Other Laboratory Studies

There have been some interesting surprises from laboratory studies. Minton et al. [1992], in a molecular-beam study that represents the first detailed look at this process at the molecular level, found ClONO photolysis at 193 nm and 248 nm to be divided roughly equally between the Cl + NO and ClO + NO pathways. They also concluded that O atoms, which had been seen in previous bulk studies, were produced by secondary dissociation of energetic NO fragments. It had been the consensus based on previous studies that Cl and NO were the primary photolysis products. Although these differences have a relatively minor impact on our understanding of the photochemical partitioning of inorganic chlorine in the stratosphere (because Cl reacts rapidly with ozone to reform ClO there), if included in models this process might alter the predicted rate of chlorine recovery in highly perturbed polar regions. In addition, because the photolysis of NO into NO and O completes an ozone-depleting cycle, there will be a small impact on ozone loss estimates in regions where ClONO abundances are enhanced. Perhaps more importantly, such differences between molecular beam and bulk experiments question the depth of our understanding of important atmospheric photochemical processes. Similar experiments are beginning to address the role of ion-molecule chemistry and should improve our understanding of the mechanisms of stratospheric cloud formation [ Nelson and Okumura, 1992].

Burkholder et al. [1993] found differences in the nitric acid absorption cross section and its temperature dependence that impact modeled photolysis rates in the lower stratosphere where the 300--325 nm wavelength region is most important. At mid-latitudes the effect is a 10% to 20%\ decrease in modeled production of NO. To some extent, this will reduce the need to rely on the heterogeneous hydrolysis of NO to reproduce observed NO to NO ratios [ Fahey et al., 1993]. It will also lead to more rapid saturation of the NO hydrolysis reaction. Studies by Schiffman et al. [1993] and Turnipseed et al. [1992] have shown that OH is the primary photolysis product at longer wavelengths, but that other products such as HONO become more important short of 250 nm. Thus, the product yield may shift somewhat with altitude. However, the full implications of these studies have yet to be explored.

DeMore [1991] has shown that the relative rates of reaction of Cl with O and CH are well understood, agreeing to within 10% with current recommendations, even at very low temperatures. This result is important for constraining calculations of partitioning between HCl and ClONO throughout the atmosphere. Poulet et al. [1992] have found that the rate constant for the BrO + HO reaction at room temperature is a factor of six larger than previously believed. The implications of this result will be discussed later. Other revisions and additions are discussed in detail by DeMore et al. [1992].

The discovery by Friedl et al. [1992] of a new compound, OClONO (chloryl nitrate), has heightened concern about weakly bound radical recombination products (or adducts). These compounds, with bond-strengths in the 10 to 20 kcal mole range, exhibit behavior intermediate between the gas- and condensed-phase kinetics of two- and three-atom species and have largely been ignored because they are difficult to study and because the adducts are expected to be important only in cold, dimly sunlit regions of the atmosphere. Although formation of chloryl nitrate requires both OClO and NO, which are rarely present simultaneously in large abundances in the stratosphere, Friedl et al. [1992] have shown that it could still be present in non-negligible amounts. The investigation of similar compounds that could represent long-lived reservoirs for otherwise reactive radical species is warranted [ Sander et al., 1994].