Validation of Odin/OSIRIS stratospheric NO₂ profiles

Abstract

[1] This paper presents the validation study of stratospheric NO 2 profiles retrieved from Odin/OSIRIS measurements of limb-scattered sunlight (version 2.4). The Optical Spectrograph and Infrared Imager System (OSIRIS) NO 2 data set is compared to coincident solar occultation measurements by the Halogen Occultation Experiment (HALOE), Stratospheric Aerosol and Gas Experiment (SAGE) II, SAGE III, and Polar Ozone and Aerosol Measurement (POAM) III during the 2002-2004 period. Comparisons with seven Systeme d'Analyse par Observation Zenithal (SAOZ) balloon measurements are also presented. All comparisons show good agreement, with differences, both random and systematic, of less than 20% between 25 km and 35 km. Inconsistencies with SAGE III below 25 km are found to be caused primarily by diurnal effects from varying NO 2 concentrations along the SAGE III line-of-sight. On the basis of the differences, the OSIRIS random uncertainty is estimated to be 16% between 15 km and 25 km, 6% between 25 km and 35 km, and 9% between 35 km and 40 km. The estimated systematic uncertainty is about 22% between 15 and 25 km, 11-21% between 25 km and 35 km, and 11-31% between 35 km and 40 km. The uncertainties for AM (sunrise) profiles are generally largest and systematic deviations are found to be larger at equatorial latitudes. The results of this validation study show that the OSIRIS NO 2 profiles are well behaved, with reasonable uncertainty estimates between 15 km and 40 km. This unique NO 2 data set, with more than hemispheric coverage and high vertical resolution will be of particular interest for studies of nitrogen chemistry in the middle atmosphere, which is closely linked to ozone depletion.

Validation of ACE-FTS version 3.5 NO<sub><i>y</i></sub> species profiles using correlative satellite measurements

“…Similar approaches have been used before in other studies, e.g. Bracher et al (2005), , Wetzel et al (2007), Brohede et al (2007a), and Wolff et al (2008).…”

“…Version 3 of the data set (used in this study) was validated by Brohede et al (2007a), who found that OSIRIS NO 2 typically agrees with correlative satellite and balloon-borne data sets within 20 % between 25 and 35 km for all seasons and latitudes. Between 35 and 45 km, the agreement was within 30 %, with smaller absolute systematic differences for comparisons in the high latitudes than for those nearer the equator.…”

“…Below 25 km, an ACE-FTS NO 2 positive bias is expected with respect to instruments that do not use the solar occultation viewing geometry due to not accounting for diurnal variations in NO 2 along the line of sight in the forward model. In solar occultation viewing geometry, not accounting for this diurnal effect is expected to lead to a ∼ 10-40 % positive bias (Brohede et al, 2007a). It can be seen from Fig.…”

“…In the simulation of the diurnal cycle for an ACE-FTS profile, the model is constrained using the corresponding ACE-FTS temperature, pressure, and O 3 profiles. The model takes into account altitude, latitude, and day of year, using NO y and N 2 O climatologies from a 3-D chemical transport model (Olsen et al, 2001), Cl y and Br y climatologies (as described by Brohede et al, 2007a), and climatological SAGE II background aerosol data. All photochemical reaction rates were obtained from Sander et al (2003).…”

“…Below 25 km, the relative differences get more negative with decreasing altitude -more negative than −100 % below 21 km. An ACE-FTS NO low bias with respect to non-solar occultation instruments, on the order of ∼ 10-40 %, is expected in this region due to not accounting for diurnal variations of NO along the line of sight (Brohede et al, 2007a). Figure 5a shows NO comparison results for data separated by local time using all available MIPAS IMK-IAA data.…”