institute of environmental physics and institute of remote sensing university of bremen
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Tropospheric NO 2 H eight Determination. S5P Verification Meeting Bremen, November 29, 2013. Andreas Richter , A. H ilboll, and J. P. Burrows. Institute of Environmental Physics and Institute of Remote Sensing University of Bremen. Introduction. - PowerPoint PPT PresentationTRANSCRIPT
Institute of Environmental Physics and Remote SensingIUP/IFE-UB Physics/Electrical Engineering
Department 1
Institute of Environmental Physics andInstitute of Remote Sensing
University of Bremen
Tropospheric NO2 Height Determination
Andreas Richter, A. Hilboll, and J. P. Burrows
S5P Verification MeetingBremen, November 29, 2013
Introduction
• Satellite observations provide nice global maps of tropospheric NO2
• Absolute values depend strongly on assumed vertical distribution• This information currently comes completely from a priori dataÞ Can‘t we do better than that?
What triggered this study?
• Monthly GOME-2 tropospheric NO2 data are missing most of the large values
• These were removed by cloud filtering as aerosol was so thick that data were classified as partially cloudy
No cloud screening
Is it only Aerosols?
• Even without cloud screening, there are data gaps over pollution hot spots on some days
• This is due to quality checking as these fits are poor
No Chisq. screening
Why are the fits poorer at strong pollution?
• There are large and clearly structured residuals in fits over pollution hot spots
• This is not random noise!
• Comparison to NO2 cross-sections shows that scaling of NO2 should change over fitting window
Wavelength dependence of Air Mass Factor
• For constant albedo, AMF of NO2 layer close to the surface increases with wavelength in a Rayleigh atmosphere
• For a surface layer, this can be a significant effect• With radiative transfer modelling and a formal inversion, this should
provide information on the altitude of the NO2
About +/- 20%
Empirical Approach
• Take standard NO2 x-section• Scale to increase amplitude with
wavelength• Orthogonalise to leave NO2
columns unchanged
When introduced in the fit, large residuals are fixed
Results Empirical Approach
• The empirical NO2 AMF proxy is found over the pollution hotspot in China
• It is not found at other locations where the NO2 slant column is large• There is some noise in the retrieval of the proxy
Results Empirical Approach: OMI
• As for GOME-2 data, the empirical NO2 AMF proxy is found over the pollution hotspot in China
• There is more noise than in GOME-2 data• Problems with row anomaly
Is there more than China?
• Fit is improved by AMF proxy everywhere over pollution hotspots
Comparison to NO2 columns
• Overall pattern similar to NO2 map
• Differences in distributions of maxima
• Artefacts over water• noise
Impact of Clouds
• On many days in winter, very large NO2 slant columns are observed over Europe and the US
• The NO2 AMF proxy picks up only very few of these signals
• This is linked to the fact that most of the events are related to cloudy scenes or snow on the surface, resulting in small wavelength dependence
Sensitivity Study
• Ratio of AMF proxy and NO2 has strong dependence on NO2 layer height
• Dependence on albedo is small between 3% and 7%
Synthetic data:• Rayleigh atmosphere• Constant albedo• NO2 layer in different altitudes• DOAS fit on spectra• NO2 temperature dependence
corrected by using 2 NO2 x-sections• AMF proxy included
• Ratio of AMF proxy / NO2 to normalise signal
Sensitivity Study: SZA
• Effect varies with SZA; larger effect at larger SZA• At large SZA, AMF proxy also found for elevated NO2
• Dependence on albedo is small between 3% and 7%
Sensitivity Study: Bright Surfaces
Þ multiple scattering over bright surfaces is stronger at shorter wavelengths
Þ wavelength dependence of AMF is inverted
• Increasing albedo reduces effect as expected for reduced importance of Rayleigh scattering
• For large albedo (> 50%), negative fit factors are found for AMF proxy => wavelength dependence is inverted and only weakly dependent on altitude
Impact of Aerosols and Clouds
Shielding
• Similar effect for both, AMF proxy and NO2 => will cancel in ratio• Ratio will give layer height for cloud free part of pixel
Light path enhancement
• Light path enhancement in clouds / aerosols depends only weakly on wavelength
• Effect on NO2 but no effect on AMF proxy• Ratio will no longer be representative of NO2 layer height
Case Study Highveld
• NO2 plume from Highveld power plants can be tracked onto the ocean• NO2 SC values increase downwind of the source• AMF Proxy also has higher values within the plume, but
– Is more narrow– Has largest values at beginning of plume, not at the end of it
What about larger wavelength difference?
• Tropospheric signal much smaller in UV fit• Ratio between two fits depends on location (=> NO2 height)• BUT: UV fit is noisy
Summary• A simple empirical pseudo-cross-section was used to detect and
correct the AMF wavelength dependence of tropospheric NO2 in GOME-2 data
• Application improves NO2 fits over pollution hotspots under clear sky conditions
• As expected, the signature is not found over clouds and bright surfaces or in cases of large stratospheric NO2
• The results can at least give an indication for where an AMF for BL NO2 is appropriate
• Tests on synthetic data suggest that for good signal to noise, an effective NO2 layer height can be determined
• Using more separated wavelengths and applying a formal inversion including aerosol properties might provide more vertical information
• Application to more data also from OMI and S5P foreseenFunding by DLR Bonnunder Contract 50EE1247