##### OMNO2 README File

## Document Version 3.1: February 15, 2008

### Overview

Nitrogen dioxide is an important chemical species in both the
stratosphere, where it plays a key role in ozone chemistry, and in the
troposphere, where it is a precursor to ozone production. In the
troposphere, it is produced in various combustion processes and in
lightning. It is a significant indicator of poor air quality, and its
measurement is a key objective of the OMI project.

This document provides a brief description of the OMI Nitrogen Dioxide
(NO~2~) data product, OMNO2. The present version of this document refers
to the Collection 3 Release (Version 1.0.5) of this data product,
November 2007. OMNO2 contains the observed slant column NO~2~ (total
amount along the optical path from the sun into the atmosphere, and then
toward the satellite), the total vertical column NO~2~ \[cm^−2^\], and
the estimated tropospheric portion of the total column NO~2~ \[cm^-2^\].
Other ancillary data are also provided in the OMNO2 file, including data
quality flags, measures of precision, and quality assurance (QA)
information.

An estimate of the NO~2~ column hidden by any clouds in the OMI field of
view has been included in the OMNO2 data product. At the user's
discretion, this "ghost column" can be added to the observed column
amounts (defined below in the section "Description of Retrieved NO~2~
Columns") to estimate total column amounts above ground. The ghost
column is not a tabulated a priori, or climatological, amount, but is
computed from the measured NO~2~ column above the clouds and the assumed
a priori NO~2~ profile *shape*. Uncertainties in the ghost column are
typically large -- on the order of 100%.

### Application

This document applies to the current public release of the OMNO2
product, version 1.0.5, November 2007. This product is archived as
Collection 3. The software versions used to produce the data are
presented in the table below.

OMCLDO2 V1.0.2

OMNO2A V1.0.5

OMNO2B V1.0.0

OMNO2 V0.9.33

### Algorithm Description

The basic algorithm for the retrieval of total column and tropospheric
NO2 from OMI data is described by *Boersma et al*. \[2001\] in *Volume
IV* of the *OMI Algorithm Theoretical Basis Document* (ATBD), available
at

<http://eospso.gsfc.nasa.gov/eos_homepage/for_scientists/atbd/docs/OMI/ATBD-OMI-04.pdf>

Further information about the algorithm and its development may also be
found in *Bucsela et al.,* \[2006\]. However, some of the data fields in
the OMNO2 product differ from those described in these references (see
"Description of Retrieved NO~2~ Columns" below).

The user is strongly advised to read the section "**Known Issues**"
below. The algorithm is divided into three processes, each performed by
a separate program, or Product-Generating Executive (*PGE)*. The end
result is the creation of the OMNO2 data product from the OMI Level 1B
product. Here we briefly describe each PGE.

The first PGE (PGE-A) performs a least-squares fit based on the ratio of
the measured earthshine radiance spectrum to a solar irradiance
spectrum. A daily OMI measurement of the solar spectrum is normally used
to compute the ratio. The closest in time available solar spectrum
measurement is used. Information about which solar spectrum was used to
process a particular OMI granule is available in the metadata. The
functions fitted to the spectral ratio are laboratory-measured trace gas
spectra, a reference Ring spectrum \[*Chance and Spurr*, 1997\] and the
coefficients of a polynomial function to model the spectrally slowly
varying components of scattering by clouds and aerosols and reflection
by the Earth's surface. The fitting algorithm uses the Differential
Optical Absorption Spectroscopy (DOAS) method, and is applied in the
spectral range 405nm to 465nm. In the current version, the only trace
gas absorption spectra considered are those of NO~2~ \[*Vandaele et
al.,* 1998\] and O~3~ \[*Burrows, et al.*, 1999\]. The trace gas
absorption spectra actually used were produced by convolving
high-resolution, laboratory-measured absorption spectra with the
measured OMI slit function. The result of the spectral fit is a slant
column density (SCD) for each OMI pixel.

The second PGE (PGE-B) computes the air-mass factors (AMFs), defined as
the ratio of the SCD to the vertical column density (VCD) of NO~2~. AMFs
depend upon a number of parameters including viewing geometry, surface
albedo and the shape (not magnitude) of the NO~2~ vertical profile. AMFs
are calculated using both ***unpolluted*** (stratospheric and a small
amount of upper tropospheric) and ***polluted*** (lower tropospheric)
NO~2~ profiles. For each location, the algorithm uses a single mean
unpolluted profile derived from the GSFC-CTM and a geographically
gridded set of annual mean polluted profiles obtained from the GEOS-CHEM
model (see Fig. 1). The method of calculation is similar to that
described by *Palmer, et al.* \[2001\], and uses pre-calculated
altitude-dependent AMFs, which are stored in look up tables. For each
profile, AMFs are derived for clear and cloudy conditions. Clouds are
represented as opaque Lambertian reflectors with an albedo of 0.8. A
radiance-weighted sum of the two is used for partly cloudy conditions.
The cloud information (cloud fraction and cloud pressure) is obtained
from the OMCLDO2 cloud product. The AMFs will be applied in the third
PGE to convert the SCDs into VCDs.

