Chemistry of alkali halide and ice surfaces: Characterization of reactions relevant to atmospheric chemistry

Abstract

Atmospherically-relevant surface reactions were studied. These reactions were investigated to provide insight into the products formed on sea salt atmospheric particle surfaces, the quantitative distribution of species on the surface of model sea salt particles, and the molecular environment of the interfacial region of HNO₃/H₂O ices. The reactions of model sea salt particles (NaCl) exposed to mineral acids (HNO₃ and H₂SO₄) were studied using Raman spectroscopy and atomic force microscopy (AFM). The reaction of powdered NaCl with HNO₃ was studied using Raman spectroscopy. NaNO₃ growth was monitored as a function of HNO₃ exposure in a flow cell. Mode-specific changes in the NO₃- vibrational mode intensities with HNO₃ exposure suggest a rearrangement of the NaNO₃ film with coverage. In the absence of H₂O, intensities of NaNO₃ bands increase with HNO₃ exposure until a capping layer of NaNO₃ forms. The capping layer prevents subsequent HNO₃ from reacting with the underlying. The reaction of NaCl with H₂SO₄ is investigated using Raman spectroscopy and atomic force microscopy (AFM). Raman spectra are consistent with the formation of NaHSO4 with no evidence for Na₂SO₄. The spectra indicate that the phase of NaHSO₄ varies with the amount of H₂O in the H₂SO₄. The reaction produces anhydrous β-NaHSO₄ which undergoes a phase change to anhydrous α-NaHSO₄. AFM measurements on NaCl (100) show the formation of two distinct types of NaHSO4 structures consistent in shape with α-NaHSO₄ and β-NaHSO₄ . Model sea salt particles were gown from solution to determine the surface Br/Cl of crystals grown from solution. These studies show surface Br concentration is 35 times that of the bulk concentration. This data is useful in the understanding of enhanced volatile Br compounds in the Arctic troposphere. Thin films of model polar stratospheric cloud (PSC) surfaces were studied in ultrahigh vacuum. Low temperature data show the preferential orientation of HNO₃ on crystalline H₂O ice. Thermodynamically-stable HNO₃ · 3H₂O is formed at ∼170 K, and subsequently desorbs from the surface. These studies show the chemical specificity of Raman spectroscopy in this chemical system. Studies of ClONO₂ adsorption onto crystalline H₂O ice suggest that ClONO₂ is weakly adsorbed.