WATER RESOURCES RESEARCH, VOL. 32, NO. 6, PAGES 1713–1718, JUNE 1996
Albedo of dirty snow during conditions of melt H. Conway, A. Gades, and C. F. Raymond Geophysics Program, University of Washington, Seattle
Abstract. The evolution of spectrally averaged albedo (wavelengths between 0.28 mm and 2.8 mm) of snow surfaces treated with known initial concentrations of particles of submicron-sized soot and air fall volcanic ash was investigated during conditions of natural melt. Depending on the particle type and concentration, the initial applications reduced the surface albedo to values ranging from 0.18 to 0.41 which were substantially lower than the albedo of the untreated natural snow (about 0.61). Many of the soot particles flushed through the snowpack with the meltwater, and surface concentrations of soot greater than about 5 3 1027 kg/kg did not persist for more than a few days. The migration of particles to depth caused the snow to brighten after the initial application, thus limiting the amount of albedo reduction and the consequent effects on melting. Nevertheless, the soot remaining near the surface had a substantial, long-term effect. The residual concentration of 5 3 1027 kg/kg persisted for several weeks and, compared to the untreated surface, reduced the albedo by about 30% and increased melting by 50%. Particles of volcanic ash with diameters larger than about 5 mm remained at or near the snow surface. Although many of the smaller particles flushed through the snow with the meltwater, the surface albedo was not changed significantly by their removal. The different behaviors of the ash and soot are probably related to the difference in their particle size distributions in relation to the thickness of water films that form the transport paths under conditions of partial saturation that are characteristic of melting snow.
Introduction Radiative fluxes dominate the surface energy balance of many snow and ice covered regions, and the surface albedo has a controlling influence on the absorption of radiative energy; for example, the amount of absorbed shortwave radiation increases by a factor of 3 when the albedo changes from 0.85 (typical for freshly deposited snow) to 0.55 (typical for coarsegrained, wet snow). Consequently, albedo has an important influence on the mass balance of glaciers and on the timing and volume of snow melt and runoff. It is well known that the albedo of snow decreases with increasing concentrations of particulate matter near the surface [Higuchi and Nagoshi, 1977; Warren and Wiscombe, 1980; Woo and Dubreuil, 1985]. Warren and Wiscombe [1985] used a radiative transfer model to show that the spectrally averaged albedo of coarse-grained melting snow is highly sensitive to the presence of particles near the surface. Particles deposited in cold polar regions are generally expected to remain near the surface after deposition and continue to affect the albedo until buried by snowfall. However, during conditions of melt, particles may be flushed by the meltwater to depths where they do not influence albedo. With present knowledge, it is difficult to model these effects because little is known about the mobility of particles in melting snow. For example, most soot in the atmosphere is submicron in size, and such small particles may be relatively mobile in melting snow. On the other hand, there is some evidence to suggest the mobility of many particles is low. There is often a visible concentration of particles on summer melting surfaces that persists to produce definable annual stratigraphy in glacier accumulation areas. Furthermore, much of the ash deposited on Copyright 1996 by the American Geophysical Union. Paper number 96WR00712. 0043-1397/96/96WR-00712$09.00
Blue Glacier during the 1980 eruption of Mount St. Helens remained at the surface throughout the ablation season and had a long-term influence on the albedo [Rhodes et al., 1987]. Here we discuss measurements of albedo of snow plots that had been artificially contaminated with known amounts of soot and ash during conditions of melt. Our interest in the effects of contaminants stems from the possible consequences on melt when aerosols from events such as volcanic eruptions, industrial pollution, or fires are deposited on snow.
Experimental Procedures Experiments were conducted on Snowdome at 2050 m on Blue Glacier (478489N; 1238429W) during July and August 1991. At noon, at this latitude and time of year, the solar zenith angle is about 308, and the incoming flux of solar radiation at the top of the atmosphere is 1145 W m22. Although clouds were common, no rain or snow fell during the measurement period. The average density of the upper meter of the snowpack was 545 kg m23. The snowpack had grain-coarsened prior to the experiments; grains were generally clustered and the mean grain diameter was 2.0 mm. Several different types of contaminants were weighed and distributed evenly over plots either 1.5 3 2 m or 3 3 2 m in area. The particles were mixed with 10 L of disaggregated snow in a bucket and then spread uniformly over the plots with a rake. Table 1 gives a summary of the type and mass of particles applied in each experiment. A summary of the characteristics of the different particles is given below. The ash, collected near Ritzville, had been deposited during the 1980 eruption of Mount St. Helens. The imaginary index of refraction of St. Helens ash is 4 3 1023 at visible wavelengths [Patterson, 1981]. Measurements using a Coulter counter indicate the mean diameter of the ash particles was 0.8 mm (by
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Table 1.
