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, 2006
G. McFiggans et al.: Aerosol effects on warm cloud activation 2603 Section 6.1 presents results from such analyses as described above from a variety of locations worldwide. In addition to the functional groupings identified above, there are a number of classes of nitrogen-containing compounds which are relatively abundant in airborne particulates (refs). Initially thought primary in origin, and hence generally concentrated in the coarse mass modes, proteins, peptides, amino acids and related amino-compounds are reasonably documented in terms of their mass loadings (Saxena 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 Uematsu, 2005; Poeschl, 2005). More recent postulations suggest that secondary processes may contribute to their transformation (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 particles 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 fingerprint of a single particle. These instruments, reviewed in detail by Noble and Prather (2000), offer a method of probing the mixing state of aerosol particle but at present cannot deliver quantitative assessments of the mass of individual 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 Spectrometer (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 ensemble mass composition distributions A large number of field experiments have now been carried out with the Aerodyne AMS in a wide variety of different 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 dominated by organic material which has been correlated with combustion sources (e.g. Allan et al., 2003b). Even in locations with large sulphate sources and significant nucleation of sulphuric acid, the organic fraction of the sub 100 nm particles approaches 90% (Zhang et al., 2005b). Where photochemistry 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; Alfarra 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 lubricating oil and fresh diesel exhaust (Canagaratna et al., 2004). The high organic content at sizes around the droplet activation 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 example, with reference to all panels other than (f) and (h) in Fig. 7, the organic to inorganic ratio below 200 nm dry diameter is clearly higher than the average ratio across the submicron distribution. It may be expected that any effect that organic components have on activation properties is exacerbated here. Since particles greater than 200 nm are likely to activate at any reasonable updraught velocity, droplet number is most likely to be influenced by composition effects for the very fine particles below 200 nm in diameter. However, 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 becomes dominated by the accumulation mode composed of organic and sulphate components, which show a widely varying ratio of between around 0.2 and 0.8 depending on source. Where sulphate is buffered by adequate ammonium, or sulphur 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 background 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
2604 G. McFiggans et al. aerosol effects on warm cloud activation 5.0 3.0 .lAngley A Manchester Semi Rural g Mancheste Remote A Winter 回 0.5 Urban Edinburgh Jeju Island p Vacuum Aerodynamic Diameter (nm) Vacuum Aerodynamic Diameter(nm) Fig. 7. AMS component mass distributions in a range of locations(after McFiggans et al., 2005 ). The primary difference between the distributions measured close to strong sources(left panels) and more remote distributions (right panels) is the presence in the polluted cases of a significant mass loading in an externally-mixed mode dominated by organic compounds around the threshold dry size for cloud activation. This mode corresponds to the less hygroscopic mode observed in urban and polluted continental air(see Sect. 3. 2. 1, Table 3)and dominates when translated into number space locations a single mass mode centred at between 350 and rial is also present. Bahreini et al. (2003)measured elevated 400 nm diameter and comprising organic and acidified sul- acidic sul phate concentrations along with a comparable load components in multiply California): Topping et al., 2004(Jeju Is, Korea), Schneider stratified layers in the lower free troposphere during ACE et al., 2004(Crete); Rupakheti et al., 2005(rural Ontario). ASIA using an airborne AMs Figure 7 shows the average The Allan et al. (2004)and Topping et al. (2004) studies size distributed mass loading of the key non refractory com- show that even in marine environments the organic-acid sul- ponents of submicron aerosol measured by AMs at a range phate aerosol usually dominates the mass loading of the ac- of sites from urban sources to the remote continental bacl cumulation mode below 500 nm but at larger sizes sea spray ground derived aerosol is increasingly important and dominates in the supermicron size range, results from a mountain station The mass spectral fingerprints of the organic fraction of in the Swiss Alps(see Sect. 6.3)have shown that whilst ni the background continental accumulation mode aerosol are trate and organic material dominates particulate driven up to ery similar from site to site. Zhang et al.