K.-U. Eichmann

The Global Ozone Monitoring Experiment (GOME): Mission Concept and First Scientific Results

The Global Ozone Monitoring Experiment (GOME) is a new instrument aboard the European Space Agencys (ESA) Second European Remote Sensing Satellite(ERS-2), which was launched in April 1995. The main scientific objective of the GOME mission is to determine the global distribution of ozone and several other trace gases, which play an important role in the ozone chemistry of the earths stratosphere and troposphere. GOME measures the sunlight scattered from the earths atmosphere and/or reflected by the surface in nadir viewing mode in the spectral region 240-790 nm at a moderate spectral resolution of between 0.2 and 0.4 nm. Using the maximum 960-km across-track swath width, the spatial resolution of a GOME ground pixel is 40 3 320 km2 for the majority of the orbit and global coverage is achieved in three days after 43 orbits. Operational data products of GOME as generated by DLR-DFD, the German Data Processing and Archiving Facility (D-PAF) for GOME, comprise absolute radiometrically calibrated earthshine radiance and solar irradiance spectra (level 1 products) and global distributions of total column amounts of ozone and NO 2 (level 2 products), which are derived using the DOAS approach (Differential Optical Absorption Spectroscopy). (Under certain conditions and some restrictions, the operational data products are publically available from the European Space Agency via the ERS Helpdesk.) In addition to the operational data products, GOME has delivered important information about other minor trace gases such as OClO, volcanic SO2 ,H 2CO from biomass burning, and tropospheric BrO. Using an iterative optimal estimation retrieval scheme, ozone vertical profiles can be derived from the inversion of the UV/VIS spectra. This paper reports on the GOME instrument, its operation mode, and the retrieval techniques, the latter with particular emphasis on DOAS (total column retrieval) and advanced optimal estimation (ozone profile retrieval). Observation of ozone depletion in the recent polar spring seasons in both hemispheres are presented. OClO observed by GOME under twilight conditions provides valuable information on the chlorine activation inside the polar vortex, which is believed to be responsible for the rapid catalytic destruction of ozone. Episodes of enhanced BrO in the Arctic, most likely contained in the marine boundary layer, were observed in early and late spring. Excess tropospheric nitrogen dioxide and ozone have been observed during the recent Indonesian fire in fall 1997. Formaldehyde could also clearly be identified by GOME and is known to be a by-product resulting from biomass burning.

Large loss of total ozone during the Arctic winter of 1999/2000

Three-dimensional model calculations are used together with total ozone observations from the Global Ozone Monitoring Experiment (GOME) and ozone sonde measurements at Ny-Alesund, Spitsbergen to quantify the chemical ozone loss inside the Arctic polar vortex in winter 1999/2000. GOME shows March 2000 mean Arctic total ozone values of 365 DU, about 100 DU less than the 1980-1989 mean from TOMS data, well reproduced by the model calculations. A comparison of the modeled ozone with a passive ozone tracer and ozone sonde observations at Ny-Alesund shows that by the end of March 2000 about 2.5 ppmv of ozone are chemically depleted in the lower stratosphere, corresponding to more than 70% ozone loss. At the same time, the inferred loss in total ozone inside or at the edge of the polar vortex is between 90 and 140 DU. The large ongoing loss during March 2000 is likely to be due to widespread denitrification, which maintains high chlorine activation during this period.

Sciatran - a new radiative transfer model for geophysical applications in the 240–2400 NM spectral region: the pseudo-spherical version

Abstract A radiative transfer model, SCIATRAN, has been developed mainly for the retrieval of atmospheric constituents from global nadir radiance measurements of the SCIAMACHY satellite spectrometer. This is a further development of the successful GOMETRAN. SCIATRAN solves the radiative transfer equation using the Finite Difference Method for a plane-parallel vertically inhomogeneous atmosphere taking into account multiple scattering. The present program version utilizes the pseudo-spherical approach, including refraction, appropriate for solar zenith angles up to about 92°. SCIATRAN comprises, amongst others, the following features: (i) quasi-analytical calculation of weighting functions of atmospheric and surface parameters, (ii) DOAS airmass factor calculations, (iii) height resolved radiation fluxes, including actinic fluxes for photolysis rate calculations, (iv) inelastic rotational Raman scattering by N 2 and O 2 molecules, (v) parameterization schemes for aerosols and clouds, (vi) a line-by-line and a correlated-k distribution mode for line absorption, and (vii) thermal emission. This paper presents an overview of the SCIATRAN model, focusing on the pseudo-spherical approach developed to consider the sphericity of the Earth. SCIATRAN will be made available via the world wide web for non-commercial scientific applications.

