Optical Spectrograph And Infrared Imager System Research Articles

The impact of sea-glint upon limb radiance

A simple radiative transfer model is developed to calculate the contribution of sea-glint to limb radiance. It is shown that the absolute sea-glint signal peaks between 70° and 80° solar zenith angle. Sea-glint can contribute 10–15% of the total limb radiance at wavelengths greater than 600 nm, which is several times brighter than an equivalent 5% reflecting Lambertian ocean surface. A test case was identified over the Arabian Sea in October 2002 and the model results compared to limb observations from the Optical Spectrograph and Infra-Red Imaging System (OSIRIS) on-board the Odin satellite. PACS Nos.: 94.10.Gb, 93.85.+q, 42.68.Ay, 42.68.Mj

Status of the Odin/OSIRIS stratospheric O3 and NO2 data products

This paper describes the status of the stratospheric ozone and nitrogen dioxide data products from the Optical Spectrograph and InfraRed Imager System (OSIRIS) instrument on the Odin satellite. The current version of the data products is 3.0, covering the period from November 2001 to the present. The O3 and NO2 retrieval methods are reviewed along with an overview of the error analyses and geophysical validation status. PACS Nos.: 07.05.Kf, 07.87.+v, 42.68.Mj, 92.60.hd, 92.60.Ta, 92.60.Vb, 92.70.Cp, 95.75.Fg, 95.75.Rs

Vertical profiles of lightning-produced NO<sub>2</sub> enhancements in the upper troposphere observed by OSIRIS

Abstract. 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.

Validation of ozone profiles retrieved from Stratospheric Aerosol and Gas Experiment III limb scatter measurements

Ozone profiles retrieved from Stratospheric Aerosol and Gas Experiment (SAGE III) limb scatter measurements are compared with correlative measurements made by occultation instruments (SAGE II, SAGE III, and HALOE), a limb scatter instrument (Optical Spectrograph and Infrared Imager System (OSIRIS)), a series of ozonesondes and lidars, and a transport model, in order to ascertain the accuracy and precision of the SAGE III instrument in limb scatter mode. The measurement relative accuracy is found to be 5–10% from the tropopause to about 45 km, whereas the relative precision is found to be less than 10% from 20 to 38 km. The height registration, which is inferred directly from radiance data profiles, is found to have a standard deviation (“relative precision”) of about 350 m. Measurement repeatability, a measure of absolute precision, which is estimated by multiple measurements over the same geographical area in the tropics on consecutive orbits, is found to be ∼3%.

Intercomparison of ozone profile measurements from ASUR, SCIAMACHY, MIPAS, OSIRIS, and SMR

The airborne submillimeter radiometer (ASUR) was deployed onboard the Falcon research aircraft during the scanning imaging absorption spectrometer for atmospheric cartography (SCIAMACHY) validation and utilization experiment (SCIAVALUE) and the European polar stratospheric cloud and lee wave experiment (EuPLEx) campaigns. A large number of ozone profile measurements were performed over a latitude band spanning from 5°S to 80°N in September 2002 and February/March 2003 during the SCIAVALUE and around the northern polar latitudes in January/February 2003 during the EuPLEx. Both missions amassed an ample microwave ozone profile data set that is used to make quantitative comparisons with satellite measurements in order to assess the quality of the satellite retrievals. In this paper, the ASUR ozone profile measurements are compared with measurements from SCIAMACHY and Michelson interferometer for passive atmospheric sounding (MIPAS) on Environmental Satellite and optical spectrograph and infrared imager system (OSIRIS) and submillimeter radiometer (SMR) on the Odin satellite. The cross comparisons with the criterion that the ASUR measurements are performed within ±1000 km and ±6 hrs of the satellite observations show a good agreement with all the four satellite sensors. The differences in data values are the following: −4 to +8% for ASUR‐SCIAMACHY (operational product, v2.1), within ±15% for ASUR‐SCIAMACHY (scientific product, v1.62), up to +6% for ASUR‐MIPAS (operational product v4.61) and ASUR‐MIPAS (scientific product v1‐O3‐1), up to 17% for ASUR‐OSIRIS (v012), and −6 to 17% for ASUR‐SMR (v222) between the 20‐ and 40‐km altitude range depending on latitude. Thus, the intercomparisons provide important quantitative information about the quality of the satellite ozone profiles, which has to be considered when using the data for scientific analyses.

Validation of Odin/OSIRIS stratospheric NO2 profiles

This paper presents the validation study of stratospheric NO2 profiles retrieved from Odin/OSIRIS measurements of limb‐scattered sunlight (version 2.4). The Optical Spectrograph and Infrared Imager System (OSIRIS) NO2 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 NO2 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 NO2 profiles are well behaved, with reasonable uncertainty estimates between 15 km and 40 km. This unique NO2 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.