A1 A2 A3 L1 L2 L3 H1
CONWAY ET AL.: ALBEDO OF DIRTY SNOW
Amount and Date of Application of Particles Contaminant
Amount, kg m22
Day of Application
natural snow volcanic ash volcanic ash volcanic ash hydrophobic soot hydrophobic soot hydrophobic soot hydrophyllic soot
zzz 0.0167 0.0167 0.0668 0.0033 0.0033 0.0330 0.0033
zzz 202 214 214 202 214 214 202
number). However, most of the mass was contained in the relatively few larger particles; assuming spherically shaped particles, the mean diameter by mass was 19 mm. The hydrophobic soot particles (commonly known as lampblack) was generated by burning hydrocarbons. The particles tend to flocculate in water. The imaginary index of refraction of soot is 0.5 [Clarke and Charlson, 1985]. Manufacturers specifications give a mean particle diameter (by number) of 0.06 mm, but the effective size is often larger because soot generated by pyrolysis usually form bead-like chains that consist of several tens of particles [Lee, 1983]. The hydrophyllic soot particles, a commercial product of Columbian Chemicals called Raven H2O, had been treated with a surfactant to prevent flocculation in water. The applications of ash were similar to the amount deposited on Blue Glacier following the 1980 eruption of Mount St. Helens but more than 2 orders of magnitude less than the concentration at which ablation is maximized [Driedger, 1981]. Most soot that falls out from the atmosphere is hydrophobic and submicron in size. The applications of soot were similar to those expected globally following a major nuclear explosion or fire, or locally after a large forest fire. Measurements of spectrally integrated albedo were made simultaneously using an upward facing Eppley (model PSB) radiometer and a downward facing Kipp and Zonen radiometer attached at the middle of a 3-m-long pole. Both instruments are cosine collectors that measure spectrally averaged radiation over wavelengths from 0.28 to 2.8 mm. Prior to the experiments, the radiometers had been calibrated against each other by taking concurrent measurements with both instruments facing upward. The pole was usually supported horizontally by tripods at either end, and the incoming and reflected shortwave radiation from natural snow was recorded hourly on a data logger. At a time close to solar noon (;1220 Pacific Standard Time (PST)), the pole was taken off the tripods, and two people (one at each end of the pole) walked it around the plots of treated snow. At each site the downlooking radiometer was positioned 1.0 m above the center of the plot. The surface was close to horizontal and special care was taken to level the instruments. Potential errors introduced by measurements made on slopes or by tilted instruments have been discussed by Grenfell et al. [1994]. The downlooking measurements require calibration because some of the measured upward irradiance was reflected from snow outside the plot, and the two operators block some of the viewing area. Geometric constructions for a radiometer 1 m above the surface show that 0.477 of the measured irradiance was reflected from the dirty snow in the case of the small (1.5 3 2 m) plots and 0.635 in the case of the large (3 3 2 m) plots. We calculate that the two operators (each 0.4 m wide and standing 1.5 m from the center of the
plot) block 0.026 of the irradiance. We correct the measurements by assumming the albedo of the snow outside the plots was the same as that measured for natural snow, and the albedo of the two operators was 0.15. The average daily surface lowering of the natural snow due to ablation was measured using a network of six stakes located within 50 m of the plots; water equivalent ablation was calculated using the average snow density. Differential lowering of the treated plots relative to the natural surface was obtained by spanning the plot with a rod and averaging measurements of the distance from the rod to the surface. Snow stratigraphy and particle profiles were obtained at the end of the experiments by sampling down the walls of snowpits. Each 100 cm3 sample was weighed and melted, and then a subvolume of the meltwater was filtered through a 0.4-mm nucleopore filter. The mass fraction of ash particles could be obtained directly by drying and weighing the filters. A correction for the mass of natural impurities was obtained by filtering and weighing samples from a nearby pit excavated in untreated snow. The error of the measurements (650 mg) was less than 10%. The residual concentration of soot in the snow was too small to be detected by weighing. Instead we used an optical technique similar to one described by Clarke et al. [1987] in which the gray density of a filter containing an unknown mass was matched to a standard filter that had been made by filtering a known mass of soot. Implicit is the assumption that the gray density varies with the mass of particles, but this is not valid if the filter contains more than a monolayer of particles. The solutions were ultrasounded prior to filtering, and care was taken to prevent particles from caking on the filter. Each filter of unknown mass was illuminated with a constant light source and photographed with the set of standards. The photographic negative was then digitized, and the gray scale of the unknown filter was matched with one of the standard filters. The error of the measurements of mass of soot on a filter was 61 mg or about 5%.
