Atmos. Chem. Phys., 6, 2593-2649, 2006 www.atmos-chem-phys.net/6/2593/2006 Atmospheri C Author(s)2006. This work is licensed Chemistry under a Creative Commons License and Physics The effect of physical and chemical aerosol properties on warm cloud droplet activation G. MeFiggans', P. Artaxo, U. Baltensperger, H. Coe, M. C. Facchini, G. Feingold, S. Fuzzi, M. Gysel 3, A. Laaksonen, U. Lohmann, T. F Mentel, D. M. Murphy, CD.O'Dowd0, J.R. Snider,and E Weingartner I Atmospheric Sciences Group, SEAES, University of Manchester, P.O. Box 88, Manchester, M60 1QD, UK iNstituto de Fisica, Universidade de Sao Paulo, Rua do Matao, TravessaR, 187, CEP 05508-900 Sao Paulo, Brazil SPaul Scherrer Institut, Labor fur Atmospharenchemie, 5232 Villigen PSI,Switzerland 4Istituto di Scienze dell'Atmosfera e del Clima, CNR, 40129 Bologna, Italy NOAA Environmental Technology Laboratory, 325 Broadway, Boulder, Colorado 80305, USA b Department of Applied Physics, University of Kuopio, P.O. Box 1627, 70211 Kuopio, Finland "Institute for Atmospheric and Climate Science, Schafmattstr. 30, ETH Zurich, 8093 Zurich, Switzerland 8Forschungszentrum Julich GmbH, ICG-II: Troposphare, 52425 Julich, Germany 9NOAA Aeronomy Laboratory, 325 Broadway, Boulder, Colorado 80305, USA 10 Department of Physics, National University of Ireland, Galway, Ireland I University of Wyoming, Department of Atmospheric Science, Laramie, WY82071,USA Received: 7 June 2005- Published in Atmos. Chem. Phys. Discuss: 12 September 2005 Revised: 13 January 2006-Accepted: 29 May 2006- Published: 5 July 2006 Abstract. The effects of atmospheric aerosol on climate coming shortwave radiation and absorb outgoing longwave forcing may be very substantial but are quantified poorly at radiation(the"aerosol direct effect" Mc Cormick and lud- present; in particular, the effects of aerosols on cloud radia- wig, 1967, Charlson and Pilat, 1969; Hay wood and Boucher, tive properties, or theindirect effects"are credited with the 2000, Charlson et al., 1992). Aerosol particles that act as greatest range of uncertainty amongst the known causes of cloud condensation nuclei hanges in droplet number radiative forcing. This manuscript explores the effects that affecting the albedo and persistence of clouds; these are re- ne composition and properties of atmospheric aerosol can spectively termed theTwomey(first) and the cloud lifetime have on the activation of droplets in warm clouds, so poten- (second) aerosol indirect effects"(Warner, 1968: Twomey tially influencing the magnitude of the indirect effect. The 1974; Albrecht, 1989; Liou and Ou, 1989; Lohmann and effects of size, composition, mixing state and various derived Feichter, 2005). The Twomey effect refers to the aerosol properties are assessed and a range of these properties pro- induced increase in cloud number droplet for vided by atmospheric measurements in a variety of locations uid water content whereas the cloud lifetime effect is a result is briefly reviewed. The suitability of a range of process-level of the reduced precipitation efficiency of the more numer- descriptions to capture these aerosol effects is investigated ous smaller cloud droplets. Absorbing aerosol has also been by assessment of their sensitivities to uncertainties in aerosol shown to cause local warming of the atmosphere, which may properties and by their performance in closure studies. The result in stabilisation of the sub-cloud layer, and large-scale treatment of these effects within global models is reviewed burn-off of clouds. This has been termed the"semi-direct and suggestions for future investigations are made. effect(Fischer and Grassl, 1975; Hansen et al. 1997, Ack erman et al 2000, Johnson et al., 2004) The aerosol indirect effect is currently credited with the 1 Introduction greatest range of uncertainty amongst the known causes of radiative forcing(Ramaswamy et al., 2001); this range Aerosol particles affect the radiation balance of the atmo- stated as being around 4 times the uncertainty associated with sphere in a number of ways. They scatter and absorb in- forcing by radiatively active gases. That its absolute magni- tude may be comparable to that from radiatively active gases Correspondence to: G. Mcl necessitates greatly improved quantification of the factors af- (g. mcfiggans(@ manchester.a fecting and contributing to the aerosol indirect effect Published by Copernicus GmbH on behalf of the European Geosciences UnionAtmos. Chem. Phys., 6, 2593–2649, 2006 