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Sodium airglow is generated when excited sodium atoms emit electromagnetic radiation while they are relaxing from an excited state into a lower energetic state. This electromagnetic radiation, the two sodium D lines at 589.0 and 589.6 nm, can usually be detected from space or from ground. Sodium nightglow occurs at times when the sun is not present and excitation of sodium atoms is a result of chemical reaction with ozone. The detection of sodium nightglow can be a means to determine the amount of sodium in the earth's mesosphere and lower thermosphere (MLT). In this study, we present time series of monthly mean sodium concentration profiles, by utilizing the large spatial and temporal coverage of satellite sodium D-line nightglow measurements. We use the OSIRIS/Odin mesospheric limb measurements to derive sodium concentration profiles and vertical column densities and compare those to measurements from SCIAMACHY/Envisat and GOMOS/Envisat. Here we show that the Na D-line limb emission rate (LER) and volume emission rate (VER) profiles calculated from the OSIRIS and SCIAMACHY measurements, although the OSIRIS LER and VER profiles are around 25 % lower, agree very well in shape and overall seasonal variation. The sodium concentration profiles also agree in shape and magnitude, although those do not show the clear semi-annual cycle which is present in the LER and VER profiles. The comparison to the GOMOS sodium vertical column densities (VCDs) shows that the OSIRIS VCDs are of the same order of magnitude although again the semi-annual cycle is not as clear. We attribute the differences in the LER, VER and sodium profiles to the differences in spatial coverage between the OSIRIS and SCIAMACHY measurements, the lower signal-to-noise ratio (SNR) of the SCIAMACHY measurements and differences in local time between the measurements of the two satellites.

The Spatial Heterodyne Observations of Water instrument (SHOW) is a limb-sounding satellite prototype that utilizes the Spatial Heterodyne Spectroscopy (SHS) technique, operating in a limb-viewing configuration, to observe limb-scattered sunlight in a vibrational band of water vapour within a spectral window from 1363 to 1366 nm. The goal is to retrieve high vertical and horizontal resolution measurements of water vapour in the upper troposphere and lower stratosphere. The prototype instrument has been configured for observations from NASA's ER-2 high-altitude airborne remote science airplane. Flying at a maximum altitude of ∼21.34 km with a maximum speed of ∼760 km h−1, the ER-2 provides a stable platform to simulate observations from a low-earth orbit satellite. Demonstration flights were performed from the ER-2 during an observation campaign from 15 to 22 July 2017. In this paper, we present the laboratory characterization work and the level 0 to level 1 processing of flight data that were obtained during an engineering flight performed on 18 July 2017. Water vapour profile retrievals are presented and compared to in situ radiosonde measurements made of the same approximate column of air. These measurements are used to validate the SHOW measurement concept and examine the sensitivity of the technique.

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.

The understanding of the excitation mechanism of the sodium D-line nightglow emission is important for many investigations of atomic sodium in the upper mesosphere region. In 1939, Sidney Chapman proposed a first scheme of the chemical reactions involving sodium and ozone to explain the yellow sodium line doublet in the terrestrial nightglow. Later, it became clear that this mechanism could not explain the observed variability in the Na D-line ratio. Hence, Slanger et al. (2005) suggested a modified Chapman mechanism. However, it is well accepted that there is still a parameter that is not well quantified, i.e., the branching ratio f A that defines how many sodium atoms are produced (via the reaction Na + O3) in the excited Na( P 2 J ) state. Here, we retrieve sodium profiles in the mesosphere region from limb measurements carried out with the OSIRIS instrument on the Odin satellite between 2002 and until recently. These profiles are compared to profiles obtained from ground-based Lidar measurements. Until 2010, the Lidar was located in Fort Collins, Colorado at 40.5° N and 105.0° W and was then moved to Logan, Utah at 41.8° N and 111.2° W. In this study, we found 20 nights with measurements suitable for comparison. By adjusting the branching ratio in the retrieval algorithm until the Lidar Na-profile and the satellite Na-profile agree best and combining the three different approaches for the comparison we propose a value of the branching ratio of: 0.064 ± 0.028 .

