Water vapor and atmospheric gas retrieval techniques
Contact Persons
Dr. Daniel Schläpfer |
Keywords
Atmospheric Trace Gases, Water Vapor Retrieval, Channel Selection, Differential Absorption, Atmospheric Correction, Sun Photometry
Abstract
The major goals in this project were:
- Evaluation of methods for imaging spectroscopy in trace gas detection
- Development and improvement of the methodology for trace gas detection
- Application of the results for atmospheric correction algorithms
- Intercomparison of ground and space based measurements
- Evaluation of quantification procedures using the MODTRAN RTC
- Application to the measurement of tropospheric water vapor
- Estimation of water vapor profiles at different slope expositions
- Investigation of the relationship between topology and water vapor amounts
The data used for this studies were flown with the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) onboard a NASA ER-2 aircraft.
The project has been completed in 1998. A printed dissertation which summarizes the results is available within the 'Remote Sensing Series' of the Remote Sensing Laboratories. Please order through RSL. The work was supported by the Swiss National Science Foundation (Nationalfonds) and the Paul Scherrer Institute (PSI). Some work was done at the Los Alamos National Laboratory in the Group NIS-2.
The technique is currently used on an operational basis within the ATCOR atmospheric correction programs in collaboration with Rolf Richter at DLR-DFD Oberpfaffenhofen.
Method
The optical measurement of atmospheric trace gases can be performed using sensor channels located in bands or lines of the absorption spectrum. To quantify the trace gas amount, the so called differential absorption technique is applied. It performs a ratioing between influenced channels within the absorption band (measurement channels) to non influenced channels besides the band (reference channels). Measurement channels are ideally sensitive to the trace gas of interest and as insensitive as possible to noise and other disturbing effects. Reference channels are located as close as possible to measurement channels, but may not be influenced by any absorbing gases. The various ratioing methods differ from one another by the number of selected channels and by the calculation technique.
Schematic view of the APDA technique in an absorption band, the differential absorption between (2) and (1) is calculated and related to water vapor contents.
© Daniel Schläpfer, Remote Sensing Laboratories, 1996
A technique called 'Atmospheric Pre-corrected Differential Absorption' (APDA) was derived directly from simplified radiative transfer equations. lt combines a partial atmospheric correction with a differential absorption technique. APDA iteratively corrects for the atmospheric path radiance term for the retrieval of water vapor. This improves the results especially over low background albedos. The error of the method for various ground reflectance spectra is simulated and is below 5% for most spectra. The channel combinations for two test cases are then defined, using a quantitative procedure, which is based on MODTRAN simulations and the image itself. An error analysis indicates, that the influence of aerosols and channel calibration is minimal. The APDA technique is then applied to two AVIRIS images acquired in 1991 and 1995. The accuracy of the measured water vapor columns is within a range of 5% compared to ground truth radiosonde data.
Performance of the APDA technique on a AVIRIS 1995 image (Camarillo, USA)
© Daniel Schläpfer, Remote Sensing Laboratories, 1996
Results
A quantitative method of channel selection yielded ranked sets of channels for imaging spectrometry of ozone and water vapor. The combinations of the evaluated measurement and reference channels in various ratio methods was evaluated. Applying the radiative transfer code MODTRAN, the selected methods were calibrated to the total trace gas columns at standard ground reflectance spectra. The evaluation was applied to the specific characteristics of the AVIRIS sensor, but the methodology can be applied to other sensors too. Furthermore, it is not restricted to ozone and water vapor measurement but contains common tools of channel selection and method evaluation for any atmospheric trace gas (e.g. carbon dioxide or methane). The methodology has the potential of being used in terrestrial applications (e.g. chlorophyll measurements, limnological applications, etc.) too.
The quantification of water vapor over land is possible with an accuracy of about ±6%. The distribution with terrain height is very similar to the profile measured by balloon sondes. Hence, the used methodology has the potential to derive water vapor profiles from the boundary layer, using the digital terrain model. In a next step a three dimensional modelling of the water vapor field in a valley will be possible. The complementary measurement of water vapor over lakes will only be possible with further improvements of the methodology.
