Homogeneous Photochemistry

New observations of OH and HO from the ER-2 aircraft during the Stratospheric Photochemistry, Aerosol, and Dynamics Experiment (SPADE) have recently been presented [ Cohen et al., 1994, Salawitch et al., 1994a,b, and Wennberg et al., 1994]. These results trace the response of HO to changing NO, O, and solar zenith angles in the lower stratosphere, and are particularly significant because HO is an important ozone-destroyer and because its reactions influence the abundances of nitrogen oxides and halogen oxides. The agreement between modeled and measured OH and HO is quite good (with some important exceptions at very high solar zenith angles). Thus, attempts to link heterogeneous hydrolysis of NO to low NO and high ClO mentioned above, which relied on modeled HO photochemistry, should be valid. As discussed by Salawitch et al. [1994a], the ER-2 measurements of OH and HO also reflect the need to include hydrolysis of NO on sulfate aerosols in models. The direct effect on OH is fairly small, but the increase in HO as a result of decreased NO is much stronger. As discussed by Wennberg et al. [1994] this effect also increases the importance of ozone destruction by HO through the HO + O reaction.

The picture from ground-based measurements of OH is perhaps more complicated, although these observations are more sensitive to OH in the upper stratosphere and the mesosphere. Comparisons between long-term measurements of OH column abundances obtained at Fritz Peak, CO and more recent ones at Lauder, New Zealand reported by Wood et al. [1994] expose interesting differences between the two hemispheres. In addition, the observed diurnal behavior is not reproduced well by models. These differences may point to deficiencies in our understanding of middle-to-upper stratospheric photochemical balance.

Observations from balloons, satellites, and aircraft continue to refine our understanding of the chemical mechanisms that partition inorganic chlorine, although the emerging picture is somewhat murky. Emission measurements of ClO and HCl from balloons by Stachnik et al. [1992] indicate that photochemical models underpredict the HCl/ClO ratio in the middle stratosphere, while aircraft observations in the lower stratosphere reported by Webster et al. [1993b,1994] indicate the opposite. The gaps between models and measurements are narrowed if additional HCl sources (in the former case) or sinks (in the latter case) are added to the models. These results are not necessarily contradictory if the missing source of HCl is photochemical and the missing sink for HCl is transport and/or heterogeneous chemistry, processes that are too slow to compete with fast gas-phase photochemistry in the middle and upper stratosphere. It is also possible that there is an error in one or more of the measurements. Limited intercomparisons of remote and in situ measurements of ClO at mid-latitudes [ Avallone et al., 1993a and Waters et al., 1995b] show good agreement. HCl intercomparisons are in preliminary stages and indicate that the aircraft results are systematically lower than remote observations [ Webster et al., 1994]. Additional new measurements and intercomparisons should shed light on this important issue.

Based on balloon observations, Toohey et al. [1993a] showed that increases in ClO during the early morning could be explained well by photolysis of ClONO over the altitude range 18 to 30 km. Measurements from the ER-2 [ Salawitch et al., 1994b, Stimpfle et al., 1994, and Wennberg et al., 1994] confirmed that the response of ClO to NO in daylight is consistent with ClONO as the primary source of ClO in the lower stratosphere [ Brune, 1991]. New simultaneous measurements of NO, NO, O, and the radiation field during SPADE [ Gao et al., 1994 and Jaeglé et al., 1994] confirmed that NO and NO are in steady-state in the lower stratosphere, and generally validate previous model calculations of NO abundances from simultaneous observations of NO and O. However, there remain important systematic differences between modeled and measured NO abundances that fall within the combined uncertainties of the measurements and models. If validated, these results could indicate a problem with kinetic parameterizations of NO/NO balance [ Jaeglé et al., 1994].