The BIRA-IASB BrO Product
Thu 29 September 2016
GOME BrO Slant column retrieval
The inversion is performed in the 344.7-359 nm spectral range and basically follows the recommendations issued in Aliwell et al. . The BrO absorption cross-sections used are those from Wilmouth et al. , convolved to the GOME resolution, which is derived as part of the retrieval algorithm. The DOAS procedure accounts for the GOME undersampling [Chance, 1998] and slant columns are corrected for the GOME diffusor plate spectral artefact according to Richter et al. . More details on the DOAS procedure applied can be found in Van Roozendael et al. .
The whole GOME data set has been reprocessed in November 2003. With respect to the GOME BrO product previously provided on the BIRA-IASB website, the DOAS analysis has been improved by adding formaldehyde (H2CO) in the fit. Indeed, in some isolate cases, an excess of BrO could be seen along a particular orbit. This effect is due to the presence of formaldehyde and is corrected by fitting H2CO.
Another change in the DOAS analysis is the use of a polynomial of order 3 rather than 2. This corrects better the problem of GOME Scan-Mirror Degradation, which manifests itself by an asymmetry between eastern and western pixels, particularly around 60° of latitude (North and South). In the case of BrO, it results in systematic higher values for eastern pixels. This effect tends to get worse with the lifetime of the GOME instrument. Order 3 polynomial is able to correct the problem until the year 2000. However, from 2001 and for the following years, this correction isn't sufficient anymore and maps are presented only for centre and backscan pixels.
last change concerns errors in geolocations that have been found in the south
hemisphere for certain orbits in 1997 and 1998. A number of pixels had slightly
too high latitudes, which gave the effect of seeing an excess of BrO in Antarctic
Ocean. This artefact is now corrected.
In the current state of the BrO algorithm product, BrO vertical columns are derived using Air Mass Factors (AMFs) calculated by means of a discrete-ordinate multiple-scattering radiative transfert model, under the assumption that BrO is located in the stratosphere and accounting for the line-of-sight angles of the GOME pixels. An improved retrieval algorithm accounting better for tropospheric BrO AMFs is currently under development (see on-going developments).
AcknowledgementsERS-2 Satellite image: © ESA