The BIRA-IASB BrO Product

Tropospheric bromine monoxide (BrO)

Reactive bromine in the atmosphere

DOAS BrO Retrievals

GOME

SCIAMACHY

Comparison GOME vs SCIA

BrO Product Validation

On-going developments

SCIAMACHY BrO Slant column retrieval

The aim of the SCIAMACHY BrO retrieval activity is to derive consistent GOME and SCIAMACHY slant column products.

While trying to achieve this goal by modifying our existing GOME algorithms and programs to SCIAMACHY, important issues like the choice of wavelength interval and cross-section and the noise problem when including formaldehyde needed to be investigated in more detail.

Detailed information about these investigations is available in a tech-note (PDF).
The remainder of the text on this page only presents the most important conclusions.

The strong Wood’s anomaly present in the channel 2 of SCIAMACHY prevents reliable BrO retrievals to be obtained in the fitting window (344.7-359 nm) used for GOME evaluations. SCIAMACHY nadir spectra are characterized in this wavelength region by strong polarization features and by a reduced signal to noise ratio around 340-360 nm. In order to bypass these difficulties, alternative BrO fitting windows have been explored. The 336-351.5 nm interval was found to offer best compromise, showing good consistency with GOME results in the usual 344.7-359 nm interval and, at the same time, optimal stability for SCIAMACHY. Similarly, for reasons of consistency with GOME retrievals, the Wahner BrO cross-section is preferred instead of Wilmouth data despite their lower resolution (see also the GOME vs SCIA comparison). Our recommendation for DOAS retrieval of BrO from SCIAMACHY is therefore to use a UV-shifted fitting interval (336-351.5 nm), with the Wahner BrO cross-sections. Remaining small polarization features can be satisfactorily eliminated by adding two empirical polarization vectors fitted as part of the DOAS process.

Fitting interval :	336-351.5 nm
Molecular absorption cross-sections :	BrO [Wahner et al., 1988] O3 at 223K V2 [Bogumil et al., 1999] O3 at 243K V2 [Bogumil et al., 1999] NO2 at 243K V1 [Bogumil et al., 1999] O4 [Greenblatt et al., 1990] OClO [Bogumil et al., 1999]
Additional terms/corrections :	Polynomial (order 2) Ring cross-sections calculated using SCIATRAN model [Vountas, 1998] Polarization cross-sections [Key Data for SCIAMACHY processing] Offset (constant + slope) Wavelength calibration based on Kurucz solar spectrum

Due to the current unavailability of reliable SCIAMACHY solar spectra, a radiance spectrum taken at the equator is currently used as reference spectrum. Further processing then follows according to GOME standard evaluations. A mean SCD value is calculated around the equator (0°±5°) for each day and then subtracted to attenuate the noise due to SCIA radiances. Afterwards an offset correction is applied, taking into account the BrO value generally admitted at the equator (5e13 mol/cm²).

This study has been performed in parallel with the Bremen BrO team (T.Wagner, O.Afe). The Bremen and IASB BrO evaluations from both GOME and SCIAMACHY data are fully consistent.

Acknowledgements