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Quantum chemical studies on atmospheric sulfuric acid nucleation Theo Kurtén Division of Atmospheric Sciences Department of Physical Sciences University of Helsinki

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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.

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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 (H 2 SO 4 ) concentrations, and sometimes also ammonia (NH 3 ). Time Diameter (m)

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Suggested nucleation mechanisms: Binary H 2 SO 4 -H 2 O Ternary H 2 SO 4 -H 2 O-NH 3 Ion-induced H 2 SO 4 -H 2 O H 2 SO 4 + organics

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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.

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NH 3 enhances formation of neutral clusters, but the effect only becomes apparent when n(H 2 SO 4 ) 2. For ionic clusters, NH 3 has little or no effect. Gibbs free energies of formation for clusters with 2-4 sulfuric acid molecules T = 265 K [H 2 SO 4 ] = 0.36 ppt [NH 3 ] = 1 ppb [HSO 4 - ] = 3000 cm -3 Blue: clusters with NH 3 Red: clusters without NH 3 Solid lines: neutral clusters Dashed lines: ionic clusters RI-CC2/aug-cc-pV(T+d)Z energies with BLYP/DZP geometries & frequencies. Data by I.K. Ortega.

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However, NH 3 :H 2 SO 4 mole ratio almost always 1:1 | typical atmospheric range| RI-MP2/aug-cc-pV(T+d)Z energies 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.

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The problem with sulfuric acid – ammonia - water nucleation It might not be (only) sulfuric acid… Threshold H 2 SO 4 concentration for nucleation (Berndt et al.): cm -3 if taken from a liquid reservoir cm -3 if produced from SO 2 + H 2 O + UV Some other SO 2 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.)

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Comparison of sulfuric acid and peroxo- disulfuric acid dimers (data by M. Salonen) H 2 SO 4 ●H 2 SO 4, H 2 SO 4 ●H 2 S 2 O 8, E 0 =-18.0, G =-6.2 kcal/mol E 0 =-20.2, G =-4.7 kcal/mol H 2 SO 4 ●H 2 SO 4 ●H 2 O, H 2 SO 4 ●H 2 S 2 O 8 ●H 2 O, E 0 =-33.0, G =-7.3 kcal/mol E 0 =-37.4, G =-8.7 kcal/mol RI-MP2/QZVPP

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Amines much more strongly bound than NH 3 to H 2 SO 4, and somewhat more strongly to HSO 4 - H 2 SO 4 ●NH 3, G =-6.6 kcal/molH 2 SO 4 ●(CH 3 ) 2 NH, G =-13.7 kcal/mol HSO 4 - ●NH 3, G =+1.8 kcal/mol HSO 4 - ●(CH 3 ) 2 NH, 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.

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Amines also promote addition of H 2 SO 4 to both neutral and ionic clusters much more effectively than NH 3 Reaction G , kcal/mol H 2 SO 4 + H 2 SO 4 (H 2 SO 4 ) H 2 SO 4 ·NH 3 + H 2 SO 4 (H 2 SO 4 ) 2 ·NH H 2 SO 4 ·(CH 3 ) 2 NH + H 2 SO 4 (H 2 SO 4 ) 2 ·(CH 3 ) 2 NH HSO H 2 SO 4 HSO 4 - ·H 2 SO HSO 4 - ·NH 3 + H 2 SO 4 HSO 4 - ·H 2 SO 4 ·NH HSO 4 - ·(CH 3 ) 2 NH + H 2 SO 4 HSO 4 - ·H 2 SO 4 ·(CH 3 ) 2 NH Computed using RI-CC2/aug-cc-pV(T+d)Z energies and RI-MP2/aug-cc-pV(D+d)Z geometries & frequencies.

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Conclusions NH 3 significantly assists the growth of atmospheric clusters in the H 2 SO 4 co-ordinate However, amines are likely to be even more effective, and might actually be the main enhancers of nucleation. The NH 3 :H 2 SO 4 mole ratio of nucleating clusters in atmospheric conditions likely to be between 1:3 and 1:1. NH 3 probably plays only a small role in ion-induced nucleation. Amines, on the other hand, might be important H 2 S 2 O 8 might also play a role in atmospheric nucleation (along with or even instead of H 2 SO 4 ).

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References Articles by us T. Kurtén et al.: Atmos. Chem. Phys. 2007, 7, 2765 (NH 3 :H 2 SO 4 mole ratio); Boreal Env. Res. 2007, 12, 431 (H 2 SO 4 hydration, ions) V. Loukonen et al.: J. Phys. Chem. A 2007, submitted (amines) M. Salonen et al.: Atmos. Res. 2007, submitted (SO 2 oxidation intermediates) L. Torpo et al.: J. Phys. Chem. A 2007, 111, (role of NH 3 ) Articles by others S. M. Ball et al.: J. Geophys. Res. 1999, D104, (experiments on NH 3 & nucleation) T. Berndt et al.: Science 2005, 307, 698; Geophys. Res. Lett. 2006, 33, L15817 (H 2 SO 4 and SO 2 nucleation experiments) D. Hanson & F. Eisele: J. Phys. Chem. A 2000, 104, 1715 (H 2 SO 4 hydration) S. M. Murpy et al.: Atmos. Chem. Phys. 2007, 7, 2313 (amines) A. Nadykto & F. Yu: Chem. Phys. Lett. 2007, 435, 14 (H 2 SO 4 -NH 3 -H 2 O 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.

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Acknowledgements CSC center for computer science Johanna Blomqvist, Nino Runeberg, Mikael Johansson Academy of Finland

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Thank you for your attention! Mange tak for er opmærksomhed!

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