Interaction between Common Organic Acids and ... - ACS Publications

J. Phys. Chem. A 2010, 114, 387–396

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Interaction between Common Organic Acids and Trace Nucleation Species in the Earth’s Atmosphere Yisheng Xu,†,‡ Alexey B. Nadykto,*,‡ Fangqun Yu,‡ J. Herb,‡ and Wei Wang† Atmospheric Chemistry and Aerosol Research DiVision, Chinese Research Academy of EnVironmental Science, Beijing 100012, China, and Atmospheric Sciences Research Center, State UniVersity of New York at Albany, 251 Fuller Road, Albany 12203, New York, USA ReceiVed: July 20, 2009; ReVised Manuscript ReceiVed: October 26, 2009

Atmospheric aerosols formed via nucleation in the Earth’s atmosphere play an important role in the aerosol radiative forcing associated directly with global climate changes and public health. Although it is well-known that atmospheric aerosol particles contain organic species, the chemical nature of and physicochemical processes behind atmospheric nucleation involving organic species remain unclear. In the present work, the interaction of common organic acids with molecular weights of 122, 116, 134, 88, 136, and 150 (benzoic, maleic, malic, pyruvic, phenylacetic, and tartaric acids) with nucleation precursors and charged trace species has been investigated. We found a moderate strong effect of the organic species on the stability of neutral and charged ionic species. In most cases, the free energies of the mixed H2SO4-organic acid dimer formation are within 1-1.5 kcal mol-1 of the (H2SO4)(NH3) formation energy. The interaction of the organic acids with trace ionic species is quite strong, and the corresponding free energies far exceed those of the (H3O+)(H2SO4) and (H3O+)(H2SO4)2 formation. These considerations lead us to conclude that the aforementioned organic acids may possess a substantial capability of stabilizing both neutral and positively charged prenucleation clusters, and thus, they should be studied further with regard to their involvement in the gas-to-particle conversion in the Earth’s atmosphere. Introduction 1-7

Aerosol particles formed in the atmosphere via nucleation influence the climate indirectly by affecting cloud properties and precipitation and may also have adverse public health impacts (including various cardiovascular deceases and lung cancer). For example, some medical studies4 have shown a clear correlation between the ultrafine particle production near busy roadways and enhanced mortality rate. Although the formation of atmospheric aerosols has been extensively studied in the past, the molecular nature of the gas-to -particle conversion in the Earth’s atmosphere remains unclear. The atmospheric nucleation can be described schematically as the H2SO4-H2O-X process, where X is a stabilizer of H2SO4-H2O clusters. Many species have been proposed as stabilizers/catalysts of the binary H2SO4-H2O nucleation; however, atmospheric nucleation mechanisms and chemical composition of nucleating particles is a subject of ongoing debates.8 Ammonia, the most common base in the atmosphere, has been considered as a principle stabilizer of binary clusters since the 1990s. The somewhat excessive enthusiasm about the ternary homogeneous H2SO4-H2O-NH3 nucleation (THN) disappeared when it was discovered that the widely used THN theory9 grossly overestimates nucleation rates and contains a number of errors.10 The original THN model has been revised; however, the revised model predicts very low THN rates in the atmosphere. Other candidate mechanisms of the gas-to-particle conversion/nucleation in the Earth’s atmosphere include ion-mediated nucleation (IMN) of H2SO4-H2Oion,11 nucleation of iodine-containing vapors,12 and organicsenhanced nucleation.13 * Corresponding author. E-mail: [email protected]. † Chinese Research Academy of Environmental Science. ‡ State University of New York at Albany.