Experimental Results Albedo Observations Figure 1 shows measurements of spectrally averaged albedo (a) of treated and untreated snow. Times and amounts of the initial applications of particles on the various plots are given in Table 1. Noontime measurements over the natural snowpack show the albedo varied considerably. We suspect most of the variability was caused by clouds which are common in the region. For example, measurements on day 216 (a 5 0.66) which was cloudy were about 13% higher than measurements the next day (a 5 0.58) which was clear. Clouds absorb light at near-infrared wavelengths and other things being equal, the spectrally averaged albedo increases under cloudy skies [Warren, 1982]. Clouds also alter the partitioning between direct and diffuse beam radiation. The ‘‘effective’’ zenith angle of diffuse radiation, about 508, is greater than that during our measurements at midday (about 308), which also contributes to increase the effective albedo during cloudy conditions [Warren, 1982]. Figure 1a shows the effects of ash on albedo. Although plots A1 and A2 had been treated with the same amount of ash, A1 had been treated 12 days before A2 on day 202 (Table 1). The albedo reduction (almost 30% less than the natural snow) was the same for both plots, indicating the ash had a long-term influence on albedo. The effect of increasing the initial appli-
CONWAY ET AL.: ALBEDO OF DIRTY SNOW
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ments on the same day the particles were distributed over plot L1, but visual observations and subsequent measurements indicate that the surface brightened. After 2 or 3 days the albedo reached a steady value that did not change significantly over the next 15 days. Measurements at plot L2 also show the albedo increased within three days to a value that was similar to that of L1 (a 5 0.41 6 0.02). Plot L3 brightened to the same steady value within 3 or 4 days, even though the initial application of soot was 10 times higher than L1 and L2 (Table 1). The steady state albedo appeared to be independent of the initial concentration of particles used in these experiments. Figure 1c shows the effects of hydrophyllic soot (Raven H2O) on albedo. Particles had been distributed during the evening of day 202. By midday the next day the plot had visibly brightened and measurements show the albedo had increased to a steady value (a 5 0.48 6 0.03) that persisted for the next 15 days. The average long-term albedo was higher than the plot containing the same initial concentration of lampblack but still 20% lower than the untreated snow (Figure 1c). Particle Stratigraphy
Figure 1. Evolution of the albedo of natural snow and snow treated with (a) ash (A1, A2, and A3), (b) hydrophobic soot (L1, L2, and L3), and (c) hydrophyllic soot (H1). Particles had been distributed over plots A1, L1, and H1 on day 202 and over plots A2, A3, L2, and L3 on day 214. The mass of particles distributed on plot A3 was 4X greater than plots A2 and A1. The mass of soot distributed on plot L3 was 10X greater than plots L2 and L1. cation of ash by a factor of 4 is shown in the measurements over plot A3 where the albedo was about 50% less than the natural snow. Figure 1b shows the effects of hydrophobic soot (lampblack) on albedo. Plot L1 had been treated in the evening of day 202, while plots L2 and L3 were started at noon 12 days later on day 214. In all cases the presence of soot caused a substantial reduction in albedo, but the surface always brightened considerably after the initial application. We do not have measure-
