Cloud multiphase processes and their impact on climate

1. Cloud multiphase processes and their impact on climate Maria Cristina Facchini Istituto di Scienze dell’Atmosfera e del Clima - C.N.R. Bologna, Italy Atmospheric Chemistry

2. Acknowledgements M. Mircea, S. Fuzzi, S. Decesari, E. Matta ISAC-CNR, Bologna, Italy R.J. Charlson University of Washington, Seattle, USA A. Nenes, J.A. Seinfeld California Institute of Technology, Pasadena, USA S.L. Clegg University of East Anglia, Norwich, UK M. Kulmala University of Helsinki, Helsinki, Finland E. Tagliavini University of Bologna, Italy Atmospheric Chemistry

3. Clouds and climate • Clouds are the most important factor controlling the Earth albedo and hence the temperature of our planet • Cloud optical properties are controlled by size/number of droplets which in turn are governed by the “availability “ of aerosol particles to serve as CCN Atmospheric Chemistry

4. Clouds and climate 2 • Changes in cloud optical properties induced by man’s activity are at the moment highly uncertain Atmospheric Chemistry

5. Parameters influencing CDN • Many years ago, Twomey suggested that the most important parameter influencing cloud droplet number (CDN) is aerosol number concentration, while aerosol chemical composition has a relatively minor effect • Recently, model and experimental results have induced to revisit this assumption and to re-examine the relative importance of the different factors influencing CDN distribution Atmospheric Chemistry

6. CDN and aerosol number • The number of CDN is not a “linear” function of aerosol number (Ramanathan et al., Science, 2001) • The large degree of variation suggests that cloud properties are controlled by many different factors Atmospheric Chemistry

7. The issue • how does the chemistry of the cloud multiphase system influence formation and evolution of the cloud droplet population ? Atmospheric Chemistry

8. …an intuitive picture of cloud chemistry gas phase R R Dry particle wet aerosol Cloud droplet s Absorbing material Soluble fraction chemical composition RH Atmospheric Chemistry

9. Cloud formation • Atmospheric thermodynamic parameters (moisture availability, updraft velocity, temperature, etc.) • Aerosol properties: classically, the controlling chemical variables are CCN size distribution and water soluble mass Atmospheric Chemistry

10. Theory of cloud formation aw = water activity s = surface tension nw = water molar volume Atmospheric Chemistry

11. Water activity ? ? Only one paper: Clegg et al., J. Aerosol Sci., 2001 for inorganic aqueous electrolytic solutions Atmospheric Chemistry

12. Raoult term Kelvin term Modified Köhler equation Atmospheric Chemistry

13. Chemical factors controlling cloud formation • Not simply inorganic soluble salts influence cloud formation • Soluble or slightly soluble organics influence equilibrium water vapor pressure and decrease surface tension of the droplets • Soluble gases condensation (Charlson et al., Science 2001) Atmospheric Chemistry

14. Aerosol chemical composition Atmospheric Chemistry

15. Organic aerosols andKöhler theory • Organic aerosols influence equilibrium supersaturation by: • “adding” soluble material • decreasing surface tension with respect to pure water or an inorganic salt solution Atmospheric Chemistry

16. Speciation of organic aerosol • The traditional analytical approach has usually been individual compound speciation, but less than 10% of OC mass has been accounted for • A new method using functional group analysis has been developed which accounts for up to 90% of OC mass (Decesari et al., J. Geophys. Res., 2000) Atmospheric Chemistry

17. Organic solutes in clouds • WSOC are a complex mixture of highly oxidised, multifunctional compounds with residual aromatic nuclei and aliphatic chains • Neutral compounds: mainly aliphatic polyols, polyethers, sugars; • Mono-/di-acids: hydroxylated aliphatic acidic compounds; • Polyacids: unsaturated polyacidic compounds both aliphatic and aromatic with a minor content of hydroxyl groups • This information can be used to construct a set of model compounds Atmospheric Chemistry

18. Why model compounds? • Too often the physical and chemical properties of atmospheric OC are simulated in models using compounds which are not representative of the physical reality • Modellers need a synthetic information of a few model compounds which can be used to simulate in a quantitative way the whole OC of aerosol and clouds Atmospheric Chemistry

19. CH2 C CH2 CH Ar O OH CH2 CH3 31 % 9 % 21 % 38 % CH2 CH2 CH2 CH2 O O CH CH2 CH CH2 Ar OH Ar OH HO HO 50 % 5 % 23 % 23 % 8 % 9 % 42 % 41 % neutral fraction mono-/di-acids polyacids (Fuzzi et al., GRL, 2001) Atmospheric Chemistry

20. Kelvin term Modified Köhler equation Atmospheric Chemistry

21. Surface tension measurements cloud Tenerife ACE-2 fog Po Valley cloud ACE-ASIA s= K - b T ln (1+a C) Atmospheric Chemistry

22. Effect of organics on Sc inorganic only 0.05 mm inorganic+organic inorganic+organic+s 0.1 mm from Mircea et al., Tellus, 2001 0.3 mm Atmospheric Chemistry

23. Trace gas dissolution Laaksonen et al., JAS, 1998 HNO3 Atmospheric Chemistry

24. 0.08 10 cm/s 0.1 m s-1 30 cm/s 0.06 0.3 m s-1 R* 100 cm/s 1.0 m s-1 300 cm/s 0.04 3.0 m s-1 Maximum albedo change, DR* 0.02 organic no Ds insoluble 0.00 5 ppb HNO3  2 conc. organic with Ds -0.02 -0.04 Modelling of chemical effects 0.1 m s-1 0.3 m s-1 1.0 m s-1 3.0 m s-1 insoluble organic no Ds organic with Ds 5 ppb HNO3  2 conc. polluted case marine case (Nenes et al., GRL in press) Atmospheric Chemistry

25. Effect of size segregated chemical composition Dotted line: bulk composition Solid line: size-segr. compostion See poster Mircea et al., Session B Atmospheric Chemistry

26. Water activity of multicomponent solutions aw Clegg treatment as above + measured s modified Koher theory data from Clegg et al., J. Aerosol Sci., 2001 Atmospheric Chemistry

27. Conclusions • Dissolution of gases, dissolution of soluble and slightly soluble organics and the associated decrease of s influence droplet population • There are many conditions in the atmosphere in which chemical factors influence/control cloud microphysics to the same extent as cloud dynamics and/or aerosol number concentration Atmospheric Chemistry

28. …still needed • data on physical and chemical properties of aerosol are needed for different areas and aerosol types • thermodynamic data and models of awfor the complex cloud droplet solutions are needed Atmospheric Chemistry

29. Model compounds molar composition (%) pinonaldehyde 16 levoglucosan 9 catechol 2 azelaic acid 14 hydroxy-benzoic acid 15 b-hydroxy-butyric acid 3 fulvic acid 41 (Fuzzi et al., GRL, in press) Atmospheric Chemistry

30. Models neutral compounds cathecol pinonaldehyde hydrated levoglucosan Atmospheric Chemistry

31. Models mono-/di-acids azelaic acid b-hydroxy-butyric acid hydroxy-benzoic acid Atmospheric Chemistry

32. Model polyacids fulvic acid Atmospheric Chemistry

33. Surface tension depends onchemical composition Atmospheric Chemistry