(2005a) have de- veloped a two component approach to interpretation of the the site by orographic winds, acidified sulphate aerosol dom- AMS mass spectra that, for localities in regions of fresh or inates in the lower free troposphere, though organic mate aged pollution sources appears to account for over 90% of tmos.Chem.Phvs.6.2593-26492006 www.atmos-chem-phys.net/6/2593/2006/
2604 G. McFiggans et al.: Aerosol effects on warm cloud activation 3.0 2.5 2.0 1.5 1.0 0.5 0.0 2 3 4 5 6 7 100 2 3 4 5 6 7 1000 2 Vacuum Aerodynamic Diameter (nm) 4.0 3.0 2.0 1.0 0.0 dM/dlogDva ( µg / m3 ) 6.0 5.0 4.0 3.0 2.0 1.0 0.0 5.0 4.0 3.0 2.0 1.0 0.0 10.0 8.0 6.0 4.0 2.0 0.0 2 3 4 5 6 7 100 2 3 4 5 6 7 1000 2 Vacuum Aerodynamic Diameter (nm) 2.5 2.0 1.5 1.0 0.5 0.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 3.0 2.5 2.0 1.5 1.0 0.5 0.0 Org SO4 NO3 NH4 Vancouver Manchester Summer Manchester Winter Edinburgh Jeju Island Jungfraujoch Sumas Langley Urban Urban Urban Urban Rural Semi Rural Remote Alpine Remote Marine A B C D E F G H Fig. 7. AMS component mass distributions in a range of locations (after McFiggans et al., 2005). The primary difference between the distributions measured close to strong sources (left panels) and more remote distributions (right panels) is the presence in the polluted cases of a significant mass loading in an externally-mixed mode dominated by organic compounds around the threshold dry size for cloud activation. This mode corresponds to the less hygroscopic mode observed in urban and polluted continental air (see Sect. 3.2.1, Table 3) and dominates when translated into number space. locations a single mass mode centred at between 350 and 400 nm diameter and comprising organic and acidified sulphate aerosol has been observed (Allan et al., 2004 (northern California); Topping et al., 2004 (Jeju Is., Korea); Schneider et al., 2004 (Crete)); Rupakheti et al., 2005 (rural Ontario)). The Allan et al. (2004) and Topping et al. (2004) studies show that even in marine environments the organic-acid sulphate aerosol usually dominates the mass loading of the accumulation mode below 500 nm but at larger sizes sea spray derived aerosol is increasingly important and dominates in the supermicron size range, results from a mountain station in the Swiss Alps (see Sect. 6.3) have shown that whilst nitrate and organic material dominates particulate driven up to the site by orographic winds, acidified sulphate aerosol dominates in the lower free troposphere, though organic material is also present. Bahreini et al. (2003) measured elevated acidic sulphate concentrations along with a comparable loading of highly oxygenated organic components in multiply stratified layers in the lower free troposphere during ACEASIA using an airborne AMS. Figure 7 shows the average size distributed mass loading of the key non refractory components of submicron aerosol measured by AMS at a range of sites from urban sources to the remote continental background. The mass spectral fingerprints of the organic fraction of the background continental accumulation mode aerosol are very similar from site to site. Zhang et al. (2005a) have developed a two component approach to interpretation of the AMS mass spectra that, for localities in regions of fresh or aged pollution sources appears to account for over 90% of Atmos. Chem. Phys., 6, 2593–2649, 2006 www.atmos-chem-phys.net/6/2593/2006/
G. McFiggans et al. Aerosol effects on warm cloud activation 2605 the variance observed. For northern mid-latitudes, where least some of both organics and inorganic materials. For ex most of these analyses have been conducted, the character- ample, in Atlanta 70 to 9g% of the measured particles con- istic component mass distributions in more aged airmasses tained organics and 90 to 98% contained sulphate Lee et al show that the accumulation mode organics are largely present 2002). Even in a very clean marine boundary layer the ma- in almost identical modal sizes as inorganic components.e. jority of the particles contained both sea salt and organics that they have some degree of internal mixing. It is precisely (Middlebrook et al., 1998). Indeed, it is extremely difficult to hese airmasses which show the mass fragmentation patterns prepare pure sulphuric acid particles in the laboratory with- with a characteristically high degree of oxygenation. This out organic contamination(Middlebrook et al., 1997). Con- implies that, when present, the oxygenated organics may versely, the widespread oxidation of So to H2 sO4(by OH mode inorganics. In contrast, urban-influenced air, exhibit- ticles ensures that any particle that has spent much time in ing mass distributions with an essentially externally-mixed the atmosphere has acquired at least a small percentage of high aitken mode contribution are those airmasses sulphate with a more hydrocarbon-like fragmentation pattern. The Figure 8 shows that a large fraction of particles through mass of such organics may probably be apportioned by num- out the troposphere contain both sulphate and organics. This ber to the Aitken mode distribution: HTDMA studies(see does not imply that the particles in a given volume of air Sect. 3.2. 