Abstract
When deposited on snow and ice, light absorbing impurities (LAIs) such as dust and black carbon (BC) reduce surface albedo and enhance melt. BC comes from incomplete combustion of fossil fuels and biomass. Local and regional sources of BC exist in High Mountain Asia, such as dry-dung burning for heat and fuel, which occurs in close proximity to snow and glaciers. Local dust or dust transported from the Indo-Gangetic Plain is also present. In the Ganges River Basin, meltwater is dominated by seasonal snow, yet relatively few observations of impurities in seasonal snow exist. To understand sources of impurities and their concentrations for seasonal snow on land, we evaluated multiple lines of evidence to scale up from ground-based measurements in the Dudh Koshi River Basin, a remote headwater basin within the Khumbu Region of Nepal. We obtained ground-based in-situ observations of refractory black carbon (rBC) measured by Single Particle Soot Photometer (SP2), including size distributions in snow on land. We interpreted these results in the context of concurrent Moderate Imaging Spectroradiometer (MODIS) satellite observations and speciated aerosol optical depth derived from reanalysis products modeled with the Navy Aerosol Analysis Prediction System (NAAPS) global aerosol model. We collected snow samples, mostly in the Gokyo Valley, at varying distances from local tea houses along a 2000 meters above sea level (m.a.s.l) elevation transect from 3250 to 5299 m.a.s.l . rBC concentrations ranged from 3.9 to 76.8 µg-rBC/L-H2O. Although previous data do not exist at these lower elevations, our findings are higher than previous surface snow results at higher elevations in the nearby Khumbu Valley. In general, rBC concentrations were lower in fresh snow than aged snow; concurrent MODIS satellite observations of snow albedo also show smaller impacts from LAIs in visible wavelengths in fresh snow. rBC decreased with elevation in aged snow samples, as did concurrent MODIS albedo observations. rBC-particle size distributions also shifted to a larger mode for aged snow samples. Results from NAAPS indicate anthropogenic and biogenic fine aerosols from biofuel (dry-dung burning) are primary aerosol species for the study period, at ~thrice the concentration of dust and smoke.
Highlights
At the headwaters of the Ganges basin, melt from seasonal snow on land, which can be distinguished from melt of exposed glacier ice or snow on ice, has been found to be the dominant source of meltwater contributions above 2000 meters above sea level (m a.s.l.) (e.g., Figure 3; Armstrong et al, 2018). Armstrong et al (2018) found that snow on land contributes discharge volumes of 12.5 km3 at 3000–4000 m a.s.l., 22.1 km3 at 4000– 5000 m a.s.l. and 30.7 km3 at 5000–6000 m a.s.l
In this study we explore the concentration and size distributions in the winter dry season of refractory black carbon (rBC) measured by SP2 in seasonal snow that remained frozen until just prior to analysis. rBC was characterized in the Gokyo Valley, which is one valley west of the Khumbu Valley studied by Kaspari et al (2014) and Jacobi et al (2015)
We analyzed data for the 1/3◦ pixel covering the Dudh Koshi River Basin shown in Figure 5A using the native Moderate Imaging Spectroradiometer (MODIS) sinusoidal projection, as well as snow covered area and vis from MODIS within the 1/3◦ NAAPS pixel (Figure 5B)
Summary
At the headwaters of the Ganges basin, melt from seasonal snow on land, which can be distinguished from melt of exposed glacier ice or snow on ice, has been found to be the dominant source of meltwater contributions above 2000 meters above sea level (m a.s.l.) (e.g., Figure 3; Armstrong et al, 2018). Armstrong et al (2018) found that snow on land contributes discharge volumes of 12.5 km at 3000–4000 m a.s.l., 22.1 km at 4000– 5000 m a.s.l. and 30.7 km at 5000–6000 m a.s.l. At the headwaters of the Ganges basin, melt from seasonal snow on land, which can be distinguished from melt of exposed glacier ice or snow on ice, has been found to be the dominant source of meltwater contributions above 2000 meters above sea level (m a.s.l.) (e.g., Figure 3; Armstrong et al, 2018). Other recently published studies show that local basin hypsometry plays an important role in high elevation meltwater contributions (Mimeau et al, 2019). Contributions of snow on land, snow on ice, and exposed glacier ice may vary from basin to basin at high elevations. Due to the large snow-covered land area at these elevations, the chemistry of the snowmelt is an important contribution to downstream water quality
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