The Ozone Hole Breakup in September 2002 as Seen by SCIAMACHY on ENVISAT

An unprecedented stratospheric warming in the Southern Hemisphere in September 2002 led to the breakup of the Antarctic ozone hole into two parts. The Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) on the European Environmental Satellite (ENVISAT ) performed continuous observations of limb-scattered solar radiance spectra throughout the stratospheric warming. Thereby, global measurements of vertical profiles of several important minor constituents are provided with a vertical resolution of about 3 km. In this study, stratospheric profiles of O3 ,N O 2, and BrO retrieved from SCIAMACHY limb-scattering observations together with polar stratospheric cloud (PSC) observations for selected days prior to (12 September), during (27 September), and after (2 October) the ozone hole split are employed to provide a picture of the temporal evolution of the Antarctic stratosphere’s three-dimensional structure.

O3 profiles from GOME satellite data—I: Comparison with ozonesonde measurements

Ozone profiles on a global scale can IX derived from GOME satellite data by minimizing the difference be- tween the measured and the corresponding simulated spec- tra as a function of the vertical distribution of 0s. For this purpose the Full Retrieval Method (FURM) was developed, which is based on the optimal estimation approach and con- tains the radiative transfer code GOMETRAN as an essential component. The quality of the GOME ozone profiles is as- sessed by comparing them with 197 coincident ozonesonde measurements at five selected European stations. The com- parison results show that the seasonal ozone variations are very well reproduced by the GOME profiles. The agree- ment between the GOME and the sonde measurements is best above 18 km altitude where the mean relative difference is below 10 8 and the root mean square of the relative dif- ferences is of the order of 10 6. Larger differences occur in the tropopause region and lowermost stratosphere where the natural ozone variability is largest. Q 1999 Elsevier Science

The cold Arctic winter 1995/96 as observed by GOME and HALOE: Tropospheric wave activity and chemical ozone loss

The Global Ozone Monitoring Experiment aboard the European Remote Sensing Satellite ERS-2 was the only satellite instrument measuring total ozone on a near-global scale during the extremely cold Arctic winter 1995/96. Extremely low total ozone was observed within the Arctic vortex during February and March. The lowest value in this winter was 178 DU (Dobson units) over Greenland on 19 February, which was about 160 DU below the February Arctic vortex mean of total ozone. Although severe chemical ozone destruction occurred in late winter 1995/96, the extremely low values in total ozone observed after the middle of February were, in all cases, related to mini-hole events, where large horizontal divergent transport of ozone from the lower-stratospheric layer above a high tropopause rapidly reduced the total column in a localized region. The observed total-ozone minima were located near the vortex edge and in the region of minimum lower-stratospheric temperatures that were, in selected cases, sufficiently low for the formation of polar stratospheric ice clouds (PSC type II) below 188 K at the isentropic level of 475 K. Coincident ozone profile observations in early March from the Halogen Occultation Experiment on the Upper Atmosphere Research Satellite indicate that the strong chemical ozone loss was mainly confined to the polar vortex region, and that the extremely low total-ozone values below 250 DU were mainly caused by short-term reversible dynamical reductions superimposed upon chemical ozone loss occurring on longer timescales. Enhanced OClO and chlorine activation, due to strong tropospheric wave activity associated with an ozone mini-hole event, was only observed in early March following a stratospheric temperature drop below the ice frost point. In general, however, the observation of very low total ozone in mini-hole events does not necessarily point to significant additional chemical depletion. Copyright

O3 Profiles from GOME satellite data—II: Observations in the Arctic Spring 1997 and 1998

Abstract Ozone observations by the Global Ozone Monitoring Experiment (GOME) on board the ERS-2 satellite during the Arctic spring periods 1997 and 1998 are presented. From the derived ozone vertical distributions, extensive regions of low ozone total column were observed and it is shown that the major decrease is dominating in the lower and middle stratosphere inside the polar vortex. The winter 1997/98 was warmer than the year before and less ozone depletion was observed. In spring 1998 an ozone mini-hole event was observed by GOME and ozone profiles under minihole conditions were derived for the first time.

