Research. Rate of Evolution of the Specific Surface Area of Surface Snow Layers. 2. Snow Sampling. 1. Introduction

Transcription

1 Research Rate of Evolution of the Specific Surface Area of Surface Snow Layers AXEL CABANES, LOÏC LEGAGNEUX, AND FLORENT DOMINEÄ * CNRS, Laboratoire de Glaciologie et Géophysique de l Environnement, B.P. 96, 54 Rue Molière, Saint Martin d Hères, Cedex, France The snowpack can impact atmospheric chemistry by exchanging adsorbed or dissolved gases with the atmosphere. Modeling this impact requires the knowledge of the specific surface area (SSA) of snow and its variations with time. We have therefore measured the evolution of the SSA of eight recent surface snow layers in the Arctic and the French Alps, using CH 4 adsorption at liquid nitrogen temperature (77 K). The SSA of fresh snow layers was found to decrease with time, from initial values in the range cm 2 /g to values as low as 257 cm 2 /g after 6 days. This is explained by snow metamorphism, which causes modifications in crystal shapes, here essentially crystal rounding and the disappearance of microstructures. A parametrization of the rate of SSA decrease is proposed. We fit the SSA decrease to an exponential law and find that the time constant R exp (day -1 ) depends on temperature according to R exp ) 76.6 exp (-1708/T), with T in kelvin. Our parametrization predicts that the SSA of a snow layer evolving at - 40 C will decrease by a factor of 2 after 14 days, while a similar decrease at -1 C will only require 5 days. Wind was found to increase the rate of SSA decrease, but insufficient data did not allow a parametrization of this effect. 1. Introduction For over a decade, numerous studies of atmospheric chemistry in polar regions have revealed that the snowpack could interact chemically with the atmosphere. Spring-time Arctic ozone depletion (1) is among the most studied aspects. Recently, studies have shown that significant exchanges of reactive gases between the snowpack and the lower troposphere were taking place, increasing the interest of studying air-snow interactions. Production of NO x by the snowpack has been observed in Greenland and Antarctica (2-5). Emission of carbonyl compounds (formaldehyde, acetaldehyde, and/or acetone) from the snowpack have also been observed in Greenland (6, 7), the Canadian Arctic (8-10), and at middle latitudes (11). Emissions of persistent organic pollutants have also been observed and modeled (12). Numerous models of atmospheric chemistry over snowcovered surfaces (13, 14) lead to calculated concentrations lower than measured ones (8, 15), which demonstrates that the impact of the snowpack must be included to yield reliable * Corresponding author phone: (33) ; florent@lgge.obs.ujf-grenoble.fr. atmospheric composition. However, the inclusion of this impact requires the knowledge of the physical processes involved, but these are insufficiently understood (16). Possible processes include sublimation/condensation of snow and solutes, adsorption/desorption of trace gases from the snow crystals surfaces, diffusion of species in the ice lattice, and heterogeneous reactions on crystals surfaces (17). Many physical parameters such as the specific surface area (SSA) of the snow must be known to quantify processes involving the surface of snow crystals. SSA is the surface area accessible to gases for a given mass of snow and is expressed in cm 2 /g rather than m 2 /g because snow SSA values are usually low (18-21). Fresh snow layers have the highest interaction potential with the atmosphere. Fresh crystals are at the surface of the snowpack and have the highest potential for physical evolution, and their SSA decrease can be rapid (19, 21). The rate of SSA decrease is an essential parameter to quantify the amounts of a given species released by desorption, as mentioned by Hutterli et al. (6) in the case of HCHO. However, there are only few data on the rate of decrease of snow SSA shortly after precipitation (19, 21), and quantitative rate parameters have not been extracted from those data. We have therefore studied the SSA evolution of eight fresh snow layers, sampled in the French Alps and in the Arctic. Two of these layers were thick enough to be divided into sublayers, so that 