G. McFiggans et al. Aerosol effects on warm cloud activation d/ax and aNa/aw or their logarithmic equivalents) Some online instruments provide single particle composition not reported, it is difficult to compare their results to information and hence information about which components those of Feingold(2003) for overlapping parameter space. co-exist in the same particles; their chemical mixing state Both studies do however agree that Nd is more sensitive to Others may provide component mass loadings with high size the size parameter rg, and that Nd is more sensitive to rg and time resolution. However, online techniques cannot cur- under polluted conditions rently provide as much detailed speciation information c The greater sensitivity of cloud droplet number to size may be available from offline techniques mpared to composition illustrates that the aerosol size must be captured as a primary pre-requisite. The sensitivity to the 3. 1. 4. I Offline bulk measurements showing the complexity compositional complexities should only be investigated in of the atmospheric aerosol composition e knowledge that the size and number information is likely to be equally important(or moreso). It should be noted that This section presents such features of aerosol composition the treatment of composition does not address the sensitivity (organic and inorganic)which may be gained from offline to composition changes with size and to varied composition analyses as relate to cloud activation, without attempting an at any one size; evidence for the prevalence of both being exhaustive review. A summary of some available size segre- provided in the forthcoming sections. The sensitivity of acti- gated chemical compositions is also provided in a form suit vation and cloud droplet number to more detailed aspects of able for cloud modelling purposes aerosol composition is discussed in Sect. 4.2 Despite the wide range of sampling and analytical tech- niques that have been employed, characterisation of aerosol 3.1. 4 The composition of ambient aerosol chemical composition as a function of size is often still in- Until recently, the vast majority of cloud modelling studies is diverse, complex and variable with location and condi- have conventionally assumed, implicitly or explicitly, that tion. The particles comprise many inorganic and organic the soluble material in aerosol particles comprises inorganic compounds ranging in solubility, density and, surface ten- components. The emerging complexity of ambient aerosol sion. Thus, comprehensive papers about the aspects of the equires that this description is revisited chemical composition relevant to cloud formation are rare It must be remembered that number of activated droplets is In order to use Eq.(2), the chemical composition must be dependent on the number distribution of particles of a given"translated"for cloud modelling purposes into concentra type and not directly on the mass loading. Although the acti- tion of molecules(organic and inorganic)dissolved in cloud vation of an individual particle is dependent on its(soluble) droplets, total insoluble mass and, if present, the concen- mass, techniques which coarsely probe component distribu- tration of some"critical" species with limited solubility(or tion loadings by mass will not provide adequate insight to slightly soluble species). The degree of dissociation and ph predict droplet number. Composition is likely to be impor- may also be needed (Laaksonen et al., 1998; Lohmann et al tant only in a limited size range: particles smaller than about 2004); this is addressed further in Sect. 4. It should be noted 40 nm diameter are unlikely to activate into cloud droplets that there are exceptions to these requirements when Eq (1) regardless of their composition and sufficiently large parti- is used, depending on how water activity is evaluated, see cles will almost always contain enough soluble material to Sect. 4.4 activate. To predict droplet activation it is necessary to de- Soluble inorganic components are relatively well un- termine size-resolved composition in the 40 to 200 nm size derstood; the majority comprising a few inorganic range coupled to information about the mixing-state of the salts (Heintzenberg, 1989), which are relatively well- population. No single technique can currently provide all characterised in terms of their hygroscopic properties( Clegg this information. This section reviews the available evidence et al., 1998; Ansari and Pandis, 2000). The insoluble inor- for the composition of ambient particles from a range of ganic fraction can also be important(as in the case of dust studies and techniques From combinations of these sources aerosol from urban or natural sources)and many different it should eventually be possible to adequately describe the component or element measurements are available. How- aerosol composition distribution for purposes of CCN and ever, this information is difficult to directly interpret in terms possibly droplet number prediction of total insoluble mass Offline analyses of bulk particulate material collected by Organic matter has been shown to represent an important filter pack and impactor sampling have conventionally been fraction of the aerosol mass in different environments, and is used to determine aerosol mass composition. Applied ana- routinely measured by means of thermal techniques liousse lytical techniques can provide information ranging from de- et al., 1996, Jacobson et al., 2000; Putaud et al., 2004) tailed molecular speciation to aggregate lumped chemical ganic carbon(OC)and Elemental carbon(EC)(or black car- functionality. These techniques have recently been supple- bon(BC)are reported in terms of carbon mass and the trans- mented by online instrumentation which may provide addi- formation to aerosol mass is problematic without knowing tional information to that available from offline techniques. the main chemical C structure( Turpin and Lim, 2001; Rus- www.atmos-chem-phys.net/6/2593/2006/ Atmos. Chem. Phys., 6, 2593-2649, 2006G. McFiggans et al.: Aerosol effects on warm