We expect that the observed changes in albedo are caused partly by changes in the particle stratigraphy. For each experiment the particles were initially distributed through the upper 2.5 cm of the snowpack. Figure 2a shows the vertical distribution of ash 13 days after particles were first introduced. At the time of sampling, the snowpack was partly saturated, and the upper 10 cm consisted of clusters of coarse (2 mm diameter) grains. Total surface lowering since the introduction of the particles (almost 127 cm) implies that about 69 cm of water had flushed through the remaining snow since that time. Although our observations indicate that most of the visible ash was confined to the upper 2 cm of the snowpack, this was not detected by the measurements because they were obtained by sampling over 5-cm depth intervals. We suspect the concentration in the upper 2 cm could be twice as high as the spatial average shown in Figure 2a. Examination of the filters with an optical microscope after the experiments showed that most visible particles remaining near the surface were larger than 5 mm; apparently, many of the smaller particles had flushed through the snow with the meltwater. However, although these small particles represent 85% of the initial number of ash particles, they make up less than 5% of the total mass. Measurements indicate that about 90% of the original mass of ash remained in the upper 10 cm of the snowpack. The loss of the large number of smaller particles had no discernable influence on the albedo. Figure 2b shows the distribution of hydrophobic soot 10 days after particles were first introduced. Total meltwater production between times of introduction and sampling was about 40 cm. Measurements at the end of the experiment showed the snowpack consisted of 10 cm of partly saturated grain clusters overlying a series of saturated ice layers separated by coarsegrained snow. The particle concentration near the surface (4.3 3 1027 kg/kg) was about 2 orders of magnitude lower than the initial concentration and more than 50% of the soot had flushed through the snow by this time. The measurements (Figure 2b) do not fully resolve all details of the vertical distribution of particles; visual observations indicated most of the soot between 7.5 and 14.5 cm was concentrated in a thin ice layer 10 cm below the surface. Figure 2c shows the distribution of hydrophyllic soot 10 days after particles were first introduced. Again the concentration
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Figure 3. Cumulative surface lowering between days 203 and 221 for natural snow and for snow treated with hydrophobic soot (L1) and ash (A1). The average rate of ablation of the contaminated snow (7.9 cm d21) was about 50% greater than that of natural snow (5.3 cm d21). Warm temperatures and strong winds on days 218 and 219 enhanced the ablation rate on those days.
Ablation Figure 3 shows the cumulative surface lowering between days 203 and 221 for natural snow and for snow treated with the lower concentrations of ash (A1) and hydrophobic soot (L1). The rate of ablation of the contaminated plots (;7.9 cm d21) was generally 50% greater than that for natural snow (5.3 cm d21). The albedo complement (1 2 a) of the contaminated plots (0.58) was also about 50% higher than the natural snow (0.39), suggesting that melt was controlled primarily by the flux of absorbed solar radiation. A notable exception occurred on days 218 and 219 when the weather was warm and windy (the 1 m air temperature was ;68C; hourly average wind speed at 2 m was greater than 10 m s21). Under these conditions, turbulent and latent fluxes are likely to dominate the surface energy exchange, and we might expect differences in albedo to be less important. In fact, the natural snow ablated much faster than the contaminated plots (Figure 3). We suspect this phenomenon may be related to differences in local topography; at the beginning of the storm the surface of the contaminated plots was 30 to 40 cm below the natural snow surface which would reduce their exposure to the wind.
Discussion
Figure 2. Particle stratigraphy of (a) snow treated with ash 12 days after the initial application, (b) hydrophobic soot measured 10 days after the initial application, and (c) hydrophyllic soot also measured 10 days after the initial application. In all cases, particles were initially distributed through the upper 2.5 cm of the snowpack. of particles near the surface (2.1 3 1027 kg/kg) was 2 orders of magnitude lower than the initial concentration, and only 1% (by mass) remained in the upper 50 cm of the snowpack. Most of the hydrophyllic soot flushed through the snow with the meltwater.