Although it is well-known that atmospheric aerosol particles contain organic species,14-24 the role of organics in the atmospheric nucleation has long been underestimated or neglected. The importance of organic species has been pointed out in the experiments of Zhang et al. (2004),13 who have found a considerable enhancement in nucleation rates due to the presence of organic acids. Another important issue is that the theoretical treatment of matter in a state, neither liquid nor gaseous but in an indeterminate state of molecular cluster, requires the application of advanced theoretical instruments other than the bulk-liquid classical nucleation theory. The importance of a rigorous, presumably quantum, treatment of nucleating and prenucleation clusters has been pointed out in Nadykto et al. (2006),3 where the classical problem of the ion sign preference has been solved. It has been shown that the quantum approach provides an adequate description of molecular interactions in nucleating vapors. The recent work of Nadykto and Yu (2007)15 has shown that common low-molecular carboxylic formic and acetic acids may be capable of stabilizing small H2SO4-H2O clusters. Their conclusion about the potential importance of organic species for the stabilization of atmospheric prenucleation clusters has also been confirmed in other quantum-chemical studies.25,26 The relevance of organic species to the nucleation in the Earth’s atmosphere has also been discussed in the recent studies.27,28 In addition to the discussion on the composition of the nucleating vapors, McGraw and Zhang (2008)28 showed how the nucleation theorem and multivariate statistical methods can be effectively combined for interpretation and parametrization of laboratory measurements of nucleation rate involving multiple vapor species. Benzoic, maleic, malic, pyruvic, phenylacetic, and tartaric acids represent well the common atmospheric organic acids,

10.1021/jp9068575 CCC: $40.75  2010 American Chemical Society Published on Web 12/03/2009

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Figure 1. Structure of most stable isomers of maleic, benzoic, and pyruvic acids.

TABLE 1: Geometric Properties of Maleic and Benzoic Acidsa maleic acid

e

parameter

nonlocal DFT

C1-O2 C1-O3 C1-C5 C5-C7 C7-C9 C9-C12 C9-C10 O2-O10 ∠O2-C1-O3 ∠O3-C1-C5 ∠O2-C1-O5 ∠C1-C5-C7 ∠C5-C7-C9 ∠C5-C7-O10 ∠O10-C9-O12 ∠C7-C9-O12

1.24 1.37 1.48 1.36 1.51 1.24 1.35

b

benzoic acid c

PW91PW91

exptl

1.23 1.35 1.47 1.35 1.51 1.22 1.33 2.61 121.3 111.3 127.4 127.9 133.6 120.1 122.1 117.8

1.20 1.27 1.44 1.43 1.46 1.21 1.27 2.46 125.5 114.8 118.4 126.7 124.7 124.1 119.2 111.3

parameter

PW91PW91

MP2d

exptle

C1-C2 C2-C3 C3-C4 C4-C5 C5-C6 C6-C1 C4-C12 C12-O14 C12-O13 ∠C1-C2-C3 ∠C2-C3-C4 ∠C3-C4-C5 ∠C4-C5-C6 ∠C5-C6-C1 ∠C6-C1-C2 ∠C3-C4-C12 ∠C5-C4-C12 ∠C4-C12-O14 ∠C4-C12-O13 ∠O13-C12-O14

1.396 1.393 1.40 1.40 1.39 1.396 1.485 1.365 1.22 120.09 119.9 119.8 120.1 120.0 120.1 122.2 117.9 113 125 122

1.406 1.404 1.409 1.408 1.402 1.407 1.494 1.358 1.218

1.36 1.42 1.39 1.39 1.41 1.37 1.48 1.29 1.24 123 118 119 120 122 118 122 119 118 122 121.9

a Bond lengths and angles are given in angstroms and degrees, respectively. Experimental geometry ref 43.

b

Ref 40.

c

Experimental geometry ref 41.

d

Ref 42.

TABLE 2: Geometric Properties of Pyruvic Acida

a

parameters

PW91PW91

B3LYPb

MP2/aug-cc-pVDZc

MP2/6311++G**d

exptle

C1-O2 C1-O3 O3-H10 C1-C4 C4-O5 C4-C6 C6-H7 C6-H8 C6-H9 C1-C4-C6 O2-C1-C4 O3-C1-C4 O5-C4-C6 H9-C6-C4 H8-C6-C4 H7-C6-C4 H10-O3-C1 O2-C1-C4-C6 O3-C1-C4-C6 O5-C4-C6-C1 H9-C6-C4-C1 H8-C6-C4-O5 H7-C6-C4-O5 H10-O3-C1-C4

