A global climatology of the mesospheric sodium layerfrom 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 allow 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

The AOTF-based NO₂ camera

The abundance of NO₂ in the boundary layer relates to air quality and pollution source monitoring. Observing the spatiotemporal distribution of NO₂ above well-delimited (flue gas stacks, volcanoes, ships) or more extended sources (cities) allows for applications such as monitoring emission fluxes or studying the plume dynamic chemistry and its transport. So far, most attempts to map the NO₂ field from the ground have been made with visible-light scanning grating spectrometers. Benefiting from a high retrieval accuracy, they only achieve a relatively low spatiotemporal resolution that hampers the detection of dynamic features. <br><br> We present a new type of passive remote sensing instrument aiming at the measurement of the 2-D distributions of NO₂ slant column densities (SCDs) with a high spatiotemporal resolution. The measurement principle has strong similarities with the popular filter-based SO₂ camera as it relies on spectral images taken at wavelengths where the molecule absorption cross section is different. Contrary to the SO₂ camera, the spectral selection is performed by an acousto-optical tunable filter (AOTF) capable of resolving the target molecule's spectral features. <br><br> The NO₂ camera capabilities are demonstrated by imaging the NO₂ abundance in the plume of a coal-fired power plant. During this experiment, the 2-D distribution of the NO₂ SCD was retrieved with a temporal resolution of 3 min and a spatial sampling of 50 cm (over a 250 × 250 m² area). The detection limit was close to 5 × 10¹⁶ molecules cm⁻², with a maximum detected SCD of 4 × 10¹⁷ molecules cm⁻². Illustrating the added value of the NO₂ camera measurements, the data reveal the dynamics of the NO to NO₂ conversion in the early plume with an unprecedent resolution: from its release in the air, and for 100 m upwards, the observed NO₂ plume concentration increased at a rate of 0.75–1.25 g s⁻¹. In joint campaigns with SO₂ cameras, the NO₂ camera could also help in removing the bias introduced by the NO₂ interference with the SO₂ spectrum

Retrieval of vertical profiles of atmospheric refraction angles by inversion of optical dilution measurements

In this paper, we consider occultations of celestial bodies through the atmospheric limb from low Earth orbit satellites and we show how the usual change of tangent altitude associated with atmospheric refraction is inseparably connected to a variation of the observed apparent intensity, for extended and pointlike sources. We demonstrate, in the regime of weak refraction angles, that atmospheric optical dilution and image deformation are strictly concomitant. The approach leads to the integration of a simple differential equation related to the observed transmittance in the absence of other absorbing molecules along the optical path. The algorithm does not rely on the absolute knowledge of the radiometer pointing angle that is related to the accurate knowledge of the satellite attitude. We successfully applied the proposed method to the measurements performed by two past occultation experiments: GOMOS for stellar and ORA for solar occultations. The developed algorithm (named ARID) will be applied to the imaging of solar occultations in a forthcoming pico-satellite mission

AerGOM, an improved algorithm for stratospheric aerosol extinction retrieval from GOMOS observations – Part 2: Intercomparisons

AerGOM is a retrieval algorithm developed for the GOMOS instrument onboard Envisat as an alternative to the operational retrieval (IPF). AerGOM enhances the quality of the stratospheric aerosol extinction retrieval due to the extension of the spectral range used, refines the aerosol spectral parameterization, the simultaneous inversion of all atmospheric species as well as an improvement of the Rayleigh scattering correction. The retrieval algorithm allows for a good characterization of the stratospheric aerosol extinction for a wide range of wavelengths.</br></br>In this work, we present the results of stratospheric aerosol extinction comparisons between AerGOM and various spaceborne instruments (SAGE II, SAGE III, POAM III, ACE-MAESTRO and OSIRIS) for different wavelengths. The aerosol extinction intercomparisons for <i>λ</i> &lt; 700 nm and above 20 km show agreements with SAGE II version 7 and SAGE III version 4.0 within ±15&thinsp;% and ±45&thinsp;%, respectively. There is a strong positive bias below 20 km at <i>λ</i> &lt; 700 nm, which suggests that cirrus clouds at these altitudes have a large impact on the extinction values. Comparisons performed with GOMOS IPF v6.01 alongside AerGOM show that at short wavelengths and altitudes below 20 km, IPF retrievals are more accurate when evaluated against SAGE II and SAGE III but are much less precise than AerGOM. A modified aerosol spectral parameterization can improve AerGOM in this spectral and altitude range and leads to results that have an accuracy similar to IPF retrievals. Comparisons of AerGOM aerosol extinction coefficients with OSIRIS and SAGE III measurements at wavelengths larger than 700 nm show a very large negative bias at altitudes above 25 km. Therefore, the use of AerGOM aerosol extinction data is not recommended for <i>λ</i> &gt; 700 nm.</br></br>Due to the unique observational technique of GOMOS, some of the results appear to be dependent on the star occultation parameters such as star apparent temperature and magnitude, solar zenith angle and latitude of observation. A systematic analysis is carried out to identify biases in the dataset, using the various spaceborne instruments as references. The quality of the aerosol retrieval is mainly influenced by the star magnitude, as well as star temperature to a lesser degree. To ensure good-quality profiles, we suggest to select occultations performed with star magnitude <i>M</i> &lt; 2.5 and star temperature <i>T</i> &gt; 6 × 10³ K. Stray-light contamination is negligible for extinction coefficients below 700 nm using occultations performed with a solar zenith angle  &gt; 110° but becomes important at larger wavelengths. Comparison of AerGOM results in the tropics shows an enhanced bias below 20 km that seem to confirm cirrus clouds as its cause. There are also differences between mid-latitude and tropical observations that cannot yet be explained, with a bias difference of up to 25 %.</br></br>This bias characterization is extremely important for data users and might prove valuable for the production of unbiased long-term merged dataset

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