Background information about the Row Anomaly in OMI

A row anomaly is an anomaly which affects all wavelengths for a particular viewing direction of OMI. This corresponds to a binned row on the CCD dectectors, and hence the term ‘Row Anomaly’. Several row anomalies have occurred in the recent past. These anomalies affect the quality of the Level 1B and Level 2 data products. Please read this information carefully prior to using OMI data. Please respect the dates mentioned as the anomalies have occurred recently.

Executive summary on processing status

• Level 1B is flagged for anomaly 1 and 2a in the forward stream, starting on October 8, 2008.
• Level 2 NO₂ slant columns (OMNO2A) and cloud O₂-O₂ (OMCLDO2) in forward stream copy over the flags, and apply a wavelength correction, starting on February 17, 2009.
• Level 2 O₃ vertical columns (OMDOAO3) in forward stream copy over the flags, and apply a wavelength correction, starting on February 27, 2009.

The rest of this document describes the occurence of the anomaly, the effects on the OMI spectra, and remedies that are currently being developed. As alluded to above, some flags and remedies are currently running in the forward stream.

This document will be updated when more information becomes available. This version was updated on Monday, 14-Dec-2009 02:11:04 UTC.

Overview of the anomalies

Anomaly 1

Since June 25th, 2007 an anomaly is present in (cross-track) scenes 53-54 (0-based). The anomaly comprises a decrease in the radiance signal, with secondary effects. The Level 1B data has been flagged for this anomaly for (cross-track) scenes 53-54 (0-based).

All Level 2 data products are affected by this anomaly.

Anomaly 2

(a)

Since May 11th, 2008 an anomaly is present in (cross-track) scenes 37-42 (0-based) towards the northern end of the OMI orbit. The anomaly comprises a decrease or increase in the radiance signal depending on position in orbit. The Level-1B data are flagged for this anomaly.

(b)

On December 3rd, 2008, this anomaly expanded to cross-track position 44 (0-based) and occurs along the full length of the orbit. The Level-1B flagging is not yet adjusted to reflect this expansion.

All Level 2 data products are affected by this anomaly.

Anomaly 3

Since January 24th, 2009 an anomaly is present in (cross-track) scenes 27-44 (0-based). The anomaly comprises a decrease and increase in the radiance signal depending on position in obit. The Level-1B data are not yet flagged for this anomaly for (cross-track) scenes 27-44 (0-based).

After the event of January 24th, 2009, the anomaly continues to change. From February 2nd, 2009, row 27 recovers while row 45 becomes affected. The anomaly now spans rows 28-45 (0-based). From March 3rd, 2009, row 46 becomes affected. From March 30th, 2009, row 47 becomes affected.

A somewhat larger event takes place on April 21st, 2009. Rows 48-50 become affected. The anomaly now spans rows 28-50, in addition to rows 53 and 54 that have been stable since the appearance of the first anomaly. All row numbers are 0-based. On May 6th, 2009 row 51 becomes affected, and on May 18th, 2009, row 52 becomes affected as well, joining the two anomaly ranges. Rows 28-54 are now affected to various degrees. The news is not all bad though, as rows 41 and 42 appear to recover at around the same time.

The next change we observed occurred on August 18, 2009, this time near nadir. Rows 25, 26, and 27 are now certainly affected, making the affected range of rows reach from 25 through 54 (inclusive, zero-based). Rows 41-45 appear to be unaffected, or only slightly affected, but we will continue to monitor these rows to make an assessment of their status.

All Level 2 data products are affected by this anomaly.

Row Anomaly Flagging

The flags for the row anomaly are stored in the Level 1B data product in the new field ‘XTrackQualityFlags’. When Level 2 products are generated, this field should be copied in the Level 2 orbit data files. In addition, the Radiance Error bit or Radiance Warning bit in the Fit Quality flags or similar field of the Level 2 products are raised, depending on the status of the Level 1B corrections. Not all Level 2 products have been adapted at this moment. The table below gives an overview for each product, and as of which software version and date the fix is included.

Additional Notes

The OMI cross track positions mentioned here are 0-based and hold for UV2 and VIS channels only. For the UV1 channel all numbers should be divided by 2 and rounded down to an integer.

Effect of the Anomaly on the OMI Radiance Spectrum

The row anomalies have four distinct effects on the OMI radiance spectra:

1. A decrease in the radiance level for several viewing directions. It is currently assumed that this is caused by a partial blocking of the OMI nadir port. The blocking object is assumed to be opaque. This is effectively a multiplicative error on the radiances.

2. An increase in the radiance level at (mostly) the same viewing directions as are affected by a decrease in the radiance for the northern part of the orbit. This occurs when the part of OMI containing the nadir port is directly illuminated by the sun. This is assumed to be caused by reflection of sunlight into the nadir port via the blocking object (outside of OMI). This is an additive error on the radiances.

   This increase in the radiance level is not observed for the first anomaly in rows 53-54 (0-based).

