1. Trapping of Hydroxyl Radical and Ozone at Salt Aerosol Surfaces: A Molecular Dynamics Study Martina Roeselová, a Douglas J. Tobias, b R. Benny Gerber, b,c and Pavel Jungwirth a a) J. Heyrovský Institute of Physical Chemistry and Center for Complex Molecular Systems and Biomolecules, Prague, Czech Republic b) Department of Chemistry, University of California, Irvine, CA92697-2025, USA c) Department of Physical Chemistry and Fritz Haber Center for Molecular Dynamics, Hebrew University, Jerusalem 91904, Israel Na Cl - I - O H Atmospheric chemistry of sea-salt aerosols Sea-salt aerosols consist of aqueous salt particles formed by evaporation of water from tiny droplets of sea water that are ejected into the air during wave breaking. They have a wide size range (ca. 0.1-10  m diameter). Reactions of sea salt aerosols with ozone produce a variety of atmospherically important halogen compounds. Molecular chlorine is formed by the photolysis of ozone in the presence of salt particles above their deliquescence point. Observed kinetics could not be modeled using well-known bulk phase chlorine chemistry  interfacial mechanism considered. Cl 2 production from sea-salt aerosols Knipping, Lakin, Foster, Jungwirth, Tobias, Gerber, Dabdub, Finlayson- Pitts, Science 288, 301 (2000) ??? Is the suggested surface mechanism feasible ??? Computer model of the open surface of an aqueous salt aerosol particle We mimic the surfaces of sea salt aerosol droplets by simulating flat slabs using 3D periodic boundary conditions All simulations were done using AMBER 6 molecular dynamics program package with polarizable force field. Slab systems studied: (1) Saturated (6.1 M) NaCl solution slab: 864 H 2 O, 96 Na +, 96 Cl – (2) Neat water slab: 864 H 2 O (3) 1.2 M NaI solution slab: 864 H 2 O, 18 Na +, 18 I – Simulation of OH and O 3 interaction with the slab surfaces (1) direct scattering (2) adsorption desorption (3) absorption OH O3O3 125 trajectories for each system ambient conditions thermal impact velocity 5 different impact angles Density profiles NaCl slab OH O3O3 water slab NaI slab trapping 8 20 33 1 0 2 NaCl slab water slab NaI slab 107 117 114 110 108 113 18 8 11 15 17 12 OH confined to interface of NaCl slab absorption enhanced in NaI slab O 3 desorbs back into gas phase much faster than OH very little absorption of O 3 Trapping probabilities (number of trajectories; total=125) Mean trapping times NaCl slab OH O3O3 water slab NaI slab NaCl slab water slab NaI slab 43 ps 50 ps 53 ps 16 ps 18 ps 21 ps no substantial difference in trapping between salt solution and neat water Orientational effect OH prefers parallel alignment with the surface or perpendicular arrangement with hydrogen facing the slab. gas phase interface bulk Contact of OH with Cl - ions g(r) Average number of particles in the first solvation shell of OH in the interface total Cl - water 6.32 0.61 5.71 Distribution of contact times (1) (2) (3) k Simulations did NOT confirm the scavenging role of surface halide ions. Trapping of OH and O 3 is not enhanced at salt solution surfaces compared to neat water. Nevertheless, trapping is rather efficient for all systems studied. k Mass accommodation coefficients are similar for both solutes. However, O 3 desorbs substantially faster than OH. Mean trapping times of OH are more than two times longer than for O 3. k Both solutes strongly prefer interface over bulk solvation. Presence of iodide ions in the solution leads to slight enhancement of uptake of the solutes into the bulk. k Orientational effect of the interface leads to partial alignment of hydroxyl radical at the surface (experimentally measurable). k OH spends more than half of the time in interface in contact with chloride ions. Thus, the results of our simulations support the proposed mechanism of Cl 2 production via the OH…Cl - complex in the surface layer of a sea salt aerosol particles. References Conclusions 6.1 M NaCl solution slab1.2 M solution NaI slab(top view) (1) E. M. Knipping, M. J. Lakin, K. L. Foster, P. Jungwirth, D. J. Tobias, R. B. Gerber, D. Dabdub, B. J. Finlayson-Pitts, Experiments and Simulations of Ion-Enhanced Interfacial Chemistry on Aqueous NaCl Aerosols, Science 288 (2000), 301-306 (2) P. Jungwirth, D. J. Tobias, Ions at the Air/Water Interface, J. Phys. Chem. B 106 (2002), 6361-6373 (3) M Roeselova, D. J. Tobias, R. B. Gerber, P. Jungwirth, Impact, Trapping and Accommodation of OH and O 3 at Salt Aerosol Surfaces: A Molecular Dynamics Study, to be submitted