Ground-Based Solar Absorption FTIR Spectroscopy: Characterization of Retrievals and First Results from a Novel Optical Design Instrument at a New NDACC Complementary Station

Abstract The authors describe the optical design of a high-resolution Fourier Transform Spectrometer (FTS), which serves as the primary instrument at the University of Toronto Atmospheric Observatory (TAO). The FTS is dedicated to ground-based infrared solar absorption atmospheric measurements from Toronto, Ontario, Canada. Instrument performance is discussed in terms of instrumental line shape (ILS) and phase error and modulation efficiency as a function of optical path difference. Typical measurement parameters are presented together with retrieval parameters used to derive total and partial column concentrations of ozone. Retrievals at TAO employ the optimal estimation method (OEM), and some impacts of the necessary a priori constraints are examined. In March 2004, after participating in a retrieval algorithm user intercomparison exercise, the TAO FTS was granted the status of a Complementary Observation Station within the international community of high-resolution FTS users in the Network for the Detection of Atmospheric Composition and Change (NDACC). During this exercise, average differences between total columns retrieved from the same spectra by different users were below 2.1% for O3, HCl, and N2O in the blind phase, and below 1% in the open phase, when all retrieval constraints were identical. Finally, a 2.5-yr time series of monthly mean stratospheric ozone columns agrees within 3% with those retrieved from Optical Spectrograph and Infrared Imager System (OSIRIS) measurements on board the Odin satellite, which is within the errors of both measurement platforms.

OSIRIS observations of OH A2Σ+-X2Π 308 nm solar resonance fluorescence at sunrise in the upper mesosphere

Since the OH molecule plays a critical role as a catalyst in atmospheric photochemistry, an accurate measurement of the OH density profile covering a broad range of latitudes and solar local times is required to quantify the major reactions involved. The optical spectrograph and infra-red imager system (OSIRIS) instrument on the Odin satellite observes scattered solar radiation at the terrestrial limb from the upper troposphere, the stratosphere, and the mesosphere. The wavelength range, 275 nm to 810 nm includes the OH A2Σ+–X2Π 0–0 band at 308 nm, which is seen in solar resonance emission superimposed upon the underlying atmospheric Rayleigh-scattering background. OSIRIS routinely detects the OH 308 nm emission in the mesosphere from sunrise through to sunset. One feature of the OH diurnal variation is a nocturnal layer in the 80–85 km region that is frequently, but not always, detectable in solar resonance for a short period following sunrise — the feature we label here as the "sunrise flash". This paper describes the observational analysis procedures involved in the quantitative measurement of the OSIRIS OH profiles together with a broad overview of the variability of the feature at sunrise. Also included is a photochemical model simulation of the OH sunrise layer using background atmospheric parameters, especially the water vapour mixing ratio, provided by the ACE/FTS instrument on the Canadian SCISAT satellite. For a number of nearly coincident measurements between OSIRIS OH and ACE/FTS water vapour, the model simulations show general agreement between the two. Agreement is improved by modifying the eddy mixing rates in the 80–85 km region, commensurate with the expected range of mixing rates.PACS Nos.: 42.68.Ay, 82.20.Pm, 82.30.Cf, 82.33.Tb, 92.60.H–, 92.60.Ta

Diurnal effects in limb scatter observations

Instruments that measure UV/visible scattered light from the Earth's limb are emerging as an important class of sensors capable of providing high‐quality profiles of aerosols and trace gases from the upper troposphere to the mesosphere. Critical to the inversion of limb scatter observations is the forward radiative transfer model. A fast and accurate radiative transfer model, VECTOR (Vector Orders‐of‐scattering Radiative transfer model), is presented that is able to account for the diurnal variation of species such as NO2 and BrO along the observing line of sight and the incoming solar beam. VECTOR has been used to quantify for the first time diurnal effect errors in NO2 and BrO with application to OSIRIS (Optical Spectrograph and Infra‐Red Imager System) and SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography), two limb viewing satellite instruments. For a solar zenith angle near 90° at the tangent point, errors can exceed 50% for NO2 and 100% for BrO in the lower stratosphere, with the largest errors generally occurring when viewing across, and at large angles to, the terminator. These results applied to OSIRIS NO2 and SCIAMACHY BrO reveal that diurnal effect errors are generally small (<10%). Yet 1 out of every 6 OSIRIS NO2 profiles experiences large (10–35%) errors and 1 out of every 11 SCIAMACHY BrO profiles experiences large (10–100%, or larger) errors in the lower stratosphere.

