G. McFiggans et al. Aerosol effects on warm cloud activation 2603 Section 6. 1 presents results from such analyses as de- 3. 1.4.2. 1 Online measurement techniques providing ensem- scribed above from a variety of locations worldwide ble mass composition distributions In addition to the functional groupings identified above, A large number of field experiments have now been car- there are a number of classes of nitrogen-containing com- ried out with the Aerodyne AMs in a wide variety of dif- pounds which are relatively abundant in airborne particu- ferent sampling environments. In urban environments across lates(refs). Initially thought primary in origin, and hence North America and Europe(Allan et al., 2003a;Drewnick generally concentrated in the coarse mass modes, proteins, et al., 2004; Boudries et al., 2004; Alfarra et al., 2004, Zhang peptides, amino acids and related amino-compounds are rea- et al., 2005b), the sub 100 nm diameter particles are dom- sonably documented in terms of their loadings(Sax inated by organic material which has been correlated with ena and Hildemann, 1996; Miguel et al., 1999 Zhang and combustion sources (e. g. Allan et al., 2003b). Even in loca- 2003a, b,c,Kuznetsova et al., 2005, Matsumoto and Ue- of sulphuric acid, the organIc fraction of the sub 100 meon Anastasio,2001, 2003; Zhang et al., 2002b; Mace et al., tions with large sulphate sources and significant nucleation matsu,2005;Poeschl, 2005). More recent postulations sug- ticles approaches 90%(Zhang et al., 2005b). Where photo- gest that secondary processes may contribute to their trans- chemistry is efficient, inorganic species are also observed at formation (Franze et al, 2005). Studies of these compounds these smaller sizes suggesting condensation is occurring onto in particulate matter, hydrometeors, and precipitation have these freshly produced particles (e.g. Zhang et al., 2004, Al- reported high concentrations, indicating that they account for farra et al., 2004), increasing their hygroscopicity. The mass a major mass fraction of water-soluble organic carbon and spectral fingerprint of these aerosols demonstrates that they may be present in significant numbers of fine particles. These are principally hydrocarbon-like in nature and, so far as AMS compounds are known to act as surfactants and are thereby fragmentation is concerned, can be represented by lubricat likely to influence the interaction of atmospheric aerosol par- ing oil and fresh diesel exhaust( Canagaratna et al., 2004) ticles with water vapor through the surface tension term in The high organic content at sizes around the droplet activa- Eq()or through other sur face effects(see Sect. 4.1.4). The tion threshold suggests that the effect of organics on warm roles of these compounds are discussed in Sect. 4.1.9 cloud activation may be much larger than their contribution to the overall mass budget of sub-micron particles. For ex- ample, with reference to all panels other than (f) and(h) Fig. 7, the organic to inorganic ratio below 200 nm dry dia- 3.1.4.2 Online techniques meter is clearly higher than the average ratio across the sub- micron distribution. It may be expected that any effect that organic components have on activation properties is exacer- bated here. Since particles greater than 200 nm are likely to Whilst useful detailed chemical composition information activate at any reasonable updraught velocity, droplet num- may be obtained by bulk sampling and offline analysis, there ber is most likely to be influenced by composition effects e notable drawbacks to the approach. Coe and Allan (2005) for the very fine particles below 200 nm in diameter. How have reviewed a range of online mass spectroscopic methods ever, even in urban environments the largest fraction of the which have been developed over the last decade. These fall non refractory particle mass arises in the accumulation mode into two principal types. The first are laser-based systems and is composed of organic and sulphate aerosol which from that ablate single particles and obtain a mass spectral fin- their modal and temporal behaviour can be considered to be gerprint of a single particle. These instruments, reviewed in largely internally mixed (Alfarra et al., 2004). This inference detail by Noble and Prather(2000), offer a method of prob- about the mixing state may be important for the activation of ing the mixing state of aerosol particle but at present can- accumulation mode particles not deliver quantitative assessments of the mass of individ- Further from the source region the sub 100 nm becomes ual components. The second type uses thermal volatilisation less important and the aerosol population in the regional to vaporise the particles followed by electron bombardment background in many mid latitude continental regions be- to ionise the neutral gas. This latter method, to-date most comes dominated by the accumulation mode composed of widely employed in the Aerodyne Aerosol Mass Spectrom- organic and sulphate components, which show a widely vary- eter(AMS), was first described by Jayne et al. (2000). It is ing ratio of between around 0. 2 and 0. 