11 evolution rates were obtained. These rates seemed to be affected by temperature, wind speed, and other environmental parameters, whose values are also reported. We then hope that this first, and necessarily incomplete, set of data will assist modelers of snow-air interactions in estimating the rate of SSA decrease of fresh snow. 2. Snow Sampling Snow layers studied in this work were sampled in the French Alps in early 1999 and 2001, hereafter referred to as winters 1999 and 2001, in the Canadian Arctic during winter and spring 2000, and in Svalbard during spring In the French Alps, snow samples were collected at three sites. Two of them were located at Col de Porte (45 12 N, 5 44 E) at an elevation of 1330 m, in the Chartreuse range, about 10 km north of Grenoble. The first sampling site was located within the meteorological station of Centre d Etude de la Neige (CEN) and is referred to as site P. Continuous monitoring of air temperature and humidity, surface snow temperature, snowpack height, liquid and solid precipitation rates, and wind speed at 10 m height are performed by CEN. Air temperature was recorded in a ventilated shelter 1.5 m above the snow surface. Snow surface temperature was measured by its IR emission (22). The second site was located 500 m west of CEN in a flat forest clearing of about 3000 m 2. This site, referred to as site P, was preferred to site P when chemical measurements were coupled to our physical measurements because it is further from the road. Meteorological parameters were assumed to be similar to those at site P. This was verified for air temperature, that was found to be within 0.5 C ofthe CEN values on several occasions. The third site was located at Col du Lautaret (45 02 N, 6 24 E), 55 km east of Grenoble, in a small south-facing sheltered basin at an elevation of 2058 m. This higher elevation site, referred to as site L, had to be used because of the unusually warm temperatures during winter 2001, that /es025880r CCC: $ American Chemical Society VOL. 37, NO. 4, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY Published on Web 12/24/2002

2 TABLE 1. Snow Layers Studied in the French Alps and the Arctic no. date of snowfall period of study sampling site initial layer thickness (cm) mean air temp initial snow density fresh snow crystal shape 1 Feb 6-9, 1999 Feb 10, 1999 Col de Porte a plates, needles and columns Feb 16, 1999 site P 2 Mar 4, 1999 Mar 4, 1999 Col de Porte plates, needles and columns Mar 9, 1999 site P 3 Feb 9, 2001 Feb 9, 2001 Col du Lautaret a dendritic crystals Feb 15, 2001 site L 4 Feb 3, 2000 Feb 4, 2000 Alert 3 to columns, bullets combination Feb 22, 2000 site A 5 Feb 7, 2000 Feb 7, 2000 Alert columns, bullets combination Feb 22, 2000 site A 6 Apr 13-14, 2000 Apr 14, 2000 Alert columns, bullets combination Apr 23, 2000 site A 7 Apr 25-28, 2000 Apr 25, 2000 Alert to 0.21 b variably rimed dendritic crystals Apr 29, 2000 site A 8 Apr 27, 2001 Apr 27, 2001 Ny-Ålesund columns, rimed dendritic crystals May 3, 2001 site N a Density measured at the bottom of the layer. b In wind-packed accumulation. resulted in rare snow events at site P. Unfortunately, there is no meteorological station at or near site L. In the Canadian Arctic, snow samples were collected near the Alert base (Ellesmere Island N, W) during the ALERT 2000 campaign. The sampling site, referred to as site A, was located 5.4 km south of the base at about 180 m above sea level (18). In Svalbard, the snow was sampled at Ny-Ålesund (78 55 N, E) during the NICE campaign in April-May The sampling site N was located about 50 m south of the Italian station, at the edge of the Ny-Ålesund village. The sampling method has been described in detail earlier (19, 21-22). Briefly, vertical faces were dug to observe the stratigraphy and to locate the snow layer of interest. For each sample, about 150 cm 3 of snow was collected in two glass vials using a stainless steel spatula thermally equilibrated with the snow. Great care was taken to pertub the snow as little as possible during sampling (21). The vials were immediately dropped in liquid nitrogen to stop metamorphism and remained immersed until being transferred into the volume used to measure SSA. Snow and air temperatures were measured at different heights with a mercury or alcohol thermometer, or a thermocouple. According to the thickness of the layer, snow was sampled at up to three levels, and density was measured using a sampler of known volume. 