Differences in the evolution of albedo of snow treated with soot (which brightened with time to a steady value) and ash (which had a constant, long-term value) are related to differences in particle mobility. The experiments with ash suggest that particles larger than about 5 mm were relatively immobile during conditions of melt. This result is similar to that of Higuchi and Nagoshi [1977], who found that dust particles larger than 4 –10 mm were immobile. Although this size is more than an order of magnitude smaller than expected by trapping during saturated flow [McDowell-Boyer et al., 1986], during natural melt the snow is more likely to be partially saturated. Under these conditions the mobility of particles is limited by the thickness of the transporting film [Tien and Payatakes, 1979]. Our measurements indicate the average daily melt rates of the dirty snow varied from 20 mm d21 to more than 100 mm
CONWAY ET AL.: ALBEDO OF DIRTY SNOW
d21, but we expect peak flows could be 5–10 times higher than these average values. If we assume Darcian flow and the hydraulic conductivity is a function of density [Colbeck, 1972], for the physical conditions during our experiments, the thickness of the water film should vary from about 5 mm to 12 mm (for flow rates ranging from 50 to 500 mm d21). Using Stokes law to calculate drag and assuming particles are mobile when the drag exceeds the gravitational forces [Stenatakis and Tien, 1993], it can be shown that particles larger than about 6 mm would be immobile at flow rates of 500 mm d21. This size is similar to the observed critical size of 5 mm. Particle mobility through melting snow is discussed in more detail in a separate paper (C. F. Raymond et al., manuscript in preparation, 1996). If particle mobility is controlled by the drag and gravitational forces, all particles smaller than 1 mm should be mobile when the flux of melt water exceeds about 20 mm d21. However, the presence of some residual soot (presumably submicron sized) in the upper snowpack (Figures 2b and 2c) suggests that this is not always the case. However, we do not rule out the possibility that some particles (or clusters of particles) may have been larger than the manufacturers specification. This possibility is supported by measurements of the size distribution of the hydrophobic soot initially distributed over the snow (using a Coulter counter) which indicated that 5% of the particles (by mass) were larger than 1 mm. We suspect the peak in the concentration of hydrophobic soot 10 cm below the surface (Figure 2b) is related to the effects of penetrating solar radiation on the distribution of liquid water through the snowpack. The presence of impurities shifts the vertical distribution of absorbed energy toward the surface which also shifts the distribution of free water. Consequently, we expect the near-surface particles to be relatively mobile and particles would tend to concentrate at a depth where the drag forces imposed by the transporting film of water are less than the gravitational forces. The surface albedo may be affected by properties such as surface roughness, grain size, liquid water content, as well as the presence or absence of light absorbing impurities, and we use a model of radiative transfer described by Grenfell [1991] to help interpret our results. Our measurements in the natural snowpack indicate that the average diameter of snow grains was 2.0 mm, and the natural impurities were optically equivalent to 2 3 1028 kg/kg of soot. Comparison of modeled and measured diffuse-sky albedo for the natural snow (using the appropriate solar zenith angle for the day of year and latitude of Blue Glacier) shows the model overestimated the albedo by about 10%. The model can be improved by using an equivalent grain size of 2.8 mm (rather than 2.0 mm) which is not unreasonable given that the grains were clustered and the presence of liquid water between grains is expected to increase the optical grain size [Warren, 1982]. Soot particles are modeled as uniform spheres (d 5 0.2 mm) evenly distributed through a layer. The model includes multiple layers to represent variations of particle and snow stratigraphy with depth [Grenfell, 1991]. We assume that the soot was initially distributed uniformly through the upper 2.5 cm of the snowpack. The measured variations in the final particle stratigraphy (shown in Figure 2) were used when simulating the final albedo. Albedo measurements and model results for snow contaminated by soot under diffuse sky conditions are compared in Table 2. The measured albedo was always much higher than the model when particles were first distributed over the snowpack. The discrepancy could be eliminated if we assume the soot was
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Table 2. Measured Versus Modeled Albedo L2
L3
H1
Albedo
Natural Snow
Initial
Final
Initial
Final
Initial
Final
Measured Modeled
0.61 0.61
0.28 0.05
0.43 0.43
0.18 0.03
zzz zzz
zzz 0.05
0.49 0.59
distributed through the upper 75 cm of the snowpack, but this is not realistic. We suspect some of the disagreement is caused by clumping of particles which is expected at high concentrations. Clumping increases the effective size of the particles and reduces the impact on albedo [Warren, 1982]. The agreement between the model and measurements at the end of the experiments was very good for the plot containing hydrophobic soot (L1, Table 2) but less satisfactory for hydrophyllic soot (H1, Table 2). We are not certain why the discrepancy is so large for H1. It is possible the coarse resolution of the particle stratigraphy contributed to the difference, but even if all the soot was contained in the upper 0.5 cm of the snowpack (rather than the upper 2.5 cm), the modeled albedo (0.58) is still much higher than the measurement (0.49).
