Didier Fussen

Atmospheric Chemistry Experiment (ACE): Mission overview

SCISAT-1, also known as the Atmospheric Chemistry Experiment (ACE), is a Canadian satellite mission for remote sensing of the Earths atmosphere. It was launched into low Earth circular orbit (altitude 650 km, inclination 74°) on 12 Aug. 2003. The primary ACE instrument is a high spectral resolution (0.02 cm-1) Fourier Transform Spectrometer (FTS) operating from 2.2 to 13.3 μm (750-4400 cm-1). The satellite also features a dual spectrophotometer known as MAESTRO with wavelength coverage of 285-1030 nm and spectral resolution of 1-2 nm. A pair of filtered CMOS detector arrays records images of the Sun at 0.525 and 1.02 μm. Working primarily in solar occultation, the satellite provides altitude profile information (typically 10-100 km) for temperature, pressure, and the volume mixing ratios for several dozen molecules of atmospheric interest, as well as atmospheric extinction profiles over the latitudes 85°N to 85°S. This paper presents a mission overview and some of the first scientific results. Copyright 2005 by the American Geophysical Union.

GOMOS on Envisat: an overview

Abstract GOMOS (Global Ozone Monitoring by Occultation of Stars) on board Envisat measures O 3 , NO 2 , NO 3 , neutral density, aerosols, H 2 O, and O 2 , in the stratosphere and mesosphere by detecting absorption of starlight in ultraviolet, visible and near-infrared wavelengths. During bright limb conditions GOMOS will also observe scattered solar radiation. GOMOS will deliver ozone concentration profiles at altitudes 15–100 km with a vertical sampling better than 1.7 km and with a global coverage. As a self-calibrating method stellar occultation measurements provide a basis for a long-term global monitoring of ozone profiles. We will present here the status of the GOMOS instrument and show samples of first results obtained in 2002.

A warm layer in Venus' cryosphere and high-altitude measurements of HF, HCl, H2O and HDO

Venus has thick clouds of H2SO4 aerosol particles extending from altitudes of 40 to 60 km. The 60–100 km region (the mesosphere) is a transition region between the 4 day retrograde superrotation at the top of the thick clouds and the solar–antisolar circulation in the thermosphere (above 100 km), which has upwelling over the subsolar point and transport to the nightside. The mesosphere has a light haze of variable optical thickness, with CO, SO2, HCl, HF, H2O and HDO as the most important minor gaseous constituents, but the vertical distribution of the haze and molecules is poorly known because previous descent probes began their measurements at or below 60 km. Here we report the detection of an extensive layer of warm air at altitudes 90–120 km on the night side that we interpret as the result of adiabatic heating during air subsidence. Such a strong temperature inversion was not expected, because the night side of Venus was otherwise so cold that it was named the ‘cryosphere’ above 100 km. We also measured the mesospheric distributions of HF, HCl, H2O and HDO. HCl is less abundant than reported 40 years ago. HDO/H2O is enhanced by a factor of ∼2.5 with respect to the lower atmosphere, and there is a general depletion of H2O around 80–90 km for which we have no explanation.

Nighttime ozone profiles in the stratosphere and mesosphere by the Global Ozone Monitoring by Occultation of Stars on Envisat

[1] The Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument on board the European Space Agency’s Envisat satellite measures ozone and a few other trace gases using the stellar occultation method. Global coverage, good vertical resolution and the self-calibrating measurement method make GOMOS observations a promising data set for building various climatologies. In this paper we present the nighttime stratospheric ozone distribution measured by GOMOS in 2003. We show monthly latitudinal distributions of the ozone number density and mixing ratio profiles, as well as the seasonal variations of profiles at several latitudes. The stratospheric profiles are compared with the Fortuin-Kelder daytime ozone climatology. Large differences are found in polar areas and they can be shown to be correlated with large increases of NO2. In the upper stratosphere, ozone values from GOMOS are systematically larger than in the Fortuin-Kelder climatology, which can be explained by the diurnal variation. In the middle and lower stratosphere, GOMOS finds a few percent less ozone than Fortuin-Kelder. In the equatorial area, at heights of around 15–22 km, GOMOS finds much less ozone than Fortuin-Kelder. For the mesosphere and lower thermosphere, there has previously been no comprehensive nighttime ozone climatology. GOMOS is one of the first new instruments able to contribute to such a climatology. We concentrate on the characterization of the ozone distribution in this region. The monthly latitudinal and seasonal distributions of ozone profiles in this altitude region are shown. The altitude of the mesospheric ozone peak and the semiannual oscillation of the number density are determined. GOMOS is also able to determine the magnitude of the ozone minimum around 80 km. The lowest seasonal mean mixing ratio values are around 0.13 ppm. The faint tertiary ozone peak at 72 km in polar regions during wintertime is observed.

In-flight performance and calibration of SPICAV SOIR onboard Venus Express

Solar occultation in the infrared, part of the Spectoscopy for Investigation of Characteristics of the Atmosphere of Venus (SPICAV) instrument onboard Venus Express, combines an echelle grating spectrometer with an acousto-optic tunable filter (AOTF). It performs solar occultation measurements in the IR region at high spectral resolution. The wavelength range probed allows a detailed chemical inventory of Venuss atmosphere above the cloud layer, highlighting the vertical distribution of gases. A general description of the instrument and its in-flight performance is given. Different calibrations and data corrections are investigated, in particular the dark current and thermal background, the nonlinearity and pixel-to-pixel variability of the detector, the sensitivity of the instrument, the AOTF properties, and the spectral calibration and resolution.

