1Service de Chimie Quantique et Photophysique, Université Libre de Bruxelles (U.L.B.), Brussels, Belgium
2UPMC Univ. Paris 06; Université Versailles St-Quentin; CNRS/INSU, LATMOS-IPSL, Paris, France
3Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium
*now at: European Commission, Joint Research Centre(JRC), 21027 Ispra, Italy
**L. Clarisse and P. F. Coheur are respectively Postdoctoral Researcher and Research Associate with FRS-FNRS, Belgium
Abstract. Methanol (CH3OH) and formic acid (HCOOH) are among the most abundant volatile organic compounds present in the atmosphere. Their role in tropospheric chemistry stems from their influence on the oxidizing capacity of the atmosphere and, in the case of HCOOH, from its influence on the acidity of clouds and precipitation. In this work, we derive the first global distributions of these two organic species using the Infrared Atmospheric Sounding Interferometer (IASI) launched onboard the MetOp-A satellite in 2006. This paper describes the method used and provides a first critical analysis of the retrieved products. The retrieval process follows a two-step approach in which global distributions are first obtained on the basis of a simple radiance indexing (transformed into brightness temperatures), and then mapped onto column abundances using suitable conversion factors. For methanol, the factors were calculated using a complete retrieval approach in selected regions. In the case of formic acid, a different approach, which uses a set of forward simulations for representative atmospheres, has been used. In both cases, the main error sources are carefully determined: the average relative error on the column for both species is estimated to be about 50%, increasing to about 100% for the least favorable conditions. The distributions for the year 2009 are discussed in terms of seasonality and source identification. Global correlations between methanol and formic acid as well as correlations between these two species and carbon monoxide are also presented and discussed.