A variant of the limb–nadir matching technique for deriving tropospheric NO2 columns is presented in which the stratospheric component of the NO2 slant column density (SCD) measured by the Ozone Monitoring Instrument (OMI) is removed using non-coincident profiles from the Optical Spectrograph and InfraRed Imaging System (OSIRIS). In order to correct their mismatch in local time and the diurnal variation of stratospheric NO2, OSIRIS profiles, which were measured just after sunrise, were mapped to the local time of OMI observations using a photochemical box model. Following the profile time adjustment, OSIRIS NO2 stratospheric vertical column densities (VCDs) were calculated. For profiles that did not reach down to the tropopause, VCDs were adjusted using the photochemical model. Using air mass factors from the OMI Standard Product (SP), a new tropospheric NO2 VCD product – referred to as OMI-minus-OSIRIS (OmO) – was generated through limb–nadir matching. To accomplish this, the OMI total SCDs were scaled using correction factors derived from the next-generation SCDs that improve upon the spectral fitting used for the current operational products. One year, 2008, of OmO was generated for 60° S to 60° N and a cursory evaluation was performed. The OmO product was found to capture the main features of tropospheric NO2, including a background value of about 0.3 × 1015 molecules cm−2 over the tropical Pacific and values comparable to the OMI operational products over anthropogenic source areas. While additional study is required, these results suggest that a limb–nadir matching approach is feasible for the removal of stratospheric NO2 measured by a polar orbiter from a nadir-viewing instrument in a geostationary orbit such as Tropospheric Emissions: Monitoring of Pollution (TEMPO) or Sentinel-4.

The Aerosol Limb Imager (ALI) is an optical remote sensing instrument designed to image scattered sunlight from the atmospheric limb. These measurements are used to retrieve spatially resolved information of the stratospheric aerosol distribution, including spectral extinction coefficient and particle size. Here we present the design, development and test results of an ALI prototype instrument. The long-term goal of this work is the eventual realization of ALI on a satellite platform in low earth orbit, where it can provide high spatial resolution observations, both in the vertical and cross-track. The instrument design uses a large-aperture acousto-optic tunable filter (AOTF) to image the sunlit stratospheric limb in a selectable narrow wavelength band ranging from the visible to the near infrared. The ALI prototype was tested on a stratospheric balloon flight from the Canadian Space Agency (CSA) launch facility in Timmins, Canada, in September 2014. Preliminary analysis of the hyperspectral images indicates that the radiance measurements are of high quality, and we have used these to retrieve vertical profiles of stratospheric aerosol extinction coefficient from 650 to 1000 nm, along with one moment of the particle size distribution. Those preliminary results are promising and development of a satellite prototype of ALI within the Canadian Space Agency is ongoing.

An anomalous red glow due to scattered sunlight was observed at Longyearbyen (78° N, 15° E) on 6 December 2002 from 07:30 UT to 13:30 UT when the solar zenith angle varied between 100.7° and 104°. A model for this red sky event using sunlight scattered in a two stage process by Polar Stratospheric Clouds (PSC) at 25km is presented and demonstrated to be feasible. The model requires a significant fraction of the polar vortex, which is cold enough for the formation of ice PSC, to be occupied with PSC with an integrated vertical extinction of approximately 0.037 at 845nm. Given these conditions, the model is able to predict, within an order of magnitude, the spatial distribution of intensities measured by meridional scanning photometers located at Longyearbyen across the visible and near infra-red spectrum. Keywords. Aerosols and particles; Transmission and scattering of radiation; Polar Meteorology

The Optical Spectrograph and InfraRed Imaging System (OSIRIS) instrument on board the Odin spacecraft has been measuring limb-scattered radiance since 2001. The vertical radiance profiles measured as the instrument nods are inverted, with the aid of the SASKTRAN radiative transfer model, to obtain vertical profiles of trace atmospheric constituents. Here we describe two newly developed modes of the SASKTRAN radiative transfer model: a high-spatial-resolution mode and a Monte Carlo mode. The high-spatial-resolution mode is a successive-orders model capable of modelling the multiply scattered radiance when the atmosphere is not spherically symmetric; the Monte Carlo mode is intended for use as a highly accurate reference model. It is shown that the two models agree in a wide variety of solar conditions to within 0.2 %. As an example case for both models, Odin–OSIRIS scans were simulated with the Monte Carlo model and retrieved using the high-resolution model. A systematic bias of up to 4 % in retrieved ozone number density between scans where the instrument is scanning up or scanning down was identified. The bias is largest when the sun is near the horizon and the solar scattering angle is far from 90°. It was found that calculating the multiply scattered diffuse field at five discrete solar zenith angles is sufficient to eliminate the bias for typical Odin–OSIRIS geometries.

Limb-scanning satellites can provide global information about the vertical structure of Polar Mesospheric Clouds. However, information about horizontal structures usually remains limited. In eighteen days during the northern hemisphere summers of 2010 and 2011, the Odin satellite was operated in a special mesospheric mode with short limb scans limited to the altitude range of Polar Mesospheric Clouds. For Odin’s Optical Spectrograph and InfraRed Imager System (OSIRIS) this provides multiple views through a given cloud volume, which forms a basis for tomographic analyses of the vertical/horizontal cloud structures. Here we present an algorithm for a tomographic analysis of mesospheric clouds based on maximum probability techniques. We also present the first simultaneously retrieved vertical and horizontal Polar Mesospheric Cloud structures. The findings show that the tomographic algorithm is able to locate detailed structures such as tilts, stratifications, or holes that cannot be analyzed by other limb, nadir, or ground-based measurements. We find a mean peak altitude of the clouds to be 83.6 km. We identify horizontal patches down to sizes of 300 km, which corresponds to a horizontal resolution that is limited by the available number of limb scans.

