Klaus Bramstedt

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.

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 Intercomparison of Top-of-Atmosphere Reflectivity Measured by MERIS and SCIAMACHY in the Spectral Range of 443–865 nm

This letter is aimed at better understanding of Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) reflectance radiometric calibration errors using the Medium Resolution Imaging Spectrometer (MERIS) onboard ENVISAT. Earlier investigations showed that the SCIAMACHY calibration error can reach 20% in the visible bands, which prevents aerosol retrievals using the SCIAMACHY data. Recent improvements of the SCIAMACHY calibration are discussed. It is found that the differences in reflectances for the wavelengths 443, 560, 665, 754, and 865 nm between MERIS and improved Processor 6 SCIAMACHY data are close to the MERIS radiometric calibration error, which is below 4%

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.

Performance degradation of GOME polarization monitoring

Abstract The Global Ozone Monitoring Experiment (GOME) is a nadir-viewing double spectrometer which measures solar radiation backscattered from the Earths atmosphere over a broad wavelength range, from the ultraviolet (UV) to the near infrared (Burrows, 1998). It has been operating since 1995 on board the ESA ERS-2 satellite, monitoring a large range of atmospheric trace constituents, with particular emphasis on ozone. The performance of the instrument is monitored in-flight by means of routine on-board calibration measurements, observing the sun and, occasionally, the moon. In this way, degradation of optical components in space can be monitored. The performance of the broad-band detectors which monitor the polarization state of the incoming light is analyzed by means of solar measurements. The measurements of the polarization detector which samples UV light show a degradation of 6% per year. The optical components affected can be (partially) identified by monitoring the fractional polarization, which is a characteristic of the light back-scattered by the Earths atmosphere. The influence of the observed degradation on Earth radiation measurements is estimated to be in the order of 1.5% per year in the UV wavelength range.

SCIAMACHY In-Orbit Operations and Performance

Since the launch in early 2002 SCIAMACHY has successfully operated in low-Earth orbit for more than 8 years. For the first several months a challenging Commissioning Phase programme was executed. It successively brought SCIAMACHY into full operation mode and verified the instrument’s functional capabilities. In early August 2002 quasi-routine measurements executing nominal mission scenarios could start. In January 2003 the routine operations phase commenced. Since then SCIAMACHY is kept under strict configuration control. Because of the harsh space environment the instrument is subject to degradation, both optically and thermally. The optical performance is described by the throughput which is a measure for how optical components in a light path age with time. It also includes characterisation of optical imperfections such as scan angle dependence, channel 7 light leak and spatial stray light. Illustrating the thermal performance includes decontaminations, used to tackle the ice layers in channels 7 and 8 and configuration of the thermal control systems to respond to degradation. Finally the improvement of the line-of-sight performance by determination of mispointing angles achieved the best possible pointing knowledge. This was especially needed for the retrieval of accurate limb data products. The current excellent status of SCIAMACHY is a prerequisite for successfully accomplishing the intended ENVISAT mission extension until 2013.

SCIAMACHY Solar Occultation: Ozone and NO 2 Profiles 2002–2007

SCIAMACHY on-board ENVISAT is measuring solar irradiances and Earthshine radiances from the UV to the NIR spectral region in nadir, limb, and lunar as well as solar occultation geometry. Solar occultation measurements are performed in Sun-scanning mode during sunset at northern latitudes between 49°N and 69°N, depending on season. The radiative transfer and retrieval code SCIATRAN 2.1 is used to derive vertical profiles of ozone and NO2 from the SCIAMACHY solar occultation measurements. The retrieval scheme is an optimal estimation approach with Twomey-Tikhonov regularization. Ozone and NO2 are simultaneously retrieved in the fit windows 524–590 nm and 425–453 nm, respectively. Here we present an almost complete dataset from August 2002 to August 2007, including validation results with independent measurements from the satellite instruments HALOE and SAGE II.