Importance of Aerosol for Clouds and Clouds for Aerosol

1. Why the aerosol are important for clouds and Why clouds are important for the aerosol Jeff Snider University of Wyoming, Laramie jsnider@uwyo.edu

2. Outline - Basic physics of cloud/aerosol and aerosol/cloud interactions Properties of aerosol particles that make them good nuclei Cloud condensation nuclei (CCN) Ice nuclei Aerosol-to-CCN closure studies Populations of aerosol and hydrometeors (i.e., droplets, drops and crystals) Importance of clouds to the aerosol

4. In the atmosphere, H2O vapor is often subsaturated • However, saturated conditions exist within clouds • Some cloud environments can be supersaturated • Vapor state (subsaturated, saturated or supersaturated) is quantified with vapor pressure or absolute humidity (or vapor mixing ratio)

5. Below 0 oC, interesting thermodynamic effects take place • Ice is the stable phase, but liquid water can coexist • Like water, ice has a temperature-dependent saturation vapor pressure • The saturation vapor pressure over liquid water exceeds that over ice

6. Below 0 oC, the saturation vapor pressure over liquid water is larger than that over ice • From the perspective of the ice hydrometeors, mixed phase clouds are supersaturated. The ice can grow by deposition • The water saturation ratio increases by ~10% for every 10 degree of supercooling

7. Bergeron, 1935 - Points to the importance mixed-phase cloud, the favored growth of ice in such environments, and consequence for precipitation

8. "The supersaturation" "The ice supersaturation" "The water supersaturation" Description of conditions in warm clouds- Thermodynamic - water vapor is in equilibrium with liquid water Dynamic - water vapor amount exceeds equilibrium Description of conditions in cold clouds- Thermodynamic - water vapor is in equilibrium with ice Dynamic - water vapor amount exceeds equilibrium (Note: often liquid water and ice coexist....so-called mixed phase clouds)

9. How to apportion the latent heat? Rate of vapor mass transfer to a droplet of diameter “D” Rate of sensible heat mass transfer to a droplet of diameter “D” Combining, and linearizing the Claussius-Clapeyron equation-> Maxwell-Mason droplet growth equation

10. Köhler theory provides two things: 1) A connection between wet diameter and saturation ratio at the interface 2) A connection between critical saturation ratio and dry diameter

11. We have two equations that account for single particle growth via condensation- 1) Droplet size is related to time and ambient conditions The Maxwell-Mason Equation 2) Bdry condition at the droplet/air interface is related to properties of the nucleus The Koehler Equation

12. Sphere equivalent dry diameter of particle Soluble mass fraction van’t Hoff factor Molecular weight of solute, density of dry particle Surface energy of solution/air interface Characteristic of an “active” cloud condensation nucleus: -Large particle size -Contain materials (solute) that dissolve in water -Contain solutes that dissociate in solution -Contain many solute molecules -Contain solutes that reduce the energetic cost of forming an interface

13. Some active ice nuclei have lattice dimensions similar to ice-

15. Crystal concentrations observed in some clouds do depend on temperature in a manner consistent with generation via nucleation But, information is needed for describing the connection between ice nuclei sources, nuclei activity spectra and ice crystal concentrations. This is lacking

16. With ice there are additional complications: Ice can form via secondary processes Collision between graupel and snow Shattering of freezing drops Ice from one cloud can also "seed" a neighboring cloud

17. Aerosol-to-CCN Closure Studies How well do the predicted and observed CCN concentrations compare?

18. Why Closure Studies? Chemical Transport Model use a mass balance, constrained by aerosol source and sink processes, to derive aerosol size spectra. Parameterization, based on observation, is often used in the models. Questions: How reliable are the observational data sets used in the parameterizations? Does systematic error in the measurements alter the sign or sensitivity of the model prediction to alterations in aerosol properties? Under what circumstances are the simplifying assumptions OK? A common assumption is that the particles are spherical, often they are not Picture from Alexei Kiselev

19. The Wyoming static diffusion CCN Instrument

20. DMT (Scrips/Caltech/GT) Continuous-flow CCN Instrument

21. Two CCN instruments: Developer Chamber type Operating principle Calibration CCN detection Non-ideality #1 Non-ideality #2 Non-ideality #3 University of Wyoming Static Non-linearity of ew(T) Snider et al. (2006) Scattering from ensemble of droplets Temp. difference between outer and inner wall Wall material may alter ew(T) Activation spectrum broadening Developer Chamber type Operating principle Calibration CCN detection Non-ideality #1 Non-ideality #2 Droplet Measurement Technologies (DMT) Continuous flow Dissimilarity of vapor and heat diffusivities Lance et al. (2006) Single particle scattering Temp. difference between outer and inner wall Concentration bias

22. Wyoming Static Thermal Diffusion CCN Instrument

23. DMT (Scrips/Caltech/GT) Continuous-flow CCN Instrument Aerosol flow stream is surrounded by sheath flow H2O vapor diffuses (inward) faster than sensible heat Maximum supersaturation is near exit to OPC Activated (growing) CCN are counted in OPC Resistance to heat flow across wall Efficiency = (Th'-Tc')/(Th-Tc) ~ 0.7 Efficiency is evaluated in laboratory studies

24. DMT Calibration: Th-Tc = 5.35 oC, Qtot=0.5 L/min, P = 0.8 atm, SAR = 10 Particles of known size and composition are produced in a DMA Ammonium sulfate is preferred, but there are issues Koehler theory used to infer particle SSc from the DMA-selected Dd Small test particles (i.e. particle SSc > maximum chamber SS) -> no activation Large test particles (i.e. particle SSc < maximum chamber SS) -> complete activation Activated fraction = 0.5 defines the maximum chamber SS Outer wall temperatures, efficiency, chamber model -> from max chamber SS