The limb-scatter satellite viewing geometry is well suited to detecting low-concentration aerosols in the upper troposphere due to its long observation path length (~ 50–100 km), high vertical resolution (~ 1–2 km) and good geographic coverage. We use the fully three-dimensional radiative transfer code SASKTRAN to simulate the sensitivity of limb-scatter viewing Odin/OSIRIS satellite measurements to absorbing mineral dust and carbonaceous aerosols (smoke and pure soot), as well as to non-absorbing sulfate aerosols and ice in the upper troposphere. At long wavelengths (813 nm) the addition of all aerosols (except soot) to an air only atmosphere produced a radiance increase as compared to air only, on account of the low Rayleigh scattering in air only at 813 nm. The radiance reduction due to soot aerosol was negligible (< 0.1%) at all heights (0–100 km). At short wavelengths (337, 377, 452 nm), we found that the addition of any aerosol species to an air only atmosphere caused a decrease in single-scattered radiation due to an extinction of Rayleigh scattering in the direction of OSIRIS. The reduction was clearly related to particle size first, with absorption responsible for second-order effects only. Multiple-scattered radiation could either increase or decrease in the presence of an aerosol species, depending both on particle size and absorption. Large scatterers (ice, mineral dust) all increased multiple-scattered radiation within, below and above the aerosol layer. Small, highly absorbing pure soot particles produced a negligible multiple-scattering response (< 0.1%) at all heights, primarily confined to within and below the soot layer. Medium-sized scatterers produced a multiple-scattering response that depended on their absorbing properties. Increased radiances were simulated as compared to air only at all short wavelengths (337, 377 and 452 nm) for sulfate aerosol particles (non-absorbing) while decreased radiances were simulated for smoke particles (absorbing) at 337 and 377 nm, where multiple scattering involving the medium-sized carbonaceous particles amplified their absorbing properties. At 452 nm, however, this effect was attenuated and albedo-dependent. At short wavelengths, the combined effect of single scattering decreases and multiple scattering increases led to complex total radiance signatures that generally could not unambiguously distinguish absorbing versus non-absorbing aerosols. Smoke aerosols led to a total radiance decrease (as compared to air only) at all altitudes above the aerosol layer (15–100 km). This unique signature was a result of the aerosols' strong absorbing properties, non-negligible scattering efficiency, and number concentrations high enough to make multiple scattering effects due to the aerosol itself significant. Thus, in the limb-scatter viewing geometry scene darkening above the aerosol layer is unambiguously due to absorption whereas scene darkening within and below the aerosol layer can simply be the result of a reduction in single-scattered radiance. Our simulations show a greater scene darkening for decreasing wavelengths, increasing surface albedo, decreasing solar zenith angle, and increasing particle number concentration, however, at 337 nm this effect did not exceed 0.2% of the total radiance due to air only, making the unique identification of medium-sized carbonaceous aerosols, i.e. smoke, difficult. Scene darkening (or brightening) varies linearly with particle number concentration over three orders of magnitude. A fortuitous, unexpected implication of our analysis is that limb-scatter retrievals of aerosol extinction are insensitive to external information about aerosol absorption.

We observed ozone (O3) in the vertical region between 250 and 0.0005 hPa (~ 12–96 km) using the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) on the Japanese Experiment Module (JEM) of the International Space Station (ISS) between 12 October 2009 and 21 April 2010. The new 4 K superconducting heterodyne receiver technology of SMILES allowed us to obtain a one order of magnitude better signal-to-noise ratio for the O3 line observation compared to past spaceborne microwave instruments. The non-sun-synchronous orbit of the ISS allowed us to observe O3 at various local times. We assessed the quality of the vertical profiles of O3 in the 100–0.001 hPa (~ 16–90 km) region for the SMILES NICT Level 2 product version 2.1.5. The evaluation is based on four components: error analysis; internal comparisons of observations targeting three different instrumental setups for the same O3 625.371 GHz transition; internal comparisons of two different retrieval algorithms; and external comparisons for various local times with ozonesonde, satellite and balloon observations (ENVISAT/MIPAS, SCISAT/ACE-FTS, Odin/OSIRIS, Odin/SMR, Aura/MLS, TELIS). SMILES O3 data have an estimated absolute accuracy of better than 0.3 ppmv (3%) with a vertical resolution of 3–4 km over the 60 to 8 hPa range. The random error for a single measurement is better than the estimated systematic error, being less than 1, 2, and 7%, in the 40–1, 80–0.1, and 100–0.004 hPa pressure regions, respectively. SMILES O3 abundance was 10–20% lower than all other satellite measurements at 8–0.1 hPa due to an error arising from uncertainties of the tangent point information and the gain calibration for the intensity of the spectrum. SMILES O3 from observation frequency Band-B had better accuracy than that from Band-A. A two month period is required to accumulate measurements covering 24 h in local time of O3 profile. However such a dataset can also contain variation due to dynamical, seasonal, and latitudinal effects.

