Total and Tropospheric NO2 columns -- publications

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Tropospheric NO2

Averaging kernels for DOAS total-column satellite retrievals
H.J. Eskes and K.F. Boersma
Atmos. Chem. Phys. 3, 1285-1291, 2003.
The Differential Optical Absorption Spectroscopy (DOAS) method is used extensively to retrieve total column amounts of trace gases based on UV-visible measurements of satellite spectrometers, such as ERS-2 GOME. In practice the sensitivity of the instrument to the tracer density is strongly height dependent, especially in the troposphere. The resulting tracer profile dependence may introduce large systematic errors in the retrieved columns that are difficult to quantify without proper additional information, as provided by the averaging kernel (AK). In this paper we discuss the DOAS retrieval method in the context of the general retrieval theory as developed by Rodgers. An expression is derived for the DOAS AK for optically thin absorbers. It is shown that the comparison with 3D chemistry-transport models and independent profile measurements, based on averaging kernels, is no longer influenced by errors resulting from a priori profile assumptions. The availability of averaging kernel information as part of the total column retrieval product is important for the interpretation of the observations, and for applications like chemical data assimilation and detailed satellite validation studies.
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Error Analysis for Tropospheric NO2 Retrieval from Space
K.F. Boersma, H.J. Eskes and E.J. Brinksma
J. Geophys. Res. 109 D04311, doi:10.1029/2003JD003962, 2004.
Retrieval uncertainty estimates for vertical tropospheric NO2 columns based on theoretical error source discussions combined with actual Global Ozone Monitoring Experiment (GOME) observations are presented. Contributions to the total retrieval uncertainty are divided into three categories: (1) errors caused by measurement noise and spectral fitting, affecting the slant column density, (2) errors related to the separation of stratospheric and tropospheric NO2 affecting the estimate of the stratospheric slant column, and (3) errors due to uncertainty in model parameters such as clouds, surface albedo and a priori profile shape, affecting the tropospheric air mass factor. Furthermore, it is shown that a correction for the effective temperature of the trace gas is essential, and that a correction for the presence of aerosols needs to be accompanied by aerosol corrections to the cloud retrieval. A discussion of the error components and total retrieval uncertainty is given for March 1997. Tropospheric NO2 columns can be retrieved with a precision of 35-60% over regions with a large contribution of the troposphere to the total column. This error estimate demonstrates the need for highly accurate albedo maps, cloud retrieval schemes, and realistic a priori NO2 profile shapes.
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last modified: 3 March 2004
data product contact: Folkert Boersma & Henk Eskes
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