www.atmos-chem-phys.net/6/2593/2006/ © Author(s) 2006. This work is licensed under a Creative Commons License. Atmospheric Chemistry and Physics The effect of physical and chemical aerosol properties on warm cloud droplet activation G. McFiggans1 , P. Artaxo2 , U. Baltensperger3 , H. Coe1 , M. C. Facchini4 , G. Feingold5 , S. Fuzzi4 , M. Gysel1,3 , A. Laaksonen6 , U. Lohmann7 , T. F. Mentel8 , D. M. Murphy9 , C. D. O’Dowd10, J. R. Snider11, and E. Weingartner3 1Atmospheric Sciences Group, SEAES, University of Manchester, P.O. Box 88, Manchester, M60 1QD, UK 2 Instituto de Fisica, Universidade de Sao Paulo, Rua do Matao, Travessa R, 187, CEP 05508-900 Sao Paulo, Brazil 3Paul Scherrer Institut, Labor fur Atmosph ¨ arenchemie, 5232 Villigen PSI, Switzerland ¨ 4 Istituto di Scienze dell’Atmosfera e del Clima, CNR, 40129 Bologna, Italy 5NOAA Environmental Technology Laboratory, 325 Broadway, Boulder, Colorado 80305, USA 6Department of Applied Physics, University of Kuopio, P.O. Box 1627, 70211 Kuopio, Finland 7 Institute for Atmospheric and Climate Science, Schafmattstr. 30, ETH Zurich, 8093 Zurich, Switzerland 8Forschungszentrum Julich GmbH, ICG-II: Troposph ¨ are, 52425 J ¨ ulich, Germany ¨ 9NOAA Aeronomy Laboratory, 325 Broadway, Boulder, Colorado 80305, USA 10Department of Physics, National University of Ireland, Galway, Ireland 11University of Wyoming, Department of Atmospheric Science, Laramie, WY 82071, USA Received: 7 June 2005 – Published in Atmos. Chem. Phys. Discuss.: 12 September 2005 Revised: 13 January 2006 – Accepted: 29 May 2006 – Published: 5 July 2006 Abstract. The effects of atmospheric aerosol on climate forcing may be very substantial but are quantified poorly at present; in particular, the effects of aerosols on cloud radia￾tive properties, or the “indirect effects” are credited with the greatest range of uncertainty amongst the known causes of radiative forcing. This manuscript explores the effects that the composition and properties of atmospheric aerosol can have on the activation of droplets in warm clouds, so poten￾tially influencing the magnitude of the indirect effect. The effects of size, composition, mixing state and various derived properties are assessed and a range of these properties pro￾vided by atmospheric measurements in a variety of locations is briefly reviewed. The suitability of a range of process-level descriptions to capture these aerosol effects is investigated by assessment of their sensitivities to uncertainties in aerosol properties and by their performance in closure studies. The treatment of these effects within global models is reviewed and suggestions for future investigations are made. 1 Introduction Aerosol particles affect the radiation balance of the atmo￾sphere in a number of ways. They scatter and absorb in￾Correspondence to: G. McFiggans (g.mcfiggans@manchester.ac.uk) coming shortwave radiation and absorb outgoing longwave radiation (the “aerosol direct effect” McCormick and Lud￾wig, 1967; Charlson and Pilat, 1969; Haywood and Boucher, 2000, Charlson et al., 1992). Aerosol particles that act as cloud condensation nuclei cause changes in droplet number affecting the albedo and persistence of clouds; these are re￾spectively termed the “Twomey (first) and the cloud lifetime (second) aerosol indirect effects” (Warner, 1968; Twomey, 1974; Albrecht, 1989; Liou and Ou, 1989; Lohmann and Feichter, 2005). The Twomey effect refers to the aerosol￾induced increase in cloud number droplet for a constant liq￾uid water content whereas the cloud lifetime effect is a result of the reduced precipitation efficiency of the more numere￾ous smaller cloud droplets. Absorbing aerosol has also been shown to cause local warming of the atmosphere, which may result in stabilisation of the sub-cloud layer, and large-scale burn-off of clouds. This has been termed the “semi-direct effect” (Fischer and Grassl, 1975; Hansen et al., 1997; Ack￾erman et al., 2000; Johnson et al., 2004). The aerosol indirect effect is currently credited with the greatest range of uncertainty amongst the known causes of radiative forcing (Ramaswamy et al., 2001); this range is stated as being around 4 times the uncertainty associated with forcing by radiatively active gases. That its absolute magni￾tude may be comparable to that from radiatively active gases necessitates greatly improved quantification of the factors af￾fecting and contributing to the aerosol indirect effect. Published by Copernicus GmbH on behalf of the European Geosciences Union