Abstract. Various instrumental or geophysical artifacts, such as
saturation, stray light or obstruction of light (either coming from the
instrument or related to solar eclipses), negatively impact satellite
measured ultraviolet and visible Earthshine radiance spectra and downstream
retrievals of atmospheric and surface properties derived from these spectra.
In addition, excessive noise such as from cosmic-ray impacts, prevalent
within the South Atlantic Anomaly, can also degrade satellite radiance
measurements. Saturation specifically pertains to observations of very
bright surfaces such as sunglint over open water or thick clouds. When
saturation occurs, additional photoelectric charge generated at the
saturated pixel may overflow to pixels adjacent to a saturated area and be
reflected as a distorted image in the final sensor output. When these
effects cannot be corrected to an acceptable level for science-quality
retrievals, flagging of the affected pixels is indicated. Here, we introduce
a straightforward detection method that is based on the correlation, r, between
the observed Earthshine radiance and solar irradiance spectra over a 10 nm
spectral range; our decorrelation index (DI for brevity) is simply
defined as a DI of 1−r. DI increases with anomalous additive effects or excessive
noise in either radiances, the most likely cause in data from the Ozone
Monitoring Instrument (OMI), or irradiances. DI is relatively
straightforward to use and interpret and can be computed for different
wavelength intervals. We developed a set of DIs for two spectral channels of
the OMI, a hyperspectral pushbroom imaging spectrometer. For each OMI
spatial measurement, we define 14 wavelength-dependent DIs within the OMI
visible channel (350–498 nm) and six DIs in its ultraviolet 2 (UV2) channel
(310–370 nm). As defined, DIs reflect a continuous range of deviations of
observed spectra from the reference irradiance spectrum that are
complementary to the binary saturation possibility warning (SPW) flags
currently provided for each individual spectral or spatial pixel in the OMI
radiance data set. Smaller values of DI are also caused by a number of
geophysical factors; this allows one to obtain interesting physical results
on the global distribution of spectral variations.