Quantum chemical studies on atmospheric sulfuric acid nucleation

1. Quantum chemical studies on atmospheric sulfuric acid nucleation Theo Kurtén Division of Atmospheric Sciences Department of Physical Sciences University of Helsinki 08.11.2007

2. 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.

3. 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). Diameter (m) Time

4. Suggested nucleation mechanisms: • Binary H2SO4-H2O • Ternary H2SO4-H2O-NH3 • Ion-induced H2SO4-H2O • H2SO4 + organics

5. 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.

6. 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. • Gibbsfree 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 RI-CC2/aug-cc-pV(T+d)Z energies with BLYP/DZP geometries & frequencies. Data by I.K. Ortega.

7. However, NH3:H2SO4 mole ratio almost always  1:1 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. |typical atmospheric range|

8. 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.)

9. Comparison of sulfuric acid and peroxo-disulfuric acid dimers (data by M. Salonen) RI-MP2/QZVPP 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

10. Amines much more strongly bound than NH3 to H2SO4, and somewhat more strongly to HSO4- H2SO4●NH3, G=-6.6 kcal/molH2SO4●(CH3)2NH, G=-13.7 kcal/mol HSO4-●NH3, G=+1.8 kcal/molHSO4-●(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.

11. Amines also promote addition of H2SO4 to both neutral and ionic clusters much more effectively than NH3 Computed using RI-CC2/aug-cc-pV(T+d)Z energies and RI-MP2/aug-cc-pV(D+d)Z geometries & frequencies.

12. 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).

13. 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 byFrisch et al. (Gaussian Inc. 2004) • SIESTA version 2.0 by Soler & Artacho et al. • Turbomole version 5.8. by Alhrichs et al.

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

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