retrieval for the airborne APEX imaging spectrometer 7 th DOAS Workshop 68 July 2015 Gerrit Kuhlmann 1 Andreas Hueni 2 Alexander Damm 2 and Dominik Brunner 1 1 Empa Swiss Federal Laboratories for Materials Science and Technology Switzerland ID: 1024426
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1. Improving the Empa NO2 retrievalfor the airborne APEX imaging spectrometer7th DOAS Workshop, 6-8 July 2015 Gerrit Kuhlmann1, Andreas Hueni2, Alexander Damm2 and Dominik Brunner11 Empa, Swiss Federal Laboratories for Materials Science and Technology, Switzerland2 Remote Sensing Laboratories, University of Zurich, Switzerland
2. Empa APEX NO2 retrieval – Zurich 2010APEX campaign over ZurichSat. 26. June 201010:00 local time17:30 local time2Kuhlmann et al., Improving the Empa APEX NO2 Retrieval, 7th DOAS Workshop, 2015Popp, C., Brunner, D., Damm, A., Van Roozendael, M., Fayt, C., and Buchmann, B.: High-resolution NO2 remote sensing from the Airborne Prism EXperiment (APEX) imaging spectrometer, Atmos. Meas. Tech., 5, 2211-2225, doi:10.5194/amt-5-2211-2012, 2012.APEX morning overflight over Zurich
3. APEX – Airborne Prism EXperimenthyperspectral imaging spectrometervisible and near-infrared (VNIR) and short wave infrared (SWIR)designed for environmental remote sensing of land surface3Kuhlmann et al., Improving the Empa APEX NO2 Retrieval, 7th DOAS Workshop, 2015VNIRSpectral range380.5 to 971.7 nmSpectral sampling interval (SSI)0.55 to 8 nmSpectral resolution (FWHM)0.6 to 6.3 nmSpatial pixels (across track)1000Field of view28 degreeSpatial resolution5×2.5 m2 at 5000 m above groundSignal-to-noise ratio(400 to 550 nm)65 to 245920 to 3455single pixel10×20 pixelsSelected characteristics of APEXAPEX instrument
4. APEX campaign: Zürich 2013Friday, 30.8.2013, 11:24-12:05 UTC (LT = UTC+2)4Kuhlmann et al., Improving the Empa APEX NO2 Retrieval, 7th DOAS Workshop, 201526 km10 kmTrue colour composite of 4 flight stripesZürichTROPOMI (7×7km2)
5. Conditions during campaign5Kuhlmann et al., Improving the Empa APEX NO2 Retrieval, 7th DOAS Workshop, 2015APEXNO230 μg m-3boundary layer height1 kmwind speed1m s-1, northerlytemperature22 ˚Cglobal radiation800 Wm-2APEXNO2 concentrations measured byground-based monitoring networkOverview over various parameter
6. Empa APEX NO2 retrieval (Version 2.0-alpha) 6Kuhlmann et al., Improving the Empa APEX NO2 Retrieval, 7th DOAS Workshop, 2015Level 0 -> Level 1 processing (RSL)Level 0 Level 1raw countsNO2 dSCDradiance spectra (spectrum & reference)NO2 dSCD fit (Empa)NO2 cross sectionO4 cross sectionRing cross sectionDegradation to APEX CW and FWHM Solar spectrumUndersampling correction440 - 510 nmpolynomial (5th order)offset (2nd order)Improvement of Level 1 (Empa)Spectral calibration usinghigh-resolution solar spectrumGaussian slit function FWHMCentral wavelength (CW)400 - 600 nm, 42 sub-windows, ~30 nm, polynomial (3rd order)Solar spectrum
7. In-flight spectral calibration: One spectrum7Kuhlmann et al., Improving the Empa APEX NO2 Retrieval, 7th DOAS Workshop, 2015Wavelength shiftremove outliner (shift > 0.20 nm or fitting error 0.20 nm)fit cubic smoothing splineExample for single spectrumFWHM of slit functionlinear scaling of lab calibration400-450nm and 500-550nm
8. In-flight spectral calibration: All spectra8Kuhlmann et al., Improving the Empa APEX NO2 Retrieval, 7th DOAS Workshop, 2015ISF FWHM: 1.7 to 2.5 nmsampling interval (SSI): 1.1 nmsampling ratio: 1.5 to 2.5Wavelength deviation from mean (488.0 nm)averaged in across- and along-track directionspectral smile in across-track directionalong-track changes likely due to pressure changes in spectrometerFWHM deviation from mean (2.0 nm)averaged in across- and along-track direction
9. NO2 differential slant column densities (dSCD)9Kuhlmann et al., Improving the Empa APEX NO2 Retrieval, 7th DOAS Workshop, 2015reference areaNO2 dSCDNO2 dSCD errors
10. Distribution of dSCDs and errorsfor all four stripesNO2 differential slant column densities (dSCD)10Kuhlmann et al., Improving the Empa APEX NO2 Retrieval, 7th DOAS Workshop, 2015
11. VCDdSCDAMFVertical column densities (VCD) and air mass factors (AMF)= 1015 cm-2 11Kuhlmann et al., Improving the Empa APEX NO2 Retrieval, 7th DOAS Workshop, 2015ancillary parameters:APEX surface reflectance product and elevationa priori NO2 profiles (Chimere model, ~5km resolution)
12. Post-processingremove bad fits (RMS > 0.003, about 2% of values or calibration failed)de-stripping by fitting a polynomial (degree 5) on across-track mean12Kuhlmann et al., Improving the Empa APEX NO2 Retrieval, 7th DOAS Workshop, 2015VCDVCD de-striped
13. NO2 VCD map over Zurich13Kuhlmann et al., Improving the Empa APEX NO2 Retrieval, 7th DOAS Workshop, 2015
14. ConclusionsAPEX combines high spatial coverage with high spatial resolution allowing mapping of NO2 at the urban scale:coverage (10×30 km2 within 30 minutes)resolution (~50×50m2)The new version of the Empa APEX NO2 retrieval:better spectral calibrationimproving DOAS fit (dSCD error: 4.5x1015 cm-2)improving AMF calculation (TBD) bridge the gap between satellite and ground-based measurements14Kuhlmann et al., Improving the Empa APEX NO2 Retrieval, 7th DOAS Workshop, 2015
15. Thank you for your attention!15Kuhlmann et al., Improving the Empa APEX NO2 Retrieval, 7th DOAS Workshop, 2015
16. Mueller et al.: Two-week NO2 maps for the City of Zurich, Switzerland, derived by statistical modelling utilizing data from a routine passive diffusion sampler network, AE, 201516Kuhlmann et al., Improving the Empa APEX NO2 Retrieval, 7th DOAS Workshop, 2015