Misure di nuclei di ghiacciamento. Problematiche e aspetti critici. F.Belosi, G.Santachiara, A.Nicosia, F.Prodi

Transcription

1 Misure di nuclei di ghiacciamento. Problematiche e aspetti critici F.Belosi, G.Santachiara, A.Nicosia, F.Prodi

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4 Nucleation of ice in the atmosphere Homogeneous nucleation of ice Heterogeneous nucleation

5 Relative humidity with respect to water RH w (%) = e / e sat,w x 100 e: the water vapour pressure at the experimental temperature T Relative humidity with respect to ice RH ice (%) = e / e sat,ice x 100 Solution water activity a w = e / e sat,w Ice saturation ratio S ice = e / e sat. ice S ice <1 ; S ice = 1 ; S ice >1 Supersaturation with respect to ice SS ice = e /e s.,ice -1 SS ice = RH ice (%) -100%. Water saturation ratio: S w = e / e sat. water S w <1; S w = 1 ; S w >1 Supersaturation with respect to water: SS w = e / e sat, w -1 SS w = RH w (%) -100%.

6 Mixed clouds: growth of ice crystals by diffusion of water vapour through Wegener-Bergeron-Findeisen. Aerosol particles (IN) catalyse the formation of a few frozen cloud droplets from supercooled droplets of liquid water. The ice crystals grow at the expense of the remaining majority of liquid droplets through transfer of water vapour. The resulting large ice particles have higher fall velocities than the small liquid droplets, and may initiate precipitation.

7 Homogeneous freezing nucleation Pure water and water solutions (inorganic/organic compounds): Polar stratospheric clouds cirrus clouds in the upper troposphere (~200mbar, K), updrafts of cumulus cloud. Homogeneous freezing temperature of pure water depends on diameter of droplets: Micrometer cloud droplets can supercool down to -38 C. Nanodroplets freeze at lower temperatures Blu symbols: Median freezing temperatures of pure water samples vs. drop diameter. Red symbols: heterogeneous freezing

8 Droplets of water solution freeze at T< than pure water The melting point curves and the freezing temperatures of droplets of aqueous solution is a function of water activity of the solution, irrespective of the nature of the solute. a w = e / e sat,w Heterogeneous freezing T of four different IN vs. water activity of the solution. Silica spheres, AgI, ATD, Zobrist, Koop, 2000;

9 Heterogeneous nucleation

10 Primary nucleation process: a) Homogeneous freezing occurs spontaneously within an aqueous droplet which may or may not contain solutes. b) Heterogeneous freezing include (1) Immersion/condensation freezing (2) Contact freezing (3) Depositional freezing c) Homogeneous nucleation requires a greater degree of metastability supercooling or supersaturation than heterogeneous nucleation

11 Contact freezing

12 INP concentration in the atmosphere - INP: Mineral dust, soot, biological particles, volcanic ash, organic compounds, - The capability to heterogeneously activate ice formation depends on: - T, supersaturation with respect to water and ice, particle size, chemical composition, crystallographic structure of surfaces. - C IN = f(t,s ice,s w ) ~ cm -3 < C CCN << C aer - Represent a small fraction of all aerosol particles (1 in 10 6 ). IN number concentration (at STP) active at water saturation or above vs. T. Locations: Arctic and Amazon Basin. Mixed clouds Fletcher, 1962: Nice= A exp(βδt); Meyers (1992): Nice= exp(a+b Sice)

13 Particles produced by various combustion: CFDC, T = -55 C; Cirrus Storm Peak Laboratory (3200 a.s.l.) Aerosol particles were either sampled directly by the particle analysis by Laser Mass Spectrometer PALMS, or were first passed to a continuous-flow diffusion chamber (CFDC) to induce ice nucleation in aerosol particles at defined temperature and humidity. [De Mott et al. PNAS, 2003; Cziczo et al., 2004].

14 Requirements for an efficient IN a) Water insolubility Water soluble species, e.g. solid ammonium sulphate (AS) particles and oxalic acid, can also act as IN in the immersion and deposition mode b) Crystallographic requirements: the surface crystal structure should be similar to that of ice. Organic aerosol, biological particles (pollen, bacteria, spores etc.) have also been shown to nucleate ice despite their amorphous non-crystalline nature. Sea-spray aerosol as source of ice nucleating aerorol DeMott, PNAS,

15 - NH 4 SO 4 2- T> 273K CCN b) T 230 K RH > RH sat ice : Deposition mode By increasing RH, AS can reach deliquescence RH. During the time it takes the crystal to fully deliquesce, ice nucleation via immersion freezing may occur. If ice deposition nucleation does not occur, the AS crystal will start to deliquesce once the appropriate deliquescence RH value (dash-dotted line) is reached. At this point the AS crystal will dissolve and intermediately form a crystalline core within an AS solution. If not, the particle will fully deliquesce homogeneous nucleation. [Zuberi, 2001; Hung, 2002].

