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Abstract
Abstract. Satellite observations of nitrogen dioxide (NO2) provide valuable information on both stratospheric and tropospheric composition. Nadir measurements from GOME, SCIAMACHY, OMI, and GOME-2 have been used in many studies on tropospheric NO2 burdens, the importance of different NOx emissions sources and their change over time. The observations made by the three GOME-2 instruments will extend the existing data set by more than a decade, and a high quality of the data as well as their good consistency with existing time series is of particular importance. In this paper, an improved GOME-2 NO2 retrieval is described which reduces the scatter of the individual NO2 columns globally but in particular in the region of the Southern Atlantic Anomaly. This is achieved by using a larger fitting window including more spectral points, and by applying a two step spike removal algorithm in the fit. The new GOME-2 data set is shown to have good consistency with SCIAMACHY NO2 columns. Remaining small differences are shown to be linked to changes in the daily solar irradiance measurements used in both GOME-2 and SCIAMACHY retrievals. In the large retrieval window, a not previously identified spectral signature was found which is linked to deserts and other regions with bare soil. Inclusion of this empirically derived pseudo cross-section significantly improves the retrievals and potentially provides information on surface properties and desert aerosols. Using the new GOME-2 NO2 data set, a long-term average of tropospheric columns was computed and high-pass filtered. The resulting map shows evidence for pollution from several additional shipping lanes, not previously identified in satellite observations. This illustrates the excellent signal to noise ratio achievable with the improved GOME-2 retrievals.
Highlights
Nitrogen dioxide (NO2) is an important trace gas in the Earth’s atmosphere
An improved Global Ozone Monitoring Experiment (GOME)-2 NO2 retrieval is described which reduces the scatter of the individual NO2 columns globally but in particular in the region of the Southern Atlantic Anomaly
The retrieval of atmospheric NO2 amounts from UV/visible measurements from space is based on the application of the Differential Optical Absorption Spectroscopy (DOAS) (Platt 1994)
Summary
Nitrogen dioxide (NO2) is an important trace gas in the Earth’s atmosphere. In the stratosphere, it is involved in ozone chemistry as a catalyst for ozone destruction and in the formation of halogen reservoirs such as chlorine nitrate. Atmospheric nitrogen dioxide can be detected by remote sensing measurements using the strong differential absorption structures of the NO2 molecule in the UV/visible part of the spectrum. Such measurements have been used extensively to monitor NO2 from the ground The first global tropospheric NO2 observations were possible with the Global Ozone Monitoring Experiment (GOME) launched in July 1995 (Burrows et al, 1999). As an example for the utility of the improved data set, an average NO2 field is computed over nearly 4 yr of GOME-2 data, which shows evidence for pollution from several shipping lanes not previously detectable from space
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