Molecular dynamics of aqueous NaNO3 and NaCl solutions: The surface propensity and thermodynamic properties of aqueous anions (Summer Research Institute, PNNL Scope #16248)
EMSL Project ID
18409
Abstract
Unique interface chemistry at the surface of sea-salt particles, comprised primarily of aqueous sodium chloride, has been postulated as an important source of photochemically active chlorine in the troposphere.1,2 Molecular chlorine then goes on to absorb sunlight and form atomic chlorine, a known oxidant of organics and an initiator of ozone formation in urban areas.3 A variety of oxides of nitrogen react with sea-salt particles, leading to replacement of chloride by nitrate ions as particles age.4-8 There are a number of factors that could impact Cl2 production from aerosols comprised of a mixture of nitrate and chloride. If chloride and nitrate are equally available at the interface, then molecular chlorine production could drop as aerosols age. Alternatively, if nitrate resides in the bulk and pushes chloride to the interface, Cl2 production would stay more constant even as the chloride concentration in the aerosol decreases. Aerosol chamber experiments to characterize the amount of Cl2 produced from mixed NaCl/NaNO3 aerosols are currently underway at University of California, Irvine.9 In coordination with these experimental studies, this work will focus on investigating aqueous NaNO3 as well as mixtures of NaNO3 and NaCl using classical molecular dynamics (MD) simulations.
First, MD simulations and analysis to validate the NO3- force field are warranted. Neutron diffraction studies10,11 on 7M aqueous NaNO3 contain information about the hydration structure around the nitrate ion. Reproducing neutron diffraction data using molecular dynamics simulations on 7M NaNO3 will confirm the accuracy of parameters used in the NO3- force field.
Second, MD simulations to investigate the interface of mixed NaNO3 and NaCl solutions, using methods outlined in prior work,12-15 will more definitively address the question, does nitrate reside at surface or in the bulk of aqueous sea-salt aerosols?
Finally, this work will identify the contributions of ion size and polarizability to the propensity of ions to reside at the air/water interface. Using thermodynamic integration methods employed in potential mean force calculations,15,16 the individual steps in ion solvation will be more clearly understood.
Project Details
Project type
Exploratory Research
Start Date
2006-06-05
End Date
2007-01-04
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Wingen LM, AC Moskun, SN Johnson, JL Thomas, M Roeselova, DJ Tobias, MT Kleinman, and BJ Finlayson-Pitts. 2008. "Enhanced surface photochemistry in chloride–nitrate ion mixtures." Physical Chemistry Chemical Physics. PCCP 10:5668–5677. doi:10.1039/b806613b