3. Experimental Method The SSA of each snow sample was determined by CH 4 adsorption at liquid nitrogen temperature (77.15 K), as detailed earlier (21, 22). CH 4 was used rather than N 2 because of its lower saturating vapor pressure at 77 K, which allows a much better precision (21). Briefly, we used a volumetric method which requires the measurement of the adsorption isotherm of CH 4 on snow, followed by a BET treatment (23). This yields the SSA of the snow sample and the net heat of adsorption of CH 4 on ice, Q CH4. This latter parameter was used to test the reliability of SSA measurements, as a value Q CH4 ) 2240 ( 200 J/mol must be obtained for the measurement to be considered reliable (21). The reproducibility of the method was within 6%, and its accuracy, taking into account systematic errors due to the BET treatment, was within 12% (21). 4. Results The characteristics of the eight snow layers studied are summed up in Table Description of the Snow Layer Fallen on February 6-9, 1999 at Col de Porte. This snowfall ( no. 1 in Table 1) FIGURE 1. Air temperature (1.5 m above the surface) and snow surface temperature (measured by IR emission) in early February 1999 at Col de Porte (site P). Data from CEN, Météo France. was studied at Col de Porte at site P. It fell in several episodes in 4 days until reaching a thickness of 105 cm on February 10 and covered a hard melt-freeze layer. According to CEN precipitation and snowpack height recordings, there were six major precipitation events from February 6 to February 9, three on February 6, one on February 7, one on February 8-9, and the last one on February 9. Sampling was done at several heights on February 10, 12, and 16, but a wind event that started late on February 10 at 4 p.m. (GMT) and lasted until the morning of February 12 perturbed the layer so that only the bottom sampling, at a height of cm above the underlying layer, is reported here. This snow was made up of a mixture of plates, needles, and columns with little or no rime. On February 10, it was sampled at 12:00 GMT, while the air temperarture was -9.4 C. The temperature of the sampled snow was -2.7 C, its density was 0.21, and its SSA was 414 cm 2 /g. From CEN precipitation and snowpack data, and from our density measurement, we estimated that the snow sampled was 90 h old. The temperature started dropping on February 12 (Figure 1), while wind speed became low (<5 m/s). The second sampling took place at 10:00 GMT on February 12, while the air temperature was C. The layer thickness had decreased to 75 cm, 25 of which was made up of wind-blown snow. The temperature of the sampled snow was -4.7 C, its density was 0.25, and its SSA was 341 cm 2 /g. This snow had visibly metamorphosed, although most original shapes could still be recognized. After February 12, the air temperature continued its overall decrease down to -15 C on February 14, with visible diurnal variations, while wind speed stayed low. Then, a warming occurred and the third sampling was performed on February 16 at 10:00, while the air temperature was -1.8 C. The snow temperature was -4.7 C, its density was 0.27, and its SSA 272 cm 2 /g. The snow was then very metamorphosed, with ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 4, 2003

3 TABLE 2. Snow Sampling Conditions and SSA for Snowfall No. 2 (March 4, 1999) at Col De Porte (Site P ) a date time weather conditions T air ( C) sublayer depth (cm) T snow ( C) density SSA (cm 2 /g) Mar 4 10:00 snowing -0.7 a b c Mar 4 16:00 slightly snowing -0.6 a b c Mar 5 10:00 cloudy no wind -1.8 a b c Mar 6 09:00 slightly snowing -2.9 a b c Mar 9 10:00 cloudy no wind +2.2 a b c a Time is GMT. FIGURE 2. Evolution of the SSA of the snow layer that fell on February 6-9, 1999 at site P ( no. 1 in Table 1) and of the layer that fell on April 9, 2001 at site L ( no. 3 in Table 1). FIGURE 3. Stratigraphy of the snowpack studied at Col de Porte during early March 1999 (snowfall no. 2). Sampling levels are shown. mostly rounded grains and few recognizable particles. The layer