# OMDOAO3 README File

Pepijn Veefkind, KNMI

Last update: 9 October 2007

## Overview

This document provides a brief description of the OMDOAO3 data product.
The OMDOAO3 Level 2 data product contains total ozone and ancillary
information produced by the OMI DOAS algorithm. Each file contains the
sunlit part of an OMI orbit, from the South Pole to the North Pole. In
the so-called global observation mode, the OMI swath is approximately
2600 wide, providing daily global coverage

The OMDOAO3 is produced as a standard product as well as in
near-real-time (within three hours of observation) and as a very fast
delivery product (within 20 minutes of observation). Although it is
expected that the differences will be very small, this document
describes the standard data product. The near-real-time products is
available from the [TEMIS web
site](http://www.temis.nl/protocols/O3total.html) and the very fast
delivery product from the [FMI web
site](http://omivfd.fmi.fi/cgi-bin/ind.pl).

You may refer to [release specific information about
OMDOA3](http://disc.sci.gsfc.nasa.gov/Aura/OMI/documents/v003/OMDOAO3ReleaseDetailsCol3.doc)
for details about software versions and known problems.

## Algorithm Description

The OMI DOAS algorithm is described in the [OMI Algorithm Theoretical
Basis Document
(ATBD)](http://eospso.gsfc.nasa.gov/eos_homepage/for_scientists/atbd/viewInstrument.php?instrument=13).
In addition, a
[paper](http://ieeexplore.ieee.org/iel5/36/34107/01624602.pdf?isnumber=34107&prod=JNL&arnumber=1624602&arSt=+1239&ared=+1244&arAuthor=Veefkind%2C+J.P.%3B+de+Haan%2C+J.F.%3B+Brinksma%2C+E.J.%3B+Kroon%2C+M.%3B+Levelt%2C+P.F.)
that contains a description of the algorithm has been published in IEEE
Transactions on Geoscience and Remote Sensing (IEEE TGRS, Vol 44, No. 5,
2006, doi: 10.1109/TGRS.2006.871204). 

In the DOAS (Differential Optical Absorption Spectroscopy)
implementation for OMI the ozone vertical column is determined in three
steps. In the first step the actual DOAS fitting is performed, resulting
in the so-called slant column density, which is the amount of ozone
along an average photon path from the Sun -through the atmosphere- to
the satellite. In the second step the air mass factor is determined,
which is needed to convert the slant column density into a vertical
column. In the last step a correction is performed for clouds. All the
results from intermediate steps are also contained in the product.

The OMI DOAS spectral fitting uses a 5 nm wide fit window centered
around 334.1 nm. This window has been selected based on the very low
temperature sensitivity for this window. In the spectral fitting, the
inelastic rotational Raman scattering, as well as the effective ozone
temperature are explicitly accounted for. The air mass factor is
determined off-line, by simulating OMI spectra using a radiative
transfer model. The cloud pressure needed to derive accurate air mass
factors and for correction factors for cloudy and partly cloudy
conditions, is obtained from the OMCLDO2 cloud product. This product
derives the cloud fraction and pressure from the O2-O2 absorption band
around 477 nm. The [OMI Algorithm Theoretical Basis Document
(ATBD)](http://eospso.gsfc.nasa.gov/eos_homepage/for_scientists/atbd/viewInstrument.php?instrument=13)
and the above mentioned paper in IEEE TGRS contain a complete
description of the algorithm. The effective cloud fraction is derived
from the fit window itself.

 This algorithm is one of the two algorithms that will be used to derive
total ozone values from OMI. The other is an algorithm based on the
well-known TOMS method (OMTO3 data product). Initial comparisons show
good overall agreement between the two algorithms, but there are
noticeable differences over clouds and snow/ice, as well as at large
solar zenith angles.

## Data Quality Assessment

In this section the general sata quality is discussed. The [release
specific information about
OMDOA3](http://disc.sci.gsfc.nasa.gov/Aura/OMI/documents/v003/OMDOAO3ReleaseDetailsCol3.doc)
contains details on specific features and problems in the data product
and this document should be read before using the data.

