A Lagrangian approach to studying Arctic polar stratospheric clouds using UARS MLS HNO3 and POAM II aerosol extinction measurements -: art. no. 4098

[1] We assess the viability of diagnosing polar stratospheric cloud (PSC) composition and denitrification using existing satellite measurements. A Lagrangian approach is used to track PSC evolution from formation through dissipation. Upper Atmosphere Research Satellite Microwave Limb Sounder observations of gas-phase HNO3 and Polar Ozone and Aerosol Measurement II observations of aerosol extinction from the Arctic late winter of 1995/1996 are correlated and compared to results from PSC composition models along air parcel trajectories. This approach is successful in capturing the broad patterns of PSC development. That is, a strong correlation is found between low-temperature, low-HNO3, and high-extinction points. In most cases the observed behavior along the Lagrangian paths falls within the range predicted by equilibrium liquid ternary aerosol and nitric acid trihydrate composition models. At no time along the trajectories (when there is data coverage) do the models display large changes in either HNO3 or aerosol extinction that are not reflected in the data. In general, however, there is a large degree of overlap in the comparisons between the models and the data. Unmeasured quantities and large uncertainties in both measurements and model calculations preclude conclusive determination of PSC composition or phase in most cases. In the majority of PSC events studied, gas-phase HNO3 fully recovers to pre-PSC abundances following cloud evaporation. We conclude that while severe denitrification (50% or greater) may have occurred in highly localized regions in the Arctic in 1996, it did not occur over spatial scales comparable to or larger than the MLS field of view (similar to400 km x 200 km x 6 km), even though it was a relatively cold winter. Improved measurements from upcoming satellite missions, such as Earth Observing System Aura, will ameliorate many of the difficulties in diagnosing PSC composition and denitrification encountered in this study.