thickness was about 70 cm, including 3 cm of recent surface hoar. The SSA decrease of this layer is reported in Figure Description of the March 4, 1999 Snow Layer at Col de Porte. The snowfall, ( no. 2 in Table 1) that precipitated at site P on March 4, 1999 was sampled five times until March 9 (Figure 3). Values of physical parameters measured during sampling are reported in Table 2. Air and snow surface temperatures measured at CEN are shown in Figure 4. CEN data show that precipitation began on March 4 at 1:00 GMT when the air temperature was fluctuating between -0.5 to C and finished around 16:00 while the air FIGURE 4. Air temperature (1.5 m above the surface) and snow surface temperature (measured by IR emission) in early March 1999 at Col de Porte (site P). Data from CEN, Météo France. temperature was also fluctuating between -0.5 and -2.0 C. The snowfall could be subdivided in two parts: First, 38 cm of snow were deposited in 10 h over a 7 cm-thick layer of wet snow, and precipitation almost stopped around 11:00, after the first sampling. Then, while the initial snow was settling, snow started falling again around 13:00, increasing snowpack height by 4 cm while a slight warming occurred (Figure 4). From March 5 to 7, several light snowfalls took place, adding about 15 cm of new snow at site P. Afterward, a temperature drop was observed during the night from March 7 to 8 followed by a warming after March 8 which forced us to stop our study on March 9. As determined with a magnifying glass and with an optical microscope, this snow consisted of a very variable mixture of stellar crystals, plates, needles, and columns. The initial density of the fresh snow ranged from at the top of the layer to at the bottom. Snow was collected at three heights for each sampling. To locate easily the top of the layer, a small aluminum sheet, that did not cover the area to be sampled, was deposited over the snow surface after the first sampling. The evolution of the snow stratigraphy over our sampling period is detailed in Figure 3. The SSA decreases of the three sublayers sampled are shown in Figure 5. The top sublayer had an initial SSA of 780 cm 2 /g, which decreased to 369 cm 2 /g in 5 days. For the middle and bottom sublayers, SSA decreased from 654 to 269 cm 2 /g and from 613 to 349 cm 2 /g, respectively. Microscopic observations and Table 2 show that the SSA decrease is correlated to crystal rounding and to a density increase Description of the February 9, 2001 Snow Layer Studied at Col du Lautaret. This snowfall ( no. 3 in Table 1) consisted of dendritic snow and fell on February 9, 2001 in the early morning at site L. It fell over a 15 cm-thick wet snow layer and was sampled three times. The first sampling took place on February 9 around 11:00 GMT, while the air temperature was -2 C. The snow layer VOL. 37, NO. 4, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 663

4 FIGURE 5. SSA evolution of the snowfall of March 4, 1999 (snowfall no. 2) at Col de Porte, site P. had a thickness of 24 cm, and its density increased from 0.10 near the surface to 0.20 at the base of the layer. The snow temperature was -2.5 C near the surface and -2.0 C ata depth of 20 cm. The layer was sampled at three depths, 0, 2, and 20 cm, but because of surface perturbations, only the lower part of the layer was sampled in subsequent trips. At 20 cm depth, the snow has a SSA of 627 cm 2 /g. The second sampling took place on February 13, around 11:00 GMT, while the air temperature was +1.2 C. The layer thickness had decreased to 13 cm. The crystals consisted of recognizable particles and of small rounded crystals.a 2 cmthick melt-freeze layer and 1 cm of surface hoar had formed on its surface. No weather station is present near site L, but weather observations near Grenoble and data from other alpine meteorological stations suggest warm temperatures, most likely above freezing, in the daytime, and fairly clear sky conditions at night, which can easily explain the formation of the melt-freeze layer and of the surface hoar. The snow was sampled near the base of the layer, where the temperature was -1.5 C. The sampled snow density was 0.19 and its SSA 329 cm 2 /g. The third sampling took place on February 15, around 11:00 GMT, while