The HOX⋯SO2 (X=F, Cl, Br, I) Binary Complexes: Implications for Atmospheric Chemistry

Sulfur dioxide and hypohalous acids (HOX, X=F, Cl, Br, I) are ubiquitous molecules in the atmosphere that are central to important processes like seasonal ozone depletion, acid rain, and cloud nucleation. We present the first theoretical examination of the HOX⋯SO₂ binary complexes and the associated trends due to halogen substitution. Reliable geometries were optimized at the CCSD(T)/aug-cc-pV(T+d)Z level of theory for HOF and HOCl complexes. The HOBr and HOI complexes were optimized at the CCSD(T)/aug-cc-pV(D+d)Z level of theory with the exception of the Br and I atoms which were modeled with an aug-cc-pwCVDZ-PP pseudopotential. 27 HOX⋯SO₂ complexes were characterized and the focal point method was employed to produce CCSDT(Q)/CBS interaction energies. Natural Bond Orbital analysis and Symmetry Adapted Perturbation Theory were used to classify the nature of each principle interaction. The interaction energies of all HOX⋯SO₂ complexes in this study ranged from 1.35 to 3.81 kcal mol^-1 . The single-interaction hydrogen bonded complexes spanned a range of 2.62 to 3.07 kcal mol^-1 , while the single-interaction halogen bonded complexes were far more sensitive to halogen substitution ranging from 1.35 to 3.06 kcal mol^-1 , indicating that the two types of interactions are extremely competitive for heavier halogens. Our results provide insight into the interactions between HOX and SO₂ which may guide further research of related systems.

Photodegradation Rate Constants for Anthracene and Pyrene Are Similar in/on Ice and in Aqueous Solution

Snowpacks contain a variety of chemicals, including organic pollutants such as toxic polycyclic aromatic hydrocarbons (PAHs). While PAHs undergo photodegradation in snow and ice, the rates of these reactions remain in debate. Some studies report that photochemical reactions in snow proceed at rates similar to those expected in a supercooled aqueous solution, but other studies report faster reaction rates, particularly at the air-ice interface (i.e., the quasi-liquid layer, or QLL). In addition, one study reported a surprising nonlinear dependence on photon flux. Here we examine the photodegradation of two common PAHs, anthracene and pyrene, in/on ice and in solution. For a given PAH, rate constants are similar in aqueous solution, in internal liquid-like regions of ice, and at the air-ice interface. In addition, we find the expected linear relationship between reaction rate constant and photon flux. Our results indicate that rate constants for the photochemical loss of PAHs in, and on, snow and ice are very similar to those in aqueous solution, with no enhancement at the air-ice interface.

Nitrate Photochemistry at the Air-Ice Interface and in Other Ice Reservoirs

The photolysis of snowpack nitrate (NO₃^-) is an important source of gaseous reactive nitrogen species that affect atmospheric oxidants, particularly in remote regions. However, it is unclear whether nitrate photochemistry differs between the three solute reservoirs in/on ice: in liquid-like regions (LLRs) in the ice; within the solid ice matrix; and in a quasi-liquid layer (QLL) at the air-ice interface, where past work indicates photolysis is enhanced. In this work, we explore the photoformation of nitrite in these reservoirs using laboratory ices. Nitrite quantum yields, Φ(NO₂^-), at 313 nm for aqueous and LLR ice samples agree with previous values, e.g., 0.65 ± 0.07% at -10 °C. For ice samples made via flash-freezing solution in liquid nitrogen, where nitrate is possibly present as a solid solution, the nitrite quantum yield is 0.57 ± 0.05% at -10 °C, similar to the LLR results. In contrast, the quantum yield at the air-ice interface is enhanced by a factor of 3.7 relative to LLRs, with a value of 2.39 ± 0.24%. These results indicate nitrate photolysis is enhanced at the air-ice interface, although the importance of this enhancement in the environment depends on the amount of nitrate present at the interface.

Persistent Water-Nitric Acid Condensate with Saturation Water Vapor Pressure Greater than That of Hexagonal Ice

A laboratory chilled mirror hygrometer (CMH), exposed to an airstream containing water vapor (H2O) and nitric acid (HNO3), has been used to demonstrate the existence of a persistent water-nitric acid condensate that has a saturation H2O vapor pressure greater than that of hexagonal ice (Ih). The condensate was routinely formed on the mirror by removing HNO3 from the airstream following the formation of an initial condensate on the mirror that resembled nitric acid trihydrate (NAT). Typical conditions for the formation of the persistent condensate were a H2O mixing ratio greater than 18 ppm, pressure of 128 hPa, and mirror temperature between 202 and 216 K. In steady-state operation, a CMH maintains a condensate of constant optical diffusivity on a mirror through control of only the mirror temperature. Maintaining the persistent condensate on the mirror required that the mirror temperature be below the H2O saturation temperature with respect to Ih by as much as 3 K, corresponding to up to 63% H2O supersaturation with respect to Ih. The condensate was observed to persist in steady state for up to 16 h. Compositional analysis of the condensate confirmed the co-condensation of H2O and HNO3 and thereby strongly supports the conclusion that the Ih supersaturation is due to residual HNO3 in the condensate. Although the exact structure or stoichiometry of the condensate could not be determined, other known stable phases of HNO3 and H2O are excluded as possible condensates. This persistent condensate, if it also forms in the upper tropical troposphere, might explain some of the high Ih supersaturations in cirrus and contrails that have been reported in the tropical tropopause region.

Water accommodation on ice and organic surfaces: insights from environmental molecular beam experiments

Water uptake on aerosol and cloud particles in the atmosphere modifies their chemistry and microphysics with important implications for climate on Earth. Here, we apply an environmental molecular beam (EMB) method to characterize water accommodation on ice and organic surfaces. The adsorption of surface-active compounds including short-chain alcohols, nitric acid, and acetic acid significantly affects accommodation of D2O on ice. n-Hexanol and n-butanol adlayers reduce water uptake by facilitating rapid desorption and function as inefficient barriers for accommodation as well as desorption of water, while the effect of adsorbed methanol is small. Water accommodation is close to unity on nitric-acid- and acetic-acid-covered ice, and accommodation is significantly more efficient than that on the bare ice surface. Water uptake is inefficient on solid alcohols and acetic acid but strongly enhanced on liquid phases including a quasi-liquid layer on solid n-butanol. The EMB method provides unique information on accommodation and rapid kinetics on volatile surfaces, and these studies suggest that adsorbed organic and acidic compounds need to be taken into account when describing water at environmental interfaces.