Comparison of the prior version of the OMI formaldehyde data (left), the new version (middle), and the new version with the reference sector correction (right), for the June-July-August 2012 time period.
The National Aeronautics and Space Administration (NASA) Goddard Earth Sciences Data and Information Services Center (GES DISC) has recently received a new version (184.108.40.206) of the formaldehyde data product (OMHCHO) from the Ozone Monitoring Instrument (OMI) Smithsonian Astrophysical Observatory (SAO) team. The new version, which indicates the amount of formaldehyde (HCHO) in the atmosphere, can now be downloaded from the GES DISC.
Atmospheric HCHO results from both of anthropogenic pollution and biomass burning. The data indicate the sources of the gas and the transport of the gas in the atmosphere. HCHO is produced when methane (CH4) reacts with atmospheric hydroxyl (OH) in the troposphere. HCHO is removed from the atmosphere by decomposition due to interaction with solar radiation (photolysis, a major source of free radicals in the atmosphere) or by reaction with atmospheric hydroxyl (OH).
The following list provides the changes made in the processing of the data, which significantly improve the data quality over Version 2.0:
- The fitting window has been changed to 328.5-356.5 nm.
- Air Mass Factor (AMF) calculations now use an altitude-resolved set of scattering weight look-up tables, which are provided in the data product under the field ScatteringWeights, together with the climatology profiles, provided in the field GasProfiles, and the vertical coordinates of both, in the ClimatologyLevels field. The latest version available of the OMLER product is used to account for the surface albedo of a particular pixel. A new data field, Albedo, reports the value used in the AMF calculations.
- Cross-Track Quality Flags (XtrackQualityFlags and XtrackQualityFlagsExpanded) have been carried over from the L1b product to provide information on pixels affected by the row anomaly.
- O2 - O2 collision complex has been added to the list of interfering absorbers during the fit.
- HCHO cross-sections and the high-resolution solar spectrum have been updated.
- A radiance reference over the Pacific is used instead of the solar irradiance.
- A reference sector re-normalization of vertical column densities is performed using a GEOS-Chem model derived monthly climatology over the remote Pacific Ocean. We recommend the use of these vertical columns to users. These columns are reported in the field ReferenceSectorCorrectedVerticalColumn in the L2 file (Gonzalez Abad et al., 2014)
The team has also provided notes regarding their data quality assessment:
- Across-track striping (systematically elevated or reduced column values at the same cross track position along the whole track) of the HCHO columns is a minor issue now after the improvements achieved in OMHCHO v3.0 (Collection/Product Version 003).
- Pixels affected by the row anomaly have been flagged in the Geolocation Field XtrackQualityFlags and XtrackQualityFlagsExpanded. Their use is discouraged.
The ‘row anomaly’ refers to a degradation of data quality in the OMI data. The row anomaly is fully described on the KNMI OMI Web site (Row Anomaly Background). A summary of the row anomaly is given below.
“A row anomaly is an anomaly which affects the quality of the level 1B radiance data at all wavelengths for a particular viewing direction of OMI. This corresponds to a row on the CCD detectors, and hence the term ‘Row Anomaly’. The OMI row anomaly is dynamic, it changes over time. The row anomaly affects the quality of the Level 1B radiance data and consequently the Level 2 data products.”
Gonzalez Abad, G., Liu, X., Chance, K., Wang, H., Kurosu, T.P., and Suleiman, R. (2014) Updated SAO OMI formaldehyde retrieval.Atmospheric Measurement Techniques Discussions, 7, 1-31, doi:10.5194/amtd-7-1-2014.
Links to OMI formaldehyde data at the GES DISC:
Aura OMI Formaldehyde Data Product
OMI/Aura Formaldehyde (HCHO)
Questions or comments? Email the NASA GES DISC Help Desk: email@example.com