First simultaneous global measurements of nighttime stratospheric NO2 and NO3 observed by Global Ozone Monitoring by Occultation of Stars (GOMOS)/Envisat in 2003

The Global Ozone Monitoring by Occultation of Stars (GOMOS) stellar occultation instrument on board the Envisat European satellite provides global coverage of ozone and other stratospheric species with good vertical resolution and a self-calibrating method. In this paper we present the first simultaneous global distribution of stratospheric NO 2 and NO 3 from 1 year of nighttime GOMOS data in 2003. Most previous NO 2 satellite observations have been made using the solar occultation technique. They are difficult to interpret due to the fast photochemical evolution of NO 2 at sunrise and sunset. There are no published observations of NO 3 from space because this constituent is rapidly photodissociated during daytime and is not observable by solar occultation. It is shown that the NO 2 mixing ratio reaches a maximum around 40 km with values between 14 and 16 ppbv at low and middle latitudes. The global distribution of NO 2 observed by GOMOS is very similar to the NO + NO 2 Halogen Occultation Experiment climatology deduced from sunset measurements from 1999 to 2004. At high latitude a high mixing ratio is observed in the north vortex in November 2003 after a strong solar proton event and in the south vortex in July 2003. The NO 3 mixing ratio peaks at 40–45 km. NO 3 follows a semiannual variation at low latitudes with maxima at equinoxes and an annual variation at middle and high latitudes with a maximum in summer. In the upper stratosphere the mixing ratio of NO 3 is strongly correlated with temperature due to the thermal dependence of its formation rate. Citation: Hauchecorne, A., et al. (2005), First simultaneous global measurements of nighttime stratospheric NO 2 and NO 3 observed by Global Ozone Monitoring by Occultation of Stars (GOMOS)/Envisat in 2003

A global climatology of the mesospheric sodium layer from GOMOS data during the 2002–2008 period

This paper presents a climatology of the mesospheric sodium layer built from the processing of 7 years of GOMOS data. With respect to preliminary results already published for the year 2003, a more careful analysis was applied to the averaging of occultations inside the climatological bins (10 in latitude-1 month). Also, the slant path absorption lines of the Na doublet around 589 nm shows evidence of partial saturation that was responsible for an underestimation of the Na concentration in our previous results. The sodium climatology has been validated with respect to the Fort Collins lidar measurements and, to a lesser extent, to the OSIRIS 2003–2004 data. Despite the important natural sodium variability, we have shown that the Na vertical column has a marked semi-annual oscillation at low latitudes that merges into an annual oscillation in the polar regions,a spatial distribution pattern that was unreported so far. The sodium layer seems to be clearly influenced by the mesospheric global circulation and the altitude of the layer shows clear signs of subsidence during polar winter. The climatology has been parameterized by time-latitude robust fits to alCorrespondence to: D. Fussen (didier.fussen@oma.be) low for easy use. Taking into account the non-linearity of the transmittance due to partial saturation, an experimental approach is proposed to derive mesospheric temperatures from limb remote sounding measurements.

Tunable acousto-optic spectral imager for atmospheric composition measurements in the visible spectral domain

We describe a new spectral imaging instrument using a TeO(2) acousto-optical tunable filter (AOTF) operating in the visible domain (450-900 nm). It allows for fast (~1 second), monochromatic (FWHM ranges from 0.6 nm at 450 nm to 3.5 nm at 800 nm) picture acquisition with good spatial resolution. This instrument was designed as a breadboard of the visible channel of a new satellite-borne atmospheric limb spectral imager, named the Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere (ALTIUS), that is currently being developed. We tested its remote sensing capabilities by observing the dense, turbulent plume exhausted by a waste incinerator stack at two wavelengths sensitive to NO(2). An average value of 6.0±0.4×10(17) molecules cm(-2) has been obtained for the NO(2) slant column density within the plume, close to the stack outlet. Although this result was obtained with a rather low accuracy, it demonstrates the potential of spectral imaging by using AOTFs in remote sensing.

Global measurement of the mesospheric sodium layer by the star occultation instrument GOMOS

We present the first global measurement of the sodium mesospheric layer obtained from the processing of about 100 000 star occultations by the GOMOS instrument onboard the ENVISAT satellite. The retrieval method is developed on the basis of a simple DOAS retrieval applied to averaged transmittances. The vertical inversion of the sodium slant path optical thickness is performed by using a modified Gaussian extinction profile. A global climatology is derived by using monthly bins of 20 degrees in latitude. The high variability of the sodium layer is confirmed on a global scale as well as the presence of an important modulation in the annual cycle. Also, we present some evidence for the existence of a diurnal cycle characterized by an increase of the sodium concentration in daylight.

A volcanism dependent model for the extinction profile of stratospheric aerosols in the UV‐visible range

We present a climatological model of the extinction coefficient of stratospheric aerosols in the UV-visible range based on SAGE II data. The model is a function of wavelength, latitude, relative altitude with respect to the tropopause and depends on the volcanism level. The vertical structure describes the tropopause region, the Junge layer and the high altitude domain. The model is shown to predict realistic extinction profiles with an reasonable accuracy.