Validation of HNO3, ClONO2, and N2O5 from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS)

The Atmospheric Chemistry Experiment (ACE) satellite was launched on 12 August 2003. Its two instruments measure vertical profiles of over 30 atmospheric trace gases by analyzing solar occultation spectra in the ultraviolet/visible and infrared wavelength regions. The reservoir gases HNO3, ClONO2, and N2O5 are three of the key species provided by the primary instrument, the ACE Fourier Transform Spectrometer (ACE-FTS). This paper describes the ACE-FTS version 2.2 data products, including the N2O5 update, for the three species and presents validation comparisons with available observations. We have compared volume mixing ratio (VMR) profiles of HNO3, ClONO2, and N2O5 with measurements by other satellite instruments (SMR, MLS, MIPAS), aircraft measurements (ASUR), and single balloon-flights (SPIRALE, FIRS-2). Partial columns of HNO3 and ClONO2 were also compared with measurements by ground-based Fourier Transform Infrared (FTIR) spectrometers. Overall the quality of the ACE-FTS v2.2 HNO3 VMR profiles is good from 18 to 35 km. For the statistical satellite comparisons, the mean absolute differences are generally within ±1 ppbv ±20%) from 18 to 35 km. For MIPAS and MLS comparisons only, mean relative differences lie within±10% between 10 and 36 km. ACE-FTS HNO3 partial columns (~15–30 km) show a slight negative bias of −1.3% relative to the ground-based FTIRs at latitudes ranging from 77.8° S–76.5° N. Good agreement between ACE-FTS ClONO2 and MIPAS, using the Institut für Meteorologie und Klimaforschung and Instituto de Astrofísica de Andalucía (IMK-IAA) data processor is seen. Mean absolute differences are typically within ±0.01 ppbv between 16 and 27 km and less than +0.09 ppbv between 27 and 34 km. The ClONO2 partial column comparisons show varying degrees of agreement, depending on the location and the quality of the FTIR measurements. Good agreement was found for the comparisons with the midlatitude Jungfraujoch partial columns for which the mean relative difference is 4.7%. ACE-FTS N2O5 has a low bias relative to MIPAS IMK-IAA, reaching −0.25 ppbv at the altitude of the N2O5 maximum (around 30 km). Mean absolute differences at lower altitudes (16–27 km) are typically −0.05 ppbv for MIPAS nighttime and ±0.02 ppbv for MIPAS daytime measurements

Analysis of the SAGE III measurements : inversion algorithm and validation of the results : comparisons of the products of the satellite mission ACE with correlative remote sensing and in situ measurements

Le dernier rapport du GIEC souligne que le rôle de la stratosphère dans le changement climatique est mal connu et invite à poursuivre son étude. Les mesures d'occultation solaire et in situ sont appropriées à cette étude mais il est nécessaire de les valider. Nous avons tout d'abord développé notre propre algorithme d'inversion des transmissions de SAGE III et avons comparé nos produits (profils verticaux des concentrations en O3 et en NO2 et coefficients d'extinction des aérosols (CEA)) à ceux issus de l'algorithme officiel et d'un troisième algorithme. De bons accords ont été obtenus entre ces inversions pour toutes les espèces. Nous avons poursuivi avec des comparaisons à des mesures corrélatives obtenues à l'aide d'instruments spatiaux et d'un instrument sous ballon (SPIRALE) déjà validés. Si on exclut les CEA, les résultats sont satisfaisants. Toutefois, la comparaison aux mesures in situ de SPIRALE obtenues aux abords du vortex polaire donne un désaccord pour NO2 démontrant ainsi les limites de l'occultation solaire dans la mesure d'espèces réactives dans des conditions dynamiques complexes. Une fois ces validations réalisées, nous nous sommes intéressés aux intrusions dans la stratosphère d'aérosols issus de feux de forêt et nous avons montré qu'elles conduisent à une forte augmentation du nombre de particules. Enfin, dans un cadre international, nous avons participé à la validation des instruments de la mission ACE (FTS, MAESTRO, Imager) à l'aide de SAGE III et de SPIRALE. Cela nous a permis de valider certains produits (O3 et NO2 de FTS et MAESTRO), d'en invalider d'autres (CEA d'Imager) et de confirmer le désaccord pour NO2 entre mesures in situ et à distance.One of the conclusions of the last IPCC reports is that the role of the stratosphere in the current climate change is not weil known. Consequently, stratospheric studies must continue. Solar occultation and in situ measurements are weil suited to these studies but it is necessary to validate them. First, we have developed our inversion algorithm of the SAGE III transmissions and we have compared our products (vertical profiles of O3 and N02 concentrations and of aerosol extinction coefficients (AEC)) to those from the officiaI algorithm and from a third algorithm. Good agreements are obtained between these inversions for ail species. Then, we have compared our products to those from correlative validated measurements obtained by satellite and balloon borne instrument (SPIRALE). Except CEA, results are satisfying. However, the comparison with in situ measurements from SPIRALE obtained on the edge of the polar vortex exhibits a disagreement for NO2 proving that the solar occultation method are not weil suited for reactive species in complex dynamical situation. Once these validations carried out, we have studied the stratospheric intrusions of aerosols resulting from forest fires and we have shown that they lead to a strong increase in the number of particles. Finally, in an international framework, we have taken part in the validation of the instruments of the ACE mission (FTS, MAESTRO and Imager) with SAGE III and SPIRALE data. That enabled us to validate sorne products (O3 and NO2 from FTS and MAESTRO), to invalidate others (CEA from Imager) and to confirm the discrepancy for NO2 between in situ and remote measurement