8 depending on source able to provide mass loadings of non refractory components Where sulphate is buffered by adequate ammonium, or sul- in the submicron range and deliver mass size distributions of phur sources are few, ammonium nitrate is observed in the key species type, Jimenez et al. (2003)first demonstrated the same mode (Alfarra et al., 2004; Allan et al., 2003a); this is methodology for mass quantification and Allan et al. (2003b) more common in the western parts of North America and in described the errors. A principal constraint of this approach Western Europe. Measurements using the AMs in the back has been the limited availability of single particle(and hence ground atmosphere have so far been limited to the northern mixing state)information Hemisphere mid-latitudes between 30 and 60N but at all www.atmos-chem-phys.net/6/2593/20 Atmos. Chem. Phys., 6, 2593-2649, 2006G. McFiggans et al.: Aerosol effects on warm cloud activation 2603 Section 6.1 presents results from such analyses as de￾scribed above from a variety of locations worldwide. In addition to the functional groupings identified above, there are a number of classes of nitrogen-containing com￾pounds which are relatively abundant in airborne particu￾lates (refs). Initially thought primary in origin, and hence generally concentrated in the coarse mass modes, proteins, peptides, amino acids and related amino-compounds are rea￾sonably documented in terms of their mass loadings (Sax￾ena and Hildemann, 1996; Miguel et al., 1999; Zhang and Anastasio, 2001, 2003; Zhang et al., 2002b; Mace et al., 2003a,b,c; Kuznetsova et al., 2005; Matsumoto and Ue￾matsu, 2005; Poeschl, 2005). More recent postulations sug￾gest that secondary processes may contribute to their trans￾formation (Franze et al., 2005). Studies of these compounds in particulate matter, hydrometeors, and precipitation have reported high concentrations, indicating that they account for a major mass fraction of water-soluble organic carbon and may be present in significant numbers of fine particles. These compounds are known to act as surfactants and are thereby likely to influence the interaction of atmospheric aerosol par￾ticles with water vapor through the surface tension term in Eq. (1) or through other surface effects (see Sect. 4.1.4). The roles of these compounds are discussed in Sect. 4.1.9. 3.1.4.2 Online techniques Whilst useful detailed chemical composition information may be obtained by bulk sampling and offline analysis, there are notable drawbacks to the approach. Coe and Allan (2005) have reviewed a range of online mass spectroscopic methods which have been developed over the last decade. These fall into two principal types. The first are laser-based systems that ablate single particles and obtain a mass spectral fin￾gerprint of a single particle. These instruments, reviewed in detail by Noble and Prather (2000), offer a method of prob￾ing the mixing state of aerosol particle but at present can￾not deliver quantitative assessments of the mass of individ￾ual components. The second type uses thermal volatilisation to vaporise the particles followed by electron bombardment to ionise the neutral gas. This latter method, to-date most widely employed in the Aerodyne Aerosol Mass Spectrom￾eter (AMS), was first described by Jayne et al. (2000). It is able to provide mass loadings of non refractory components in the submicron range and deliver mass size distributions of key species type, Jimenez et al. (2003) first demonstrated the methodology for mass quantification and Allan et al. (2003b) described the errors. A principal constraint of this approach has been the limited availability of single particle (and hence mixing state) information. 3.1.4.2.1 Online measurement techniques providing ensem￾ble mass composition distributions A large number of field experiments have now been car￾ried out with the Aerodyne AMS in a wide variety of dif￾ferent sampling environments. In urban environments across North America and Europe (Allan et al., 2003a; Drewnick et al., 2004; Boudries et al., 2004; Alfarra et al., 2004; Zhang et al., 2005b), the sub 100 nm diameter particles are dom￾inated by organic material which has been correlated with combustion sources (e.g. Allan et al., 2003b). Even in loca￾tions with large sulphate sources and significant nucleation of sulphuric acid, the organic fraction of the sub 100 nm par￾ticles approaches 90% (Zhang et al., 2005b). Where photo￾chemistry is efficient, inorganic species are also observed at these smaller sizes suggesting condensation is occurring onto these freshly produced particles (e.g. Zhang et al., 2004; Al￾farra et al., 2004), increasing their hygroscopicity. The mass spectral fingerprint of these aerosols demonstrates that they are principally hydrocarbon-like in nature and, so far as AMS fragmentation is concerned, can be represented by lubricat￾ing oil and fresh diesel exhaust (Canagaratna et al., 2004). The high organic content at sizes around the droplet activa￾tion threshold suggests that the effect of organics on warm cloud activation may be much larger than their contribution to the overall mass budget of sub-micron particles. For ex￾ample, with reference to all panels other than (f) and (h) in Fig. 7, the organic to inorganic ratio below 200 nm dry dia￾meter is clearly higher than the average ratio across the sub￾micron distribution. It may be expected that any effect that organic components have on activation properties is exacer￾bated here. Since particles greater than 200 nm are likely to activate at any reasonable updraught velocity, droplet num￾ber is most likely to be influenced by composition effects for the very fine particles below 200 nm in diameter. How￾ever, even in urban environments the largest fraction of the non refractory particle mass arises in the accumulation mode and is composed of organic and sulphate aerosol which from their modal and temporal behaviour can be considered to be largely internally mixed (Alfarra et al., 2004). This inference about the mixing state may be important for the activation of accumulation mode particles. Further from the source region the sub 100 nm becomes less important and the aerosol population in the regional background in many mid latitude continental regions be￾comes dominated by the accumulation mode composed of organic and sulphate components, which show a widely vary￾ing ratio of between around 0.2 and 0.8 depending on source. Where sulphate is buffered by adequate ammonium, or sul￾phur sources are few, ammonium nitrate is observed in the same mode (Alfarra et al., 2004; Allan et al., 2003a); this is more common in the western parts of North America and in Western Europe. Measurements using the AMS in the back￾ground atmosphere have so far been limited to the Northern Hemisphere mid-latitudes between 30◦ and 60◦ N but at all www.atmos-chem-phys.net/6/2593/2006/ Atmos. Chem. Phys., 6, 2593–2649, 2006