Stratospheric Chemistry

Heterogeneous Stratospheric Chemistry

H₂SO₄/H₂O solutions are key players in stratospheric ozone loss. It is now widely accepted that sulfate aerosols participate in ozone depletion by: (1) serving as surfaces for chlorine activation, (2) affecting the NO_x budget, and (3) being the starting material for Polar Stratospheric Clouds (PSCs). Although some field observations and laboratory work corroborate this general picture, a fundamental understanding of these heterogeneous processes is lacking. The primary goal of this research is to study surfaces of H₂SO₄/H₂O solutions in contact with atmospherically relevant gases, e.g. HCl, HNO₃, and HOCl, to provide molecular details of heterogeneous reactions. Incorporating our results into atmospheric models will lead to a fuller and more accurate picture of ozone depletion. This information will aid researchers, and thereby policy makers, in making vital decisions about issues such as deploying a fleet of civilian Super-Sonic-Transports (SSTs) and choosing CFC substitutes.

Recently, we have used SFG to study surfaces of H₂SO₄/H₂O solutions at various mole fractions, including the relevant 0.1-0.4x, characteristic of stratospheric sulfate aerosols at mid-latitudes. At 0.1x H₂SO₄, water molecules are fairly ordered on the surface; at 0.4x, the surface resembles pure H₂SO₄. These findings indicate that reactions occurring at the surface of stratospheric sulfate aerosols may be inhibited at the higher sulfuric acid concentrations. Experiments with ionic sulfate solutions were performed to further investigate how sulfates orient surface water molecules. Results from both studies indicate that negative and positive ions set up a double layer and reorient water molecules. Water orientation and concentration greatly depends on the ability of solutions to form complexes.

Now that we have characterized the surface of sulfuric acid solutions, we are in the process of introducing HCl into the H₂SO₄/H₂O system. The goal of this research is to identify chemical species responsible for heterogeneous activation of odd chlorine at mid-latitudes, and to determine how effectively these sequester odd chlorine. It is generally believed that sulfate aerosol surfaces serve to convert relatively stable reservoir species, e.g. HCl and ClONO₂, into photolabile species like Cl₂and HOCl, e.g.:

HCl + ClONO₂----> Cl₂ + HNO₃ (1)

Photolysis of Cl₂ then directly destroys ozone in catalytic cycles. Concentration and orientation of both surface HCl and HOCl molecules are being characterized to determine if heterogeneous reactions occur on the surface of aerosols at the gas-liquid interface. These results will help develop a reaction mechanism that indicates if these molecules adsorb onto the surface, or diffuse into the bulk. For more information about these atmospheric processes, see: 1,2

REFERENCES:

1)Molina, M. J.; Molina, L. T.; Kolb, C. E., "Gas-Phase and Heterogeneous Chemical Kinetics of the Troposphere and Stratosphere" Annu. Rev. Phys. Chem. 1996, 47, 327-67.

2)Zhang, R.; Leu, M.-T.; Keyser, L. F., "Heterogeneous Reactions of ClONO2, HCl, and HOCl on Liquid Sulfuric Acid Surfaces" J. Phys. Chem. 1994, 98, 13563-13574.