ABSOLUTE CALIBRATION AND DEGRADATION OF SCIAMACHY/GOME REFLECTANCES

Transcription

1 ABSOLUTE CALIBRATION AND DEGRADATION OF SCIAMACHY/GOME REFLECTANCES J.M.Krijger, R.Snel, I.Aben, and J.Landgraf SRON, Netherlands Institute for Space Research ABSTRACT The goal of this study is to develop a method to quantify SCIAMACHY degradation and calibration in the UV, where the focus is on the O 3 profile wavelength range nm. Awaiting SCIAMACHY data reprocessing, the method has been performed on GOME observations, SCIAMACHY predecessor. The approach followed is to compare GOME reflectivity spectra with simulated spectra using collocated independent measurements of O 3, temperature and pressure. For this a vector radiative transfer model (RTM) was used to avoid errors introduced by the GOME polarisation correction algorithm, (degradation of) the GOME polarisation measurements and errors in the forward radiances introduced when using a scalar RTM. Key words: GOME, SCIAMACHY, Degradation, Calibration, Scan-mirror. GOME sensor shows degradation in all wavelength regions due to damages in its optical path. In practice it is impossible to isolate all individual (optical) components as only the overall effect can be monitored through specific observations such as the daily solar observations of GOME. Currently several changes in sensitivity of the instrument are simultaneous monitored: degradation, geometric changes in optical paths, changes of coatings. This study was funded by NIVR (SCIAMACHY Validation) and ESA (SCIAMACHY CHEOPS and GOME CHEOPS). First we describe the radiative model in section 2. The different measurements (from GOME and collocated independent measurements) are described in section 3, following with the results in section 4. The main conclusions are summarised in section INTRODUCTION 2. MODEL DESCRIPTION Satellite-based passive remote sensing is commonly used to derive global information about the composition of the Earth s atmosphere. Information about the total column or even vertical profiles of different gases in the earth atmosphere can be obtained by measuring the radiance (intensity) spectrum of sunlight reflected by the Earth s atmosphere, since these spectra contain absorption bands of gases present in the atmosphere, such as ozone. The Global Ozone Monitoring Experiment (GOME) is an operational space-based spectrometer that measures the radiance of reflected sunlight in the ultraviolet, visible and near-infrared wavelength range ( nm) with modest spectral resolution ( nm) (Burrows et al. 1999). GOME is on ESA s second European Remote Sensing satellite (ERS-2) which was launched on 21 April Comparison of the solar irradiance spectra measured by GOME through the lifetime of the sensor with early GOME solar irradiance spectra and other space instruments, showed that the pre-flight radiance parameters were no longer applicable to the GOME in-flight situation (Eisinger et al. 1996; Peeters et al. 1996; Peeters & Simon 1997; Aben et al. 2000; Hegels et al. 2000). The 2.1. Radiative Model In order to model the reflectance to compare with GOME measurements, it is necessary to use a radiative transfer model. The radiative transfer model developed at SRON solves the plane parallel radiative transfer equation using the Gauss-Seidel iterative method. Multiple scattering and polarisation are fully included in the model. A detailed description of the model is given by Landgraf et al. (2001) for the scalar case and the extension to polarisation is described by Hasekamp & Landgraf (2002). The inclusion of polarisation in the radiative transfer calculations omits errors of up to 10% in the UV on the radiance made by the commonly used scalar radiative transfer models, that neglect polarisation properties of light. Another, advantage of a radiative transfer model including polarisation is that it allows a direct modelling of the polarisation sensitive GOME measurement using the Mueller matrix formalism. In this way errors of up to 8% in the GOME data processor polarisation correction and uncorrected separate degradation effects of the Polarisation Monitoring Devices (Krijger et al. 2005) are omitted. Proc. Envisat Symposium 2007, Montreux, Switzerland April 2007 (ESA SP-636, July 2007)

