Atmospheric Chemistry 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 Acknowledgements M. Mircea, S. Fuzzi, S. Decesari, E. Matta ISAC-CNR, Bologna, Italy ISAC-CNR, Bologna, Italy R.J. Charlson University of Washington, Seattle, USA A. Nenes, J.A. Seinfeld California Institute of Technology, Pasadena, USA 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 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 Clouds and climate 2  Changes in cloud optical properties induced by man’s activity are at the moment highly uncertain

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

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

Atmospheric Chemistry 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 Theory of cloud formation a w = water activity  = surface tension w = water molar volume

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

Atmospheric Chemistry Modified Köhler equation Kelvin term Raoult term

Atmospheric Chemistry 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) (Charlson et al., Science 2001)

Atmospheric Chemistry Aerosol chemical composition

Atmospheric Chemistry Organic aerosols and Köhler 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 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 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 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 CH 2 CH 3 38 % CH CH OH 31 % 9 % O C CH 2 21 % Ar 8 % CH CH HO 9 % 9 % CH 2 42 % CH 2 OH O 41 % Ar 50 % CH CH HO 5 % 5 % CH 2 23 % CH 2 OH O 23 % Ar neutral fraction mono-/di-acids polyacids (Fuzzi et al., GRL, 2001)

Atmospheric Chemistry Modified Köhler equation Kelvin term

Atmospheric Chemistry Surface tension measurements cloud Tenerife ACE-2 fog Po Valley cloud ACE- ASIA  = K -  T ln (1+  C)

Atmospheric Chemistry 0.05  m 0.1  m 0.3  m inorganic only inorganic+organic inorganic+organic+  Effect of organics on S c from Mircea et al., Tellus, 2001

Atmospheric Chemistry Trace gas dissolution Laaksonen et al., JAS, 1998 HNO 3

Atmospheric Chemistry Modelling of chemical effects marine case polluted case insoluble organic no  organic with  5 ppb HNO 3  2 conc. 0.1 m s m s m s m s R* 10 cm/s 30 cm/s 100 cm/s 300 cm/s insoluble organic no  organic with  5 ppb HNO 3  2 conc. Maximum albedo change,  R * 0.1 m s m s m s m s -1 (Nenes et al., GRL in press)

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

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

Atmospheric ChemistryConclusions  Dissolution of gases, dissolution of soluble and slightly soluble organics and the associated decrease of  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 …still needed  data on physical and chemical properties of aerosol are needed for different areas and aerosol types  thermodynamic data and models of a w for the complex cloud droplet solutions are needed

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

Atmospheric Chemistry Models neutral compounds pinonaldehydehydrated levoglucosan cathecol

Atmospheric Chemistry Models mono-/di-acids azelaic acid  -hydroxy-butyric acid hydroxy-benzoic acid

Atmospheric Chemistry Model polyacids fulvic acid

Atmospheric Chemistry Surface tension depends on chemical composition