1)confirm that this is frequently the case in urban- are all uniformly mixed, just that most contain measurable influenced air with the observation of a separate externally- amounts of both organics and ionic species. The similarity mixed less hygroscopic or even hydrophobic mode Unfortu- of the organic peaks from one mass spectrum to the next nately, this representation of the aerosol distribution around suggests that most particles contain a mixture of organics the threshold dry size for droplet activation is highly complex as well. Entropy also provides a thermodynamic incentive and at any size where the Aitken and accumulation modes for at least some of the thousands of organic compounds to overlap, one is likely to find an external mixture of hydro- mix together in single particles. The single particle data are carbon like, organic dominated, low hygroscopicity particles quite consistent with mountaintop tandem mobility analyser and oxygenated organic compounds largely co-existing in the data. Nessler et al. (2003)reported"Most of the time, the same particles as the inorganic components. The number station was located in the free troposphere, and the shape of ratio of these particle populations will change with size as the HTDMA spectra was primarily characterized by a nar the local Aitken and background accumulation mode contri- row monomodal size distribution(only 7% of the particles butions vary. An AMS, fitted with a Time of Flight Mass experienced a smaller hygroscopic growth than the particles Spectrometer, has been shown to be able to provide quanti- in the dominant mode)....Exceptions from this behaviour tative measurement of single particle chemical composition were limited to short time periods when the station was in- as a function of size for the non-refractory component of the fluenced by local pollution due to construction work or dust particles with mass loading in agreement with a quadrupole plumes from North Africa(Saharan dust version of the same instrument(Drewnick et al, 2005 ). This These measurements have definite implications for warm and other similar advancements may enhance our future ca- cloud activation. If almost all particles contain some ionic pability to interpret aerosol-cloud interactions species then completely hydrophobic particles will be rare except in special regions. This is borne out by tandem dif- 3.1.4.2.2 Online measurement techniques providing sing ferential mobility studies(see Sect. 3.2.1). Even relatively particle information small amounts of ionic material can allow the activation of single-particle techniques may exhibit many of the see Sect. 3. 2. 1, though the abundance and nature of organics same intrinsic advantages as those described above. In addi- will affect both equilibrium water content and droplet growth tion they can provide single particle mixing-state informa- kinetics(Sect. 4.1) tion at diameters close to the activation threshold dry ra dius, single particle cluster ensemble information and, using 3.1.4.3 Use of combinations of techniques upstream separation, single particle information on droplet residuals and interstitial aerosol to derive number scaveng- It is evident that each of the techniques described above may ratios by composition. However, the single particle tech- provide distinct useful contributions to our understanding of niques are not as quantitative in describing bulk composition the effect of aerosol composition on droplet activation and as the ams results described above that considerable advantage may be derived from simultane- On-line single particle measurements have been made of ous interpretation of data from combinations of offline and opospheric particles larger than about 0. 15 um diameter: online composition analyses. Several field studies have de- a little larger than the sizes where particle composition ployed impactor and AMS measurements of composition in most important for activation(see Sect. 3. 1. 1. Except in a a variety of locations. Topping et al. (2004)demonstrated ew source regions, most tropospheric particles contained at that the ams organic mass was in agreement with both the www.atmos-chem-phys.net/6/2593/2006/ Atmos. Chem. Phys., 6, 2593-2649, 2006
