Cloud Condensation Nuclei Hygroscopic Parameter Kappa

Transcription

1 Cloud Condensation Nuclei Hygroscopic Parameter Kappa Covers Reading Material in Chapter 17.5 Atmospheric Sciences 5200 Physical Meteorology III: Cloud Physics

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3 Cloud Condensation Nuclei (CCN) Form a cloud droplet under atmospheric saturations Reported CCN concentrations always refer to specific supersaturations, for example CCN(1%) or CCN(0.5%) + Number distribution of aerosol population CCN s = & n D ) dd ) supersaturation, - Activation diameter for supersaturation

4 Cloud Condensation Nuclei (CCN) + CCN s = & f / D ) n(d ) )dd ), - Fraction of the aerosol particles of D p that are activated at supersaturation s% f / D ) is will be either 0 or 1, if the aerosol particles are internally mixed (all particles of the same diameter have the same chemical composition)

5 Realistic

6 Relative humidity RH = e e / (T) Supersaturation e > e / (T) S RH 100% S 0.5 2% in convective updrafts

7 CCN data

8 Ambient CCN Depends on both Size and Composition

9 Results from a spruce wood fire Average CCN/CN ratios as function of supersaturation and particle diameter. P. Frank, G & Ulrike, Dusek & Andreae, Meinrat. (2006). Technical note: A method for measuring sizeresolved CCN in the atmosphere. Atmospheric Chemistry and Physics Discussions /acpd

10 P. Frank, G & Ulrike, Dusek & Andreae, Meinrat. (2006). Technical note: A method for measuring sizeresolved CCN in the atmosphere. Atmospheric Chemistry and Physics Discussions /acpd Results from a peat fire. Average CCN/CN ratios as function of supersaturation. The 50 and 100 nm particles show similar behavior to the wood fire, whereas the 250 and 325 nm particles behave different

11 Ambient CCN Depends on both Size and Composition Size Dominates

12 Fig. 1. (A) An example of size-resolved 6-hour averaged CCN spectra for particle diameters between 40 and 120 nm. U. Dusek et al. Science 2006;312: Published by AAAS

13 Fig. 2. (A to D) Particle number size distributions and CCN size distributions for the four chosen case studies. U. Dusek et al. Science 2006;312: Published by AAAS

14 Are scenarios where composition changes do matter Aging of dust

15 Giant CCN Small fraction of particles with relatively large dry diameters (>1um) that activate at low supersaturations (<0.02%) These particles can grow to a large size (>10um) even before becoming actually activated. First to activate, grow rapidly, reduce the supersaturation reached in the cloud. Sea spray over ocean Large Ash from biomass burning, mineral dust, salt particles from evaporating lakes

16 Köhler Curve Koehler curve for two drops: N s =1x10-17 moles (blue solid line), N s =5x10-17 moles (red dashed line). Credit: W. Brune

17 Hygroscopic Parameter Kappa Many many organic compounds in aerosols are unknown and not individually measured Difficult to predict CCN behavior of these ambient aerosols from 1 st principals (Köhler theory)

18 Hygroscopic Parameter Kappa Kroll et al., Nature Chemistry 3, (2011)

19 Hygroscopic Parameter Kappa Kroll et al., Nature Chemistry 3, (2011)

20 Hygroscopic Parameter Kappa Many many organic compounds in aerosols are unknown and not individually measured Difficult to predict CCN behavior of these ambient aerosols from 1 st principals (Köhler theory) Petter and Kreidenweis (2007) introduced a hygroscopicity parameter, kappa Relates the volume of water taken up by a particle to the water activity. Values of kappa determined experimentally by fitting CCN activity κ

21 Köhler Curve e / (D ) ) e / y D x D = exp 4M Dσ D R L Tρ D D ) x D = Mole fraction of pure water e / = saturation vapor pressure of water adjacent to a pure water droplet of the same size and at the same temperature. y D = Activity Coefficient, accounts for non ideal behavior y D 1, for dilute solution M w = Molecular Weight of substance ρ D = water density

22 Hygroscopic Parameter Kappa e / (D ) ) e / y D x D = exp 4M Dσ D R L Tρ D D ) S = a D exp 4v Dσ D R L TD ) a D = water activity in the solution

23 Hygroscopic Parameter Kappa e / (D ) ) e / y D x D = exp 4M Dσ D R L Tρ D D ) S = a D exp 4v Dσ D R L TD ) a D = water activity in the solution Need to put this in something that we can measure

24 Hygroscopic Parameter Kappa S = a D exp 4v Dσ D R L TD ) a D = water activity in the solution 1 a D = 1 + κ V / V D Volume of the dry particle Volume of the water S = D )` D à D )` D à (1 κ) exp 4v Dσ D R L TD )

25 Hygroscopic Tandem Differential Mobility Analyser

26 Hygroscopic Tandem Differential Mobility Analyser

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