1.208 1.340 0.988 1.554 1.221 1.490 1.098 1.098 1.092 117.04 123.55 111.85 125.42 110.55 109.5 109.5 105.15 0 179.95 179.97 179.98 -122.50 122.38 0

1.207 1.338 0.983 1.549 1.219 1.499 1.096 1.096 1.091 117.2 123.1 112.3 125.3 110.1 109.8 109.8 106.1 0 180 180 180 -121.9 121.9 0

1.218 1.347 0.980 1.545 1.233 1.499 1.101 1.101 1.096 117.0 123.0 113.0 125.2 109.8 109.2 109.2 105.6 0 180 180 180 -121.9 121.9 0

1.207 1.337 0.973 1.545 1.221 1.494 1.093 1.093 1.089 117.9 122.8 112.9 125.4 109.9 109.4 109.4 105.7

1.215 1.328 0.983 1.523 1.231 1.486 1.106 1.106 1.074 118.6 122.0 114.5 125.0 110.7 109.0 109.0 105.2 -

121.8

-

Bond lengths and angles are given in angstroms and degrees, respectively. b Ref 44. c Ref 45. d Ref 46. e Experimental geometry ref 47.

which appear in the Earth’s atmosphere due to fossil fuel and biomass burning, veg, etation and various anthropogenic emissions,29,30 and is found in the gas-phase, aerosol particulate matter, and rainwater.31-36 While the presence of the aforemen-

tioned acids in the particulate matter and rainwater is well established, the precession of the in situ measurements of the gas-phase concentrations is not high enough to make a definitive conclusion about the typical concentration ranges in the Earth’s

Interaction of Organic Acids and Nucleation Species

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TABLE 3: Vibrational Fundamentals (cm-1) of Acid Monomers maleic acid PW91PW91 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 a

3637 3132 3116 3091 1729 1678 1626 1424 1297 1188

tartaric acid exptl

a

1705 1635 1587 1565 1459 1432 1261 1218

pyruvic acid b

PW91PW91

exptl

3551 2962 2923 1774 1731 1421 1346 1333 1311 1270 1230 1187 1125 1082 1007 914

3425 2980 2930 1745 1730 1455 1365 1340 1310 1260 1225 1195 1135 1090 995 925 878 830 794 740 675 619 605 576 528

849 757 645 621 598 566 527

malic acid c

PW91PW91

exptl

3632 3108 2987 1752 1738 1406 1343 1325 1177 1100 995 716 617 577 375 238

3432 3033 2936 1800 1729 1424 1384 1355 1214 1137 969 762 604 535 388 258

benzoic acid d

PW91PW91

exptl

3630 3053 2995 2902 1731 1412 1388 1356 1303 1242 1181 1120 1098 1017 911 866

3500 2940 2895 2930 1721 ∼1440 1406 1350 1280 1230 1188 1110 1040 985 948 890

PW91PW91

exptle

3653 3134 1732 1598 1577 1478 1438 1349 1327 1175 1152 1058 1021 928 703 616 587 483 418

3567 3098 1752 1606 1590 1493 1430 1347 1322 1169 1149 1066 1027 937 711 628 568 491 421

Ref 48. b Ref 49. c Ref 50. d Ref 51. e Ref 52.

atmosphere. Their estimated concentrations are in the range of ∼108-1011 cm-3, which is close to that of ammonia. In the present paper, the interaction of benzoic, maleic, malic, pyruvic, phenylacetic, and tartaric acids with key atmospheric nucleation precursors and positively charged ionic species have been studied using the Density Functional Theory (DFT)37 at the PW91PW91/6-311++G(3df,3pd)37 level. The formation of mixed dimers and trimers containing H2SO4, NH3, H3O+, and organic acids has been investigated, and the key properties of the prenucleation clusters such as stepwise changes in enthalpies, entropies, and Gibbs free energies have been calculated. The thermochemical analysis of possible nucleation pathways has been performed, and the possible involvement of the common organic acids in the atmospheric nucleation has been discussed. Methods The choice of the adequate computational strategy and an appropriate method is very important because both ab initio and DFT results are somewhat method- and basis set-dependent. However, obtaining valuable information for the nucleation research does not necessarily require very accurate free energies. Fortunately, the relative importance of different nucleation pathways is primarily related to the difference in stepwise free energy changes associated with the formation of different types of clusters, which is predicted by different methods in good agreement with each other. For example, it has been shown that the difference in binding energies of H2SO4 · NH3 and H2SO4 · H2O complexes given by different ab initio and DFT methods, with and without counterpoise corrections, is several times lower than the difference in the absolute values of the binding energy of H2SO4 · NH3 or H2SO4 · H2O clusters.38 In the present study, the initial/generated geometries have been treated using the semiempirical PM33 method and then optimized at the PW91PW91/6-31+G* level of theory. The most stable isomers (within 2 kcal mol-1) of the most stable

