First Validation of Carbon Monoxide Measurements from remote sensor IASI
Currently the evolution of the atmospheric greenhouse gases and of the atmospheric ozone distributions is not completely understood. Under these circumstances we can not expect that climate models are able to precisely predict global warming. In this context, space-based remote sensing observations have an outstanding importance, since only observations from space can give a global perspective. Among the existing thermal infrared space-borne instruments devoted to atmospheric remote sensing, the Infrared Atmospheric Sounding Interferometer (IASI) has a special status as it combines the meteorology requirements to weather forecasting (high spatial coverage and a relatively good temporal resolution) and the atmospheric chemistry needs (high spectral resolution thereby allowing for numerous atmospheric gases retrievals).
The scientific value of satellite-based measurements strongly depends on the documentation of their quality and consistence. In this context a new paper, published recently in the Atmospheric Measurement Techniques Discussion, presents the first study in which individual IASI carbon monoxide (CO) data are compared to co-located high-quality ground-based data. Although CO is not considered as greenhouse gas, it has a relevant importance on atmospheric chemistry. CO has an indirect effect on the greenhouse gas radiative forcing by elevating concentrations of methane and tropospheric ozone through chemical reactions with other atmospheric constituents (e.g., the hydroxyl radical, OH) that would otherwise destroy them. Likewise, it also participate actively in the carbon dioxide cycle, since through natural processes in the atmosphere, it is eventually oxidized to carbon dioxide.
This paper presents the first validation results of CO total columns retrieved daily and globally from space-borne IASI radiance spectra, using the FORLI software (Fast Optimal Retrievals on Layers for IASI) developed at the Université Libre de Bruxelles (ULB, Belgium). The IASI CO total column product for 2008 is evaluated using correlative CO profile products retrieved from ground-based solar absorption Fourier transform infrared (FTIR) observations at the following sites: Ny Ålesund, Kiruna, Bremen, Jungfraujoch, Izaña and Wollongong. These FTIR sites span a latitude band from 79ºN to 34ºS and all of them participate in the Network for the Detection of Atmospheric Composition Change (NDACC).
Compared to the FTIR CO total columns, the IASI total columns generally show an equal distribution of under- and overestimation. The mean differences, which vary between −4.5% for Wollongong and 10.8% for Bremen, have been shown not to be significant since the standard deviations of the differences are greater than the differences themselves. No significant impact of the co-location criteria (spatial: 50 km to 100 km; and temporal: 1 h to 24 h) on the differences could be observed. As example, figure 1 shows the annual evolution of the relative differences between FTIR and IASI CO total columns at the Izaña station.
Assuming that the NDACC FTIR data and associated error budgets are a reliable reference, and that the co-location criteria have been chosen appropriately, this work concludes that on individual IASI retrievals the reported IASI error may be underestimated. It has to be kept in mind that IASI might not probe exactly the same air mass as the FTIR instruments. On average we find that there is no bias between the IASI data and the ground-based NDACC data for the CO total columns above the 6 stations that have been investigated in this study. Further work, and in particular detailed comparisons of the FORLI-CO retrieved profiles along with the NDACC profiles, should help to identify the source of discrepancies observed on individual measurements, such as for instance specific contaminations due to fire or pollution plumes.
For more details about this study please refer to: Kerzenmacher, T., Dils, B., Kumps, N., Blumenstock, T., Clerbaux, C., Coheur, P.-F., Demoulin, P., García, O., George, M., Griffith, D. W. T., Hase, F., Hadji-Lazaro, J., Hurtmans, D., Jones, N., Mahieu, E., Notholt, J., Paton-Walsh, C., Raffalski, U., Ridder, T., Schneider, M., Servais, C., and De Mazière, M.: Validation of IASI FORLI carbon monoxide retrievals using FTIR data from NDACC, Atmos. Meas. Tech. Discuss., 5, 3973-4002, doi:10.5194/amtd-5-3973-2012, 2012. Link.
Figure 1. Relative differences of the FTIR CO total column and the IASI CO total column of all the comparison pairs for Izaña. The red lines indicate ±10% difference.