2 2.2. Studied wavelength range GOME measures between nm, however the low signal shortwave radiances measured by GOME below 270 nm do not allow a comparison of high enough signal/noise ratio with model-calculations. As such the focus here is on wavelengths above 270 nm. Towards longer wavelengths the solar light travels further through the atmosphere before being scattered (towards GOME) as ozone absorption decreases at longer wavelengths above 300 nm. As such physical parameters as surface albedo and aerosols have to be included in the calculations. These parameters vary over the globe and have to be known in order to include them in the calculations. The radiative transfer model used does not include aerosols as no measured information about atmospheric aerosol content was available. By fitting the model spectra to the measured spectra at a specific wavelength range (here: nm) an effective Lambertian reflectivity is derived for these selected wavelengths, including both surface albedo and aerosols. The individual effect of both parameters cannot be distinguished, while both parameters have a different behaviour over wavelength. In the RTM-model a constant effective reflectivity is assumed over wavelength, causing errors due to the ignoring of the different behaviour over wavelength. Therefore a sensitivity study has been performed to estimate the effect of not accounting for aerosols and surface albedo wavelength dependent behaviour in the model. This study showed that this effect already becomes non-neglectable above 330 nm, which reduces the relevant wavelength range for this study to below 330 nm. 3. DATA DESCRIPTION 3.1. NNORSY database & Climatology In order to simulate the measured reflectance various physical parameters at the location and time of measurement are needed. In this study ozone, temperature and pressure profiles are used by the RTM calculation. These collocated profiles were provided by A. Kaifel (private communication, 2004) as part of the GOME-NNORSY training-set [CHEOPS/ZSW/ATBD/001]. For a detailed description of the dataset, see [CHEOPS/ZSW/ATBD/001]. For this study only sonde measurements with at least information between 1 and 25 km in the period between June 1995 and December 2003 with a GOME measurement within 11 hrs and 300 km were selected. A total of collocations were eventually available for this study (including collocations over clouded ground sites). All ozone profiles were provided on a 1-km resolution grid. As sondes become unreliable above 30 km, the information was cut off above a randomly determined height between 25 and 30 km, in order to avoid any systematic effects. Above this cut off height, climatological ozone values are added from Fortuin & Kelder (1998) Figure 1. Time series of fitted LER-values over the Libyan Desert (23-30 N, E), at 325 nm and 335 nm. climatology and UKMO temperature profiles. The two 1-km layers below the cut off height are replaced with an average of the sonde-measured values and climatological values, in order to avoid jumps in the applied profiles. The ground pressure measured by the sonde is used to derive a pressure grid for the RTM-model GOME data The GOME data used in this study was extracted with the DLR-GOME-data-extractor version 2.3 (Slijkhuis 1999). The period studied starts in May 1996 till December The measurements were corrected with the standard options (leakage, FPA, Fixed, Straylight, Normalized, Intensity), but no polarisation correction was applied as this is compensated with the polarisation RTM calculations. Other options used, all improvements to earlier versions of the DLR-gome-data-extractor, are the performance of a wavelength cross-correlation, a correction for the Peltier Cooler effect and an improved BSDF correction (Slijkhuis 2004). During the period the GOME data processor suffered from a problem that no new solar spectra were written to the GOME data files. As such we used a special ESA dataset containing updated and correct daily solar reference spectra during this period. In order to increase signal-to-noise ratio the GOME instrument co-adds measurements at wavelengths below 283 nm over 12 sec, while measurements with wavelengths higher than 283 nm are integrated over 1.5 sec. For this study only ground pixels of 12 sec integration time are used, co-adding 1.5 sec ground-pixels ourselves where necessary Degradation at 325 nm The fitting of an effective reflectivity at 325 nm is used to compensate for various effects, like albedo, aerosols and clouds. However the work of (Koelemeijer et al. 2003) showed already degradation at 335 nm starting around the

