The BIRA-IASB BrO Product

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Tropospheric bromine monoxide (BrO)


  

GOME on board of ERS-2

Reactive bromine in the atmosphere

DOAS BrO Retrievals

GOME
SCIAMACHY
Comparison
GOME vs SCIA

BrO Product Validation

On-going developments


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page last modified:
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. [2002]. The BrO absorption cross-sections used are those from Wilmouth et al. [1999], 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. [2002]. More details on the DOAS procedure applied can be found in Van Roozendael et al. [1999].

Fitting interval :344.7-359 nm
Molecular absorption cross-sections : BrO [Wilmouth et al., 1999]
O3 221K [Burrows et al., 1999]
O3 241K [Burrows et al., 1999]
NO2 [Burrows et al., 1998]
O4 [Greenblatt et al., 1990]
OClO [H. Kromminga et al., 1999]
HCHO [Cantrell et al., 1989]
Additional terms/corrections :Polynomial (order 3)
Ring cross-sections [Vountas, 1998]
Undersampling [Chance, 1998]
Offset (constant + slope)
Shift of O3 and NO2 cross-sections (0.03nm)

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.

The 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.

AMFs Calculation

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).



Acknowledgements

ERS-2 Satellite image: © ESA