isomer/global minimum obtained at the PW91PW91/6-31+G* level have been optimized using the PW91PW91/6311+G(3df,3pd) method to get the final results. The choice of the computational method is based on the satisfactory performance of PW91PW91 on atmospheric clusters, including predictions of the Gibbs free energies, structural characteristics and vibrational spectra, and availability of a large amount of data for different atmospheric species/clusters computed at the PW91PW91/6-311+G(3df,3pd) level of theory. The computations have been carried out using the Gaussian03 suite of programs.39 Results and Discussion It is well-known that the Gibbs free energy changes, the key properties controlling nucleation rates, are computed using the optimized geometries and calculated vibrational frequencies. Therefore, the accurate prediction of both structure and vibrational fundamentals is critically important. The structure and geometric properties of the organic acids considered in the present study have been investigated theoretically in the past. In contrast, only a few experimental studies are available, and they are limited to the crystal structures. Figure 1 and Tables 1 and 2 present the comparison of the optimized geometries of the acid monomers with available experimental data and other theoretical studies. As it may be seen from Table 1, the degree of agreement of MP2, B3LYP, and PW91 with experimental geometries is very close. As a matter of fact, all the aforementioned methods agree quite well with experiments; however, none of the methods provide a perfect agreement with experiments. In the case of maleic acid, the maximum deviation in the bond lengths and angles is ∼5.7% and 6.2%, respectively. The maximum difference in the bond lengths of benzoic acid between both MP2 and PW91 and experiment exceeds 4.5%. In the case of pyruvic acid all the computational methods are in agreement with each

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Figure 2. Equilibrium geometries of most stable isomers of hydrogen-bonded complexes of organic acids (OA) with H2SO4 and NH3. Bond lengths and angles are given in angstroms and degrees, respectively.

other and the experimental data. The predictivity of PW91PW91 (1.7 and 2.1% difference in bonding lengths and angles, respectively) is close to that of MP2 (1.4 and 2.3%, respectively). Table 3 presents the comparison of the PW91PW91 vibrational spectra with the experimental data and other theoretical studies. As it may be seen from Table 2, the vibrational fundamentals computed at the PW91PW91/6-311++G(3df,3pd) level of theory are in excellent agreement with the experimental data. Another important detail is that in all the cases studied the low frequencies largely responsible for the vibrational contribution to the Zero Point Energy (ZPE) are predicted by PW91PW91/6-311++G(3df,3pd)withahighdegreeofconfidence. As may be seen from Figure 2, the intermolecular OH-bonds in (H2SO4)-OA complexes are typically on average slightly shorter than those in the sulfuric acid dimer. In contrast, the OH and HN bonding lengths in OA-NH3 complexes are substantially longer than those in the H2SO4-NH3 complex. The N-H bond length in OA-H2SO4-NH3 complexes is in the range of 0.161-0.172 nm which is ∼0.05-0.1 nm longer than that in the (H2SO4)(NH3) complex. Most of the transformations considered here are accompanied by a proton transfer. As seen from Table 3, the highest proton transfer rates for NH3-OA, (H2SO4)-OA, and NH3-H2SO4-OA complexes are found in the cases of maleic acid, benzoic acid, and benzoic acid, respectively. Table 4 presents a comparison of the thermochemical properties associated with the formation of OA-H2SO4, OA-NH3, and OA-NH3-H2SO4 complexes with those associated with the formation of ammonia bisulfate (H2SO4)(NH3) and (H2SO4)2(NH3) complexes. As it may be seen from Table 4, the affinities of the malic and maleic acids to ammonia are higher than those of the formic and acetic acids;15 however,

