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Home , Carbon monoxide, Methane

, AND RETRIEVED FROM SCIAMACHY NEAR-INFRARED NADIR OBSERVATIONS USING WFM-DOAS

M. Buchwitz 1), R. de Beek 1), J. P. Burrows 1), H. Bovensmann 1), B. Dils 2), and M. De Mazière 2)

1) Institute of Environmental Physics (iup) / Institute of Remote Sensing (ife), University of Bremen FB1, Otto Hahn Allee 1, 28334 Bremen, Germany, E-mail: [email protected] 2) Belgian Institute for Space Aeronomy (BIRA-IASB), 3, Avenue Circulaire, 1180, Brussels, Belgium

ABSTRACT

The three “carbon ” (CO), methane (CH4) and carbon dioxide (CO2) are important atmospheric constituents affecting air quality and climate. The near-infrared nadir spectra of reflected solar radiation measured by SCIAMACHY on-board ENVISAT contain information on the vertical columns of these gases which we retrieve using the scientific algorithm WFM-DOAS. For CH4 and CO2 our main data products are dry air column averaged mixing ratios (XCH4 in ppbv and XCO2 in ppmv) determined by simultaneous measurements of the dry air mass obtained from 2 (O2) for CO2 and CO2 for methane. Our CO data product is the CO vertical column in /cm . The SCIAMACHY data set is unique because of the high sensitivity of the near-infrared measurements with respect to concentration changes in the atmospheric boundary layer. This sensitivity is a pre-requisite to get information on regional surface sources and sinks which are currently only poorly constrained globally by atmospheric measurements. We present a short overview about the latest versions of the retrieval algorithm (WFM-DOAS version 0.5 for CO and CH4 and version 0.4 for CO2) and processed data (year 2003 data set based on Level 1 version 4 spectra) including comparisons with global and local reference data (MOPITT, ground based FTS, and global models).

1. INTRODUCTION

Carbon monoxide (CO) is an important air affecting local air quality and carbon dioxide (CO2) and methane (CH4) are the two most important anthropogenic greenhouse gases and contribute to global . The near- infrared nadir spectra of reflected solar radiation measured by SCIAMACHY [1] on-board ENVISAT contain information on the vertical columns of these gases which we retrieve using the scientific algorithm WFM-DOAS [2- 6,8]. Here we present the latest results for CO and methane obtained with version 0.5 of the WFM-DOAS retrieval method [6,8]. The CO2 results have been obtained with WFM-DOAS version 0.4 [4-6,8]. This v0.5 CO and methane and v0.4 CO2 data set has recently been compared with a network of ground based FTS measurements [9]. A few days of v0.4 CO2 data has also been compared earlier with ship-based FTS measurements [11]. Here we present a short summary of the current status of this activity.

2. TRACE RESULTS

We have processed all available SCIAMACHY Level 1 version 4 spectra of the year 2003 [6,8]. Because of the low reflectivity of (oceans, great ) in the near-infrared the quality of the measurements over water is typically reduced and only measurements over land are discussed here.

2.1 Carbon monoxide (CO)

Carbon monoxide columns are retrieved from a small spectral fitting window located in SCIAMACHY channel 8. The latest version of the retrieval algorithm is WFM-DOAS version 0.5 [6,8]. Figure 1 shows a comparison of SCIAMACHY CO with the operational CO column data product of MOPITT [7]. On average the SCIAMACHY CO columns are higher by about 10% but regionally, especially over , the differences can be much larger. More details are given in [6,8]. Fig. 4 shows an overview about the comparison with ground based FTS measurements (11 NDSC stations). As can be seen, the differences between WFM-DOAS version 0.5 CO and ground based FTS over the northern hemisphere are typically less than 10%. Over the southern hemisphere differences are typically around 20%. More details are given in Tab. 1 and [9].

Fig. 1: Top left panel: Year 2003 carbon monoxide vertical columns as retrieved from SCIAMACHY using the WFM- DOAS version 0.5 retrieval algorithm. Spatially co-located SCIAMACHY and MOPITT CO columns: Top right for SCIAMACHY and bottom left for MOPITT. The percentage difference SCIAMACHY – MOPITT is shown in the bottom right panel.

2.2 Methane (CH4)

Methane dry air column averaged mixing ratios are retrieved from a small spectral fitting window located in SCIAMACHY channel 6 [6,8]. CO2 from channel 6 is used as a for the light path and the CO2 column is used to estimate the air column necessary to compute a methane column averaged mixing ratio (see also [10]). The latest version of the retrieval algorithm is WFM-DOAS version 0.5 [6,8]. Figure 2 shows SCIAMACHY v0.5 methane over various source regions. Fig. 4 shows an overview about the comparison with ground based FTS measurements (11 NDSC stations). As can be seen, the differences between WFM-DOAS version 0.5 methane and ground based FTS are within about 2% (standard deviation). More details are given in Tab. 1 and [9].

