The NASA SOLVE Arctic Ozone Campaign
People: Prof Ken Carslaw and Dr Rongming Hu
Funded by: NERC
Abstract
The NASA-coordinated measurement campaign SOLVE was designed to examine the processes controlling ozone levels at mid- to high-latitudes. Our contribution to this measurement campaign was twofold: Firstly, we were providing forecasts of mesoscale polar stratospheric clouds (PSCs) necessary to direct aircraft operations for optimum in situ and remote process studies. Secondly, we were analysing PSCs using a suite of microphysical, optical and chemical models in order to better understand their properties and formation mechanisms.
Ozone loss in the Arctic is the direct result of the conversion of inorganic chlorine reservoir species (such as ClONO2 and HCl) into photochemically active forms on the surface of PSCs and liquid aerosols. While our understanding of stratospheric polar processes is well advanced, there remain several significant uncertainties. For example, several studies have indicated that chemical processing on synoptic-scale PSCs and aerosols may be insufficient to explain observations of active chlorine and ozone loss in the Arctic. In addition, the factors that control the formation of nitric acid hydrate particles in the polar regions, which predispose the Arctic to denitrification and stronger ozone loss in a colder future stratosphere, are poorly described in models.
Our recent work has led us to believe that mesoscale cooling events induced by airflow over mountains might lead to an increase in active chlorine levels beyond that predicted by large scale models (Carslaw et al., 1998a).
As well as the potential importance of mesoscale PSCs in perturbing synoptic scale chemical and particle fields, such clouds also offer a unique natural laboratory for studying cloud microphysics and for quantitatively determining the rate of chlorine activation of air as it is processed through UT/LS clouds. The European APE-POLECAT mission (Airborne Polar Experiment-Polar Stratospheric Clouds, Leewaves, Chemistry and Transport) conducted from Rovaniemi in Finland during the winter 1996-97 demonstrated the feasibility of forecasting mesoscale PSCs and probing them in quasi-Lagrangian flights from their leading to their trailing edge, thus allowing the full PSC lifecycle to be observed. Such results cannot be obtained by studying non-stationary synoptic clouds.