The Optical Spectrograph and InfraRed Imaging System (OSIRIS) on board the Odin spacecraft has been taking limb-scattered measurements of ozone number density profiles from 2001–present. The Stratospheric Aerosol and Gas Experiment II (SAGE II) took solar occultation measurements of ozone number densities from 1984–2005 and has been used in many studies of long-term ozone trends. We present the characterization of OSIRIS SaskMART v5.0× against the new SAGE II v7.00 ozone profiles for 2001–2005, the period over which these two missions had overlap. This information can be used to merge OSIRIS with SAGE II into a single ozone record from 1984 to the present, if other satellite ozone measurements are included to account for gaps in the OSIRIS dataset in the winter hemisphere. Coincident measurement pairs were selected for ±1 h, ±1° latitude, and ±500 km. The absolute value of the resulting mean relative difference profile is 0.9 were calculated for 13.5–49.5 km, demonstrating excellent overall agreement between the two datasets. Coincidence criteria were relaxed to maximize the number of measurement pairs and the conditions under which measurements were taken. With the broad coincidence criteria, good agreement (< 5%) was observed under most conditions for 20.5–40.5 km. However, mean relative differences do exceed 5% for several cases. Above 50 km, differences between OSIRIS and SAGE II are partly attributed to the diurnal variation of ozone. OSIRIS data are biased high compared with SAGE II at 22.5 km, particularly at high latitudes. Dynamical coincidence criteria, using derived meteorological products, were also tested and yielded similar overall results, with slight improvements to the correlation at high latitudes. The OSIRIS optics temperature is low (

We present a technique for estimating the optical thickness of subvisual cirrus clouds detected by OSIRIS (Optical Spectrograph and Infrared Imaging System), a limb-viewing satellite instrument that measures scattered radiances from the UV to the near-IR. The measurement set is composed of a ratio of limb radiance profiles at two wavelengths that indicates the presence of cloud-scattering regions. Cross-sections and phase functions from an in situ database are used to simulate scattering by cloud-particles. With appropriate configurations discussed in this paper, the SASKTRAN successive-orders of scatter radiative transfer model is able to simulate accurately the in-cloud radiances from OSIRIS. Configured in this way, the model is used with a multiplicative algebraic reconstruction technique (MART) to retrieve the cloud extinction profile for an assumed effective cloud particle size. The sensitivity of these retrievals to key auxiliary model parameters is shown, and it is shown that the retrieved extinction profile, for an assumed effective cloud particle size, models well the measured in-cloud radiances from OSIRIS. The greatest sensitivity of the retrieved optical thickness is to the effective cloud particle size. Since OSIRIS has an 11-yr record of subvisual cirrus cloud detections, the work described in this manuscript provides a very useful method for providing a long-term global record of the properties of these clouds.

On 20 February 2001, a converted Russian ICBM delivered Odin, a small Swedish satellite, into low Earth orbit. One of the sensors onboard is a small Canadian spectrometer called OSIRIS. By measuring scattered sunlight from Earth's horizon, or limb, OSIRIS is able to deduce the abundance of trace gases and particles from the upper troposphere into the lower thermosphere. Designed and built on a modest budget, OSIRIS has exceeded not only its 2-yr lifetime but also all expectations. With more than a decade of continuous data, OSIRIS has recorded over 1.8 million limb scans. The complexities associated with unraveling scattered light in order to convert OSIRIS spectra into highquality geophysical profiles have forced the OSIRIS team to develop leading-edge algorithms and computer models. These profiles are being used to help address many science questions, including the coupling of atmospheric regions (e.g., stratosphere–troposphere exchange) and the budgets and trends in ozone, nitrogen, bromine, and other species. One specific example is the distribution and abundance of upper-tropospheric, lightning-produced reactive nitrogen and ozone. Arguably OSIRIS's most important contributions come from its aerosol measurements, including detection and characterization of subvisual cirrus and polar stratospheric and mesospheric clouds. OSIRIS also provides a unique view of the stratospheric aerosol layer, and it is able to identify and track perturbations from volcanic activity and biomass burning.