the air temperature was +1.0 C. The layer was 10 cm thick, and the top of the layer consisted of a 1 cm-thick melt-freeze layer. The presence of wind-deposited snow in a sheltered area indicated that wind had blown between February 13 and 15. The snow was sampled near the base of the layer, where the temperature was -0.5 C. The sampled snow density was 0.20, and its SSA was 257 cm 2 /g. The SSA evolution of snow sampled at the base of this layer is reported in Figure Winter and Spring Snow Layers Studied near Alert. In the Canadian Arctic, four fresh snow layers were studied near Alert at site A. Two fell on February 3 and 7, 2000, and another two fell on April and (Table 1). The description of the snow layers and the evolution of their SSA have been detailed recently (19, 20), and only the main data are briefly recalled here Description of the February 3 and 7, 2000 Snow Layers. The February 3 fall ( no. 4 in Table 1) fell by -19 C at the end of a southwesterly wind event. Snow formed a wind-blown discontinuous layer 3-8 cm thick. It consisted of columns and combination of bullets, as usually observed in polar regions at low temperature (24). The density was New snow ( no. 5 in Table 1) consisting of columns and combination of bullets fell on February 7 by -38 C under calm conditions and covered the February 3 layer with a continuous 1 cm-thick layer of density The air temperature then remained between -43 and -34 C, and surface hoar grew continuously on the snow surface until February 22 when a northerly wind event remobilized both layers and the surface hoar. Both layers were sampled several times between February 3 and 22, FIGURE 6. Evolution of the SSA of several snow layers studied in the Arctic. As observed for Alpine snow, the SSA of Arctic snow essentially decreased with time. The SSA of the February 3 snow shows an overall decreasing trend: from 770 cm 2 /g to 460 cm 2 /g in 17 days (Figure 6). Unlike for other snow layers, this decrease is not perfectly monotonic. As detailed by Cabanes et al. (20), this is due to different intensity of windcompaction within the same snowbank, while the snow was falling. Since wind compaction of fresh snow seems to accelerate SSA decrease (20), this added effect of variable importance yielded a nonmonotonic plot. For the February 7 layer, the SSA decrease was by a factor of 2.7 from 1460 cm 2 /g to 550 cm 2 /g in 13 days (Figure 6). Snow photomacrographs were taken to attempt to understand the processes responsible for the SSA decrease. They showed (20) that crystal rounding was the main change observed in the February 3 layer. For the February 7 layer, the SSA decrease was explained by two processes: (i) rounding of crystals and (ii) dilution by surface hoar of lower SSA (590 cm 2 /g) Description of the April Snow Layer. On April 13 and 14, 2000, two snowfalls ( no. 6 in Table 1) deposited 2 and 1 mm of snow, under temperatures of -27 and -23 C, respectively, under calm conditions. As in winter, snow was made up of columns and bullet combinations, and its density was We studied the SSA evolution of both snowfalls sampled together until April 25, while the average air temperature was -27 C. Until then, no precipitation or wind occurred. The initial SSA was 779 cm 2 /g, and it decreased by a factor of cm 2 /g in 9 days (Figure 6). As in winter, surface hoar of low SSA, 354 cm 2 /g, was observed to grow over the snow layer and, together with crystal rounding, contributed to the SSA decrease Description of the April Snow Layer. This snow layer ( no. 7 in Table 1) fell from April 25 to 28 by -15 C under strong continuous northerly winds and consisted of variably rimed dendritic crystals. It formed a 5 cm-thick layer of variable density. Values were as low as in windsheltered areas and reached 0.21 in wind-packed accumulations. The SSA decrease was also dependent on the wind conditions. In wind-exposed accumulations, the SSA decrease was very rapid: from 1530 to 540 cm 2 /g in just 1.5 day. In wind-sheltered areas, the decrease was from 1530 to only 1015 cm 2 /g in 1.5 day (Figure 6). Photomacrographs (20) show that the quick decrease was associated with crystal rounding and to the disappearance of microstructures such as small subdendrites with diameters in the µm size range. From geometrical considerations (18), we estimate that such structures have a SSA of about 4000 cm 2 /g Description of the April 27, 2001 Snow Layer Studied at Ny-Ålesund. This 6 cm-thick snow layer (no. 8, Table 1) formed on April 27 at site N. It precipitated by a temperature around -0.3 C under calm conditions and was mostly made up of columns and rimed dendritic crystals. The density of fresh snow was This layer was then sampled on April 28 and 30 and May ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 4, 2003

