C. Fayt

Ten years of GOME/ERS-2 total ozone data- : The new GOME data processor (GDP) version 4: 1. Algorithm description

The Global Ozone Monitoring Instrument (GOME) was launched on European Space Agencys ERS-2 platform in April 1995. The GOME data processor (GDP) operational retrieval algorithm has generated total ozone columns since July 1995. In 2004 the GDP system was given a major upgrade to version 4.0, a new validation was performed, and the 10-year GOME level 1 data record was reprocessed. In two papers, we describe the GDP 4.0 retrieval algorithm and present an error budget and sensitivity analysis (paper 1) and validation of the GDP total ozone product and the overall accuracy of the entire GOME ozone record (paper 2). GDP 4.0 uses an optimized differential optical absorption spectroscopy (DOAS) algorithm, with air mass factor (AMF) conversions calculated using the radiative transfer code linearized discrete ordinate radiative transfer (LIDORT). AMF computation is based on the TOMS version 8 ozone profile climatology, classified by total column, and AMFs are adjusted iteratively to reflect the DOAS slant column result. GDP 4.0 has improved wavelength calibration and reference spectra and includes a new molecular Ring correction to deal with distortion of ozone absorption features due to inelastic rotational Raman scattering effects. Preprocessing for cloud parameter estimation in GDP 4.0 is done using two new cloud correction algorithms: OCRA and ROCINN. For clear and cloudy scenes the precision of the ozone column product is better than 2.4 and 3.3%, respectively, for solar zenith angles up to 80°. Comparisons with ground-based data are generally at the 1-1.5% level or better for all regions outside the poles.

Intercomparison of BrO measurements from ERS-2 GOME, ground-based and balloon platforms

The consistency of BrO column amounts derived from GOME spectra and from correlative ground-based and balloon measurements performed in 1998-1999 during the Third European Stratospheric Experiment on Ozone (THESEO) has been investigated. The study relies on W-visible observations at several mid- and high latitude ground-based stations in both hemispheres, complemented by balloon-borne solar occultation profile measurements and 3D chemical transport model simulations. Previous investigations have reported GOME BrO columns systematically larger than those deduced from balloon, suggesting BrO being present, possibly ubiquitously, in the free troposphere. The robustness of this hypothesis has been further tested based on the presently available correlative data set. It is shown that when accounting for the BrO diurnal variation and the solar zenith angle dependency of the sensitivity of correlative data to the troposphere, measurements from all platforms are consistent with the presence of a tropospheric BrO background of 1-3 ~10’~ mole&m’ extending over mid- and high

One-decade trend analysis of stratospheric BrO over Harestua (60°N) and Lauder (45°S) reveals a decline

A trend analysis is performed of stratospheric BrO from ground-based UV-visible observations at Harestua (60°N, 11°E) and Lauder (45°S, 170°E) from 1995 through 2005. At both stations, a positive trend of about +2.5% per year is found for the 1995-2001 period, while a negative trend of about -1% per year is obtained between 2001 and 2005. Given a mean age of air of about 4 ± 1 years, the decline in stratospheric bromine since 2002 follows the decline of tropospheric organic bromine observed since the second half of 1998, as a result of the Montreal Protocol. These findings confirm that the impact of the Montreal Protocol restrictions on brominated substances have now reached the stratosphere. From our study, we have also derived a contribution of 6 ± 4 ppt of the brominated very short-lived substances and inorganic bromine tropospheric sources to the total bromine loading.

Retrieval of Tropospheric BrO and NO2 from UV-Visible Observations

This contribution has been concerned with three different activities. First, global N02 data records from HALOE, POAM-II and POAM-III satellite instruments complemented by ground-based data sets from the NDSC, have been investigated for mutual consistency and exploited to build a global time-latitude stratospheric NO2 profile climatology that may be used for tropospheric NO2 retrieval from ERS2-G0ME. Second, a BrO total column product derived from GOME has been created over the total present lifetime of the ERS-2 satellite (1996 to 2002) and made available to the scientific community through a dedicated web site. In addition, a prototype improved GOME BrO retrieval algorithm has been designed with the aim to determine better global tropospheric BrO columns from the combined use of GOME data and three-dimensional model data, and to provide the stratospheric reference needed in a tropospheric residual approach. Third, ground-based multi-axis DOAS measurements have been conducted at the Observatoire de Haute Provence, (OHP, 44 °N) between January 2001 and June 2002 in support of satellite retrieval activities. Results obtained demonstrate the high sensitivity of the measurements to tropospheric amounts of NO2 and HCHO, as well as the potential of the technique to unravel small mid-latitude free-tropospheric BrO contents. As to the latter point, comparisons between combined GOME-SLIMCAT and MAX-DOAS tropospheric BrO evaluations suggest that approximately half of the mid-latitude BrO total column resides in the troposphere.