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Measuring the specific surface area of snow using methane adsorption and IR reflectance

Measuring the specific surface area of snow using methane adsorption and IR reflectance -- difficulties and some progress --. Florent Domine Laurent Arnaud Carlo Carmagnola Nicolas Champollion Anne Dufour Frédéric Flin Jean-Charles Gallet Bernard Lesaffre Samuel Morin Ghislain Picard.

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Measuring the specific surface area of snow using methane adsorption and IR reflectance

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  1. Measuring the specific surface area of snow using methane adsorption and IR reflectance -- difficulties and some progress -- Florent Domine Laurent Arnaud Carlo Carmagnola Nicolas Champollion Anne Dufour Frédéric Flin Jean-Charles Gallet Bernard Lesaffre Samuel Morin Ghislain Picard Takuvik joint international Laboratory, Québec Glaciology Laboratory, Grenoble Météo France, Grenoble Glaciology Laboratory, Grenoble Météo France, Grenoble Météo France, Grenoble Norwegian Polar Institute, Tromsø Météo France, Grenoble Météo France, Grenoble Glaciology Laboratory, Grenoble

  2. 50 100 150 200 0 250 m2 kg-1 Specific Surface Area (SSA) • light scattering and albedo • radiation e-folding depth • amounts of adsorbed gases • rate of surface chemical reactions Determines : Central physical and chemical variable Currently known range of values: Fresh snow Decomposing particles Fine-grain snow Faceted crystals Depth hoar Refrozen snow

  3. Specific Surface Area (SSA) Thousands of values have been measured using: • CH4 adsorption • NIR or SWIR reflectance • X-Ray tomography • Stereology • … Claimed Precision of a given method is 5-12% Claimed accuracy a given method is 10-20% Not always Do all methods agree with each other ? What are the various sources of error ? The method itself andsnow sampling

  4. Objectives of this talk Detailed presentation of the CH4 adsorption technique Reliability of the method and comparison with X-Ray tomography Detailed presentation of the SWIR reflectance method using the DUFISSS instrument Comparison between the DUFISSS and ASSSAP instruments Conclusions on possible sources of errors and artifacts How to improve the reliability of SSA measurements Focus of this presentation is on current difficulties

  5. Specific Surface Area (SSA) Currently known range of values: Early measurements: 50 – 223 m2 kg-1 for dry fresh snow 15 – 90 m2 kg-1 for decomposing grains 10 - 40 m2 kg-1 for fine-grained snow 8 - 45 m2 kg-1 for faceted crystals 7 - 22 m2 kg-1 for depth hoar 2 – 25 m2 kg-1 for rerfrozen snow 200 - 1300 m2 kg-1, fresh snow, Adamson 1967 7770 m2 kg-1 Jellinek 1967 57 m2 kg-1 fresh snow, Chaix et al. 1996 200 - 1300 m2 kg-1, fresh snow, Adamson 1967 7770 m2 kg-1 , Jellinek 1967 57 m2 kg-1, fresh snow, Chaix 1996 N2 adsorption at 77 K N2 adsorption at 77 K CH4 adsorption at 77 K Measuring snow SSA using gas adsorption is delicate, and CH4 is better than N2

  6. Principle of gas adsorption method

  7. l-N2 77 K First step : snow sampling Can any transformation take place in the sample during those operations ? 260 K

  8. P P Vintro Th Th Th expansion Th Vexpansion Tc Tc Tc Liquid N2 snow Tc n1 P1 Before expansion: n1 moles of gaz in Vintro Second step : Measuring the isotherm n2  P2 After expansion: n2 moles of gaz in Vintro + Vexp Nads = n1 - n2  adsorption isotherm:Nads = f (P2)

  9. P Nads = n1 - n2 Vintro Th Vmh After n increments of adding CH4 in Vintro : Vmc Tc Liquid N2 snow Obtain adsorption isotherm Nads=f(P2) accuracy of volume measurement is critical

  10. Principle of gas adsorption method BET hypotheses (Brunauer, Emmet, Teller, 1938) First layer adsorbs with enthalpy Qa Subsequent layers adsorb with enthapy QL= enthalpy of condensation QL Qa

  11. Analysis of adsorption isotherm: BET transform B.E.T. Equation, linear section 0.07 - 0.22 P/P0 Surface area of 1 molecule Surface area of sample, m2 CH4: 19.18 Å2 (Chaix et al., 1996) mass of sample, kg SSA of sample, m2 kg-1 (3 h of work)