Fig. 2: Methane column averaged mixing ratios as retrieved from SCIAMACHY using WFM-DOAS version 0.5 globally (a) and for various regions (b-f). The SCIAMACHY measurements are solar zenith angle bias corrected (see [6]) and scaled with 1.02.

2.3 Carbon dioxide (CO2)

CO2 dry air column averaged mixing ratios are retrieved from a small spectral fitting window located in SCIAMACHY channel 6 [4,5,6,8]. O2 from channel 4 is used as a proxy for the light path and the O2 column is used to estimate the air column necessary to compute a CO2 column averaged mixing ratio. The latest version of the retrieval algorithm is WFM-DOAS version 0.4 [4,5,6,8]. Fig. 3 shows a comparison with global model simulations. Tab. 1 summarizes the results of the comparison with FTS. Note that only a few stations measure column averaged CO2. The comparison with the FTS is based on three non-ideal stations (Jungfraujoch: mountain station not observing the boundary layer; Ny Alesund: high latitude stations (low , low /ice albedo), and Egbert (strong influence of local sources)). More details are given in [9].

Fig. 3: Top: Carbon dioxide column averaged mixing ratios as retrieved from SCIAMACHY using WFM-DOAS version 0.4. Bottom: Corresponding TM3 model simulations (courtesy S. Körner and M. Heimann, MPI-, Jena, Germany). The SCIAMACHY averaging kernels have not been considered for TM3.

3. SUMMARY COMPARISON WITH GROUND-BASED FTS

Recently the SCIAMACHY WFM-DOAS year 2003 data set has been compared with a network of ground based FTS stations [9]. Figure 4 gives an overview about the stations and the mean relative difference of SCIAMACHY CO and methane with respect to FTS. The main results of this comparison are summarized in Tab. 1.

SCIAMACHY Data Product N Bias [%] Scatter [%] R [-] CO column WFM-DOAS v0.5 22362 -0.01 25.1 0.86 Methane column averaged mixing ratio XCH4 WFM-DOAS v0.5 (SZA bias corrected) 42072 -3.28 1.93 0.80 WFM-DOAS v0.5 (not corrected) 42072 -4.09 3.36 0.72 CO2 column averaged mixing ratio XCO2 WFM-DOAS v0.4 7704 -6.95 3.78 0.42 (limited comparison with three non-ideal stations) Tab. 1: Summary of the comparison of the SCIAMACHY WFM-DOAS v0.5/v0.4 CO, methane and CO2 data set with ground based FTS measurements. N = number of SCIAMACHY measurements compared with the FTS measurements, Bias is the relative difference SCIA-FTS in percent, Scatter is the standard deviation of the difference in percent, and R is the correlation coefficient (more details are given in [9]).

Fig. 4: Comparison of SCIAMACHY CO columns (blue: WFM-DOAS version 0.5 = WFMD) and methane column averaged mixing ratios (blue: WFMD) with ground based FTS measurements (top right: list of stations). IMAP and IMLM are two independent retrieval algorithms developed by University of Heidelberg and SRON, respectively (see [9] for details)

5. CONCLUSIONS AND OUTLOOK

We have presented a short overview about the latest status of the retrieval of CO, methane and carbon dioxide columns from SCIAMACHY near-infrared nadir spectra (Level 1 version 4) using the WFM-DOAS algorithm. Our next steps are: Reprocessing of the year 2003 data set using Level 1 version 5 spectra (improved calibration) and processing and analysis of more data (2004, 2005, …). Information about the latest status is given on our SCIAMACHY/WFM-DOAS web page: http://www.iup.physik.uni-bremen.de/sciamachy/NIR_NADIR_WFM_DOAS/index.html .

ACKNOWLEDGEMENTS

We thank ESA and DLR for providing us with the SCIAMACHY spectra (operational Level 1 data products). The MOPITT data have been obtained from the NASA Langley DAAC. The TM3 model data have been obtained from S. Körner and M. Heimann, MPI Biogeochemistry, Jena, Germany. Funding for this study came from the German Ministry for Research and Education (BMBF) via DLR-Bonn (project SADOS), the EC 5th FP on Energy, Environment and Sustainable Development (EVERGREEN, see http://www.knmi.nl/evergreen ), from ESA (GMES-PROMOTE, see http://www.gse-promote.org ) and from the University and the State of Bremen. We acknowledged exchange of information within the EU 6th FP Network of Excellence ACCENT (http://www.accent-network.org ).

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