3 Figure 2. Degradation for different GOME viewing angles (assuming a constant LER) over the Libyan Desert (23-30 N, E), at 325 nm. The average for each time series is shown in orange. Figure 3. GOME reflectance degradation as function of wavelength and time (averaged per month). Note the erroneous channel boundaries (around 314 nm) and the Fe +, Mg +, Mg and Si lines around 276, 280, 285 and 288 nm respectively. year Therefore it is likely that degradation has also affected the 325 nm wavelength range during the studied period ( ). By fitting an effective reflectivity at wavelengths around 325 nm any degradation at these wavelengths is compensated for, which is undesirable for this study. In order to determine the degradation at these wavelengths a study similar to Koelemeijer et al. (2003) was done by determining the effective reflectivity over the Libyan Desert. The effective reflectivity of the Libyan Desert (23-30 North, East) is very constant over time and the Libyan Desert is rarely clouded. Only GOME channel 2 pixels (1.5 sec integration) are selected instead of the larger 12 sec integration pixels. No backscan pixels were used. All GOME measurements over the Libyan Desert with a cloud fraction smaller then 0.20, as provided by TEMIS, were selected and an effective reflectivity fit was made using the RTM-model. For ozone, temperature and pressure profiles the AFGL mid-latitude summer profiles were used. The ozone profile was scaled with the total ozone column from TEMIS. Fig. 1 shows the effective reflectivities resulting from a fit at 335 nm and 325 nm as a function of time. The results at 335 nm are identical to those found by Koelemeijer et al. (2003) and shows that degradation clearly starts around the year For the results at 325 nm, beside an offset, the temporal behaviour is the same, showing that degradation started around the same time for both wavelengths. These results show that fitting an effective reflectivity to the measurements at 325 nm cannot be done without degradation correction. By assuming the effective reflectivity of the Libyan Desert does not change with time, an average effective reflectivity before 1999 was determined and used for all model calculations to determine the degradation at 325 nm by comparing measured with modelled reflectance at 325 nm. The results are shown in Fig. 2. No degradation is present until After 1999 degradation causes an overestimation of the GOME reflectance and a strong viewing angle dependent degradation is visible. However in the rest of this study only the longer integration (12 sec) pixels are used, which are 8 co-added 1.5 sec integration pixel. Backscan pixels use the same angles as East, Nadir and West and will thus Figure 4. GOME reflectance degradation as function of wavelength and time. The degradation is averaged over a year and shown colour-coded as indicated in the legend. Note the erroneous channel boundaries (around 314 nm) and the Fe +, Mg +, Mg and Si lines around 276, 280, 285 and 288 nm respectively. have a similar angle-dependent degradation. Therefore the average degradation of East, Nadir and West viewing angles was determined and used in the remainder of this study to correct for degradation at 325 nm. At special request from DLR also the degradation at 385 nm was determined with the same method, to avoid sharp transitions for degradation corrections beyond 325 nm. 4. RESULTS 4.1. Observed degradation Each modelled spectrum is individually compared to each corresponding collocated spectrum measured by GOME, using all earlier discussed corrections. The comparison spectra are then averaged over a month and the standard deviation over this period is determined. All comparison spectra outside of three times the standard deviation are removed from the dataset and the monthly average and

4 Figure 5. Initial correction for the radiometric calibration, determined over the period June 1995 Dec In black the standard deviation on the correction factor. Figure 7. Similar for Fig. 6, but now with degradation for 385 nm added and removal of yearly oscillations. Figure 6. GOME reflectance degradation since June 1995 for different wavelength ranges as function of time. standard deviation are re-determined from this cleaned dataset. From these comparison spectra the degradation can be determined. The resulting monthly averaged degradation is shown in Fig. 3 as function of wavelength and time, with the colour indicating the amount of degradation. Erroneous results can be seen around the channel boundaries