26. Snider et al., in press, Journal of Atmospheric and Oceanic Technology

27. Snider et al., in press, Journal of Atmospheric and Oceanic Technology

28. ΔT SSnom K/M, 1975 Chamber Model Köhler Model D50 Seff Wyoming calibration: ΔT = 2.2 oC, Tt=20 oC, P = 0.8 atm From measurement of activated fraction versus sphere equivalent diameter, the size at 50% activation is determined - > D50 Snider et al., in press, Journal of Atmospheric and Oceanic Technology

29. We concluded that the supersaturation determined from temperature measurement, and a model of the chamber, that the nominal supersaturation is a factor of 1.6 larger than the supersaturation evaluated from particle size and a Koehler model The cause of this discrepancy... Snider et al., in press, Journal of Atmospheric and Oceanic Technology

30. DMT / Wyoming CCN comparison experiments in Leipzig, November 2005 Simple (ammonium sulfate) and complicated (soot-coated) test particles Determinations of Critical Supersaturation in both instruments Table shows statistics (average and standard dev) for the DMT / Wyoming Ratio

31. Droplets=1000 and 50 cm-3, No Ice, zbase = 500 m, Tbase = -10 oC

32. Vertical velocity Positive constant Positive constant Liquid Mixing ratio The physics of S(t) in a parcel model - LWC lags the adiabatic liquid mixing ratio, and more so when there are fewer droplets, i.e., 50 cm-3 In an adiabatic parcel their is no supersaturation, vapor and liquid are at equilibrium. In other words the supersaturating effect of the cooling is exactly balanced by the formation of liquid (dS/dt = 0, and S=1) It follows that the initial rate of increase of S is larger for the parcel with fewer droplets, compared to the parcel with more droplets

33. Vertical velocity Positive constant Positive constant Liquid Mixing ratio The physics of S(t) in a parcel model (continued) Also, the characteristic time for adjustment to a steady state is longer in the case of the of the simulation with fewer droplets, i.e., 50 cm-3 Hence, the peak saturation ratio is larger, and it occurs higher in the cloud when there are fewer droplets, i.e., 50 cm-3

34. unpolluted polluted polluted unpolluted Have I contradicted myself? Large cloud droplet concentration -> small maximum saturation ratio Small cloud droplet concentration -> large maximum saturation ratio Does this imply that an increase in nuclei (i.e., pollution) will decrease the maximum saturation ratio enough to decrease the droplet concentration? I.e., causing a reverse of the first indirect effect of aerosol on climate? from Snider et al., JGR, 2003

35. Parcel Model w Parcel Model Calculation - Sensitivity of droplet concentration to aerosol and updraft Parcel Model w Mcfiggans et al., Atmospheric Chemistry and Physics, 2006

36. Droplets=50 cm-3, Ice=100 L-1, zbase = 500 m Tbase = -10 oC

37. Cloud, and especially precipitation associated with clouds, has a profound impact on the aerosol! Aerosol are removed from the atmosphere by precipitation Coalescence scavenging Aerosol scavenging by precipitation falling below cloud Aerosol attachment to cloud and precip via brownian motion Aerosol number concentration can be decreased even if precipitation evaporates On average there is a steady state between aerosol source and aerosol removal In some cloud regimes there is an imbalance between source and sink Marine stratocumulus

38. Marine summertime clouds - DYCOMS-II (2001) Marine stratus, July, cloud top temperatures > 0 oC, 300 km west of California Aerosol Source Processes - Wind speeds in the marine boundary layer (MBL) < 10 m/s Characteristic time for sea salt aerosol source ~ 10 day Entrainment of free troposphere (FT) into MBL characterized using tracers Characteristic time for entrainment of FT aerosol into MBL ~ 10 day Aerosol Sink Processes - Drizzle rates were surprisingly large (~10 mm/day, 100 mm/day locally!) Coalescence scavenging thought to dominate Aerosol source rates < Aerosol sink rates Aerosol concentrations decrease Aerosol surface area decreases, a threshold is reached, new particle formation occurs Evidence for new particle formation in the MBL on July 11, 2001 (RF02) Heavy drizzle, open-cell cloud structure also documented in new particle region

39. Leon et al., Journal of Atmospheric and Oceanic Technology, 2006

40. Drizzle is most intense in regions of rising air motion (w ~ 1 m/s) Ascent is driven by horizontal convergence at the base of the MBL Coupling of ascent and drizzle implies longer drizzle growth times and enhanced removal of cloud droplets (drizzle scavenging) compared to drizzle formation elsewhere in the MBL Leon et al., Journal of Atmospheric and Oceanic Technology, 2006

41. We concluded: 1) New aerosol particles were formed in response to a depletion of the preexisting aerosol surface area by heavy drizzle. 2) Organization of the cloud into open cell structures may be either necessary for new particle formation or a consequence of it. Petters et al., Journal of Geophysical Research, 2006

42. Concluding Remarks - 1. Aerosol size spectra and number concentration are influenced by precipitation and this in turn influences the properties of clouds 2. No measurement is perfect, but through intercomparison instrument bias uncovered and accounted for. 3. Models need data sets for parameterization and also for initialization. These data sets should be as free of measurement bias as is possible. 4. Through collaboration we reach our objectives sooner and with greater understanding of the consequence of our efforts. Acknowledgements - The group at Warsaw (Hanna, Tymon, etc.) Markus Petters (Colorado State University) David Leon (University of Wyoming)