they are substantially lower than that of the sulfuric acid. The affinities of the benzoic, phenylacetic, tartaric, and malic acids to the sulfuric acid are stronger than the sulfuric acid dimerization free energy. This implies that all the aforementioned species can stabilize sulfate clusters. Moreover, the bonding of these species is stronger than that in the ammonia bisulfate cluster. The formation of OA-(H3O+) and OA-(H3O+)-(H2SO4) is relevant directly to the nucleation of positive ions, which is an important unresolved problem of the atmospheric nucleation theory.8,14 Recent state-of-the-art measurements of ion mobility and charging state of freshly nucleated particles indicate that ions are involved in more than 90% of the particle formation events.53,54 While both experimental and theoretical studies show that the nucleation of negative binary ions is generally favorable,14 positive ions dominate some of the nucleation events in the atmosphere.55 This implies that other species, including the common organics, may be involved in the nucleation of positive ions. No explanation to this phenomenon is available at the present time. Figure 3 presents equilibrium geometries of most stable isomers of positively charged hydrogen-bonded complexes of organic acids with H2SO4 and H3O+, and Table 5 presents the comparison of the changes in the Gibbs free energy associated with their formation with those associated with the formation of (H2SO4)(H3O+) and (H2SO4)2(H3O+). As may be seen from Figure 3, in the most stable isomers the hydronium ion is connected to organic acids via a single OH bond, and the OA + H3O+ S (OA)(H3O+) process is accompanied by a proton transfer from H3O+ to organic acid. The bonding in OA-H2SO4-H3O+ complexes is similar and is accompanied by a proton transfer leading to the formation of the protonated organic acid-H2SO4-H2O complexes. In all

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TABLE 4: Enthalpy (kcal mol-1) and Gibbs Free Energy Changes (kcal mol-1) Associated with Reactions among Neutral Clusters Composed of Atmospheric Precursors (Sulfuric Acid and Ammonia) and Organic Acids (Benzoic Acid, C6H5COOH; Malic Acid, C4H6O5; Maleic Acid, C4H4O4; Phenylacetic Acid, C6H5CH2COOH; Pyruvic Acid, C3H4O3; and Tartaric Acid, C4H6O6) at a Temperature of 298.15 K and Pressure of 101.3 KPa reaction

∆H

∆G

C6H5COOH + NH3 S (C6H5COOH)(NH3)

-11.20 (-10.46)a

-2.23 (-1.28)a

C4H6O5 + NH3 S (C4H6O5)(NH3)

-12.30

-3.62

C4H4O4 + NH3 S (C4H4O4)(NH3)

-14.08

-5.34

C6H5CH2COOH + NH3 S (C6H5CH2COOH)(NH3)

-11.46

-1.96

C3H4O3 + NH3 S (C3H4O3)(NH3)

-8.94

-0.96

C4H6O6 + NH3 S (C4H6O6)(NH3)

-14.64

C6H5COOH + H2SO4 S (C6H5COOH)(H2SO4)

-20.71 (-16.25)

C4H4O4 + H2SO4 S (C4H4O4)(H2SO4)

-16.55

-4.89


-19.22

-7.46

C6H5CH2COOH + H2SO4 S (C6H5CH2COOH)(H2SO4)

-20.54

-8.09


-15.19

-4.17


-19.92

-7.82

(C6H5COOH)(NH3) + H2SO4 S (C6H5COOH)(NH3)(H2SO4)

-25.22

-13.33

(C4H4O4)(NH3) + H2SO4 S (C4H4O4)(NH3)(H2SO4)

-16.85

-5.98


-23.0

-11.25

(C6H5CH2COOH)(NH3) + H2SO4 S (C6H5CH2COOH)(NH3)(H2SO4)

-18.76

-5.54


-23.21

-10.89


-21.23

-8.51

(C6H5COOH)(H2SO4) + NH3 S (C6H5COOH)(NH3)(H2SO4)