This paper presents extensive validation analyses of ozone observations from the Atmospheric Chemistry Experiment (ACE) satellite instruments: the ACE Fourier Transform Spectrometer (ACE-FTS) and the Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation (ACE-MAESTRO) instrument. The ACE satellite instruments operate in the mid-infrared and ultraviolet-visible-near-infrared spectral regions using the solar occultation technique. In order to continue the long-standing record of solar occultation measurements from space, a detailed quality assessment is required to evaluate the ACE data and validate their use for scientific purposes. Here we compare the latest ozone data products from ACE-FTS and ACE-MAESTRO with coincident observations from satellite-borne, airborne, balloon-borne and ground-based instruments, by analysing volume mixing ratio profiles and partial column densities. The ACE-FTS version 2.2 Ozone Update product reports more ozone than most correlative measurements from the upper troposphere to the lower mesosphere. At altitude levels from 16 to 44 km, the mean differences range generally between 0 and +10% with a slight but systematic positive bias (typically +5%). At higher altitudes (45-60 km), the ACE-FTS ozone amounts are significantly larger than those of the comparison instruments by up to ~40% (typically +20%). For the ACE-MAESTRO version 1.2 ozone data product, agreement within ±10% (generally better than ±5%) is found between 18 and 40 km for the sunrise and sunset measurements. At higher altitudes (45-55 km), systematic biases of opposite sign are found between the ACE-MAESTRO sunrise and sunset observations. While ozone amounts derived from the ACE-MAESTRO sunrise occultation data are often smaller than the coincident observations (by as much as -10%), the sunset occultation profiles for ACE-MAESTRO show results that are qualitatively similar to ACE-FTS and indicate a large positive bias (+10 to +30%) in this altitude range. In contrast, there is no significant difference in bias found for the ACE-FTS sunrise and sunset measurements. These systematic effects in the ozone profiles retrieved from the measurements of ACE-FTS and ACE-MAESTRO are being investigated. This work shows that the ACE instruments provide reliable, high quality measurements from the tropopause to the upper stratosphere and can be used with confidence in this vertical domain

The purpose of this study is to perform a global search of the upper troposphere (z≥10 km) for enhancements of nitrogen dioxide and determine their sources. This is the first application of satellite-based limb scattering to study upper tropospheric NO2. We have searched two years (May 2003–May 2005) of OSIRIS (Optical Spectrograph and Infrared Imager System) operational NO2 concentrations (version 2.3/2.4) to find large enhancements in the observations by comparing with photochemical box model calculations and by identifying local maxima in NO2 volume mixing ratio. We find that lightning is the main production mechanism responsible for the large enhancements in OSIRIS NO2 observations as expected. Similar patterns in the abundances and spatial distribution of the NO2 enhancements are obtained by perturbing the lightning within the GEOS-Chem 3-dimensional chemical transport model. In most cases, the presence of lightning is confirmed with coincident imagery from LIS (Lightning Imaging Sensor) and the spatial extent of the NO2 enhancement is mapped using nadir observations of tropospheric NO2 at high spatial resolution from SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography) and OMI (Ozone Monitoring Instrument). The combination of the lightning and chemical sensors allows us to investigate globally the role of lightning to the abundance of NO2 in the upper troposphere (UT). Lightning contributes 60% of the tropical upper tropospheric NO2 in GEOS-Chem simulations. The spatial and temporal distribution of NO2 enhancements from lightning (May 2003–May 2005) is investigated. The enhancements generally occur at 12 to 13 km more frequently than at 10 to 11 km. This is consistent with the notion that most of the NO2 is forming and persisting near the cloud top altitude in the tropical upper troposphere. The latitudinal distribution is mostly as expected. In general, the thunderstorms exhibiting weaker vertical development (e.g. 11≤z≤13 km) extend latitudinally as far poleward as 45° but the thunderstorms with stronger vertical development (z≥14 km) tend to be located within 33° of the equator. There is also the expected hemispheric asymmetry in the frequency of the NO2 enhancements, as most were observed in the northern hemisphere for the period analyzed.

Two years of Odin/OSIRIS measurements of the limb-scattered solar radiance are analyzed with respect to polar mesospheric clouds (PMCs) in both the southern hemisphere (SH) and the northern hemisphere (NH). Spatial and temporal distributions, tangent altitudes (TAs) and the brightness of PMCs during two SH summers of 2001/2002 and 2002/2003, and two NH summers of 2002 and 2003 are presented. The Odin/OSIRIS data clearly indicate inter-annual and inter-hemispheric differences in both PMC altitudes and brightness. Most of the SH clouds were detected between 70°S and 82°S, the lowest PMC latitude was 61.4°S in 2001/2002 and 54.7°S in 2002/2003, and only eight clouds were observed equatorward of 70°S in 2001/2002 compared to 33 clouds in 2002/2003. In 2002, NH clouds were observed later (by about 9 days), were in general less frequent and their maximum detection frequency was registered 2 weeks later than in 2003. The number of PMCs observed equatorward of 60°N in 2003 was almost twice as high as in 2002. The inter-annual variability of PMC properties in the NH was generally lower than in the SH. The PMC detection frequency in the SH was about 40% lower than in the NH. The inter-hemispheric difference in the mean PMC TA was between 1.1 and 2.3 km with NH clouds lower than SH clouds. The inter-hemispheric difference in the mean PMC brightness was about ∼35–40% with the NH clouds being brighter.