Validation was performed by comparing the OMDOAO3 total ozone data
version 1.0.1  with collocated Brewer data for the Northern hemisphere,
for November 2004 (see above mentioned paper in IEEE TGRS) as well as
for the period between March 28 and June 11, 2005. On average, the
retrieved ozone column is biased by 2 % with respect to the Brewers
(\[OMI-Brewer\]/Brewer), the standard deviation on this number is 3.5%.
The validation results show that the errors increase with increasing
Solar zenith angle. In the current version of OMDOAO3 (1.0.5) the Solar
zenith angle dependent bias is expected to be much smaller. This is
confirmed by comparing to the OMTO3 Collection 2 dataset.

Several papers on the validation of the OMDAO3 data product v1.0.1 will
be part of a special issue of JGR on the validation of Aura data
products, to appear early 2008. We will continue to work on the
validation of the Collection 3 version 1.0.5 data.

To assess the quality of the individual retrievals it is very important
to lok at the quality flags fields in the data products, as described in
the [Product Format
Specification](http://disc.sci.gsfc.nasa.gov/Aura/OMI/documents/v003/OMDOAO3_fs.doc).
Especially the ProcesingQualityFlags are important to filter for bad
quality data. The best quality data have a ProcesingQualityFlags of 0.
It is recommended to apply a bitwise AND on the ProcesingQualityFlags
field using a  value of 10911 to filter the data. This will filter all
data for which the ProcesingQualityFlags bit 0, 1, 2, 3, 4, 7, 9, 11
and/or 13 are set.

## Product Description

A single OMDOAO3 product file contains all OMI measurements on the
sunlit portion of the Earth, for one Aura orbit. During one orbit OMI
performs approximately 1650 measurements, which take 2 seconds each. In
the global observation mode, 60 across track ground pixels are measured
simultaneously during a measurement. These 60 measurements cover a swath
of approximately 2600 km wide. During so-called zoom-in measurements the
swath width maybe reduced. The operational baseline includes zoom-in
measurements for 1 day a month. In case of zoom-in measurements 30 of
the 60 across track pixels contain data, the other 30 contain fill
values.

A complete description of the format of the OMDOAO3 data products is
provided
[here](http://disc.sci.gsfc.nasa.gov/Aura/OMI/documents/v003/L2_ozone_spec_issue11_draft.pdf).
A more condensed listing of the data fields is available in the [Product
Format
Specification.](http://disc.sci.gsfc.nasa.gov/Aura/OMI/documents/v003/OMDOAO3_fs.doc)

[The OMDOAO3 product is written as an HDF-EOS5 swath file. For a list of
tools that read HDF-EOS5 data files, please visit this
link:](http://disc.sci.gsfc.nasa.gov/Aura/OMI/documents/v003/OMDOAO3_fs.doc)
<http://disc.gsfc.nasa.gov/Aura/tools.shtml>.

## Data Availability

The OMDOAO3 product can be obtained from the [NASA Goddard Earth
Sciences Data and Information Service Center
(DISC).](http://disc.sci.gsfc.nasa.gov/Aura/OMI/omdoao3_v003.shtml) Full
OMDOAO3 data, as well as subsets of these data over many ground stations
and along Aura validation aircraft flights paths are also available
through the [Aura Validation Data Center (AVDC)
website](http://avdc.gsfc.nasa.gov/)) to those investigators who are
associated with the various Aura science teams. [Bojan
Bojkov](mailto:Bojan.Bojkov@avdc.gsfc.nasa.gov") is the point of contact
at the AVDC.

 For users not interested in the detailed information provided on
OMDOAO3 dataset we are developing several gridded products. In addition,
we intend to make OMDOAO3 data available in a geographically ordered
(rather than time-ordered) format that can be more easily subsetted and
manipulated on-line prior to ordering. Please check the [Goddard Earth
Sciences (GES) Data and Information Service
Center](http://disc.gsfc.nasa.gov/Aura/OMI/) for current information on
these products.

## Contact Information

 Questions related to the OMDOAO3 dataset should be directed to the [GES
DISC](mailto:help-disc@listserv.gsfc.nasa.gov). For questions and
comments related to the OMDOAO3 algorithm and data quality please send
mail to contact <omdoao3@ltpmail.gsfc.nasa.gov>.