Odin/OSIRIS observations of stratospheric NO₃ through sunrise and sunset

Abstract. The nitrate radical (NO₃) has been detected in visible limb-scattered spectra measured by the Optical Spectrograph and InfraRed Imager System (OSIRIS) on-board the Odin satellite when observing at large solar zenith angles (91–97°). Apparent slant column densities of NO₃ at tangent heights between 10 and 45 km are derived via spectral fitting in the 610–680 nm window. Using observations from multiple scans spanning solar zenith angles of 91–97°, the rapid evolution of NO₃ through sunrise and sunset can be traced. Slant column densities are found to be generally consistent with those simulated using a radiative transfer model with coupled photochemistry. In addition, a strong dependence of NO₃ with temperature is observed. These results indicate that our current knowledge of NO₃ photochemistry is generally consistent with OSIRIS observations to within the limitations of the radiative transfer modeling. Furthermore, they reveal that OSIRIS possesses signal-to-noise sufficient to make useful measurements of scattered sunlight out to solar zenith angles of 91–97° and suggest the possibility of retrieving profile information for NO₃ and other species at large solar zenith angles.