DOI: 10.1126/science.1189732
, 1366 (2010);
328 Science Renyi Zhang
Getting to the Critical Nucleus of Aerosol Formation
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11 JUNE 2010    VOL 328    SCIENCE   
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PERSPECTIVES
Getting to the Critical Nucleus of Aerosol Formation
ATMOSPHERIC SCIENCE
Renyi Zhang 1 ,2, 3
A better understanding of how aerosols form in the atmosphere could greatly improve climate models.
A
tmospheric aerosols—microscopic particles suspended in Earth’s atmo-sphere—are a major environmen-tal problem. They degrade visibility, nega-tively affect human health, and directly and indirectly influence climate by absorbing and refl ecting solar radiation and modifying cloud formation. Researchers do not, how-ever, fully understand at the molecular level how aerosols form, creating one of the largest sources of uncertainty in atmospheric mod-els and climate predictions ( 1). Recent fi nd-ings suggest a path to a better understanding of aerosol formation ( 2– 4).
Aerosols can be directly emitted into the atmosphere—for example by plants, com-bustion, or sea spray—or form through a chemical process known as nucleation, in which gaseous molecules bond. Nucleation produces a large fraction of atmospheric aerosols, and investigators have frequently observ
ed nucleation in various environments, including urban, forested, and marine areas ( 5). New particle formation is commonly considered to be a two-step process: First,
nucleation forms a “critical nucleus,” which then grows to a detectable size ( 6). Classi-cal nucleation theory reveals that when the critical nucleus forms, the free energy of the nucleating system reaches a maximum—“the nucleation barrier”—beyond which aero-sol growth becomes spontaneous. The rate at which nucleation occurs is related to the chemical makeup of the critical nucleus and the gaseous concentrations of the nucleating species ( 7). That rate is an important variable in simulations of aerosol formation in atmo-spheric models ( 8).
Previous studies, however, have not been able to directly measure nucleation rates
1
State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China.  2College of Environmental Science and Engineer-ing, Fudan University, Shanghai, 200433, China.  3Depart-ments of Atmospheric Sciences and Chemistry, Center for Atmospheric Chemistry and the Environment, Texas A&M University, College Station, TX 77843, USA. E-mail: renyi-zhang@tamu.edu whose by-pro
duct after reaction is water.The products of the catalytic reaction described by Uyanik et al . create the frame-work for a number of natural products with varied biological effects. The benzofuran products can form the starting point for the synthesis of more complex pharmaceutical candidates. For example, tremetone has both antifungal and insecticidal properties and is derived from a plant extract. A similar plant
isolate, rotenone, is a potent anti-leukemic drug candidate as well ( 14). In 2005, the Merck Company reported the synthesis of a drug candidate with this same backbone that modulates the levels of serum triglyc-erides and high-density lipoprotein in the blood ( 15). A synthesis of this target with this new method could be accomplished in fewer steps than the 2005 method, produce less waste, and reduce cost.
The future of hypervalent iodine is likely to be as varied as the chemists working in this area. Elucidating the mechanism, includ-ing the steps that lead to a chiral product, will allow for further improvements in selectivity. Ideally, this catalyst system, like any catalyst developed, will be tested on other substrates and reactions involving iodine-containing catalysts. Iodine chemistry, with its versa-tile reactivity, is an excellent area to discover new, more environmentally friendly, greener organocatalysts. The chemistry described by Uyanik et al . is but a taste of what is to come.
References
1. T. Katsuki, K. B. Sharpless, J. Am. Chem. Soc. 102, 5974
(1980).
2. M. Uyanik, H. Okamoto, T. Yasui, K. Ishihara, Science
328, 1376 (2010).
3. T. Wirth, Angew. Chem. Int. Ed. 44, 3656 (2005).
4. R. M. Moriarty,  J. Org. Chem. 70, 2893 (2005).
5. T. Dohi  et al ., Chem. Commun. (Camb.) 2005, 2205
(2005).
6. T. Dohi  et al ., Angew. Chem. Int. Ed. 44, 6193 (2005).
7. C. I. Herrerías, T. Y. Zhang, C.-J. Li, Tetrahedron Lett. 47,
13 (2006).
8. Y. Yamamoto, H. Togo, Synlett  2006, 798 (2006).    9. R. D. Richardson, T. Wirth, Angew. Chem. Int. Ed. 45,
4402 (2006).
10. R. D. Richardson  et al ., Synlett  2007, 0538 (2007).  11. T. Dohi  et al ., Angew Chem. Int. Ed. 47, 3787 (2008).  12. M. Uyanik, T. Yasui, K. Ishihara, Angew. Chem. Int. Ed.
49, 2175 (2010).
13. K. Maruoka, Org. Process Res. Dev. 12, 679 (2008).  14. M. Abou-Shoer, F. E. Boettner, C.-J. Chang, J. M. Cassady,
Phytochemistry  27, 2795 (1988).
15. G. Q. Shi  et al ., J. Med. Chem. 48, 5589 (2005).
10.1126/science.1191408
Guiding iodine catalysts to their targets. (A ) In the reac-tion described by Uyanik et al ., hydrogen peroxide reacts
with the salt formed by a chi-ral ammonium cation (R 4N +) and iodide, making water and
oxidized iodine. This hyperva-lent (hypo)iodite then reacts with the ketophenol to gen-erate the chiral benzofuran skeleton, where X can be one of many different functional groups. The reaction shows excellent selectivity for one of the two products that differ in handedness (in this case,
the favored product has the
COX group behind the plane formed by the rest of the mol-ecule; the other enantiomer
has this group in front). The compound is a starting point for a host of pharmaceutical candidates. (B ) The structural formula of the chiral ammonium cation is shown on the left. The three-dimensional rendering of the chiral ammonium salt on the right has the nitrogen atom in blue and carbon atoms in gray; hydrogen and fl
uorine atoms are omitted for clarity.
A B R 4N + IO – or R 4N + O=I–O –R 4N + I –
H 2
H 2
O Catalyst (just a pinch)
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    SCIENCE    VOL 328    11 JUNE 2010
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PERSPECTIVES
or the  che mical com-position of the  criti-cal nucleus in binary or multicompone nt sys-te ms like  those  found in the atmosphere. The-ore tical me thods have  also faile d to re liably 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 t
he 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 re ce nt laboratory expe rime nts, 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
bioge nic and anthropoge nic 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 e ngage  in he te roge ne ous re actions 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-me nting advance d the ore tical approache s (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 [H 2SO 4]m s  [A]n A , where m s  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, n A , 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., n A  ≈ ∂In J /∂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 predi
ct 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
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