Optical Spectrograph and Infra‐Red Imaging System (OSIRIS) observations of mesospheric OH A2Σ+‐X2Π 0‐0 and 1‐1 band resonance emissions

Although only a minor species, the OH molecule plays an important role in the photochemical control of mesospheric ozone density and has been the target of a number of observational programs, principally through the OH A2Σ+‐X2Π 0‐0 band emission at 308 nm. This emission band arises from solar resonance fluorescence excitation of OH X2Π ground state molecules, and its observation is complicated by the presence of an underlying atmospheric Rayleigh scattering spectrum. We show that the OH A2Σ+‐X2Π 0‐0 band emission has been reliably and routinely detected with the moderately low, 0.9 nm, spectral resolution Optical Spectrograph and Infra‐Red Imaging System (OSIRIS) limb scanning spectrograph. Changes in upper mesospheric water vapor observed by the Halogen Occultation Experiment (HALOE) are readily detected as changes in the OH density profiles seen by OSIRIS. Altitude profiles of OH density in the middle and upper mesosphere are in good agreement with model results that incorporate coordinated HALOE water vapor measurements. The agreement is within the HALOE and OSIRIS error limits when the recommended standard reaction rates are assumed. Conversely, model calculations of OH density using the revised reaction rates proposed to explain the Middle Atmosphere High‐Resolution Spectrograph Investigation (MAHRSI) OH profiles typically fall outside the observed OSIRIS error limits. The OSIRIS results suggest that the probable difference between the observed and modeled OH densities is less than 15% from 55 to 80 km. Diurnal and seasonal OH variations observed by OSIRIS are in good agreement with model predictions. These successful comparisons suggest that the OSIRIS OH database, spanning more than 4 years of operation and broad ranges of latitude and local time, can contribute significantly to studies of OH photochemistry and upper mesospheric water vapor.

Comparison of Odin-OSIRIS OH A2Σ+-X2Π 0-0 mesospheric observations and ACE-FTS water vapor observations

[1] As the production of hydroxyl (OH) in the mesosphere is driven by solar photodissociation of water vapor, the OH vertical profile is thus influenced by the water vapor profile. The details of this coupling are investigated by using experimental water vapor profiles observed by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) instrument as input to a photochemical model to predict OH profiles for comparison with experimental OH profiles observed by the Optical Spectrograph and Infra-Red Imager System (OSIRIS) instrument. We show that the model OH profiles in the 60 km to 80 km altitude region employing standard reaction rates are in agreement within 15% with the experimental OH profiles over a range of longitudes and local times, whereas the MAHRSI observations indicate the model OH densities in the middle mesosphere are approximately 30% too large. The quality of the OSIRIS OH observations and the ACE-FTS water vapor observations are both sufficiently accurate in the upper mesosphere to yield satisfactory agreement for individual profile comparisons. An example of the hemispheric maps of OH density measured by OSIRIS is shown for an altitude of 80 km.

Correlation of PMC relative brightness and altitudes observed by Odin/OSIRIS in the Northern Hemisphere in 2002–2003

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.

Shape and composition of PMC particles derived from satellite remote sensing measurements

The Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE‐FTS) on the SciSat satellite measured nearly 30 spectra of polar mesospheric clouds (PMCs) between 65° and 70°N from July 5 to 14, 2004. The ACE‐FTS measurements are augmented by UV observations made at the same latitude and time period by the Optical Spectrograph and Infrared Imager System (OSIRIS) on the Odin satellite. Our analysis of these measurements shows that PMC particles are composed of nonspherical ice crystals with mean (equivalent spherical) particle radii of 59 ± 5 nm.

Validation of ACE‐FTS stratospheric ozone profiles against Odin/OSIRIS measurements

Stratospheric ozone profiles measured by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE‐FTS) have been extensively compared with the coincident ozone profiles measured by the Optical Spectrograph and InfraRed Imager System (OSIRIS) on Odin. All coincidences took place between January 31 and August 5, 2004, with the majority in the Northern Hemisphere. ACE‐OSIRIS statistical comparisons were performed in the altitude range of 16–32 km where OSIRIS ozone data are in a good agreement, 5–7%, with another satellite and ground‐based instruments. At altitudes 3–5 km above and below the ozone peak, ACE‐FTS and OSIRIS profiles agree to within ∼4–7%, while in the vicinity of the peak the ACE‐FTS profiles are systematically lower than OSIRIS profiles by about 10%. This result is consistent for all latitudes between 30°S and 82°N.

