Abstract
Abstract. We developed a new CO vertical column density product from near IR observations of the SCIAMACHY instrument onboard ENVISAT. For the correction of a temporally and spatially variable offset of the CO vertical column densities we apply a normalisation procedure based on coincident MOPITT (version 4) observations over the oceans. The resulting normalised SCIAMACHY CO data is well suited for the investigation of the CO distribution over continents, where important emission sources are located. We use only SCIAMACHY observations for effective cloud fractions below 20 %. Since the remaining effects of clouds can still be large (up to 100 %), we applied a cloud correction scheme which explicitly considers the cloud fraction, cloud top height and surface albedo of individual observations. The normalisation procedure using MOPITT data and the cloud correction substantially improve the agreement with independent data sets. We compared our new SCIAMACHY CO data set, and also observations from the MOPITT instrument, to the results from three global atmospheric chemistry models (MATCH, EMAC at low and high resolution, and GEOS-Chem); the focus of this comparison is on regions with strong CO emissions (from biomass burning or anthropogenic sources). The comparison indicates that over most of these regions the seasonal cycle is generally captured well but the simulated CO vertical column densities are systematically smaller than those from the satellite observations, in particular with respect to SCIAMACHY observations. Because SCIAMACHY is more sensitive to the lowest part of the atmosphere compared to MOPITT, this indicates that especially close to the surface the model simulations systematically underestimate the true atmospheric CO concentrations, probably caused by an underestimation of CO emissions by current emission inventories. For some biomass burning regions, however, such as Central Africa in July–August, model results are also found to be higher than the satellite observations.
Highlights
Atmospheric carbon monoxide CO is released by natural and anthropogenic sources such as biomass burning or fossil fuel combustion
We developed a normalisation procedure of SCIAMACHY CO vertical column density (VCD) using simultaneous observations of the MOPITT instrument over the ocean
We considered the influence of the actual effective cloud fraction and effective cloud top height using simultaneously retrieved cloud properties from the FRESCO+ algorithm (Koelemeijer et al, 2001; Wang et al, 2008)
Summary
Atmospheric carbon monoxide CO is released by natural and anthropogenic sources such as biomass burning or fossil fuel combustion. In the comparison studies with model simulations the CO VCDs from SCIAMACHY were typically found to be higher than the corresponding model data, especially over polluted regions This finding is in agreement with recent studies suggesting that current emission inventories underestimate the true emissions (e.g., Gloudemans et al, 2006; de Laat et al, 2006, 2007, 2010; Gloudemans et al, 2009; Kopacz et al, 2010). (b) Some comparison studies were performed in a quantitative way (e.g., Turquety et al, 2008; Tangborn et al, 2009; de Laat et al, 2010; Kopacz et al, 2010): Only collocated observations were used for these comparisons and the different height-dependencies of near IR and thermal IR sensors were adequately considered (e.g., using model simulations as transfer tools).
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