The Optical Spectrograph and Infrared Imager System (OSIRIS) instrument onboard the Odin satellite detects polar mesospheric clouds (PMCs) using limb scattered sunlight and is able to routinely measure the relative brightness and altitude of each detected cloud. OSIRIS has created a significant database of PMCs detections and cloud parameters collected during two intensive campaigns in the northern hemisphere summer months of 2002 and 2003 (days 9–24 from solstice). These observations have revealed a definite anti-correlation between the cloud's relative brightness and altitude. We have measured that, on average, the PMCs descend by 22.5 m for every unit increase in relative brightness. No systematic difference has been observed between the tangent altitudes of the morning (∼07:30 local time) and afternoon (∼17:30 local time) PMCs but the morning clouds were found to be at least 30% brighter than those detected in the afternoon. We have also found a strong 10–14 day oscillation in the global PMC relative brightness in both 2002 and 2003 that is possibly related to a dynamical activity within the mesosphere.

The OSIRIS instrument onboard the Odin spacecraft has recently made observations of the Oxygen InfraRed Atmospheric band and OH Meinel band volume emission rate profiles in the mesosphere. Features similar to the tertiary ozone peak reported earlier by other investigators are clearly present in the observed emission profiles. This peak, caused by a photochemical imbalance between the destruction of odd-oxygen by odd-hydrogen and the production of odd-oxygen, was previously believed to be confined to the winter hemisphere just equatorward of the polar terminator. However, northern hemisphere measurements made by OSIRIS in the spring of 2002 indicate that the tertiary peak is also present in the afternoon and early evening throughout the springtime, with the altitude of the peak rising steadily from around 70 km in the polar regions near local noon to about 80 km near the equator at sunset. These new observations of the extended tertiary peak are presented and it is shown that they are directly related to the previously observed tertiary ozone peak and are consistent with a simple one-dimensional time-dependent photochemical model.

In spring 2011, the Arctic polar vortex was stronger than in any other year on record. As the polar vortex started to break up in April, ozone and NO2 columns were measured with UV-visible spectrometers above the Polar Environment Atmospheric Research Laboratory (PEARL) in Eureka, Canada (80.05° N, 86.42° W) using the differential optical absorption spectroscopy (DOAS) technique. These ground-based column measurements were complemented by Ozone Monitoring Instrument (OMI) and Optical Spectrograph and Infra-Red Imager System (OSIRIS) satellite measurements, Global Modeling Initiative (GMI) simulations, and meteorological quantities. On 8 April 2011, NO2 columns above PEARL from the DOAS, OMI, and GMI datasets were approximately twice as large as in previous years. On this day, temperatures and ozone volume mixing ratios above Eureka were high, suggesting enhanced chemical production of NO2 from NO. Additionally, GMI NOx (NO + NO2) and N2O fields suggest that downward transport along the vortex edge and horizontal transport from lower latitudes also contributed to the enhanced NO2. The anticyclone that transported lower-latitude NOx above PEARL became frozen-in and persisted in dynamical and GMI N2O fields until the end of the measurement period on 31 May 2011. Ozone isolated within this frozen-in anticyclone (FrIAC) in the middle stratosphere was lost due to reactions with the enhanced NOx. Below the FrIAC (from the tropopause to 700 K), NOx driven ozone loss above Eureka was larger than in previous years, according to GMI monthly average ozone loss rates. Using the passive tracer technique, with passive ozone profiles from the Lagrangian Chemistry and Transport Model, ATLAS, ozone losses since 1 December 2010 were calculated at 600 K. In the air mass that was above Eureka on 20 May 2011, ozone losses reached 4.2 parts per million by volume (ppmv) (58%) and 4.4 ppmv (61%), when calculated using GMI and OSIRIS ozone profiles, respectively. This gas-phase ozone loss led to a more rapid decrease in ozone column amounts above Eureka in April/May 2011 compared with previous years. Ground-based, OMI, and GMI ozone total columns all decreased by more than 100 DU from 15 April to 20 May. Two lows in the ozone columns were also investigated and were attributed to a vortex remnant passing above Eureka at ~500 K on 12/13 May and an ozone mini-hole on 22/23 May.

