2602 G. McFiggans et al. aerosol effects on warm cloud activation sell, 2003). The assumption that BC belong to the insoluble Functional group analytical techniques provide an alter fraction of the aerosol has been questioned by recent experi- native approach to traditional individual compound specia- ments showing that thermally refractory fractions of TC can tion methods. These techniques analyse the different types of be efficiently extracted with water (Yu et al., 2004, Mayol- chemical structures(as for example total carboxylic groups, Bracero et al., 2002). Furthermore, OC/BC concentrations total carbonyls, etc. ) but provide little or no information on the individual molecules(Decesari et al., 2000; Maria Experimental studies indicate that, in addition to the et al., 2002). Functional group methods have sometime inorganic components, water-soluble organic compounds been coupled to extraction-classification or separation tech wSOC)in atmospheric aerosol particles are also potentially niques, providing a more comprehensive description of oC important in clouds, and an understanding of organic par- and being able to account for up to 90% of the wSoc titioning in cloud droplets(whether dissolved or present as mass(Decesari et al., 2001; Varga et al., 2001). In partic insoluble inclusions) is crucial to our understanding of their ular, in the functional group analysis approach proposed by possible effects on cloud activation(see for example Fac- Decesari et al. (2000), wsoC is separated into three main chini et al. 1999b Jacobson et al. 2000: Kiss et al.. 200 classes of compounds: neutral compounds Maria et al., 2003). WSOC, as opposed to inorganic aerosol /di-carboxylic acid(MDA)and polycarboxylic acids(Pa) components, comprise hundreds(or even thousands) of in- Quantitative measurements of wsoc by Total Organic Ca dividual species(Saxena and Hildemann, 1996: Maria et al., bon(ToC) analyser and of proton concentration of the 2004, Hamilton et al., 2004, Murphy, 2005; Kanakidou et al., main functional groups contained in each of the three above 2005), each contributing a small fraction of the mass. Sev- mentioned classes by Proton Nuclear Magnetic Resonance eral studies of aerosol WSOC concentration and composition (HNMR)can be used to formulate a set of a few "model have been carried out(Zappoli et al., 1999; Facchini et al., compounds representative of the whole wSoC (Fuzzi et al 1999b, Kiss et al., 2001, 2002; Mayol- Bracero et al., 2002; 2001). A systematic technique for deriving model com Cavalli et al., 2004a, b, Putaud et al., 2004, Sullivan et al., pounds for biomass burning aerosol collected in the Ama- 2004: Xiao and Liu, 2004). Molecular level identification zon has recently been submitted for publication(Decesari and analysis is the traditional goal of aerosol organic analysis et al., 2006). Since the model compounds derived in this (for example IC: Falkovich et al., 2005; IEC-UV: Schkolnik way reproduce quantitatively the average chemical structure et al., 2005; GC-MS: Graham et al., 2002; Pashynska et al., of wSoc it can be argued that they may be used as best 2002; Carvalho et al., 2003; lon et al., 2005), but such indi- guess surrogates in microphysical models involving biomass vidual component approaches only account for a small frac- burning aerosol. Likewise, model mixtures of wSoc for tion of the total aerosol and a long list of compounds present many different types of aerosol in a range of locations are in very small concentration is usually provided. In addition available or their definition is in progress to the analytical procedure, bulk sampling techniques which are frequently employed for such analyses are inappropriate Urban aerosol, Bologna, Italy(Matta et al., 2003; Fuzzi for cloud activation purposes and size-segregated determin etal,2001), tion is necessary( Carvalho et al., 2003; Matta et al., 2003 Dust aerosol, Monte Cimone, Italy(Putaud et al., 2004) Cavalli et al. 2004b: Putaud et al.. 2004: Falkovich et al 2005) Clean marine aerosol, Mace Head, Ireland(Cavalli The representation of aerosol composition therefore et al., 2004b) presents a dilemma; it is evident that the aerosol wSoc Biomass burning aerosol, Rondonia, Brazil (Decesari cannot be correctly represented by molecules accounting fo etal,2006), only a small fraction of the total carbon mass, but a repre- sentation of participating species is required for a fundamen ACE Asia, Chinese outflow, Gosan, Jeju Island, Korea tal prediction of cloud activation. Frequently, due to the Topping et al., 2004), complexity, the wSoC chemical composition is reduced fo modelling purposes to one or two"representative"species or Boreal forest aerosol, Hyytiala, Finland( Cavalli et al surrogate molecules selected from the long list of compounds 2004a, Decesari et al., 2006) Ity of wSOC and the wide range of physical properties rele- tion concerning both inw studies have provided informa- and organic aerosol chemi vant to activation, an arbitrary choice of representative com- cal composition which can be directly used by cloud mod- pounds can fail in reproducing relevant physical and chemi- els. These papers provide a comprehensive description of the al properties. For the above reasons, a"realistic"represen- chemical composition of different aerosol types as a function tation of wsoc is necessary for cloud modelling purposes, of size( Chan et al., 1999; Zappoli et al., 1999, Pakkanen but it is difficult to achieve through any individual analyti- et al., 2001; Putaud et al., 2000; Temesi et al., 2001; Maria cal methodology or by choosing surrogate chemical compo- et al., 2003; Sellegri et al., 2003; Cabada et al., 2004, Chic sitions from a list of compounds detected in the aerosols et al., 2004; Sardar et al., 2005) tmos.Chem.Phvs.6.2593-26492006 www.atmos-chem-phys.net/6/2593/2006/2602 G. McFiggans et al.: Aerosol effects on warm cloud activation sell, 2003). The assumption