Abstract
Abstract. A new retrieval algorithm for OSIRIS (Optical Spectrograph and Infrared Imager System) nitrogen dioxide (NO2) profiles is described and validated. The algorithm relies on spectral fitting to obtain slant column densities of NO2, followed by inversion using an algebraic reconstruction technique and the SaskTran spherical radiative transfer model (RTM) to obtain vertical profiles of local number density. The validation covers different latitudes (tropical to polar), years (2002–2012), all seasons (winter, spring, summer, and autumn), different concentrations of nitrogen dioxide (from denoxified polar vortex to polar summer), a range of solar zenith angles (68.6–90.5°), and altitudes between 10.5 and 39 km, thereby covering the full retrieval range of a typical OSIRIS NO2 profile. The use of a larger spectral fitting window than used in previous retrievals reduces retrieval uncertainties and the scatter in the retrieved profiles due to noisy radiances. Improvements are also demonstrated through the validation in terms of bias reduction at 15–17 km relative to the OSIRIS operational v3.0 algorithm. The diurnal variation of NO2 along the line of sight is included in a fully spherical multiple scattering RTM for the first time. Using this forward model with built-in photochemistry, the scatter of the differences relative to the correlative balloon NO2 profile data is reduced.
Highlights
Nitrogen oxides, such as NO and NO2, are the reactive nitrogen-containing species in the middle atmosphere and are produced mainly from the breakdown of nitrous oxide in the stratosphere (Crutzen, 1971)
We provide a detailed description of the new retrieval algorithm whose heritage is the “fast” algorithm (Bourassa et al, 2011) as well as the algorithm developed in a series of papers (Sioris et al, 2003, 2004, 2007)
Spectral fitting refers to a multiple linear regression including the following basis functions: a fourthorder closure polynomial which is justified based on an adjusted R2 test, and temperature-dependent NO2 (Vandaele et al, 1998) and O3 (Serdyuchenkov et al, 2014) absolute absorption cross sections interpolated to the temperature (T ) of the tangent layer using the European Centre for MediumRange Weather Forecasting (ECMWF) analysis and convolved with a Gaussian to OSIRIS spectral resolution
Summary
Nitrogen oxides, such as NO and NO2, are the reactive nitrogen-containing species in the middle atmosphere and are produced mainly from the breakdown of nitrous oxide in the stratosphere (Crutzen, 1971). The partitioning between NO and NO2 depends on several factors such as the local ozone concentration and the photolysis frequency of NO2. NO2 increases steadily during daylight hours due to the UV photolysis of N2O5 (e.g. Wetzel et al, 2012). The photochemistry of NO2, which is rapid near the day–night terminator, leads to horizontal gradients within the field of view, for limb sounders such as OSIRIS (Optical Spectrograph and Infrared Imager System) (Llewellyn et al, 2004) on the Odin satellite or for solar occultation instruments operating in either the UV– visible or mid-infrared (e.g. Kerzenmacher et al, 2008). Besides OSIRIS, other space-borne limb scattering instruments
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