Conclusions Differences in the evolution of albedo of snow treated with ash and soot can be attributed to differences in the mobility and the optical properties of particles. Ash particles larger than about 5 mm were relatively immobile during conditions of melt and the mass concentration of ash near the surface remained at or near original levels. Ash had a long-term influence on the albedo that increased with concentration. On the other hand, very high concentrations of submicron soot did not persist at the surface of coarse-grained, melting snow; the mass fraction of soot at the surface decreased to about 5 3 1027 kg/kg within three days because of downward migration of particles with the meltwater. The residual concentration near the surface appeared to be independent of the initial concentration of particles. The migration of soot particles to depth caused the snow to brighten but even the reduced concentration of soot remaining near the surface had a long-term effect on albedo. The reduction in albedo (about 30% less than the natural snow) increased the ablation rate by about 50% during the experiments. The partial mobility of submicron-sized soot through melting snow limits the potential impact of large fallouts of soot on albedo (suggested by Warren and Wiscombe [1985, Figure 1] to just a few days. However, a large fallout might promote an early onset of melt and accelerate the subsequent melt rate by about 50%. Since the residual effects from soot and the longterm effects from ash probably depend on the presence of large-diameter (d . 5 mm) particles, one might expect depositions of particles from very remote sources (likely to consist of very small particles in low concentrations) would wash out more quickly than the artificial applications used in these experiments. Consequently, the long-term effect of particles on albedo may be even less persistent in practice than the effects measured in our experiments. Acknowledgments. This research was supported in part by NSF grant DPP 9024254. We also wish to thank the Olympic National Park for logistical support, Chemical Distributors Inc. (Seattle) for supply-
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ing the soot, and Mary Davis, Ellen Mosley-Thompson, and Lonnie Thompson (Byrd Polar Research Center) for particle size analyses. We also wish to thank three anonymous referees whose comments greatly improved the original manuscript.
References Clarke, A. D., and R. J. Charlson, Radioactive properties of background aerosol: Absorption component of extinction, Science, 229, 263–265, 1985. Clarke, A. D., K. J. Noone, J. Heintzenberg, S. G. Warren, and D. S. Covert, Aerosol light absorption measurement techniques: Analysis and intercomparisons, Atmos. Environ., 21, 1455–1465, 1987. Colbeck, S. C., A theory of water percolation in snow. J. Glaciol., 2(63), 369 –385, 1972. Driedger, C. L., The 1980 eruptions of Mount St. Helens, Washington: Effect of ash thickness on snow ablation. U.S. Geol. Surv. Prof. Pap., 1250, 757–760, 1981. Grenfell, T. C., A radiative transfer model for sea ice with vertical structure variations, J. Geophys. Res., 96(C9), 16,991–17,001, 1991. Grenfell, T. C., S. G. Warren, and P. C. Mullen, Reflection of solar radiation by the Antarctic snow surface at ultraviolet, visible, and near-infrared wavelengths, J. Geophys. Res., 99(D9), 18,699 –18,684, 1994. Higuchi, K., and A. Nagoshi, Effect of particulate matter in surface snow layers on the albedo of perennial snow patches, IAHS AISH Publ., 118, 95–97, 1977. Lee, K.-T., Generation of soot particles and studies controlling soot light absorption, Ph.D. thesis, 162 pp., Univ. of Wash., Seattle, 1983. McDowell-Boyer, L. M., J. R. Hunt, and N. Sitar, Particle transport through porous media, Water Resour. Res., 22(13), 1901–1921, 1986.
Patterson, E. M., Measurements of the imaginary part of the refractive index between 300 and 700 nanometers for Mount St. Helens, Science, 211, 836 – 838, 1981. Rhodes, J. J., R. L. Armstrong, and S. G. Warren, Mode of formation of ‘‘ablation hollows’’ controlled by dirt content of snow, J. Glaciol., 33(114), 135–139, 1987. Stenatakis, K., and C. Tien, A simple model of cross-flow filtration based on particle adhesion, AIChE J., 39(8), 1292–1302, 1993. Tien, C., and A. C. Payatakes, Advances in deep bed filtration, AIChE J., 25(5), 737–759, 1979. Warren, S. G., Optical properties of snow, Rev. Geophys., 20(1), 67– 89, 1982. Warren, S. G., and W. J. Wiscombe, A model for the spectral albedo of snow, II, Snow containing atmospheric aerosols, J. Atmos. Sci., 37, 2734 –2745, 1980. Warren, S. G., and W. J. Wiscombe, Dirty snow after nuclear war, Nature, 313, 467– 470, 1985. Woo, M.-K., and M.-A. Dubreuil, Empirical relationship between dust content and arctic snow albedo, Cold Reg. Sci. Technol., 10, 125–132, 1985. H. Conway, A. Gades, and C. F. Raymond, Geophysics Program, Box 351650, University of Washington, Seattle, WA 98195. (e-mail:
[email protected];
[email protected];
[email protected])
(Received November 30, 1995; revised February 29, 1996; accepted March 5, 1996.)