5 TABLE 3. Correlation Coefficients R 2 and Factors r Obtained by Fitting the Rate of SSA Decrease to Eqs 1 (Linear), 2 (Exponential), and 3 (Logarithmic) snow layer R 2 linear R 2 exponential R 2 log T, C average, range r lin, cm 2 g -1 day -1 r exp day -1 r log cm 2 /g Feb 10, 1999 bottom (-15, 0) Mar 4, 1999 top (-8, 2) Mar 4, 1999 middle (-8, 2) Mar 4, 1999 bottom (-8, 2) Feb 3, (-45, -19) Feb 7, (-43, -34) 59, Apr 13-14, (-33, -22) Apr 25-28, 2000 a (-18, -13) Apr 25-28, 2000 b (-18, -13) Feb 9, (-6, 1) Apr 27, (-4, 0) a Wind sheltered. b Wind exposed. On April 28, a melt-freeze crust formed on the surface. The lower part of the layer was sampled and consisted of still recognizable particles. A new snowfall deposited about 55 mm of fresh snow by a temperature around 0 C on April 29. Temperature then decreased to around -2.5 C and new snow fell on April 30. From April 30 to May 2, temperature evolved between -2.0 C and -3.5 C, while the density of the layer studied increased up to 0.29 on May 2. Initial snow SSA was 853 cm 2 /g and decreased to 290 cm 2 /g in 6 days (Figure 6). 5. Discussion The main purpose of this study is to quantify the rate of SSA decrease of fresh snow and to propose a parametrization of this decrease that can be used in models of air-snow interactions. The SSA evolutions of snow layers reported in Figures 2, 5, and 6 always show a monotonic decrease with time except in the case of the February 3 layer at Alert, where wind is thought to be responsible for the noise in the decrease. In the other case where wind was involved (April 25-28, 2000 snowfall at Alert), wind was also observed to have an important effect, as it considerably accelerated SSA decrease. Observations discussed here and earlier (20) indicate that the observed SSA decreases are due to modifications in crystal shapes, essentially rounding due to metamorphism, and also, for the very surface layers in the Canadian Arctic, to dilution by surface hoar of lower SSA. Quantifying SSA decrease rigorously would then require the physical modeling of crystal growth and rounding, which are governed by numerous parameters such as temperature, temperature gradient, ventilation, permeability, air relative humidity, density increase, etc. Such an enterprise is well beyond the scope of this first paper on the subject. Moreover, besides the recent work of Cabanes et al. (20) whose data are used here, there are almost no previous data on the rate of decrease of snow SSA, and there is no previous attempt to quantify this rate. Hanot and Dominé (22) have discussed the rate of decrease of a snow layer, but subsequent reinterpretation (21) concluded that those measurements were perturbed by an artifact. Jellinek and Ibrahim (25) studied the rate of decrease of the SSA of powdered ice and fitted the decrease curves with an exponential equation. Dominé et al. (26) and Legagneux et al. (21) discussed in great detail criterions to evaluate the validity of snow and ice SSA measurements using CH 4 or N 2 adsorption. They concluded that almost all previous measurements used experimental systems inadequate for the measurements of low SSA values and that therefore those previous measurements were unreliable. It is thus not possible to base any quantification on data or theoretical developments available in the literature. At this point, considering the limited scope of this paper, we will therefore only seek empirical correlations between SSA and time and investigate whether empirical parameters can be related to measurable environmental variables. Many possible equations can be used to fit our SSA decrease plots. Considering the complexity of the physics involved, and the preliminary character