G. McFiggans et al.: Aerosol effects on warm cloud activation 2605 the variance observed. For northern mid-latitudes, where most of these analyses have been conducted, the characteristic component mass distributions in more aged airmasses show that the accumulation mode organics are largely present in almost identical modal sizes as inorganic components – i.e. that they have some degree of internal mixing. It is precisely these airmasses which show the mass fragmentation patterns with a characteristically high degree of oxygenation. This implies that, when present, the oxygenated organics may be approximated as internally mixed with the accumulation mode inorganics. In contrast, urban-influenced air, exhibiting mass distributions with an essentially externally-mixed high Aitken mode organic contribution are those airmasses with a more hydrocarbon-like fragmentation pattern. The mass of such organics may probably be apportioned by number to the Aitken mode distribution: HTDMA studies (see Sect. 3.2.1) confirm that this is frequently the case in urbaninfluenced air with the observation of a separate externallymixed less hygroscopic or even hydrophobic mode. Unfortunately, this representation of the aerosol distribution around the threshold dry size for droplet activation is highly complex and at any size where the Aitken and accumulation modes overlap, one is likely to find an external mixture of hydrocarbon like, organic dominated, low hygroscopicity particles and oxygenated organic compounds largely co-existing in the same particles as the inorganic components. The number ratio of these particle populations will change with size as the local Aitken and background accumulation mode contributions vary. An AMS, fitted with a Time of Flight Mass Spectrometer, has been shown to be able to provide quantitative measurement of single particle chemical composition as a function of size for the non-refractory component of the particles with mass loading in agreement with a quadrupole version of the same instrument (Drewnick et al., 2005). This and other similar advancements may enhance our future capability to interpret aerosol-cloud interactions. 3.1.4.2.2 Online measurement techniques providing single particle information Online single-particle techniques may exhibit many of the same intrinsic advantages as those described above. In addition they can provide single particle mixing-state information at diameters close to the activation threshold dry radius, single particle cluster ensemble information and, using upstream separation, single particle information on droplet residuals and interstitial aerosol to derive number scavenging ratios by composition. However, the single particle techniques are not as quantitative in describing bulk composition as the AMS results described above. On-line single particle measurements have been made of tropospheric particles larger than about 0.15 µm diameter: a little larger than the sizes where particle composition is most important for activation (see Sect. 3.1.1. Except in a few source regions, most tropospheric particles contained at least some of both organics and inorganic materials. For example, in Atlanta 70 to 99% of the measured particles contained organics and 90 to 98% contained sulphate (Lee et al., 2002). Even in a very clean marine boundary layer the majority of the particles contained both sea salt and organics (Middlebrook et al., 1998). Indeed, it is extremely difficult to prepare pure sulphuric acid particles in the laboratory without organic contamination (Middlebrook et al., 1997). Conversely, the widespread oxidation of SO2 to H2SO4 (by OH in the sunlit atmosphere) followed by condensation onto particles ensures that any particle that has spent much time in the atmosphere has acquired at least a small percentage of sulphate. Figure 8 shows that a large fraction of particles throughout the troposphere contain both sulphate and organics. This does not imply that the particles in a given volume of air are all uniformly mixed, just that most contain measurable amounts of both organics and ionic species. The similarity of the organic peaks from one mass spectrum to the next suggests that most particles contain a mixture of organics as well. Entropy also provides a thermodynamic incentive for at least some of the thousands of organic compounds to mix together in single particles. The single particle data are quite consistent with mountaintop tandem mobility analyser data. Nessler et al. (2003) reported “Most of the time, the station was located in the free troposphere, and the shape of the HTDMA spectra was primarily characterized by a narrow monomodal size distribution (only 7% of the particles experienced a smaller hygroscopic growth than the particles in the