PERSPE or the chemical com position of the criti. B because of photochemical oxi- dation of volatile organic com- cal nucleus in binary or multicomponent sys- 张 pounds abundantly emitted by biogenic and anthropogenic tems like those found sources(8, 14). The presence in the atmosphere. The- of organic acids could enable oretical methods have fewer sulfuric acid molecules also failed to reliably to form a critical nucleus(se identify the nucleation the figure), and would expla barrier(9, 10). As a result, investigators have the weaker dependence of the nucleation indirectly inferred the composition of the crit rate on sulfuric acid concentration found ical nucleus by measuring the dependence of atmospheric measurements(3, 8).Moreover, he nucleation rate on the gaseous concentra- Nucleation Growth direct analyses of the chemical compositions tions of the nucleating species(2-5, 11-13). of nanoparticles have suggested that organ- Sulfuric acid, for instance, is a ma Nucleation ics engage in heterogeneous reactions to nucleating component in the atmosphere. Its r form nonvolatile compounds that contribute presence in gaseous concentrations of 10to to particle growth(15). 0 molecules cm"or more is a necessary ove models used to assess the envi- condition for new particle formation (5). Organics-assisted aerosols In aerosol formation, ronmental and climate impacts of aerosols, it Atmospheric measurements have suggested bonding particles must cross an energy thresh- is imperative for future studies to precisely hat nucleation rates depend weakly on sulfu- old-the nucleation barrier-beyond which aerosol quantify the chemical makeup of the critical ric acid concentrations, implying that just one growth becomes spontaneous (A). Organic acids(B) nucleus. This may be accomplished by aug- menting advanced theoretical approaches studies suggest that nucleation rates depend ing of the barrier by ue a moles composition of the ments of the size and chemical composition one or two sulfuric aci more strongly on sulfuric acid concentra- aerosol growth(D). Knowing the tions, corresponding to a critical nucleus of critical nucleus would enable researchers to predict of freshly nucleated nanoparticles in the labo- four to nine sulfuric acid molecules(11-13). the nucleation rate, an important variable in atmo- ratory and in the field. This larger number agrees with predictions spheric models (16) Eoa6EgEoo under classical nucleation theory(7) In recent laboratory experiments, Sip- tions of sulfuric acid (10 to 10 molecules ila et al. reported rapid binary nucleation of cm")(9, 10) (Cambridge sulfuric acid at concentrations comparable Metzgera et al and others, however, sug- Univ.PressCambridgeUk),www.ipcc.ch/ipccreports ar4-wgl. htm(2007) to those found in the atmosphere; their find- gest that it is highly plausible that the answer ef al. proc. Notl Acad. sci. USA 107 one or two sulfuric acid molecules(2). The cies are involved in nucleation and are pres- 4. R Zhang et al, Proc. NatL Acad Sci. U.S.A. 106, 17650 difference between these results and previ- ent in the critical nucleus(3, 8). One candi ous laboratory measurements is explained by date is organic acids, because they form larger 5. P. H McMurry et al. /. Geophys. Res. 110, D22502 the authors'use of an improved instrument and more stable heterodimers with sulfuric 6. M Kulmala, L Pirjola. , M Makela, Nature 404, 66 that can count particles as small as 1.5 nm. acid (4, 9, 10, 12). The interaction between Previous measurements were limited by a an organic acid and sulfuric acid involves 7. R. MCGraw, R Zhang, /. Chem. Phys. 128,064508 low counting efficiency for particles smaller one strong and one medium-strength hydro- 8. 1. Fan, R. Zhang, D. Collins, G. Li, Geophys. Res. Lett. 33 timates of the nucleation rate. Sipila et al.'s 3 kcal mol larger than that of the sulfuric 9. 1. Zha0, A Khalzov, R Zhang, R. McGraw, /. Phys. Chem conclusion that nucleation weakly depends acid dimer(9, 10), and the heterodimer has 10. A B. Nadykto, F Yu, Chem. Phys. Lett.435,14(200n on the concentration of sulfuric acid raises an a vacant Oh group in the sulfuric acid moi- 11. L.H. Young et al, Atmos. Chem. Phys. 8, 4997(2008) portant question: Are one or two sulfuric ety to allow further growth through hydro- 12. &. Zhang et al., cience 304, 1487(2004) acid molecules(a monomer or dimer)enough gen-bond formation a dimer of two organic 14. 1 Fan, R Zhang, Environ. Chem. 1. 