16 Static vapour diffusion chamber FRIDGE diffusion cell: deposition-nucleation, condensation-freezing Dynamic filter processing chamber (DFPC) 1: air inlet; 2: minced ice; 3:slit and air temperature thermocouple 4: filter; 5: filter temperature thermocouple; 6: Peltier cooling device; 7: thermocouple; 8: air outlet; 9: plexiglass cover; 10: observation slit; 11:aluminium plate.

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18 Continuous flow diffusion chamber (CFDC) : CSU-IS, PINC, FINCH, LACIS, IMCA-ZINC Can detect: deposition, condensation/immersion freezing, homogeneous freezing, but not contact freezing. - Expansion chamber- AIDA - Drop freezing technique

19 Drop-freezing assay [immersion freezing] A) Sampling of aerosol on a filter In-line filter (0.2 µm pore) Filter in tube, + 5 ml H 2 O Filter containing the collected aerosol placed in distilled water aerosol removed by ultrasonic shaker drops tested for immersion freezing Budke Koop, 2015 B) Sampling of aerosol with impinger Shake for 20 min

20 Dry dispersion Brush disperser filter DFPC Water suspension Nebulization Silica gel CFDC Water suspension Immersion freezing Drop-freezing technique Dry dispersion CFDC, Expansion chamber

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22 For material with the same chemical composition, the threshold temperature of ice formation depends on particle diameter Montmorillonite; Deposition nucleation below water saturation was found only below -30 C or -35 C dependent on particle size. - Smaller particles require a higher RH ice to be activated [Welti, ACP, 2009]

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24 Preactivation memory effect - reproducibility : Particles that acted as ice nuclei can give different results in further cicles, i.e. change in the ice nucleation efficiency. The threshold temperature of ice formation for material with the same chemical composition depends on nucleation mode, i.e. if the aerosol act as deposition, condensation or immersion freezing, or contact freezing. Inconclusive results even per similar compounds: minerals from different sources can have different physical and chemical properties, dispersion method (wet, dry) changes size distribution, hygroscopicity. Powder samples have usually undergone mechanical processing (miling, etc.), are chemically treated.

25 Overview of ice nucleation onset temperatures and saturation ratios with respect to ice: different particle species, nucleation modes, sample preparation, particle sizes, etc.

26 A calibration ice nucleating material does not exist, and there is no general agreement on a standard technique for measuring IN. INTERCOMPARISON CAMPAIGNS First Workshop on INP: Lannemezan, France Second International Workshop on Condensation and Ice Nuclei: 1971, Colorado State University Third International Workshop on Ice Nucleus Measurements: 1975, Laramie, Wyoming) Fourth International Workshop on comparing ice nucleation measuring systems (ICIS-2007): Comparison of activated conditions of ATD particles in various instruments. ZINC results deviated from others and this discrepancy remains unexplained.

27 Fifth International Ice Nucleation Workshop, March 9-27, 2015 Karlsruhe Institute of Technology (KIT, Germany) Tunesian soil dust Illite Microcline k-feldspar Soil dust from Argentina Snomax Blind-test (no. 4): Arizona Test Dust; Argentine soil dust

28 Summary Even after 50 years of ice nuclei studies, a calibration ice nucleating material does not exist, and there is no general agreement on a standard technique for measuring IN. Similar compounds (e.g. montmorillonite) can give different results depending on impurity, coating, hygroscopicity, size, generation. Preactivation memory effect reproducibility Secondary organic coating increase the RH for ice nucleation. Intercomparison campaigns More work should be done to represent ice nucleation of dust in numerical simulations in both cirrus and mixed phase clouds.

29 P1: Concentrazione dei nuclei di ghiacciamento a San Pietro Capofiume (BO): sorgenti locali e trasporto da lunghe distanze M.Rinaldi, F.Belosi, A.Nicosia, G.Santachiara, S.Decesari, M.C.Facchini P6: Particelle di cellulosa come nuclei di ghiacciamento in nube M.Piazza, A.Nicosia, G.Santachiara, F.Prodi, F.Belosi