Analyse des mesures de l'expérience satellitaire SAGE III : algorithme d'inversion et validation des résultats. Comparaison des produits des instruments de la mission spatiale ACE avec des mesures corrélatives à distance et in situ.

One of the conclusions of the last IPCC report (2007) is that the comprehension of climate change is still incomplete. In particular, the role of the stratosphere is not well known ; that is why it is important to study its composition and the physical and chemical processes in the stratosphere. Solar occultation measurements (like SAGE III) and in situ measurements are particularly appropriate for these studies. In this thesis, we have studied the consistency of the existing measurements. My work consists of the inversion of SAGE III transmissions in order to obtain vertical profiles of ozone and nitrogen dioxide concentrations and of aerosol extinction coefficients in nine channels between 385 and 1545 nm. In the channels around 450 nm, a vertical smoothing of the tangent transmissions is required to compensate for the problem of spectral neutrality of the attenuator. In the 1545 nm channel, CO2 absorption is computed using MODTRAN 5. The uncertainties are evaluated using a Monte Carlo method. SAGE III products obtained by our algorithm are compared to official products (NASA), to products from another algorithm developped in St. Petersburg and to coincident measurements from the SAGE II and POAM III sensors. Globally, these comparisons show that the SAGE III products from my LOA algorithm are of good quality. However, a comparison performed with data from the balloon-borne instrument SPIRALE (in situ measurements) on the edge of the polar vortex shows a good agreement between ozone and a disagreement for NO2. This disagreement shows that in complex dynamical situations, the solar occultation method for reactive species (such as NO2) is not well suited. Furthermore, diurnal variations of NO2 complicate the comparisons between remote sensing measurements and in situ ones. A specific study concerning aerosol resulting from forest fires in the western Canada (August 2003) has been done using official SAGE III data. Intrusions of biomass burning aerosols in the lower stratosphere by pyroconvection could be the reason for the increase in aerosol extinction coefficients in the lower stratosphere observed in SAGE III events. We have deduced the microphysical properties of these aerosols from the SAGE III measurements and showed that the increase in the aerosol extinction coefficient was caused by an increase in the number of particles in the lower stratosphere. Nevertheless, the chemical composition of these aerosols could not be deduced from the SAGE III measurements because extinction measurements are not sensitive enough to the refraction index. Since the end of the SAGE II, SAGE III and POAM III missions, the ACE-SCISAT instruments along with the SOFIE instrument are the only solar occultation instruments providing informations on the stratosphere. Thus, we have studied the consistency between ACE data and SAGE II, SAGE III and SPIRALE data. This work is carried out within the framework of the international validation campaign. We have shown that aerosol extinction coefficients retrieved using IMAGER data are in disagreement with those retrieved using SAGE II and SAGE III and that the ozone and NO2 volume mixing ratio from FTS and MAESTRO are in a good agreement with SAGE III data. However, we find also discrepancies between NO2 retrieved by ACE and that retrieved by SPIRALE although the other species retrieved using FTS (CH4, N2O, HNO3, O3, HCl) and MAESTRO (O3) are in a good agreement with SPIRALE data.Le dernier rapport du GIEC (2007) souligne que la compréhension du changement climatique en cours est encore incomplète. Le rôle de la stratosphère est notamment mal connu. C'est pourquoi il est important d'étudier sa composition et les processus physicochimiques s'y déroulant. Les mesures d'occultation solaire telles celles de l'instrument satellitaire SAGE III et les mesures in situ sont particulièrement bien adaptées à l'étude de la stratosphère. J'ai étudié dans cette thèse la cohérence entre les mesures existantes.Mon travail a consisté à inverser les transmissions atmosphériques de SAGE III pour obtenir les profils verticaux des concentrations en ozone et en dioxyde d'azote ainsi que des coefficients d'extinction des aérosols dans neuf canaux entre 