140(2004) to form a critical nucleus? acids also has a large binding energy(9), but15LWang Nat.Geo.3,238(2010 Several lines of evidence suggest that the no hydrogen acceptor or donor group is avail- 16. For nudeatione is expressed by /= - the answer should be"no " Molecular dynam- able for subsequent growth, so organic acid where m is the number of sulfuric acid molecules in the ics simulation, for instance, suggests that dimers contribute negligibly to new particle critical cluster and k is a constant The nucleation theorem a hydrated sulfuric acid dimer has a diam- formation(4) eter of 0.7 nm, which is commonly believed Several studies indicate that the critical to be too small to overcome the nucleation nucleus consists of only one molecule of irithm of the nucleation rate. as a barrier(7). Quantum chemical calculations the organic species(3, 4). The presence of show the existence of two medium-strength organic acids in laboratory-produced nano- 17. Supported by the National Natural science Foundation hydrogen bonds in the sulfuric acid dimer, particles has been confirmed in particles as and the available thermodynamic data pre- small as 4 nm (4). In addition, atmospheric 1417), and the U.S. National Science Foundation (grants AGS-0938352 and CBE1-0932705 dict that the dimers would rapidly decom- concentrations of organic acids are expected pose under typical atmospheric concentra- to be much higher than that of sulfuric acid 10.1126/ science1189732 www.sciencemag.orgscIencEVol32811june2010 1367 Published by AAAswww.sciencemag.org SCIENCE VOL 328 11 JUNE 2010 1367 PERSPECTIVES or the chemical com￾position of the criti￾cal nucleus in binary or multicomponent sys￾tems like those found in the atmosphere. The￾oretical methods have also failed to reliably identify the nucleation barrier ( 9, 10). As a result, investigators have indirectly inferred the composition of the crit￾ical nucleus by measuring the dependence of the nucleation rate on the gaseous concentra￾tions of the nucleating species ( 2– 5, 11– 13). Sulfuric acid, for instance, is a major nucleating component in the atmosphere. Its presence in gaseous concentrations of 106 to 107 molecules cm−3 or more is a necessary condition for new particle formation ( 5). Atmospheric measurements have suggested that nucleation rates depend weakly on sulfu￾ric acid concentrations, implying that just one or two sulfuric acid molecules are present in the critical nucleus ( 5). In contrast, laboratory studies suggest that nucleation rates depend more strongly on sulfuric acid concentra￾tions, corresponding to a critical nucleus of four to nine sulfuric acid molecules ( 11– 13). This larger number agrees with predictions under classical nucleation theory ( 7). In recent laboratory experiments, Sip￾ilä et al. reported rapid binary nucleation of sulfuric acid at concentrations comparable to those found in the atmosphere; their fi nd￾ing implicated a critical nucleus consisting of one or two sulfuric acid molecules ( 2). The difference between these results and previ￾ous laboratory measurements is explained by the authors’ use of an improved instrument that can count particles as small as 1.5 nm. Previous measurements were limited by a low counting effi ciency for particles smaller than 3 nm, resulting in appreciable underes￾timates of the nucleation rate. Sipilä et al.’s conclusion that nucleation weakly depends on the concentration of sulfuric acid raises an important question: Are one or two sulfuric acid molecules (a monomer or dimer) enough to form a critical nucleus? Several lines of evidence suggest that the answer should be “no.” Molecular dynam￾ics simulation, for instance, suggests that a hydrated sulfuric acid dimer has a diam￾eter of 0.7 nm, which is commonly believed to be too small to overcome the nucleation barrier ( 7). Quantum chemical calculations show the existence of two medium-strength hydrogen bonds in the sulfuric acid dimer, and the available thermodynamic data pre￾dict that the dimers would rapidly decom￾pose under typical atmospheric concentra￾tions of sulfuric acid (106 to 108 molecules cm−3) ( 9, 10). Metzgera et al. and others, however, sug￾gest that it is highly plausible that the answer is “yes” if other sulfuric acid–stabilizing spe￾cies are involved in nucleation and are pres￾ent in the critical nucleus ( 3, 8). One candi￾date is organic acids, because they form larger and more stable heterodimers with sulfuric acid ( 4, 9, 10, 12). The interaction between an organic acid and sulfuric acid involves