Trapping of Hydroxyl Radical and Ozone at Salt Aerosol Surfaces: A Molecular Dynamics Study

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 Jungwirtha 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 Atmospheric chemistry of sea-salt aerosols Cl2 production from 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. ??? Is the suggested surface mechanism feasible ??? Knipping, Lakin, Foster, Jungwirth, Tobias, Gerber, Dabdub, Finlayson- Pitts, Science 288, 301 (2000) Computer model of the open surface of an aqueous salt aerosol particle 6.1 M NaCl solution slab (top view) 1.2 M solution NaI slab • 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 polarizableforce field. • Slab systems studied: (1) Saturated (6.1 M) NaCl solution slab: 864 H2O, 96 Na+, 96 Cl– (2) Neat water slab: 864 H2O (3) 1.2 M NaI solution slab: 864 H2O, 18 Na+, 18 I– Na Cl- I- O H Simulation of OH and O3 interaction with the slab surfaces • 125 trajectories for each system • ambient conditions • thermal impact velocity • 5 different impact angles OH O3 OH O3 (1) direct scattering NaCl slab water slab NaI slab NaCl slab water slab NaI slab OH confined to interface of NaCl slab (1) 18 8 11 15 17 12 (2) adsorption desorption O3 desorbs back into gas phase much faster than OH (2) 107 117 114 110 108 113 trapping (3) absorption (3) 8 20 33 1 0 2 (number of trajectories; total=125) absorption enhanced in NaI slab Density profiles Trapping probabilities Mean trapping times very little absorption of O3 OH O3 NaCl slab water slab NaI slab NaCl slab water slab NaI slab no substantial difference in trapping between salt solution and neat water 43 ps 50 ps 53 ps 16 ps 18 ps 21 ps Conclusions Orientational effect Contact of OH with Cl- ions g(r) Distribution of contact times • Simulations did NOT confirm the scavenging role of surface halide ions. Trapping of OH and O3 is not enhanced at salt solution surfaces compared to neat water. Nevertheless, trapping is rather efficient for all systems studied. • Mass accommodation coefficients are similar for both solutes. However, O3 desorbs substantially faster than OH. Mean trapping times of OH are more than two times longer than for O3. • 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. • Orientational effect of the interface leads to partial alignment of hydroxyl radical at the surface (experimentally measurable). • 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 Cl2 production via the OH…Cl- complex in the surface layer of a sea salt aerosol particles. gas phase interface bulk OH prefers parallel alignment with the surface or perpendicular arrangement with hydrogen facing the slab. Average number of particles in the first solvation shell of OH in the interface total Cl-water 6.32 0.615.71 References (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 O3 at Salt Aerosol Surfaces: A Molecular Dynamics Study, to be submitted