cloud activation 2601 (∂Nd /∂χ and ∂Nd /∂w or their logarithmic equivalents) were not reported, it is difficult to compare their results to those of Feingold (2003) for overlapping parameter space. Both studies do however agree that Nd is more sensitive to the size parameter rg, and that Nd is more sensitive to rg under polluted conditions. The greater sensitivity of cloud droplet number to size compared to composition illustrates that the aerosol size must be captured as a primary pre-requisite. The sensitivity to the compositional complexities should only be investigated in the knowledge that the size and number information is likely to be equally important (or moreso). It should be noted that the treatment of composition does not address the sensitivity to composition changes with size and to varied composition at any one size; evidence for the prevalence of both being provided in the forthcoming sections. The sensitivity of acti￾vation and cloud droplet number to more detailed aspects of aerosol composition is discussed in Sect. 4.2. 3.1.4 The composition of ambient aerosol Until recently, the vast majority of cloud modelling studies have conventionally assumed, implicitly or explicitly, that the soluble material in aerosol particles comprises inorganic components. The emerging complexity of ambient aerosol requires that this description is revisited. It must be remembered that number of activated droplets is dependent on the number distribution of particles of a given type and not directly on the mass loading. Although the acti￾vation of an individual particle is dependent on its (soluble) mass, techniques which coarsely probe component distribu￾tion loadings by mass will not provide adequate insight to predict droplet number. Composition is likely to be impor￾tant only in a limited size range: particles smaller than about 40 nm diameter are unlikely to activate into cloud droplets regardless of their composition and sufficiently large parti￾cles will almost always contain enough soluble material to activate. To predict droplet activation it is necessary to de￾termine size-resolved composition in the 40 to 200 nm size range coupled to information about the mixing-state of the population. No single technique can currently provide all this information. This section reviews the available evidence for the composition of ambient particles from a range of studies and techniques. From combinations of these sources it should eventually be possible to adequately describe the aerosol composition distribution for purposes of CCN and possibly droplet number prediction. Offline analyses of bulk particulate material collected by filter pack and impactor sampling have conventionally been used to determine aerosol mass composition. Applied ana￾lytical techniques can provide information ranging from de￾tailed molecular speciation to aggregate lumped chemical functionality. These techniques have recently been supple￾mented by online instrumentation which may provide addi￾tional information to that available from offline techniques. Some online instruments provide single particle composition information and hence information about which components co-exist in the same particles; their chemical mixing state. Others may provide component mass loadings with high size and time resolution. However, online techniques cannot cur￾rently provide as much detailed speciation information as may be available from offline techniques. 3.1.4.1 Offline bulk measurements showing the complexity of the atmospheric aerosol composition This section presents such features of aerosol composition (organic and inorganic) which may be gained from offline analyses as relate to cloud activation, without attempting an exhaustive review. A summary of some available size segre￾gated chemical compositions is also provided in a form suit￾able for cloud modelling purposes. Despite the wide range of sampling and analytical tech￾niques that have been employed, characterisation of aerosol chemical composition as a function of size is often still in￾complete (Putaud et al., 2004). The chemical composition is diverse, complex and variable with location and condi￾tion. The particles comprise many inorganic and organic compounds ranging in solubility, density and, surface ten￾sion. Thus, comprehensive papers about the aspects of the chemical composition relevant to cloud formation are rare. In order to use Eq. (2), the chemical composition must be “translated” for cloud modelling purposes into concentra￾tion of molecules (organic and inorganic) dissolved in cloud droplets, total insoluble mass and, if present, the concen￾tration of some “critical” species with limited solubility (or slightly soluble species). The degree of dissociation and pH may also be needed (Laaksonen et al., 1998; Lohmann et al., 2004); this is addressed further in Sect. 4. It should be noted that there are exceptions to these requirements when Eq. (1) is used, depending on how water activity is evaluated, see Sect. 4.4. Soluble inorganic components are relatively well un￾derstood; the majority comprising a few inorganic salts (Heintzenberg, 1989), which are relatively well￾characterised in terms of their hygroscopic properties (Clegg et al., 1998; Ansari and Pandis, 2000). The insoluble inor￾ganic fraction can also be important (as in the case of dust aerosol from urban or natural sources) and many different component or element measurements are available. How￾ever, this information is difficult to directly interpret in terms of total insoluble mass. Organic matter has been shown to represent an important fraction of the aerosol mass in different environments, and is routinely measured by means of thermal techniques (Liousse et al., 1996; Jacobson et al., 2000; Putaud et al., 2004). Or￾ganic carbon (OC) and Elemental carbon (EC) (or black car￾bon (BC)) are reported in terms of carbon mass and the trans￾formation to aerosol mass is problematic without knowing the main chemical C structure (Turpin and Lim, 2001; Rus￾www.atmos-chem-phys.net/6/2593/2006/ Atmos. Chem. Phys., 6, 2593–2649, 2006