The scattered sunlight measurements made by the Optical Spectrograph and InfraRed Imaging System (OSIRIS) on the Odin spacecraft are used to retrieve vertical profiles of stratospheric aerosol extinction at 750 nm. The recently released OSIRIS Version 5 data product contains the first publicly released stratospheric aerosol extinction retrievals, and these are now available for the entire Odin mission, which extends from the present day back to launch in 2001. A proof-of-concept study for the retrieval of stratospheric aerosol extinction from limb scatter measurements was previously published and the Version 5 data product retrievals are based on this work, but incorporate several important improvements to the algorithm. One of the primary changes is the use of a new retrieval vector that greatly improves the sensitivity to aerosol scattering by incorporating a forward modeled calculation of the radiance from a Rayleigh atmosphere. Additional improvements include a coupled retrieval of the effective albedo, a new method for normalization of the retrieval vector to improve signal-to-noise, and the use of an initial guess that is representative of very low background aerosol loading conditions, which allows for maximal retrieval range. Furthermore, the Version 5 data set is compared to Stratospheric Aerosol and Gas Experiment (SAGE) III 755 nm extinction profiles during the almost four years of mission overlap from 2002 to late 2005. The vertical structure in coincident profile measurements is well correlated and the statistics on a relatively large set of tight coincident measurements show agreement between the measurements from the two instruments to within approximately 10% throughout the 15 to 25 km altitude range, which covers the bulk of the stratospheric aerosol layer for the mid and high latitude cases studied here.

The GOMOS (Global Ozone Monitoring by Occultation of Stars) instrument on board the Envisat satellite measures the vertical composition of the atmosphere using the stellar occultation technique. While the night-time occultations of GOMOS have been proven to be of good quality, the daytime occultations are more challenging due to weaker signal-to-noise ratio. During daytime GOMOS measures limb scattered solar radiation in addition to stellar radiation. In this paper we introduce a retrieval method that determines ozone profiles between 20–60 km from GOMOS limb scattered solar radiances. GOMOS observations contain a considerable amount of stray light at high altitudes. We introduce a method for removing stray light and demonstrate its feasibility by comparing the corrected radiances against those measured by the OSIRIS (Optical Spectrograph & Infra Red Imaging System) instrument. For the retrieval of ozone profiles, a standard onion peeling method is used. The first comparisons with other data sets suggest that the retrieved ozone profiles in 22–50 km are within 10% compared with the GOMOS night-time occultations and within 15% compared with OSIRIS. GOMOS has measured about 350 000 daytime profiles since 2002. The retrieval method presented here makes this large amount of data available for scientific use.

The H Balmer emissions were first identified in terrestrial aurora by Vegard (1939). The earliest photographic spectral observations are reviewed. In the subsequent decade, the intensity ratios for H alpha, H beta, and H gamma were measured, and the well-known line broadening and blue shift were established. Recently, the H alpha, H gamma, H delta, and H epsilon features have been measured by OSIRIS on Odin. The Balmer components are resolved from other auroral features using sets of synthetic spectra. The measured intensity ratios are in good agreement with an extensive set of published model calculations. The presented observations are in the polar region averaged over limb tangent altitudes from 100 to 105 km, approximately perpendicular to the terrestrial magnetic field lines, for this geometry showing Doppler broadening without obvious Doppler shifts. The OSIRIS-measured full-width at half-height of the Ha feature is 2.2 nm corresponding to an H atom velocity of 500 km s(-1) and energy approximately 1.3 keV.

International audience; A 7+ year (2001–2008) data set of stratospheric BrO profiles measured by the Optical Spectrograph and Infra-Red Imager System (OSIRIS) instrument, a UV-visible spectrometer measuring limb-scattered sunlight from the Odin satellite, is presented. Zonal mean radiance spectra are computed for each day and inverted to yield effective daily zonal mean BrO profiles from 16 to 36 km. A detailed description of the retrieval methodology and error analysis is presented. Single-profile precision and effective resolution are found to be about 30% and 3–5 km, respectively, throughout much of the retrieval range. Individual profile and monthly mean comparisons with ground-based, balloon, and satellite instruments are found to agree to about 30%. A BrO climatology is presented, and its morphology and correlation with NO2 is consistent with our current understanding of bromine chemistry. Monthly mean Bry maps are derived. Two methods of calculating total Bry in the stratosphere are used and suggest (21.0 ± 5.0) pptv with a contribution from very short lived substances of (5.0 ± 5.0) pptv, consistent with other recent estimates.

