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SO2 total column based on OMI data

The SO2 data product based on observations of the SCIAMACHY and GOME-2 instruments are determined using a DOAS algorithm to determine the slant column density followed by the use of an AMF pre-determined with a radiative transfer model to find the vertical (total) column density.

The SO2 data product based on OMI data is derived quite differently -- namely with a "band residual method" using the residuals of the DOAS-based ozone retrieval -- and it does not give a comparable slant column density as intermediate step. The method used for OMI should in principle lead to the same SO2 total column as those derived with the other methods.

The OMI data product file contains three estimates of the total SO2 column in Dobson Units. These correspond to three a-priori vertical profiles for the SO2 vertical distribution used in the retrieval algorithm. The three vertical profiles were selected to represent typical SO2 vertical distributions for three SO2 source regimes: SO2 in the Planetary Boundary Layer (PBL, below 2 km) from anthropogenic sources, SO2 distributed between 5 and 10 km emitted by passive volcanic degassing in the free troposphere, and SO2 distributed between 15 and 20 km representing injection from explosive volcanic eruptions.

SO2 plume from eruption of Piton de la Fournaise
SO2 total column based on OMI measurements on 7 April 2007. The SO2 plume is related to the eruption of the Piton de la Fournaise volcano on Reunion Island, which started a few days earlier and continued for several days. [Image courtesy Simon Carn, OMI/NASA.]

In the OMSO2 product, all PBL data are processed with the Band Residual Difference (BRD) method, while all 5 km and 15 km data are processed with the Linear Fit (LF) algorithm. The algorithms use different subsets of calibrated residuals produced by the NASA operational ozone algorithm. This set contains residuals centered at the Earth Probe (EP) TOMS wavelengths, four residuals at extrema of the SO2 absorption cross-section from 310.8 nm to 314.4 nm, and two non-absorbing wavelengths at 345 nm and 370 nm. Both OMSO2 algorithms use the temperature-dependent SO2 cross-sections data, residual correction for background regions, and different parameterizations of the AMF.

The BRD algorithm uses differential residuals at 3 SO2 sensitive OMI UV2 wavelength pairs, and the pair average is used to produce all PBL data. A constant AMF of 0.36 is used to estimate total SO2 (vertical column). For strong volcanic degassing and eruptions, SO2 loading can be very large and the BRD algorithm may fail. The LF algorithm has been developed to optimally select residuals from the set of available OMTO3 bands to retrieve SO2 under these conditions. The LF algorithm minimizes a subset of the residuals by simultaneously adjusting total SO2, total column ozone, reflectivity at 331 nm, and polynomial coefficients (linear and quadratic) to account for the wavelength dependent effect of surface albedo and aerosol on the effective reflectivity.

More information, references and links regarding the OMI SO2 data product can be found at the website of the Aura OMI Sulphur Dioxide Data Product See also the website of the OMI Sulphur Dioxide Group


OMI SO2 data presented on this web site

                SO2 over DR Congo on 11 December 2007

This website contains maps (plots) of the SO2 distribution based on OMI measurements. The maps presented here use exactly the same colour bar as is used for data of other satellites, so that it is easy to compare the results.

The image here shows the SO2 distribution over North Occidental Europe. The times of the measurement are plotted at the middle of each orbit, once every 50 swaths. The image also shows bands of no data.

Note that in the NRT service only images are shown; data files are not supplied. The archive service provides images and links to data files.

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