figure 5: Map of the Cordillera Blancamountain range. The relative concentration
of light absorbing particles for each of the three expeditions is shown below.
Huaraz, the largest city in the region is indicated. Note that the most polluted
glaciers are nearest to Huaraz.
