Abstract
Abstract. Methanol (CH3OH) and formic acid (HCOOH) are among the most abundant volatile organic compounds present in the atmosphere. In this work, we derive the global distributions of these two organic species using for the first time 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. Time series comparing methanol, formic acid and carbon monoxide in different regions are also presented.
Highlights
Volatile Organic Compounds (VOCs) includes thousands of different carbon-containing gases present in our atmosphere at concentrations ranging from less than a pptv to more than a ppmv
The quasi-global distributions obtained from ACE-FTS (Dufour et al, 2007) have further revealed that the surface sources of methanol have a significant impact on its upper tropospheric concentrations, which are mostly driven by biogenic and biomass burning emissions in the Northern and Southern Hemisphere, respectively
In order to take advantage of the very good spatial coverage of Infrared Atmospheric Sounding Interferometer (IASI) we have chosen a simple, fast and robust approach based on brightness temperature differences ( Tb), similar to the method already used for the retrieval of sulfur dioxide (Clarisse et al, 2008) and ammonia (Clarisse et al, 2009)
Summary
Volatile Organic Compounds (VOCs) includes thousands of different carbon-containing gases present in our atmosphere at concentrations ranging from less than a pptv to more than a ppmv (only methane exceeds 1 ppmv and is usually excluded from the VOC definition). Emitted from a large variety of processes (biogenic or anthropogenic) at the Earth’s surface, they have an important influence on the atmospheric composition and climate. VOCs are precursors to tropospheric ozone (Houweling et al, 1998), they play an important role on the oxidizing capacity of the troposphere (Atkinson and Arey, 2003; Monks, 2005), they lead to the formation of secondary organic aerosols (Tsigaridis and Kanakidou, 2007; Heald et al, 2008) and they impact on climate change in different indirect ways (Meinshausen et al, 2006; Feingold et al, 2003; Charlson et al, 1987). In order to better understand and quantify their emissions and the role they play in the Earth’s system, it is important to assess their atmospheric distribution at the global scale
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