at nm, due to the very low GOME sensitivity at these wavelengths. As the RTM-model does not include any Earth atmospheric emission lines and solar Fraunhofer lines (through their Ring-effect) from Fe +, Mg +, Mg and Si lines around 276, 280, 285 and 288 nm respectively, are visible in the results. Fig. 4 shows the degradation as a function of wavelength but now averaged over a whole year. Similarly as the work of van der A et al. (2002) the figure shows that already at the beginning of the GOME lifetime, before any degradation, GOME overestimates the radiance by about 10% below 300 nm. This initial re-calibration of GOME is shown in Fig. 5. The shown curve is determined by averaging all comparison spectra between June 1995 and December During this period the comparison spectra stay relatively constant and show no sign of degradation. The derived standard deviation over this period also illustrates this. The initial re-calibration can be explained by either a systematic deviation in ozone climatology, as wavelengths below 290 nm are determined by ozone above 35 km, for which climatological values have been used. Or the initial GOME calibration was erroneous. Fig. 4 and Fig. 3 show all averaged comparison spectra corrected by the initial re-calibration, thus showing only the actual degradation since Note that many of the sharp features from Fig. 4 disappear in Fig. 3, as they are divided out by the initial recalibration. This can be an indication that many of these features are GOME calibration problems, most likely calibration differences between Earth radiances and solar irradiance. The remaining strong line is the result of the absorption line from Mg at 285 nm in the GOME measured radiance. GOME measures only a few binary units in this absorption feature in the Earth Radiance, decreasing even more in time due to degradation. At these low read-outs GOME corrections become uncertain (e.g., due the Peltier Cooler effect). Any small erroneous absolute correction will result in a large relative difference. As degradation increases so does this effect, resulting in a increasingly stronger Mg line feature. Fig. 6 shows the same information but now degradation as a function of time for different selected wavelengths bins. In the period a slight yearly variation is visible. These variations are likely a result from the method (solar angle or ozone variations) and not real GOME degradation. As such a sinoid has been fitted to each wavelength during this period and then subtracted for the full period Any residuals during were attributed to noise and set to zero as not to affect any GOME data that will be corrected using this degradation. Finally the degradation was interpolated over the Earth atmospheric emission lines from Fe +, Mg +, Mg and Si lines around 276, 280, 285 and 288 nm respectively. Fig. 7 shows the resulting actual GOME degradation since The degradation for 385 nm, as determined over the Libyan Desert has been added, allowing interpolation of degradation for wavelengths in GOME channel 2 beyond 325 nm. These final degradation correction data and the initial recalibration have been made available for integration into the next version of the DLR-GOME-dataextractor.

5 Figure 8. Modelled degradation by changes in the reflective properties of the scan mirror. Figure 9. Contaminant layer thickness as derived from a fit to the solar observations Modelled degradation The temporal behaviour of the degradation can be qualitatively explained by changes in the reflective properties of the scan mirror. The GOME scan mirror is coated with MgF 2 in order to mechanically protect the mirror surface and increase the reflectivity at low wavelengths. The layer thickness is approximately 100 nm. When MgF 2 is vacuum deposited on a substrate, it is not possible to achieve a perfectly dense layer. The layer contains voids, which change the effective refractive index of the layer. Depending on the environmental conditions, the voids may be filled with other substances, including absorbing materials. A model was constructed by (Snel 2000) at SRON describing the mirror and coating optical properties. The degradation model used for the scan mirror assumes a pure aluminium mirror, covered with a porous MgF 2 layer. The layer thickness and porosity are model parameters. Indices of refraction for aluminium and MgF 2 are taken from (Palik 1985). With the Fresnel equations (Azzam & Bashara 1987, e.g.,), the reflection coefficient