UTLS water vapour from SCIAMACHY limb measurements V3.01 (2002–2012)

The SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) aboard the Envisat satellite provided measurements from August 2002 until April 2012. SCIAMACHY measured the scattered or direct sunlight using different observation geometries. The limb viewing geometry allows the retrieval of water vapour at about 10-25 km height from the near-infrared spectral range (1353-1410 nm). These data cover the upper troposphere and lower stratosphere (UTLS), a region in the atmosphere which is of special interest for a variety of dynamical and chemical processes as well as for the radiative forcing. Here, the latest data version of water vapour (V3.01) from SCIAMACHY limb measurements is presented and validated by comparisons with data sets from other satellite and in situ measurements. Considering retrieval tests and the results of these comparisons, the V3.01 data are reliable from about 11 to 23 km and the best results are found in the middle of the profiles between about 14 and 20 km. Above 20 km in the extra tropics V3.01 is drier than all other data sets. Additionally, for altitudes above about 19 km, the vertical resolution of the retrieved profile is not sufficient to resolve signals with a short vertical structure like the tape recorder. Below 14 km, SCIAMACHY water vapour V3.01 is wetter than most collocated data sets, but the high variability of water vapour in the troposphere complicates the comparison. For 14-20 km height, the expected errors from the retrieval and simulations and the mean differences to collocated data sets are usually smaller than 10 % when the resolution of the SCIAMACHY data is taken into account. In general, the temporal changes agree well with collocated data sets except for the Northern Hemisphere extratropical stratosphere, where larger differences are observed. This indicates a possible drift in V3.01 most probably caused by the incomplete treatment of volcanic aerosols in the retrieval. In all other regions a good temporal stability is shown. In the tropical stratosphere an increase in water vapour is found between 2002 and 2012, which is in agreement with other satellite data sets for overlapping time periods.

GOME ozone profiles: a global validation with HALOE measurements

Abstract The Global Ozone Monitoring Experiment (GOME) aboard ESAs ERS-2 satellite measures the reflected and backscattered radiation from the earth in the UV/visible spectral range at moderate spectral resolution. Vertical ozone profiles can be derived from top-of-atmosphere (TOA) nadir observations using the FUll Retrieval Method FURM, which is based upon an advanced Optimal Estimation inversion scheme. These ozone profiles are validated with profiles from the HALogen Occultation Experiment (HALOE). For the year 1998, over 2100 coincident measurements of the instruments were found. These measurements are divided into 20 subsets of five zonal bands and four seasons. For each subset, the mean relative deviation between the corresponding profiles are calculated. In most cases the mean deviations between the HALOE and GOME profiles are below 10 % for the altitude range from 15 to 35 km.

Ozone profile distributions in the Arctic from GOME satellite observations during spring 1997 and 1998

The Global Ozone Monitoring Experiment (GOME) aboard ESAs ERS-2 satellite measures the reflected and backscattered radiation from the Earth in the UV/visible spectral range at moderate spectral resolution. Vertical ozone profile distributions can be derived form top-of-atmosphere nadir observations using the Full Retrieval Method FURM, which is based upon an advanced Optimal Estimation inversion scheme. During the Arctic spring seasons 1997 and 1998 hemispheric ozone profile distributions have been derived from GOME observations. In 1997 the polar vortex formed late in winter and record low temperatures were reached in late March. In the lower stratosphere depleted levels of ozone were observed by GOME. From vertical ozone distributions inside the polar vortex obtained on 24 days between 9 March and 16 May 1997, chemical ozone loss rates in the lower stratosphere are estimated. The lower stratospheric temperatures in spring 1998 were comparable to the longterm mean and the polar vortex was relatively weak. Preliminary results from this period are also presented. In both spring seasons ozone mini-hole events, which are characterized by intrusion of ozone poor subtropical air into mid and polar latitudes, were observed. From transects of GOME orbits the 2D structure of the zone mini-hole can be studied in detail.