that BC belong to the insoluble fraction of the aerosol has been questioned by recent experi￾ments showing that thermally refractory fractions of TC can be efficiently extracted with water (Yu et al., 2004; Mayol￾Bracero et al., 2002). Furthermore, OC/BC concentrations are strongly size dependent. Experimental studies indicate that, in addition to the inorganic components, water-soluble organic compounds (WSOC) in atmospheric aerosol particles are also potentially important in clouds, and an understanding of organic par￾titioning in cloud droplets (whether dissolved or present as insoluble inclusions) is crucial to our understanding of their possible effects on cloud activation (see for example Fac￾chini et al., 1999b; Jacobson et al., 2000; Kiss et al., 2001; Maria et al., 2003). WSOC, as opposed to inorganic aerosol components, comprise hundreds (or even thousands) of in￾dividual species (Saxena and Hildemann, 1996; Maria et al., 2004; Hamilton et al., 2004; Murphy, 2005; Kanakidou et al., 2005), each contributing a small fraction of the mass. Sev￾eral studies of aerosol WSOC concentration and composition have been carried out (Zappoli et al., 1999; Facchini et al., 1999b; Kiss et al., 2001, 2002; Mayol-Bracero et al., 2002; Cavalli et al., 2004a,b; Putaud et al., 2004; Sullivan et al., 2004; Xiao and Liu, 2004). Molecular level identification and analysis is the traditional goal of aerosol organic analysis (for example IC: Falkovich et al., 2005; IEC-UV: Schkolnik et al., 2005; GC-MS: Graham et al., 2002; Pashynska et al., 2002; Carvalho et al., 2003; Ion et al., 2005), but such indi￾vidual component approaches only account for a small frac￾tion of the total aerosol and a long list of compounds present in very small concentration is usually provided. In addition to the analytical procedure, bulk sampling techniques which are frequently employed for such analyses are inappropriate for cloud activation purposes and size-segregated determina￾tion is necessary (Carvalho et al., 2003; Matta et al., 2003; Cavalli et al., 2004b; Putaud et al., 2004; Falkovich et al., 2005). The representation of aerosol composition therefore presents a dilemma; it is evident that the aerosol WSOC cannot be correctly represented by molecules accounting for only a small fraction of the total carbon mass, but a repre￾sentation of participating species is required for a fundamen￾tal prediction of cloud activation. Frequently, due to the its complexity, the WSOC chemical composition is reduced for modelling purposes to one or two “representative” species or surrogate molecules selected from the long list of compounds detected in the atmosphere. However, due to the complex￾ity of WSOC and the wide range of physical properties rele￾vant to activation, an arbitrary choice of representative com￾pounds can fail in reproducing relevant physical and chemi￾cal properties. For the above reasons, a “realistic” represen￾tation of WSOC is necessary for cloud modelling purposes, but it is difficult to achieve through any individual analyti￾cal methodology or by choosing surrogate chemical compo￾sitions from a list of compounds detected in the aerosols. Functional group analytical techniques provide an alter￾native approach to traditional individual compound specia￾tion methods. These techniques analyse the different types of chemical structures (as for example total carboxylic groups, total carbonyls, etc.), but provide little or no information on the individual molecules (Decesari et al., 2000; Maria et al., 2002). Functional group methods have sometime been coupled to extraction-classification or separation tech￾niques, providing a more comprehensive description of OC and being able to account for up to 90% of the WSOC mass (Decesari et al., 2001; Varga et al., 2001). In partic￾ular, in the functional group analysis approach proposed by Decesari et al. (2000), WSOC is separated into three main classes of compounds: neutral compounds (NC), mono- /di-carboxylic acid (MDA) and polycarboxylic acids (PA). Quantitative measurements of WSOC by Total Organic Car￾bon (TOC) analyser and of proton concentration of the main functional groups contained in each of the three above mentioned classes by Proton Nuclear Magnetic Resonance (HNMR) can be used to formulate a set of a few “model” compounds representative of the whole WSOC (Fuzzi et al., 2001). A systematic technique for deriving model com￾pounds for biomass burning aerosol collected in the Ama￾zon has recently been submitted for publication (Decesari et al., 2006). Since the model compounds derived in this way reproduce quantitatively the average chemical structure of WSOC it can be argued that they may be used as best￾guess surrogates in microphysical models involving biomass burning aerosol. Likewise, model mixtures of WSOC for many different types of aerosol in a range of locations are available or their definition is in progress: . Urban aerosol, Bologna, Italy (Matta et al., 2003; Fuzzi et al., 2001), . Dust aerosol, Monte Cimone, Italy (Putaud et al., 2004), . Clean marine aerosol, Mace Head, Ireland (Cavalli et al., 2004b), . Biomass burning aerosol, Rondonia, Brazil (Decesari et al., 2006), . ACE Asia, Chinese outflow, Gosan, Jeju Island, Korea (Topping et al., 2004), . Boreal forest aerosol, Hyytial¨ a, Finland ( ¨ Cavalli et al., 2004a; Decesari et al., 2006). In summary, only a few studies have provided informa￾tion concerning both inorganic and organic aerosol chemi￾cal composition which can be directly used by cloud mod￾els. These papers provide a comprehensive description of the chemical composition of different aerosol types as a function of size (Chan et al., 1999; Zappoli et al., 1999; Pakkanen et al., 2001; Putaud et al., 2000; Temesi et al., 2001; Maria et al., 2003; Sellegri et al., 2003; Cabada et al., 2004; Chio et al., 2004; Sardar et al., 2005). Atmos. Chem. Phys., 6, 2593–2649, 2006 www.atmos-chem-phys.net/6/2593/2006/