-15.72

-6.85

(C4H4O4)(H2SO4) + NH3 S (C4H4O4)(NH3)(H2SO4)

-14.39

-6.43


-16.14

-7.41

(C6H5CH2COOH)(H2SO4) + NH3 S (C6H5CH2COOH)(NH3)(H2SO4)

-15.86

-6.65


-16.96

-7.69


-15.95

-6.10

(C6H5COOH) + (H2SO4)(NH3) S (C6H5COOH)(NH3)(H2SO4)

-18.55

-7.34

(C4H4O4) + (H2SO4)(NH3) S (C4H4O4)(NH3)(H2SO4)

-13.06

-3.10


-17.42

-6.65

(C6H5CH2COOH) + (H2SO4)(NH3) S (C6H5CH2COOH)(NH3)(H2SO4)

-18.53

-6.52


-14.28

-3.63


-18.00

-6.52

H2SO4 + NH3 S (H2SO4)(NH3)

-16.72b

-7.77b

H2SO4 + H2SO4 S (H2SO4)2

-16.16b

-5.59b

(H2SO4) + (H2SO4)(NH3) S (NH3)(H2SO4)2

-25.11b

-11.65b

CH2O2 + NH3 S (CH2O2)(NH3)

-11.63b

-2.82b

(C2H4O2) + NH3 S (C2H4O2)(NH3)

-25.11b

-2.35b

a

Ref 25 BSSE (Basis Set Superposition Error) corrected B3LYP/6-31G(d, p). b Ref 15.

-5.41 a

-8.72 (-5.21)a



Figure 3. Equilibrium geometries of most stable isomers of hydrogen-bonded complexes of organic acids (OA) with H2SO4 and H3O+. Bond lengths and angles are given in angstroms and degrees, respectively.

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TABLE 5: Enthalpy (kcal mol-1) and Gibbs Free Energy Changes (kcal mol-1) Associated with Reactions among Neutral Clusters Composed of Atmospheric Precursors (Sulfuric Acid and Ammonia) and Organic Acids (Benzoic Acid, C6H5COOH; Malic Acid, C4H6O5; Maleic Acid, C4H4O4; Phenylacetic Acid, C6H5CH2COOH; Pyruvic Acid, C3H4O3; Tartaric Acid, C4H6O6) and Positively Charged Hydronium Ion H3O+ at a Temperature of 298.15 K and Pressure of 101.3 KPa reaction

a

∆H

∆G

C6H5COOH + (H3O+) S (C6H5COOH)(H3O+)

-55.80

-47.90

C4H4O4 + (H3O+) S (C4H4O4)(H3O+)

-63.62

-54.32

C4H6O5 + (H3O+) S (C4H6O5)(H3O+)

-59.43

-49.73

C6H5CH2COOH + (H3O+) S (C6H5CH2COOH)(H3O+)

-58.55

-48.76

C3H4O3 + (H3O+) S (C3H4O3)(H3O+)

-47.19

-38.38

C4H6O6 + (H3O+) S (C4H6O6)(H3O+)

-60.06

-49.92

H2SO4 + H3O+ S (H2SO4)(H3O+)

-38.18

-29.24

(C6H5COOH)(H3O+) + H2SO4 S (C6H5COOH)(H2SO4)(H3O+)

-22.01

-9.43

(C4H4O4)(H3O+) + H2SO4 S (C4H4O4)(H2SO4)(H3O+)

-14.57

-3.87


-12.84

-2.42

(C6H5CH2COOH)(H3O+) + H2SO4 S (C6H5CH2COOH)(H2SO4)(H3O+)

-14.52

-6.24


-17.98

-9.11


-13.51

-5.49

(C6H5COOH)(H2SO4) + H3O+ S (C6H5COOH)(H2SO4)(H3O+)

-57.09

-48.61

(C4H4O4)(H2SO4) + H3O+ S (C4H4O4)(H2SO4)(H3O+)

-61.64

-53.30


-53.1

-44.68

(C6H5CH2COOH)(H2SO4) + H3O+ S (C6H5CH2COOH)(H2SO4)(H3O+)