dominant mode). . . . Exceptions from this behaviour were limited to short time periods when the station was in- fluenced by local pollution due to construction work or dust plumes from North Africa (Saharan dust).” These measurements have definite implications for warm cloud activation. If almost all particles contain some ionic species then completely hydrophobic particles will be rare except in special regions. This is borne out by tandem differential mobility studies (see Sect. 3.2.1). Even relatively small amounts of ionic material can allow the activation of organic particles (Ervans et al., 2004; Lohmann et al., 2004), see Sect. 3.2.1, though the abundance and nature of organics will affect both equilibrium water content and droplet growth kinetics (Sect. 4.1). 3.1.4.3 Use of combinations of techniques It is evident that each of the techniques described above may provide distinct useful contributions to our understanding of the effect of aerosol composition on droplet activation and that considerable advantage may be derived from simultaneous interpretation of data from combinations of offline and online composition analyses. Several field studies have deployed impactor and AMS measurements of composition in a variety of locations. Topping et al. (2004) demonstrated that the AMS organic mass was in agreement with both the www.atmos-chem-phys.net/6/2593/2006/ Atmos. Chem. Phys., 6, 2593–2649, 2006
2606 G. McFiggans et al. aerosol effects on warm cloud activation Negative Ion Spectra Positive Ion Spectra 20 Averages of 100 to 2000 spectra particles that contained either Excludes minerals, salt ontinental uS OB571998-99 Oceanic Los Angeles area:◆ 10 0.00.20.40.60 0.00.20.4 60.81.0 With >58 sulfate ions With >5 carbonaceous ions Fraction of mass spectra Fig. 8. The fraction of single particle mass spectra containing signals from sulfate and organic compounds. Most of the measured particles were between 0.2 and 0.9 um diameter. The stratosphere is the only place where most of the particles give no organic signal. Many of the articles below 3 km with no sulfate signal were nitrate particles, so they also were organic-inorganic mixtures. Negative ion spectra are very sensitive to sulfate and positive ion spectra are sensitive to organics, so particles with as little as 1% sulfates or organics may contribute 5% of the ions impactor-derived submicron water soluble organic carbon using a model incorporating the primitive form of the Kohler and total organic carbon, within experimental uncertainties Eq (1)(see Sect. 4.4) in the clean and continentally-influenced layer. However, given the methodology for extracting the 3.2 Measurements of derived properties of atmospheric water soluble component is relatively vigorous, it may not be aerosol particles and populations that the water extracted organic carbon is in water solution in be partially immiscible. These properties, whilst crucial tor be separated into those which are intrinsic properties of the cloud activation are poorly understood at present (see section constituent particles of a population, depending on their fun- on theoretical treatment of limited solubility, Sect. 4.1.3). damental properties such as composition and size and those McFiggans et al.(2005)likewise demonstrated that AMS which are composite distribution properties. We shall con- and impactor-derived inorganic and organic aerosol loadings centrate on two derived properties; i)hygroscopicity, an ex- were of comparable magnitude in the polluted continental ample of the former and ii) the ability to act as a CCn,an ex- boundary layer. The authors combined the techniques using ample of a distribution-dependent property when provided as mobility and aerodynamic sizing instrumentation to improve size and time resolution of the impactor-derived detailed or state is not strictly a derived property, when probed in terms ganic functionality analyses. This was combined with HT- of hygroscopicity it may provide useful information on the DMA analyses(see Sect. 3.2. D )to attempt to reconcile the effect of the aerosol distribution on cloud activation and is composition and sub-saturated water content(see Sect. 5.1) briefly considered here tmos.Chem.Phvs.6.2593-26492006 www.atmos-chem-phys.net/6/2593/2006/