385 et 1545 nm. Dans les canaux situés autour de 450 nm, un lissage vertical a été effectué sur les transmissions tangentes pour pallier un défaut de neutralité spectrale de l'atténuateur. Dans le canal à 1545 nm, la prise en compte de l'absorption du CO2 a été effectuée avec le modèle MODTRAN 5. Les incertitudes ont été évaluées par une méthode de Monte Carlo. Nous avons alors validé nos produits à l'aide des produits SAGE III officiels (NASA), ceux d'un troisième algorithme d´eveloppé par une équipe de Saint Petersbourg et à l'aide de mesures coïncidentes des instruments SAGE II et POAM III. Ces comparaisons montrent que les produits LOA sont de bonne qualité. Cependant, une étude effectuée à l'aide des mesures in situ de l'instrument sous ballon SPIRALE aux abords du vortex polaire a montré un bon accord pour O3 et un désaccord pour NO2. Ce désaccord montre que la méthode d'occultation solaire pour la mesure d'espèces réactives (tel NO2) dans des conditions dynamiques complexes n'est pas bien adaptée. De plus, les variations diurnes de NO2 rendent les comparaisons directes entre mesures à distance et in situ difficiles.Une étude spécifique sur les aérosols des feux de forêt de l'ouest du Canada (août 2003) a été menée avec les produits SAGE III officiels. Des intrusions d'aérosols issus des feux de forêt dans la basse stratosphère par pyroconvection sont suspectées d'être à l'origine des pics d'extinction observés par SAGE III. Nous avons déduit des mesures SAGE III les propriétés microphysiques de ces aérosols et montré que ces coefficients d'extinction anormalement élevés étaient dus à une augmentation du nombre de particules dans la basse stratosphère. Cependant, la nature chimique de ces aérosols n'a pu être déterminée car les mesures d'extinction ne sont pas assez sensibles à l'indice de réfraction.Depuis la fin des missions SAGE II, SAGE III et POAM III, les instruments satellitaires de la mission ACE-SCISAT sont les seuls instruments d'occultation solaire (hormis SOFIE) fournissant des informations sur la stratosphère. Nous nous sommes ainsi intéressés à la validation de leurs mesures à l'aide des données de SAGE II, SAGE III et SPIRALE. Ce travail s'inscrit dans le cadre de la campagne de validation internationale. Nous avons montré que les coefficients d'extinction des aérosols déduits des mesures de IMAGER sont en désaccord avec ceux de SAGE II et SAGE III et que les rapports de mélange en ozone et en dioxyde d'azote de FTS et de MAESTRO sont en bon accord avec les produits SAGE III. Cependant, nous obtenons également un désaccord concernant NO2 en comparaison avec SPIRALE bien que les autres espèces (CH4, N2O, HNO3, O3, HCl) déduites de FTS ainsi que l'ozone MAESTRO sont en bon accord avec les données SPIRALE

AerGOM, an improved algorithm for stratospheric aerosol extinction retrieval from GOMOS observations – Part 1: Algorithm description

International audienceThe GOMOS instrument on Envisat has successfully demonstrated that a UV–Vis–NIR spaceborne stellar occultation instrument is capable of delivering quality data on the gaseous and particulate composition of Earth's atmosphere. Still, some problems related to data inversion remained to be examined. In the past, it was found that the aerosol extinction profile retrievals in the upper troposphere and stratosphere are of good quality at a reference wavelength of 500 nm but suffer from anomalous, retrieval-related perturbations at other wavelengths. Identification of algo-rithmic problems and subsequent improvement was therefore necessary. This work has been carried out; the resulting AerGOM Level 2 retrieval algorithm together with the first data version AerGOMv1.0 forms the subject of this paper. The AerGOM algorithm differs from the standard GOMOS IPF processor in a number of important ways: more accurate physical laws have been implemented, all retrieval-related covariances are taken into account, and the aerosol extinction spectral model is strongly improved. Retrieval examples demonstrate that the previously observed profile perturbations have disappeared, and the obtained extinction spectra look in general more consistent. We present a detailed validation study in a companion paper; here, to give a first idea of the data quality, a worst-case comparison at 386 nm shows SAGE II–AerGOM correlation coefficients that are up to 1 order of magnitude larger than the ones obtained with the GOMOS IPFv6.01 data set