can be calculated. The effective refractive index of the coating is a function of the porosity and the medium used to fill the pores with. Degradation is modelled by gradually filling the pores and then covering the mirror with the contaminant. The refractive index of the contaminant is fitted for each detector pixel wavelength using the observed signal in the daily solar measurements (See Fig. 9). The amount of contaminant as a function of time is shown in Fig. 10. Shown in Fig. 8 is the model degradation as a function of contaminant thickness for several wavelengths. The Figure 10. Complex Refractive index of the mirror contaminant as derived from a fit to the solar observations. MgF 2 layer thickness was adjusted to 140 nm (somewhat more than the 100 nm the mirror was supposedly coated with, but still reasonable). The density of the layer was set at 80%. The case of (constant) linearly increasing contaminant thickness is shown. The similarity with Fig. 7 is striking. Both model and observations show temporal cyclic behaviour, with degradation increasing and decreasing over time. Also the behaviour for different wavelengths (shorter wavelengths degrading before longer wavelengths) is found in both model and observations. Note that the modelled degradation ratio becomes smaller then 1, while this is never observed. Varying different model parameters does not change this. Therefore it is likely another degradation factor beside mirrorcontaminant is present. 5. CONCLUSIONS This work shows several important issues concerning the degradation of the GOME instrument and the effect on the GOME reflectances. An initial offset was found, with measured reflectances in the UV below 300 nm overestimated by around 10 %, already during the beginning of the mission when no degradation is expected. The exact cause could be either ozone climatology biases or GOME calibration problems. We believe the latter is the mostly prime cause. Significant degradation for the short UV wavelengths started in 1998, while in the beginning of 2000 degradation becomes also significant at wavelengths at least up to 335 nm. Degradation after January 2000 shows that the GOME degradation is viewing-angle dependent at wavelengths around nm. Due to the co-adding of wavelengths shorter then 283 nm in this study, this angle-dependence cannot be determined for shorter wavelengths. The effect of GOME degradation on the measured reflectances is apparently of a cyclic nature and can be

6 qualitatively explained by a small dirt layer forming on the scan mirror. These successful results also show the possibility of applying the method to SCIAMACHY reflectances (work currently under progress). REFERENCES Aben I., Eisinger M., Hegels E., Snel R., Tanzi C.P., 2000, GDAQI Final report, Report TN-GDAQI- 003SR/2000, ESA Azzam M. R., Bashara N., 1987, Ellipsometry and Polarized Light, Elsevier Burrows J.P., Dehn A., Deters B., et al., 1999, J. Quant. Spectrosc. Radiat. Transfer, 61, 509 Eisinger J. M., Burrows J., Richter A., 1996, In: GOME Geophysical Validation Campaign, ESA WPP-108, Fortuin J.P.F., Kelder H., Dec. 1998, J. Geophys. Res., 103, Hasekamp O., Landgraf J., 2002, J. Quant. Spectrosc. Radiat. Transfer, 75, 221 Hegels E., Loyola D., Hummel S., Slijkhuis S., Thomas W., 2000, In: ERS-ENVISAT Symposium Looking down to Earth in the New Millennium, CD ROM Koelemeijer R.B.A., de Haan J.F., Stammes P., Jan. 2003, Journal of Geophysical Research (Atmospheres), 108, 8 Krijger J.M., Tanzi C.P., Aben I., Paul F., Apr. 2005, Journal of Geophysical Research (Atmospheres), 110, 7305 Landgraf J., Hasekamp O., Trautmann T., Box M., 2001, J. Geophys. Res., 106, 27,291 Palik E.D., 1985, Handbook of optical constants of solids, Academic Press Handbook Series, New York: Academic Press, 1985, edited by Palik, Edward D. Peeters P., Simon P.C., 1997, In: ESA SP-414: Third ERS Symposium on Space at the service of our Environment, Peeters P., Simon G., Rottman G., Woods T., 1996, In: GOME Geophysical Validation Campaign, ESA WPP- 108, Slijkhuis S., 1999, GOME Date Processor - Extraction Software User s Manual, Report ER-SUM-DLR-GO- 0045, DLR Slijkhuis S., 2004, CHEOPS-GOME Study on Seasonal effects on the ERS-2/GOME Diffuser BSDF, Report CH-TN-DLR-GO-0001, DLR Snel R., 2000, In: ERS-ENVISAT Symposium Looking down to Earth in the New Millennium, CD ROM van der A R.J., van Oss R.F., Piters A.J.M., et al., Aug. 2002, Journal of Geophysical Research (Atmospheres), 107, 2