2606 G. McFiggans et al.: Aerosol effects on warm cloud activation 20 15 10 5 0 Altitude (km) 0.0 0.2 0.4 0.6 0.8 1.0 With >5% sulfate ions Continental US: B57 1998-99 P3 2002 Oceanic: P3 2002 Los Angeles area: Averages of 100 to 2000 spectra particles that contained either sulfate or organics Excludes minerals, salt, ... 20 15 10 5 0 Altitude (km) 0.0 0.2 0.4 0.6 0.8 1.0 With >5% carbonaceous ions Fraction of mass spectra Negative Ion Spectra Positive Ion Spectra Fig. 8. The fraction of single particle mass spectra containing signals from sulfate and organic compounds. Most of the measured particles were between 0.2 and 0.9 µm diameter. The stratosphere is the only place where most of the particles give no organic signal. Many of the particles below 3 km with no sulfate signal were nitrate particles, so they also were organic-inorganic mixtures. Negative ion spectra are very sensitive to sulfate and positive ion spectra are sensitive to organics, so particles with as little as 1% sulfates or organics may contribute 5% of the ions. 127 Fig. 8. The fraction of single particle mass spectra containing signals from sulfate and organic compounds. Most of the measured particles were between 0.2 and 0.9 µm diameter. The stratosphere is the only place where most of the particles give no organic signal. Many of the particles below 3 km with no sulfate signal were nitrate particles, so they also were organic-inorganic mixtures. Negative ion spectra are very sensitive to sulfate and positive ion spectra are sensitive to organics, so particles with as little as 1% sulfates or organics may contribute 5% of the ions. impactor-derived submicron water soluble organic carbon and total organic carbon, within experimental uncertainties in the clean and continentally-influenced marine boundary layer. However, given the methodology for extracting the water soluble component is relatively vigorous, it may not be that the water extracted organic carbon is in water solution in the ambient aerosol but may be concentrated at the surface or be partially immiscible. These properties, whilst crucial for cloud activation are poorly understood at present (see section on theoretical treatment of limited solubility, Sect. 4.1.3). McFiggans et al. (2005) likewise demonstrated that AMSand impactor-derived inorganic and organic aerosol loadings were of comparable magnitude in the polluted continental boundary layer. The authors combined the techniques using mobility and aerodynamic sizing instrumentation to improve size and time resolution of the impactor-derived detailed organic functionality analyses. This was combined with HTDMA analyses (see Sect. 3.2.1) to attempt to reconcile the composition and sub-saturated water content (see Sect. 5.1) using a model incorporating the primitive form of the Kohler ¨ Eq. (1) (see Sect. 4.4). 3.2 Measurements of derived properties of atmospheric aerosol particles and populations Derived properties directly relevant to cloud activation may be separated into those which are intrinsic properties of the constituent particles of a population, depending on their fundamental properties such as composition and size and those which are composite distribution properties. We shall concentrate on two derived properties; i) hygroscopicity, an example of the former and ii) the ability to act as a CCN, an example of a distribution-dependent property when provided as a CCN activity spectrum. Though the composition mixingstate is not strictly a derived property, when probed in terms of hygroscopicity it may provide useful information on the effect of the aerosol distribution on cloud activation and is briefly considered here. Atmos. Chem. Phys., 6, 2593–2649, 2006 www.atmos-chem-phys.net/6/2593/2006/
G. McFiggans et al. Aerosol effects on warm cloud activation 2607 Table 2. Hygroscopic growth factors of various"pure"substances Substance 80 nm Flat surface References (NH4)2SO4 1.68 Topping et al. (2004) H2SO4 1661 NH4NO3 1.85 2.41 fresh mineral dust <1.05 Vlasenko et al. (2005) fresh diesel engine exhaust Weingartner et al. (1997) <1.05 Weingartner et al. (1995 dominated by volatile organic compounds secondary organic aerosol 1.07-1.14 Saathoff et al. (2003) (reaction chamber experiments) (at 85%RH) Virkkala et al. (1999) asperger et al. (2005) Cocker et al.(2001) humic-like substances 1.08-1.20 Chan and Chan(2003) ysel et al. (2004) Brooks et al. (2004) organic acids 100-1.703 Wise et al. (2003)and references therein I Divided by GFp=1.22 for H2 SO4 and 1.03 for NH4HSO4. at RH=10% 2 Exhaust from large(1 to 6 MW) high efficiency grate burners leave particles composed of the inorganic residue only. Particles from open or smouldering flames with significant amounts of organic and elemental carbon are expected to have smaller G Fp values 3 GFp similar to inorganic salts are reported for multifunctional organic acids and organic salts, whereas monoacids are generally not or ly slightly hygroscopic 3.2. 1 Hygrosco to a particle counter across an appropriate size range. In- version algorithms must be applied to the measured dis- The ratio of the diameter of a particle at a known elevated tributions to obtain the effective growth distribution of the relative humidity(RH) to that when nominally dry is the hy- aerosol owing to the finite width of the DMa transfer func groscopic diameter growth factor(G Fp). Particle G Fp val- tions during dry particle selection and grown particle detec- ues at Rh below saturation are related to their CCn activa- tion. Most widespread are the tdmafit algorithm (liu tion behaviour(Sect. 3. 