  12. Vintro Th P P Th Th expansion Vexpansion Th Tc Liquid N2 snow Tc Tc Tc Why CH4 works better than N2 n2  P2 n1 P1 Accuracy depends on (P1 - P2)/P1 N2 : P0=Psat 1000 hPa  (P1 - P2)/P1 low Constraints on pressure sensor and field use CH4 : P0=Psat 13 hPa  (P1 - P2)/P1 high Kr : P0=Psat 1 hPa  (P1 - P2)/P1 very high Precision: 6 % Accuracy: 12 %

  13. Can we check the validity of the CH4 adsorption method ? Kerbrat et al. (2008) ACP 8, 1261. Compare SSA from CH4 adsorption and from X-Ray tomography Fig. 5. The correlation between adsorption measurements and tomography was found to be SSAμCT =1.03(±0.03) SSABET. Both methods agree within 3% Swiss comparison

  14. Possible artifacts Formation of amorphous ice with super high SSA Adsorption of CH4 onto container surfaces Evidenced and corrected in: Domine et al. (2007) JGR, F02031. Correction up to 20% in low SSA samples Jellinek and Ibrahim (1967) SSA= 7770 m2 kg-1 260 K 77 K H2O

  15. Comparison of various SSA methods at La Grave, French Alps, 3100 m a.s.l., 1st and 17th April 2009 Experiment coordination: Ghislain Picard

  16. Sample preparation for X-Ray tomography: Frédéric Flin

  17. Sample preparation for X-Ray tomography: Frédéric Flin

  18. Sampling vials for CH4 adsorption

  19. Comparison CH4 adsorption - X-Ray Microtomography in Grenoble Lab measurements of SSA from samples taken in pits at La Grave X-Ray tomography values are 10-25% lower than CH4 adsorption values French comparison

  20. From our point of view, it is today difficult to resolve the discrepancy between both methods Differences can be due to : The methods themselves X-Ray Tomography CH4 adsorption Image treatment protocol Intrinsic approximations The sampling protocol X-Ray Tomography CH4 adsorption Making the sample Taking the sample Thermal cycling Thermal cycling

  21. Calculations Another illustration of SSA measurement difficulties: Measuring the SSA of snow using IR reflectance Measurements Domine et al., CRST 2006

  22. InGaAs Photodiode Signal, mV Snow Standards Reflectance DUal Frequency Integating Sphere for SSA measurement Laser diode 1310 nm Integrating sphere Gallet et al. (2009) The Cryosphere, 3, 167-182

  23. DUFISSS : SSA - reflectance calibration at 1310 nm Measure (1) SSA by CH4 adsorption and (2) Reflectance with the sphere

  24. La Grave intercomparison, 1st and 17th April 2009

  25. La Grave intercomparison, 1st and 17th April 2009

  26. Conclusion on CH4 – DUFISSS intercomparison Essentially good agreement Expected since DUFISSS has been calibrated with CH4 adsorption

  27. Laser 635 nm Laser 1310 nm InGAs photodiodes Si photodiode InGAs photodiodes Profilers of specific surface area (POSSSUM & ASSSAP) Calibrated with CH4, DUFISSS and radiative transfer modeling

  28. POSSSUM Distance measurement Electronics with pre-amplifier and laser power supply Optical module Winch (25m-long cable) Bore and its motor Profiles down to 20 m depths

  29. La Grave intercomparison, 1st and 17th April 2009

  30. La Grave intercomparison, 1st and 17th April 2009

  31. La Grave intercomparison, 1st and 17th April 2009 On 17th April 1 - POSSSUM bore hole 2 - POSSSUM measurements 4 – DUFISSS measurements 3 – Dig pit right on borehole

  32. La Grave intercomparison, 1st and 17th April 2009

  33. Conclusion on CH4 – DUFISSS - POSSSUM intercomparison Essentially good agreement But 0-15% differences observed POSSSUM has been calibrated with CH4 adsorption and DUFISSS Differences could be due to : Lateral variations in snow properties Modifications of snow properties after surface has been cut

  34. ASSSAP : Alpine Snow Specific surface Area Profiler Smaller version of POSSSUM SSA profiles down to depths of 2 m Used with DUFISSS at Summit, Greenland

  35. 10 May stratigraphy

  36. T= - 25°C H20 Small ice crystal condensation ?? T= - 45°C

  37. Conclusion on DUFISSS – ASSSAP comparison at Summit Preparing a sample for SSA measurements perturbs the snow Values obtained with 2 intercalibrated methods can differ by up to 70 % At Summit, being able to examine the sample proved critical Minimizing the time between sample preparation and measurement may also be critical a thoroughly tested sampling protocol is critical

  38. General conclusion Many methods exist to measure snow SSA These methods can be compared and intercalibrated in the laboratory In the field, sample preparation may be the main cause of differences between methods efforts at comparing sample preparation methods appear essential

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