of our approach, any choice of an analytical expression will at this point be largely arbitrary. We will here investigate three equations: linear (eq 1) exponential (eq 2), and logarithmic (eq 3): SSA ) SSA 0 -R lin t (1) SSA ) SSA 0.e -R expt In eqs 1 and 2, SSA 0 is the initial SSA, at time t )0, i.e., when the snow is deposited, and R lin and R exp are the time constants. In eq 3, SSA is undefined at t ) 0. SSA 1 is the SSA after unit time evolution, and R log, with units of cm 2 /g, is related to the rate of SSA decrease. Table 3 shows the correlation coefficients R 2 as well as R values obtained for each snow layer. R 2 values are >0.95 in 3 cases out of 11 with the linear equation, in 5 cases for the exponential equation, and in 8 cases with the logarithmic equation. The average R 2 values for eqns 1-3 are 0.873, 0.911, and 0.936, respectively. So the best fits appear to be obtained with the logarithmic equation, but the differences in R 2 with the other equations are small and may not be significant. Considering the empirical character of our approach, it is clear that there is no strong justification to actually prefer one equation over another one at this stage. Indeed, the physical reason why eq 3 works best is unclear. Other physical transformations of solids, such as creep (27), follow logarithmic laws, but no physical explanation has been given for these processes either. For model parametrization, it is desirable to relate R values to environmental variables. Wind clearly has an effect, as the fastest decreases were observed for the April 25, 2000 layer at Alert, which was exposed to strong winds (up to 11 m/s). However, most other layers were subjected to low or negligible windspeeds (<4 m/s, when measurements were available), and we clearly have too few data under windy conditions to quantify the effect of wind. Temperature also seems to have an effect. For example, the February 3, 2000 layer evolved by around -40 C at Alert and its SSA decreased by only a factor of 1.5 in 17 days, while the SSA of the March 4, 1999 top sublayer, evolving around -1.5 C, decreased by a similar ratio in just 5 days. If the data for the April 25, 2000 are (2) SSA )-R log log t + SSA 1 (3) VOL. 37, NO. 4, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 665

6 FIGURE 7. Evolution of the parameter r exp as a function of 1/T. Values of r exp are summed up in Table 3. The April 25-28, 2000 snow layer affected by wind has not been included in the data. excluded, the values of R lin, R exp, and R log indeed seem to be correlated with temperature, and this needs to be explored further. SSA decrease appears strongly linked to metamorphism, as suggested by observations mentioned here and by the numerous photomacrographs shown by Cabanes et al. (20), and sublimation and condensation of water molecules is then a crucial process, which is thermally activated. A rate dependence in 1/T is then expected. Plots of R lin, R exp, and R log vs 1/T, again excluding the April 25, 2000 layer, show correlations with R 2 values of 0.27 for R lin, 0.63 for R exp, and 0.36 for R log. Thus, even though the logarithmic equation describes better the SSA decrease of a given snow layer, the exponential equation is better suited to describe the whole data set and to provide a parametrization of the effect of temperature. Thermally activated processes actually obey an Arrhenius law, and we have tested in Figure 7 the correlation between Ln R exp and 1/T. The correlation coefficient is 0.68, slightly better than when R exp is plotted vs 1/T. From Figure 7, we propose that the rate of SSA decrease of fresh snow can be parametrized by eq 2, with R exp given by R exp (day -1 ) ) 76.6 exp T(K) According to the limited data of Figure 7, eq 4 will give the rate within 50% at the 1σ confidence level. Thus, for snow layers evolving by -1 and -40 C, under low wind speeds, we can deduce that the SSA will decrease by a factor of 2 after 5 and 14 days, respectively. More observations are necessary before we can include wind speed in a more general parametrization. Yet other factors influence SSA decrease. Increases in snow density, either caused by wind or by the weight of subsequent layers, will increase contact points between crystals and