2.2)through Kohler theory(Sect. 2) et al., 1978; Rader and MeMurry, 1986)(Stolzenburg and and hence are important in understanding the cloud activa- McMurry, 1988), accounting for the transfer function width tion of aerosol particles. Furthermore, G FD measurements of both DMAs and algorithms, where only the second DMA also provide insight into the aerosol chemical mixing state (and particle counter response) is inverted( Stratmann et al Ambient G Fp measurements are commonly determined 1997, Voutilainen et al., 2000; Collins et al., 2002). Re- sing a Hygroscopicity Tandem Differential Mobility Anal- cently, Cubison et al.(2005)have introduced an alterna- yser(HTDMA, Liu et al., 1978; Rader and McMurry, 1986). tive approach, where the TDMA measurement is inverted A size fraction, selected from the dried ambient aerosol into contributions from fixed classes of narrow growth fac- with a Differential Mobility Analyser(DMA, Liu and Pui, tor ranges. The resolution of inverted growth distributions is 1974)is exposed to well-defined elevated RH in a humid- limited by measurement uncertainties and knowledge of the ifier and the grown equilibrium size distribution is deter- exact TDMA transfer functi ned by scanning a second humidified DMA connected www.atmos-chem-phys.net/6/2593/2006/ Atmos. Chem. Phys., 6, 2593-2649, 2006
G. McFiggans et al.: Aerosol effects on warm cloud activation 2607 Table 2. Hygroscopic growth factors of various “pure” substances. GF at 90% RH Substance 80 nm Flat surface References (NH4)2SO4 1.68 1.74 Topping et al. (2004) NH4HSO4 1.721 1.771 H2SO4 1.661 1.701 NH4NO3 1.77 1.85 NaCl 2.34 2.41 fresh mineral dust <1.05 – Vlasenko et al. (2005) fresh diesel engine exhaust <1.05 – Weingartner et al. (1997) (soot) fresh petrol engine exhaust <1.05 – Weingartner et al. (1995) dominated by volatile organic compounds biomass burner exhaust up to 1.652 – Pagels et al. (2003) secondary organic aerosol 1.07–1.14 – Saathoff et al. (2003) (reaction chamber experiments) (at 85% RH) Virkkula et al. (1999) Baltensperger et al. (2005) Cocker et al. (2001) humic-like substances 1.08–1.20 – Chan and Chan (2003) Gysel et al. (2004) Brooks et al. (2004) organic acids 1.00-1.703 – Wise et al. (2003) and references therein 1 Divided by GFD=1.22 for H2SO4 and 1.03 for NH4HSO4. at RH=10%. 2 Exhaust from large (1 to 6 MW) high efficiency grate burners leave particles composed of the inorganic residue only. Particles from open or smouldering flames with significant amounts of organic and elemental carbon are expected to have smaller GFD values. 3 GFD similar to inorganic salts are reported for multifunctional organic acids and organic salts, whereas monoacids are generally not or only slightly hygroscopic. 3.2.1 Hygroscopicity The ratio of the diameter of a particle at a known elevated relative humidity (RH) to that when nominally dry is the hygroscopic diameter growth factor (GFD). Particle GFD values at RH below saturation are related to their CCN activation behaviour (Sect. 3.2.2) through Kohler theory (Sect. ¨ 2) and hence are important in understanding the cloud activation of aerosol particles. Furthermore, GFD measurements also provide insight into the aerosol chemical mixing state. Ambient GFD measurements are commonly determined using a Hygroscopicity Tandem Differential Mobility Analyser (HTDMA, Liu et al., 1978; Rader and McMurry, 1986). A size fraction, selected from the dried ambient aerosol with a Differential Mobility Analyser (DMA, Liu and Pui, 1974) is exposed to well-defined elevated RH in a humidifier and the grown equilibrium size distribution is determined by scanning a second humidified DMA connected to a particle counter across an appropriate size range. Inversion algorithms must be applied to the measured distributions to obtain the effective growth distribution of the aerosol owing to the finite width of the DMA transfer functions during dry particle selection and grown particle detection. Most widespread are the TDMAFIT algorithm (Liu et al., 1978; Rader and McMurry, 1986) (Stolzenburg and McMurry, 1988), accounting for the transfer function width of both DMAs and algorithms, where only the second DMA (and particle counter response) is inverted (Stratmann et al., 1997; Voutilainen et al., 2000; Collins et al., 2002). Recently, Cubison et al. (2005) have introduced an alternative approach, where the TDMA measurement is inverted into contributions from fixed classes of narrow growth factor ranges. The resolution of inverted growth distributions is limited by measurement uncertainties and knowledge of the exact TDMA transfer function. www.atmos-chem-phys.net/6/2593/2006/ Atmos. Chem. Phys., 6, 2593–2649, 2006