Abstract
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.
Highlights
An important player in nighttime stratospheric photochemistry is the nitrate radical, NO3
It is the range of solar zenith angles (SZAs) along a LOS that effectively smoothes the variation of slant column densities (SCDs) with tangent height, even when the local terminator is crossed
The results indicate that our current knowledge of NO3 photochemistry is generally consistent with Optical Spectrograph and InfraRed Imager System (OSIRIS) observations to within the limitations of the radiative transfer modeling
Summary
An important player in nighttime stratospheric photochemistry is the nitrate radical, NO3. The key with all methods that measure Sunlight is to observe light that has traversed a range of SZAs in order to both have a large enough photon signal to overcome noise and to sample local night where NO3 is abundant. The amount by which the SZA varies along the LOS depends heavily on the particular geometry, but representative numbers are 3◦ for the sunrise period and 2◦ for the sunset period (McLinden et al, 2006).
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