reduce SSA. This is consistent with the anticorrelation between SSA and density obtained by Narita (28) and Legagneux et al. (21). Snow permeability is also an important parameter, as it will affect the motion of water vapor throughout the snowpack and therefore the crystal transformations that lead to SSA decrease. Permeability varies greatly as a function of density and snow type (29) and should also be measured to accurately predict the rate of SSA decrease. Acknowledgments Work in the Arctic was funded by the French Polar Institute (IPEV). Work in the Alps was funded by CNRS through (4) Program National de Chimie Atmosphérique (PNCA). Meteorological data from Col de Porte were kindly supplied by Yves Lejeune, Centre d Etude de la Neige, operated by Météo- France. The Alert 2000 campaign was coordinated by Jan Bottenheim (Meteorological Service of Canada) and Paul Shepson (Purdue University). We thank CFS Alert staff and Al Gallant (MSC) for logistical and technical support and Peter Brickell (MSC) for his efforts to supply us with liquid nitrogen. Work in Svalbard was done in collaboration with the NICE campaign, coordinated by Harry Beine (CNR Rome). The efforts of Roberto Sparapani (CNR Rome) to ensure that we had optimal working conditions are gratefully acknowledged. Liquid nitrogen in Svalbard was very graciously supplied by Kay Krüger (AWI). Literature Cited (1) Barrie, L. A.; Bottenheim, J. W.; Schnell, R. C.; Crutzen, P. J.; Rasmussen, R. A. Nature 1988, 334, 138. (2) Dibb, J. E.; Talbot, D.; Munger, D.; Jacob, S.-M.; Fan J. Geophys. Res. 1998, 103, (3) Weller, R.; Minikin, A.; König-Langlo, G.; Schrems, O.; Jones, A. E.; Wolff, E. W.; Anderson, P. S. Geophys. Res. Lett. 1999, 26, 601. (4) Honrath, R. E.; Peterson, M. C.; Guo, S.; Dibb, J. E.; Shepson, P. B.; Campbell, B. Geophys. Res. Lett. 1999, 26, 695. (5) Jones, A. E.; Weller, R.; Anderson, P. S.; Jacobi, H.-W.; Wolff, E. W.; Schrems, O.; Miller, H. Geophys. Res. Lett. 2001, 28, (6) Hutterli, M. A.; Rothlisberger, R.; Bales, R. C. Geophys. Res. Lett. 1999, 26, (7) Jacobi, H.-W.; Frey, M. M.; Hutterli, M. A.; Bales, R. C.; Schrems, O.; Cullen, N. J.; Steffen, K.; Koehler, C. Atmos. Environ. 2002, 36, (8) Sumner, A. L.; Shepson P. B. Nature 1999, 398, 230. (9) Sumner, A. L.; Shepson, P. B.; Grannas, A.; Bottenheim, J.; Anlauf, K.; Worthy, D.; Schroeder, W.; Domine, F.; Perrier, S.; Houdier, S. Atmos. Environ. 2002, 36, (10) Houdier, S.; Perrier, S.; Dominé, F.; Grannas, A. M.; Guimbaud, C.; Shepson, P. B.; Boudries, H.; Bottenheim, J. W. Atmos. Environ. 2002, 36, (11) Couch, T. L.; Sumner, A.-L.; Dassau, T. M.; Shepson, P. B.; Honrath, R. E. Geophys. Res. Lett. 2000, 27, (12) Wania, F. Chemosphere 1997, 35, (13) Sander, R.; Vogt, R.; Harris, G. W.; Crutzen, P. J. Tellus 1997, 49B, 522. (14) Michalowski, B. A.; Francisco, J. S.; Li, S.-M.; Barrie, L. A.; Bottenheim, J. W.; Shepson, P. B. J. Geophys. Res. 2000, 105, (15) De Serves, C. J. Geophys. Res. 1994, 99D, (16) Grannas, A. M.; Shepson, P. B.; Guimbaud, C.; Sumner, A. L.; Albert, M.; Simpson, W.; Dominé, F.; Boudries, H.; Bottenheim, J. W.; Beine, H. J.; Honrath, R.; Zhou, X. Atmos. Environ. 2002, 36, (17) Dominé, F.; Shepson, P. B. Science 2002, 297, (18) Dominé, F.; Cabanes, A.; Taillandier, A.-S.; Legagneux, L. Environ. Sci. Technol. 2001, 35, 771. (19) Dominé, F.; Cabanes, A.; Legagneux, L. Atmos. Environ. 2002, 36, (20) Cabanes, A.; Legagneux, L.; Dominé, F. Atmos. Environ. 2002, 36, (21) Legagneux, L.; Cabanes, A.; Dominé, F.J. Geophys. Res., in press. (22) Hanot, L.; Dominé, F. Environ. Sci. Technol. 1999, 33, (23) Brunauer, S.; Emmet, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (24) Gow, A. J. Snow studies in Antarctica. CRREL Res. Rep. 177, (25) Jellinek, K.; Ibrahim, S. J. Colloid Interface Sci. 1967, 25, 245. (26) Dominé, F.; Chaix, L.; Hanot, L. J. Colloid. Interface Sci. 2000, 227, 104. (27) François, D.; Pineau, A.; Zaoui, A. Comportement mécanique des matériaux; Hermès, Paris, (28) Narita, H. Low Temp. Sci. 1971, A29, 69. (29) Albert, M. R.; Shultz, E. F.; Perron, F. E. Ann. Glacio. 2000, 31, 353. Received for review June 14, Revised manuscript received November 18, Accepted November 25, ES025880R ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 4, 2003