Vertical variation of NLC particle sizes retrieved from Odin/OSIRIS limb scattering observations

The vertical variation of noctilucent cloud (NLC) particle sizes is derived from limb scattering observations in the UV‐B spectral range performed with the Optical Spectrograph and InfraRed Imager System (OSIRIS) on the Odin satellite. The particle sizes are determined under the assumptions that NLC particles can be treated as Mie particles with a known refractive index, and that their size distribution can be approximated by a log‐normal distribution. A sample data set of 16 bright NLCs observed during the 2003 Northern Hemisphere NLC season and the 2003/2004 Southern Hemisphere NLC season was selected for this study. In almost all the cases the derived NLC particle sizes increase with decreasing altitudes. This is in agreement with the standard assumption of larger particles sedimenting out of the cloud bottom. The derived particle radii are between about 10–50 nm.

Polar vortex evolution during the 2002 Antarctic major warming as observed by the Odin satellite

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.

Tomographic retrieval of the oxygen infrared atmospheric band with the OSIRIS infrared imager

The infrared imager (IRI) component of the optical spectrograph and infrared imager system (OSIRIS) onboard the Odin spacecraft provides a set of line-of-sight brightness measurements of the oxygen infrared atmospheric (OIRA) band. This set of measurements is unique in the fact that they are ideal inputs to a two-dimensional retrieval scheme that accurately recovers the volume emission rate of the OIRA band. The retrieval is done simultaneously in two dimensions, the angle along the satellite track and the distance from the centre of the Earth. The latter is easily converted to altitude above the surface of the Earth. In this work, we present the measurement set, the retrieval technique, and some preliminary results. We clearly demonstrate that the OSIRIS infrared imager provides maps of the OIRA band volume emission rate with unprecedented spatial resolution. PACS Nos.: 07.05.Pj, 07.60.Dq, 94.10.Fa, 94.10.Gb, 94.10.Rk

The OSIRIS instrument on the Odin spacecraft

The optical spectrograph and infrared imager system (OSIRIS) on board the Odin spacecraft is designed to retrieve altitude profiles of terrestrial atmospheric minor species by observing limb-radiance profiles. The grating optical spectrograph (OS) obtains spectra of scattered sunlight over the range 280–800 nm with a spectral resolution of approximately 1 nm. The Odin spacecraft performs a repetitive vertical limb scan to sweep the OS 1 km vertical field of view over selected altitude ranges from approximately 10 to 100 km. The terrestrial absorption features that are superimposed on the scattered solar spectrum are monitored to derive the minor species altitude profiles. The spectrograph also detects the airglow, which can be used to study the mesosphere and lower thermosphere. The other part of OSIRIS is a three-channel infrared imager (IRI) that uses linear array detectors to image the vertical limb radiance over an altitude range of approximately 100 km. The IRI observes both scattered sunlight and the airglow emissions from the oxygen infrared atmospheric band at 1.27 µm and the OH (3-1) Meinel band at 1.53 µm. A tomographic inversion technique is used with a series of these vertical images to derive the two-dimensional distribution of the emissions within the orbit plane. PACS Nos.: 07.05.Pj, 07.60.Dq, 07.60.Rd, 07.87, 94.10.Dy, 94.10.Fa, 94.10.Gb, 94.10.Rk

Stratospheric and tropospheric NO 2 observed by SCIAMACHY: first results

Observations from the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) onboard Envisat are used to quantifying the vertical distribution of stratospheric nitrogen dioxide (11–41 km) from limb scattering geometry and the tropospheric column from limb–nadir matching. SCIAMACHY is validated with spatially and temporally coincident observations from Optical Spectrograph and Infrared Imager System (OSIRIS) and from Halogen Occultation Experiment (HALOE). Comparison with the Canadian Middle Atmosphere Model provides a further check of consistency. Errors in pointing are detected and corrected using the recently developed `spectral knee' technique. An instrumental artifact, presumably a tangent-height-dependent wavelength drift, is causing fine spectral structure but can be taken into account as a pseudo-absorber in the least squares fitting. Extending the fitting window to longer wavelengths than those currently employed by other optical satellite-borne NO 2 sensors allows for the retrieval to penetrate the lower stratosphere. This extended fitting window is used to retrieve lower stratospheric NO 2 in the denoxified Antarctic polar vortex. Finally, tropospheric NO 2 columns, retrieved from limb–nadir matching, are presented.

A comparison of methods for retrieving stratospheric ozone profiles from OSIRIS limb-scatter measurements

Vertical profiles of stratospheric ozone number density derived from optical spectrograph and infrared imager system (OSIRIS) measurements of limb-scattered sunlight are presented. Ozone profiles are inferred using a method based on the analysis of normalised and paired limb radiance profiles at a set of wavelengths covering the Chappuis–Wulf bands of ozone, and using differential optical absorption spectroscopy (DOAS) in the Chappuis region. Both methods incorporate the pseudo-spherical multiple scattering radiative transfer model LIMBTRAN and an optimal estimation (OE) approach. The results of both methods are compared with ozone sonde and other satellite measurements.