3. The blocking object causes an inhomogeneous illumination of the spectral slit in OMI. This causes a change in the slit function, shifting the center of weight away from the nominal center. This causes light of a specific wavelength to hit the detector in a slightly different location than expected.

   In the OMI level 0 to level 1 software, the wavelengths are not fitted, but assigned. Corrections are made based on the homogeneity of clouds and the temperature of the optical bench. Therefore the effect of an object blocking part of the incoming light is not included in the nominal level 1B wavelength assignment.

4. Light reflected by the earth from outside the nominal field of view is coupled into the nadir port. This light is collected over a large area, giving an additive error on the radiances, with a not quite constant term.

Effect on OMI Level 2 products

Some products rely directly on the radiance levels, for instance cloud fractions (both from OMCLDO2 and OMCLDRR), aerosol properties (OMAERO and OMAERUV), and others. These are affected by the first and second effect of the row anomaly.

Other products rely on the spectral information, and are affected by the wavelength shift. This is especially true for DOAS type retrievals, such as OMCLDO2, OMNO2, OMDOAO3 and others.

Some products are affected by both effects: Tropospheric NO₂ requires accurate cloud information, and is therefore affected by both effects of the anomaly.

The change in the radiance spectrum has a pronounced effect on the fit quality. Affected pixels in level 2 products can be recognized by significantly elevated χ² values, stored in the ‘ChiSquaredOfFit’ field, or other fit quality parameters in the Level 2 output file.

Provisional solutions

Here we show provisional solutions to mitigate the effect of the anomaly on level 2 products. All examples shown here are from before the appearance of row anomaly 3.

Provisional solutions for the Reduced Radiance Levels (first effect)

Analysis has shown that the blocking of the entrance port depends on the position of OMI in the orbit, the day of year, and the wavelength. Using a week of data it is possible to determine blocking factors so that corrected radiances can be produced. The figure below shows the effect of the correction on the OMI O₂-O₂ cloud fraction.

Figure 1.

In figure 1 some residual effect can be seen in row 41 (zero-based), but the anomaly at row 53 is no longer apparent. This is even more clearly shown by taking the average along the flight direction for the southern most part of an orbit, as is shown in figure 2.

The parameters and procedure with which to derive the transmission factors to correct for the radiance levels are currently investigated.

Figure 2.

Provisional solutions for the Additional Sunlight (second effect)

At this moment the focus is on finding a correction for the reduction in the radiance levels (first effect) and the wavelength shift (third effect).

Provisional solutions for the wavelength shift (third effect)

The wavelength of the OMI radiances can be fitted on-line with the position of Fraunhofer lines throughout the OMI wavelength range. For level 2 retrieval algorithms we can assume a single shift value for the respective retrieval windows. The latter is a relatively minor change, and has been implemented in OMNO2A and OMCLDO2 in the retrieval software version 1.1.0 and later. Figure 3 shows the fitted wavelength shift for the OMNO2A product.

Figure 3.

Including this shift makes a remarkable difference to the NO₂ slant columns, as is shown in figure 4. Note that uncorrected radiances were used, showing that for the slant columns of NO₂ the wavelength shift is dominant.

The χ² values show a clear reduction with the wavelength shift applied to the spectrum. A slightly higher χ² value for the affected rows is expected, as the signal to noise ratio of the Level 1B has been reduced. Further optimizations to the wavelength grid may further improve the correction. The χ² values of the fit without and with wavelength shift is shown in figure 5.

Figure 4.

If the wavelength assignments in the level 1B themselves are to be corrected, then we must assume a wavelength dependent shift. This requires more care to implement. For the moment we have chosen to leave this correction up to the individual level 2 algorithm developers.

Further Actions

Several actions have already been initiated to limit the effect of the row anomaly on OMI Level 2 and higher.

Figure 5.

A flagging field ‘XTrackQualityFlags’ has been added to the level 1B product. These flags are carried over to the level 2 products. This change has been implemented for OMNO2 and OMCLDO2, others are to follow. Starting dates when these flags are available are given in the table.

1. Because the anomaly at times changes more quickly than our flagging, the flags may miss part of the anomaly. The table will be adjusted to keep track of what is flagged.
2. The cloud fraction is not correct, as the radiance levels are uncorrected.
3. The cloud fractions are not corrected yet, so the tropospheric columns are sub-optimal.
4. The radiance levels are not yet corrected, so the air mass factor is still off, leading to a less accurate value of the O3 columns in the affected rows.
5. The radiance levels are not yet corrected, so the aerosol properties are incorrect for the affected rows.

The meaning of the values stored in the ‘XTrackQualityFlags’ flagging field is documented in GDPS Input/Output Data Specification (IODS) Volume 2 manual, available from the OMI Level 1B DISC page. A summary is given in the table below. For level 2 products where this field is copied, the radiance warning or error bits are set according to the value in the last column.

Other level 2 algorithm developers are working to copy the ‘XTrackQualityFlags’ from level 1B to the output product, and summarize the value of this field in the fit quality flags mask as is done in OMNO2A and OMCLDO2. Contact the lead for the level 2 product you are interested in to inquire about the current status if the “README” file is not clear.