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Transcript of Quantum chemical studies on atmospheric sulfuric acid nucleation Theo Kurtén Division of...
Quantum chemical studies on
atmospheric sulfuric acid nucleation
Theo Kurtén
Division of Atmospheric Sciences
Department of Physical Sciences
University of Helsinki
08.11.2007
Co-authors
University of Helsinki, Department of Physical Sciences:
Hanna Vehkamäki, Ismael Kenneth Ortega, Ville
Loukonen, Martta Salonen, Leena Torpo, Markku
Kulmala. Finnish Meteorological Institute: Veli-Matti Kerminen. University of Helsinki, Department of Chemistry: Markku
Sundberg. University of Oulu, Department of Chemistry: Kari
Laasonen, Chang-Geng Ding. University of Tartu: Madis Noppel.
New-particle formation is observed frequently in the atmosphere…
…but the molecular-level mechanisms behind these nucleation
events are unknown.
Events seem to be connected with sulfuric acid (H2SO4)
concentrations, and sometimes also ammonia (NH3).
Time
Dia
mete
r (m
)
Suggested nucleation mechanisms:
Binary H2SO4-H2O
Ternary H2SO4-H2O-NH3
Ion-induced H2SO4-H2O
H2SO4 + organics
Quantum Chemistry
= The numerical solution of Schrödinger’s equation for
a system of atomic nuclei and electrons subject to
various approximations. Approximations are made e.g. regarding the shape of the
wavefunction and the treatment of electron-electron
correlation
Different sets of approximations different model
chemistries
- denoted by a bewildering multitude of acronyms
We have recently used quantum chemistry to investigate
sulfuric acid – water – ammonia nucleation in the
atmosphere.
NH3 enhances formation of neutral clusters, but the effect only becomes apparent when n(H2SO4) 2. For ionic clusters, NH3 has little or no effect.
Gibbs free energies of
formation for clusters with
2-4 sulfuric acid
molecules
T = 265 K
[H2SO4] = 0.36 ppt
[NH3] = 1 ppb
[HSO4-] = 3000 cm-3
Blue: clusters with NH3
Red: clusters without NH3
Solid lines: neutral clusters Dashed lines: ionic clusters
-10
0
10
20
30
40
2 3 4
number of acids
RI-CC2/aug-cc-pV(T+d)Z energies with BLYP/DZP geometries & frequencies.Data by I.K. Ortega.
However, NH3:H2SO4 mole ratio almost always 1:1
|typical atmospheric range|
RI-MP2/aug-cc-pV(T+d)Zenergies with RI-MP2/aug-cc-pV(D+d)Z geometries & frequencies.
s.a. = sensitivity analysis; frequencies scaled by 0.75 and -2 kcal/mol added to the energy of each ammonia addition step.
The problem with sulfuric acid – ammonia - water nucleation
It might not be (only) sulfuric acid… Threshold H2SO4 concentration for nucleation (Berndt et al.):
- 1010 cm-3 if taken from a liquid reservoir
- 107 cm-3 if produced from SO2 + H2O + UV
Some other SO2 oxidation products participate!
…and it might not be ammonia, either. Measurements and calculations (e.g. Murphy et al.) show
that amines, rather than ammonia, may be the primary
enhancers of atmospheric nitric acid nucleation
Our calculations indicate that this is likely to be the case for
sulfuric acid nucleation, too.
(Water is probably still a safe bet, though.)
Comparison of sulfuric acid and peroxo-disulfuric acid dimers (data by M. Salonen)
H2SO4●H2SO4, H2SO4●H2S2O8, E0=-18.0, G=-6.2 kcal/mol E0=-20.2, G=-4.7 kcal/mol
H2SO4●H2SO4●H2O, H2SO4●H2S2O8●H2O, E0=-33.0, G=-7.3 kcal/mol E0=-37.4, G=-8.7 kcal/mol
RI-MP2/QZVPP
Amines much more strongly bound than NH3 to H2SO4, and somewhat more strongly to HSO4
-
H2SO4●NH3, G=-6.6 kcal/mol H2SO4●(CH3)2NH, G=-13.7 kcal/mol
HSO4-●NH3, G=+1.8 kcal/mol HSO4
-●(CH3)2NH, G=-0.7 kcal/mol
Computed using RI-CC2/aug-cc-pV(T+d)Z energies and RI-MP2/aug-cc-pV(D+d)Z geometries & frequencies. Data provided by V. Loukonen.
Amines also promote addition of H2SO4 to both
neutral and ionic clusters much more effectively than NH3
Reaction G, kcal/mol
H2SO4 + H2SO4 (H2SO4)2 -6.9
H2SO4·NH3 + H2SO4 (H2SO4)2·NH3 -14.4
H2SO4·(CH3)2NH + H2SO4 (H2SO4)2·(CH3)2NH -19.3
HSO4- + H2SO4 HSO4
-·H2SO4 -34.1
HSO4-·NH3 + H2SO4 HSO4
-·H2SO4·NH3 -34.7
HSO4-·(CH3)2NH + H2SO4 HSO4
-
·H2SO4·(CH3)2NH
-42.0
Computed using RI-CC2/aug-cc-pV(T+d)Z energies and RI-MP2/aug-cc-pV(D+d)Z geometries & frequencies.
Conclusions
NH3 significantly assists the growth of atmospheric
clusters in the H2SO4 co-ordinate
However, amines are likely to be even more effective, and
might actually be the main enhancers of nucleation.
The NH3:H2SO4 mole ratio of nucleating clusters in
atmospheric conditions likely to be between 1:3 and 1:1. NH3 probably plays only a small role in ion-induced
nucleation. Amines, on the other hand, might be important
H2S2O8 might also play a role in atmospheric nucleation
(along with or even instead of H2SO4).
References
Articles by us T. Kurtén et al.: Atmos. Chem. Phys. 2007, 7, 2765 (NH3:H2SO4 mole ratio); Boreal Env. Res.
2007, 12, 431 (H2SO4 hydration, ions)
V. Loukonen et al.: J. Phys. Chem. A 2007, submitted (amines) M. Salonen et al.: Atmos. Res. 2007, submitted (SO2 oxidation intermediates)
L. Torpo et al.: J. Phys. Chem. A 2007, 111, 10671 (role of NH3)
Articles by others S. M. Ball et al.: J. Geophys. Res. 1999, D104, 237098. (experiments on NH3 & nucleation)
T. Berndt et al.: Science 2005, 307, 698; Geophys. Res. Lett. 2006, 33, L15817 (H2SO4 and
SO2 nucleation experiments)
D. Hanson & F. Eisele: J. Phys. Chem. A 2000, 104, 1715 (H2SO4 hydration)
S. M. Murpy et al.: Atmos. Chem. Phys. 2007, 7, 2313 (amines) A. Nadykto & F. Yu: Chem. Phys. Lett. 2007, 435, 14 (H2SO4-NH3-H2O clusters)
Programs used Gaussian 03 by Frisch et al. (Gaussian Inc. 2004) SIESTA version 2.0 by Soler & Artacho et al. Turbomole version 5.8. by Alhrichs et al.
Acknowledgements
CSC center for computer scienceJohanna Blomqvist, Nino Runeberg, Mikael
Johansson
Academy of Finland
Thank you for your attention!
Mange tak for er opmærksomhed!