Isotope Effects in Photodissociation: Chemical Reaction Dynamics and Implications for Atmospheres

Is either direct photolysis or photocatalysed H-shift of peroxyl radicals a competitive pathway in the troposphere?

Peroxyl radicals (RO O . ) are key intermediates in atmospheric chemistry, with relatively long lifetimes compared to most other radical species. In this study, we use multireference quantum chemical methods to investigate whether photolysis can compete with well-established RO O . sink reactions. We assume that the photolysis channel is always RO O . + hν => RO + O(³P). Our results show that the maximal value of the cross-section for this channel is σ = 1.3 × 10^-18 cm² at 240 nm for five atmospherically representative peroxyl radicals: CH₃O O . , C(O)HCH₂O O . , CH₃CH₂O O . , HC(O)O O . and CH₃C(O)O O . . These values agree with experiments to within a factor of 2. The rate constant of photolysis in the troposphere is around 10^-5 s^-1 for all five RO O . . As the lifetime of peroxyl radicals in the troposphere is typically less than 100 s, photolysis is thus not a competitive process. Furthermore, we investigate whether or not electronic excitation to the first excited state (D₁) by infrared radiation can facilitate various H-shift reactions, leading, for example, in the case of CH₃O O . to formation of O . H and CH₂O or HO O . and CH₂ products. While the activation barriers for H-shifts in the D₁ state may be lower than in the ground state (D₀), we find that H-shifts are unlikely to be competitive with decay back to the D₀ state through internal conversion, as this has a rate of the order of 10¹³ s^-1 for all studied systems.

SO2 photoexcitation mechanism links mass-independent sulfur isotopic fractionation in cryospheric sulfate to climate impacting volcanism

Natural climate variation, such as that caused by volcanoes, is the basis for identifying anthropogenic climate change. However, knowledge of the history of volcanic activity is inadequate, particularly concerning the explosivity of specific events. Some material is deposited in ice cores, but the concentration of glacial sulfate does not distinguish between tropospheric and stratospheric eruptions. Stable sulfur isotope abundances contain additional information, and recent studies show a correlation between volcanic plumes that reach the stratosphere and mass-independent anomalies in sulfur isotopes in glacial sulfate. We describe a mechanism, photoexcitation of SO2, that links the two, yielding a useful metric of the explosivity of historic volcanic events. A plume model of S(IV) to S(VI) conversion was constructed including photochemistry, entrainment of background air, and sulfate deposition. Isotopologue-specific photoexcitation rates were calculated based on the UV absorption cross-sections of (32)SO2, (33)SO2, (34)SO2, and (36)SO2 from 250 to 320 nm. The model shows that UV photoexcitation is enhanced with altitude, whereas mass-dependent oxidation, such as SO2 + OH, is suppressed by in situ plume chemistry, allowing the production and preservation of a mass-independent sulfur isotope anomaly in the sulfate product. The model accounts for the amplitude, phases, and time development of Δ(33)S/δ(34)S and Δ(36)S/Δ(33)S found in glacial samples. We are able to identify the process controlling mass-independent sulfur isotope anomalies in the modern atmosphere. This mechanism is the basis of identifying the magnitude of historic volcanic events.