-52.53

-46.91


-49.99

-43.33


-53.65

-47.58

C6H5COOH + (H2SO4)(H3O+) S (C6H5COOH)(H2SO4)(H3O+)

-39.62

-28.09

C4H4O4 + (H2SO4)(H3O+) S (C4H4O4)(H2SO4)(H3O+)

-40.01

-28.95


-34.09

-22.91

C6H5CH2COOH + (H2SO4)(H3O+) S (C6H5CH2COOH)(H2SO4)(H3O+)

-34.89

-25.76


-30.34

-20.69


-35.39

-26.17

(H2SO4)(H3O+) + H2SO4 S (H3O+)(H2SO4)2

-23.8a

-10.6a

Ref 14.

the cases studied, the lengths of intermolecular OH bonds in organic acid-H3O+ and organic acid-H2SO4-H3O+ clusters are in the range of 1.75-1.82 Å and on average are much shorter

than the corresponding bond lengths in H2SO4-H3O+ and (H2SO4)2(H3O+) complexes. As it may be seen from Table 4, the affinity of all the organic acids to H3O+ far exceed that of

Interaction of Organic Acids and Nucleation Species H2SO4 to H3O+. They are 9-16 kcal mol-1 higher than the energy of the (H2SO4)(H3O+) formation. The free energy of (H2SO4)(H3O+)-OA formation via the addition of the organic acid is much higher than the free energy of (H2SO4)2(H3O+) formation in all the cases studied here. These considerations lead us to conclude that the organic acids studied in the present work may have a potential for being an efficient catalyst of the nucleation of positive ions in the atmosphere. Conclusion In the paper, the formation of hydrogen-bonded complexes containing H2SO4, and NH3 and trace ionic H3O+ species and benzoic, maleic, malic, pyruvic, phenylacetic, and tartaric acids representing common atmospheric organics has been investigated using the quantum-chemical methods. New thermochemical data have been reported, and the analysis of the relative cluster stability has been carried out. The present study leads us to the following conclusions: (a) Benzoic, maleic, phenylacetic, and tartaric acids are capable of stabilizing the small neutral cluster containing the sulfuric acid. Moreover, the stabilizing efficiency of these species appears to be close to or higher than that of ammonia, the commonly accepted principal stabilizer of binary sulfuric acid clusters in the Earth’s atmosphere. (b) The interaction of all the studied acids with common atmospheric positive hydronuim ion and (H2SO4)(H3O+) complexes is very strong. The affinities of these organic acids to (H3O+) and (H2SO4)(H3O+) far exceed those of H2SO4 to H3O+ and (H2SO4)(H3O+). (c) On the basis of the analysis of the first steps of the formation of prenucleation clusters, one can conclude that most of the organic acids studied in the present paper are likely to have a considerable potential in stabilizing both neutrals and positive ions. However, further research is needed to determine whether this conclusion is applicable to larger prenucleation clusters and hydrate complexes. The particle formation events frequently observed in the atmosphere may involve multiple-component nucleation processes. Existing relatively well-established nucleation theories only consider the binary H2SO4-H2O or ternary H2SO4H2O-NH3 system. Some organic acids have long been suggested to be involved in the nucleation process (at least under certain conditions), but the nucleation model of H2SO4-H2Oorganics does not exist. To develop a reliable H2SO4-H2Oorganics nucleation theory, proper thermodynamic data of the nucleation system are needed. Thermochemical data for the prenucleation clusters obtained using computational quantum methods in this study indicate that a number of organic acids may be involved in the atmospheric nucleation process (especially positive ion nucleation). Further research is needed to extend the present study to larger prenucleation clusters containing more organic acid, sulfuric acid, and water molecules so that the derived thermodynamic data can be effectively incorporated in the framework of kinetic nucleation models providing explicit simulations of cluster formation and evolution.8 The nucleation models constrained by quantum-derived thermochemical data for prenucleation clusters can be then implemented, after the proper validation, in global models to study the new particle formation in the planetary atmosphere7 and various implications associated with the aerosol radiative forcing and climate change. Acknowledgment. This work was supported by the Science Foundation of Chinese Research Academy of Environmental

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