The Odin satellite carries two instruments capable of determining stratospheric ozone profiles by limb sounding: the Sub-Millimetre Radiometer (SMR) and the UV-visible spectrograph of the OSIRIS (Optical Spectrograph and InfraRed Imager System) instrument. A large number of ozone profiles measurements were performed during six years from November 2001 to present. This ozone dataset is here used to make quantitative comparisons with satellite measurements in order to assess the quality of the Odin/SMR ozone measurements. In a first step, we compare Swedish SMR retrievals version 2.1, French SMR ozone retrievals version 222 (both from the 501.8 GHz band), and the OSIRIS retrievals version 3.0, with the operational version 4.0 ozone product from POAM III (Polar Ozone Atmospheric Measurement). In a second step, we refine the Odin/SMR validation by comparisons with ground-based instruments and balloon-borne observations. We use observations carried out within the framework of the Network for Detection of Atmospheric Composition Change (NDACC) and balloon flight missions conducted by the Canadian Space Agency (CSA), the Laboratoire de Physique et de Chimie de l'{}Environnement (LPCE, Orléans, France), and the Service d'Aéronomie (SA, Paris, France). Coincidence criteria were 5° in latitude×10° in longitude, and 5 h in time in Odin/POAM III comparisons, 12 h in Odin/NDACC comparisons, and 72 h in Odin/balloons comparisons. An agreement is found with the POAM III experiment (10–60 km) within −0.3±0.2 ppmv (bias±standard deviation) for SMR (v222, v2.1) and within −0.5±0.2 ppmv for OSIRIS (v3.0). Odin ozone mixing ratio products are systematically slightly lower than the POAM III data and show an ozone maximum lower by 1–5 km in altitude. The comparisons with the NDACC data (10–34 km for ozonesonde, 10–50 km for lidar, 10–60 for microwave instruments) yield a good agreement within −0.15±0.3 ppmv for the SMR data and −0.3±0.3 ppmv for the OSIRIS data. Finally the comparisons with instruments on large balloons (10–31 km) show a good agreement, within −0.7±1 ppmv. The official SMR v2.1 dataset is consistent in all altitude ranges with POAM III, NDACC and large balloon-borne instruments measurements. In the SMR v2.1 data, no different systematic error has been found in the 0–35km range in comparison with the 35–60 km range. The same feature has been highlighted in both hemispheres in SMR v2.1/POAM III intercomparisons, and no latitudinal dependence has been revealed in SMR v2.1/NDACC intercomparisons.

In September 2002 the Antarctic polar vortex split in two under the influence of a sudden warming. During this event, the Odin satellite was able to measure both ozone (O3) and chlorine monoxide (ClO), a key constituent responsible for the so-called “ozone hole”, together with nitrous oxide (N2O), a dynamical tracer, and nitric acid (HNO3) and nitrogen dioxide (NO2), tracers of denitrification. The submillimeter radiometer (SMR) microwave instrument and the Optical Spectrograph and Infrared Imager System (OSIRIS) UV-visible light spectrometer (VIS) and IR instrument on board Odin have sounded the polar vortex during three different periods: before (19–20 September), during (24–25 September), and after (1–2 and 4–5 October) the vortex split. Odin observations coupled with the Reactive Processes Ruling the Ozone Budget in the Stratosphere (REPROBUS) chemical transport model at and above 500 K isentropic surfaces (heights above 18 km) reveal that on 19–20 September the Antarctic vortex was dynamically stable and chemically nominal: denitrified, with a nearly complete chlorine activation, and a 70% O3 loss at 500 K. On 25–26 September the unusual morphology of the vortex is monitored by the N2O observations. The measured ClO decay is consistent with other observations performed in 2002 and in the past. The vortex split episode is followed by a nearly complete deactivation of the ClO radicals on 1–2 October, leading to the end of the chemical O3 loss, while HNO3 and NO2 fields start increasing. This acceleration of the chlorine deactivation results from the warming of the Antarctic vortex in 2002, putting an early end to the polar stratospheric cloud season. The model simulation suggests that the vortex elongation toward regions of strong solar irradiance also favored the rapid reformation of ClONO2. The observed dynamical and chemical evolution of the 2002 polar vortex is qualitatively well reproduced by REPROBUS. Quantitative differences are mainly attributable to the too weak amounts of HNO3 in the model, which do not produce enough NO2 in presence of sunlight to deactivate chlorine as fast as observed by Odin.

[1] Vertical profiles of nitrogen dioxide in the 19–40 km altitude range are successfully retrieved over the globe from Optical Spectrograph and Infrared Imager System (OSIRIS) limb scatter observations in late 2001 and early 2002. The inclusion of multiple scattering in the radiative transfer model used in the inversion algorithm allows for the retrieval of NO2 down to 19 km. The slant column densities, which represent the observations in the inversion, are obtained by fitting the fine structure in normalized radiance spectra over the 435–449 nm range, where NO2 electronic absorption is readily observable because of long light paths through stratospheric layers rich in this constituent. Details of the spectral fitting and inversion algorithm are discussed, including the discovery of a pseudo-absorber associated with pixelated detectors and a new method to verify altitude registration. Comparisons are made with spatially and temporally coincident profile measurements of this photochemically active trace gas. Better than 20% agreement is obtained with all correlative measurements over the common retrieval altitude range, confirming the validity of OSIRIS NO2 profiles. Systematic biases in the number densities are not observed at any altitude. A ‘‘snapshot’’ meridional cross section between 40� N and 70� S is shown from observations during a fraction of an orbit. INDEX TERMS: 0340 Atmospheric Composition and Structure: Middle atmosphere—composition and chemistry; 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 0394 Atmospheric Composition and Structure: Instruments and techniques; 3334 Meteorology and Atmospheric Dynamics: Middle atmosphere dynamics (0341, 0342); KEYWORDS: optical, Sun-synchronous, polar-orbiting, Fraunhofer, Ring effect, iterative onion peel