one strong and one medium-strength hydro￾gen bond, with a binding energy that is 2 to 3 kcal mol−1 larger than that of the sulfuric acid dimer ( 9, 10), and the heterodimer has a vacant OH group in the sulfuric acid moi￾ety to allow further growth through hydro￾gen-bond formation. A dimer of two organic acids also has a large binding energy ( 9), but no hydrogen acceptor or donor group is avail￾able for subsequent growth, so organic acid dimers contribute negligibly to new particle formation ( 4). Several studies indicate that the critical nucleus consists of only one molecule of the organic species ( 3, 4). The presence of organic acids in laboratory-produced nano￾particles has been confi rmed in particles as small as 4 nm ( 4). In addition, atmospheric concentrations of organic acids are expected to be much higher than that of sulfuric acid, because of photochemical oxi￾dation of volatile organic com￾pounds abundantly emitted by biogenic and anthropogenic sources ( 8, 14). The presence of organic acids could enable fewer sulfuric acid molecules to form a critical nucleus (see the fi gure), and would explain the weaker dependence of the nucleation rate on sulfuric acid concentration found in atmospheric measurements ( 3, 8). Moreover, direct analyses of the chemical compositions of nanoparticles have suggested that organ￾ics engage in heterogeneous reactions to form nonvolatile compounds that contribute to particle growth ( 15). To improve models used to assess the envi￾ronmental and climate impacts of aerosols, it is imperative for future studies to precisely quantify the chemical makeup of the critical nucleus. This may be accomplished by aug￾menting advanced theoretical approaches (i.e., quantum chemical or molecular dynam￾ics simulations) with simultaneous measure￾ments of the size and chemical composition of freshly nucleated nanoparticles in the labo￾ratory and in the fi eld. References and Notes 1. IPCC, Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, S. Solomon et al., Eds. (Cambridge Univ. Press, Cambridge, UK), www.ipcc.ch/ipccreports/ ar4-wg1.htm (2007). 2. M. Sipilä et al., Science 327, 1243 (2010). 3. A. Metzgera et al., Proc. Natl. Acad. Sci. U.S.A. 107, 6646 (2010). 4. R. Zhang et al., Proc. Natl. Acad. Sci. U.S.A. 106, 17650 (2009). 5. P. H. McMurry et al., J. Geophys. Res. 110, D22S02 (2005). 6. M. Kulmala, L. Pirjola, J. M. Makela, Nature 404, 66 (2000). 7. R. McGraw, R. Zhang, J. Chem. Phys. 128, 064508 (2008). 8. J. Fan, R. Zhang, D. Collins, G. Li, Geophys. Res. Lett. 33, L15802 (2006). 9. J. Zhao, A. Khalizov, R. Zhang, R. McGraw, J. Phys. Chem. A 113, 680 (2009). 10. A. B. Nadykto, F. Yu, Chem. Phys. Lett. 435, 14 (2007). 11. L. H. Young et al., Atmos. Chem. Phys. 8, 4997 (2008). 12. R. Zhang et al., Science 304, 1487 (2004). 13. T. Berndt et al., Science 307, 698 (2005). 14. J. Fan, R. Zhang, Environ. Chem. 1, 140 (2004). 15. L. Wang et al., Nat. Geosci. 3, 238 (2010). 16. For nucleation involving sulfuric acid and species A, the nucleation rate, J, is expressed by J = k[H2 SO4 ] ms [A]n A , where ms is the number of sulfuric acid molecules in the critical cluster and k is a constant. The nucleation theorem is derived with the Gibb’s free energy reaching the maxi￾mum (∆G*) at the critical nucleus (r*), relating the number of molecules, nA , of the species, A, in the critical nucleus to the slope of the logarithm of the nucleation rate, as a function of the logarithm of the gaseous concentration of the nucleating species, [A], i.e., nA ≈ ∆InJ/∆In[A]. 17. Supported by the National Natural Science Foundation of China Grant (grant 40728006), the Robert A. Welch Foundation (grant A-1417), and the U.S. National Science Foundation (grants AGS-0938352 and CBET-0932705). 10.1126/science.1189732 Organics-assisted aerosols. In aerosol formation, bonding particles must cross an energy thresh￾old—the nucleation barrier—beyond which aerosol growth becomes spontaneous (A) . Organic acids (B) (carbon in green) that mingle with gaseous sulfu￾ric acid (sulfur in yellow) could facilitate the cross￾ing of the barrier by creating a critical nucleus with one or two sulfuric acid molecules (C), leading to aerosol growth (D). Knowing the composition of the critical nucleus would enable researchers to predict the nucleation rate, an important variable in atmo￾spheric models ( 16). B A C D 1.4 nm Nucleation Nucleation barrier Growth Size (r) r* ∆G* G Published byAAAS on June 11, 2010 www.sciencemag.org Downloaded from