Highlights of GOMOS stellar occultation measurements and recent improvements of the data

International audienceEuropean Space Agency’s GOMOS (Global Ozone Monitoring by Occultation of Stars) instrument on-board Envisat satellite operated during 2002 - 2012. During the ten years of the mission, GOMOS instrument successfully measured vertical profiles of O3, NO2, NO3 and aerosols, including polar mesospheric clouds and polar stratospheric clouds, using UV-VIS channel and O2 and H2O using two near--IR channels. The best quality of data was obtained during the night time, but recently also the data processing of the day time observations have been improved. The two photometers of GOMOS have been used to retrieve high vertical resolution temperature profiles as well as for studying turbulence and gravity waves. Constituents with weak spectral signatures, such as OClO and Na, can be detected using temporal averaging. GOMOS measurements provide global coverage with about 200-400 daily profile measurements. Vertically the measurements extend from 5 km to 100 km with varying valid altitude range depending on constituent. Stellar occultation technique ensures high vertical resolution of 2-4 km, very accurate altitude registration and relatively simple data retrieval. The self-calibrating feature of the occultation technique is particularly suitable for long term trend analysis and therefore useful for studying climate-chemistry interactions. In particular, the high quality night time observations of ozone, NO2 and NO3 have been used in several time series analysis and studies related to middle atmosphere chemistry and dynamics including effects of the energetic particles. The joint SAGE II and GOMOS ozone profile time series have shown upper stratospheric ozone recovery at most latitudes outside the polar regions.In this presentation we discuss the highlights of GOMOS measurements and introduce the recent improvements on the data quality, including, in particular, day time measurements and ozone profiles in the very interesting upper troposphere - lower stratosphere region

Validation of ACE-FTS v2.2 methane profiles from the upper troposphere to the lower mesosphere

The ACE-FTS (Atmospheric Chemistry Experiment – Fourier Transform Spectrometer) solar occultation instrument that was launched onboard the Canadian SCISAT-1 satellite in August 2003 is measuring vertical profiles from the upper troposphere to the lower mesosphere for a large number of atmospheric constituents. Methane is one of the key species. The version v2.2 data of the ACE-FTS CH4 data have been compared to correlative satellite, balloon-borne and ground-based Fourier transform infrared remote sensing data to assess their quality. The comparison results indicate that the accuracy of the data is within 10% in the upper troposphere – lower stratosphere, and within 25% in the middle and higher stratosphere up to the lower mesosphere (<60 km). The observed differences are generally consistent with reported systematic uncertainties. ACE-FTS is also shown to reproduce the variability of methane in the stratosphere and lower mesosphere

Simultaneous measurements of OClO, NO₂ and O₃ in the Arctic polar vortex by the GOMOS instrument

International audienceWe present the first nighttime measurements of OClO from a limb-viewing satellite instrument in the Arctic polar vortex. The relationship between OClO, NO2 and O3 slant column densities in the Arctic polar vortex are analyzed from the GOMOS measurements. The retrieval process is based on a differential optical absorption spectroscopy (DOAS) method applied on the weighted median GOMOS transmittances. A study of the longitudinal distributions of OClO, NO2 and O3 above 65° north in January 2008 is presented. It shows a strong halogen activation in the lower stratosphere and a strong denoxification in the entire stratosphere inside the Arctic polar vortex. Time series of temperatures and OClO, NO2 and O3 slant column densities for the winters 2002/2003 to 2007/2008 are also presented. They highlight the correlation between temperature, OClO and NO2. The GOMOS instrument appears to be a very suitable instrument for the monitoring of OClO, NO2 and O3 in the stratosphere during nighttime

OClO slant column densities derived from GOMOS averaged transmittance measurements

The Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument on board the European platform ENVISAT (ENVironment SATellite) was dedicated to the study of the of Earth's atmosphere using the stellar occultation technique. The spectral range of the GOMOS spectrometer extends from the UV (ultra violet) to the near infrared, allowing for the retrieval of species such as O3, NO2, NO3, H2O, O2, air density, aerosol extinction and OClO. Nevertheless, OClO cannot be retrieved using a single GOMOS measurement because of the weak signal-to-noise ratio and the small optical thickness associated with this molecule. We present here the method used to detect this molecule by using several GOMOS measurements. It is based on a two-step approach. First, several co-located measurements are combined in a statistical way to build an averaged measurement with a higher signal-to-noise ratio; then, a differential optical absorption spectroscopy (DOAS) method is applied to retrieve OClO slant column densities (SCD). The statistics of the sets of GOMOS measurements used to build the averaged measurement and the spectral window selection are analyzed. The obtained retrievals are compared to results from two balloon-borne instruments. It appears that the inter-comparisons of OClO are generally satisfying (relative differences are about 15-60%). Two nighttime climatologies of OClO based on GOMOS averaged measurements are presented. The first depicts annual global pictures of OClO from 2003 to 2011. From this climatology, the presence of an OClO SCD peak in the equatorial region at about 35 km is confirmed and strong OClO SCD in both polar regions are observed (more than 1016 cm−2 in the Antarctic region and slightly less in the Arctic region), a sign of chlorine activation. The second climatology is a monthly time series. It clearly shows the chlorine activation of the lower stratosphere during winter. Moreover the equatorial OClO SCD peak is observed during all years without any significant variations. This very promising method, applied on GOMOS measurements, allowed us to build the first nighttime climatology of OClO using limb-viewing instruments

Optical extinction by upper tropospheric/stratospheric aerosols and clouds: GOMOS observations for the period 2002-2008

International audienceAlthough the retrieval of aerosol extinction coefficients from satellite remote measurements is notoriously difficult (in comparison with gaseous species) due to the lack of typical spectral signatures, important information can be obtained. In this paper we present an overview of the current operational nighttime UV/Vis aerosol extinction profile results for the GOMOS star occultation instrument, spanning the period from August 2002 to May 2008. Some problems still remain, such as the ones associated with incomplete scintillation correction and the aerosol spectral law implementation, but good quality extinction values can be expected at a wavelength of 500 nm. Typical phenomena associated with atmospheric particulate matter in the Upper Troposphere/Lower Stratosphere (UTLS) are easily identified: Polar Stratospheric Clouds, tropical subvisual cirrus clouds, background stratospheric aerosols, and post-eruption volcanic aerosols (with their subsequent dispersion around the globe). In this overview paper we will give a summary of the current results

Validation of ACE-FTS N₂O measurements

International audienceThe Atmospheric Chemistry Experiment (ACE), also known as SCISAT, was launched on 12 August 2003, carrying two instruments that measure vertical profiles of atmospheric constituents using the solar occultation technique. One of these instruments, the ACE Fourier Transform Spectrometer (ACE-FTS), is measuring volume mixing ratio (VMR) profiles of nitrous oxide (N2O) from the upper troposphere to the lower mesosphere at a vertical resolution of about 3–4 km. In this study, the quality of the ACE-FTS version 2.2 N2O data is assessed through comparisons with coincident measurements made by other satellite, balloon-borne, aircraft, and ground-based instruments. These consist of vertical profile comparisons with the SMR, MLS, and MIPAS satellite instruments, multiple aircraft flights of ASUR, and single balloon flights of SPIRALE and FIRS-2, and partial column comparisons with a network of ground-based Fourier Transform InfraRed spectrometers (FTIRs). Overall, the quality of the ACE-FTS version 2.2 N2O VMR profiles is good over the entire altitude range from 5 to 60 km. Between 6 and 30 km, the mean absolute differences for the satellite comparisons lie between -42 ppbv and +17 ppbv, with most within ±20 ppbv. This corresponds to relative deviations from the mean that are within ±15%, except for comparisons with MIPAS near 30 km, for which they are as large as 22.5%. Between 18 and 30 km, the mean absolute differences are generally within ±10 ppbv, again excluding the aircraft and balloon comparisons. From 30 to 60 km, the mean absolute differences are within ±4 ppbv, and are mostly between -2 and +1 ppbv. Given the small N2O VMR in this region, the relative deviations from the mean are therefore large at these altitudes, with most suggesting a negative bias in the ACE-FTS data between 30 and 50 km. In the comparisons with the FTIRs, the mean relative differences between the ACE-FTS and FTIR partial columns are within ±6.6% for eleven of the twelve contributing stations. This mean relative difference is negative at ten stations, suggesting a small negative bias in the ACE-FTS partial columns over the altitude regions compared. Excellent correlation (R=0.964) is observed between the ACE-FTS and FTIR partial columns, with a slope of 1.01 and an intercept of -0.20 on the line fitted to the data