**(a)**![](media/image1.png){width="2.9277777777777776in"
height="3.1034722222222224in"}
**(b)**![](media/image2.png){width="2.9944444444444445in"
height="3.123611111111111in"}

***Figure 1**: NO2 concentration profiles used in the calculation of the
air mass factors (AMFs). Shown are the unpolluted profile (a) and three
examples of polluted profiles (b) for low, moderate and high levels of
pollution.*

The third PGE (PGE-C) derives an initial estimate of the NO2 VCD,
*V~init~*, by dividing the SCD by an unpolluted AMF, and assembles all
the data collected within ±12 hours of the target orbital data granule.
Areas shown by GEOS-CHEM to contain climatologically high tropospheric
NO~2~ amounts are then masked, and the remaining regions are smoothed in
latitude bands. The smoothing algorithm employs a "boxcar" smoothing
over a 9° latitude band, followed by a zonal planetary wave smoothing up
to wave-2. At a given geographic location, the OMI pixels selected for
the smoothing are those with the smallest AMFs and slant-column
uncertainties. The result is the column amount *V~u~,* which represents
the smoothly varying background level of NO~2~,including the
tropospheric background contribution, and is used in the algorithm as an
upper limit on the unpolluted NO~2~ column. Pixels for which *V~init~*
exceeds *V~u~* are assumed to be polluted. The polluted VCD, *V~p~*, is
calculated as the difference *V~init~* *V~u~* times the ratio of the
unpolluted AMF to the polluted AMF for the pixel. The total column for
that pixel is then the sum of *V~p~* and *V~u~*. For all other pixels,
the total column is taken to be V~init~.

### 

### Description of Retrieved NO~2~ Columns

The Level-2 data file in the Public Release contains four main types of
NO~2~ vertical column amounts.

(1) *ColumnAmountNO2* (*V^obs^*)= the total observed vertical column
    density above ground.

(2) *ColumnAmountNO2Trop* (*V~trop~^obs^*)= tropospheric observed
    vertical column density (integrated from ground to 150 mb).

(3) *ColumnAmountNO2Polluted* (*V~p~^obs^*)= polluted observed vertical
    column density (integrated from ground to 250 mb).

(4) *ColumnAmountNO2BelowCloud (V~ghost~) = vertical column density
    hidden by clouds.*

In these definitions, the term *observed* refers to the amount of NO~2~
that can be seen from above the atmosphere. The observed amount depends
on the vertical distribution of NO~2~, the pressure level of the cloud
top and the effective cloud fraction. For example, if the effective
cloud fraction (based on an assumed 80% cloud albedo) is 50%, then the
observed column is half the column density above ground plus half the
column density above the cloud top. The Level-2 data also contains an
estimate of the column amount below any clouds in the field of view.
This amount is sometimes called the "ghost column", and is named
*ColumnAmountNO2BelowCloud* in the OMNO2 data product. Adding the ghost
column, V~ghost~, to any observed column amount gives the corresponding
complete column between the ground and the satellite, e.g.

*V~trop~ = V~trop~^obs^ + V~ghost~*

The quantity *ColumnAmountNO2Polluted* is given only in cases where
NO~2~ exceeds background levels (*V~init~* *V~smooth~* \> 0). In all
other cases, fill values are stored. The fill value is the OMI standard
fill value, equal to -2^100^. *ColumnAmountNO2Polluted* is the magnitude
of the column above the background, computed assuming that the origin of
the excess NO~2~ is pollution. Models show pollution affects altitudes
mainly below about 250 mb. *ColumnAmountNO2Trop* is defined as the NO~2~
vertical column between ground and the estimated mean tropopause
pressure of 150 mb. It is obtained by adding 5%
(*TropFractionUnpolluted*) of the difference between the total and
polluted columns to the polluted column.

Although not given explicitly, the *unpolluted* and *stratospheric*
columns can be found by subtracting, respectively, the polluted and
tropospheric columns from the total column. Note that for cases of
measurable pollution (*V~init~* *V~smooth~* \> 0), the unpolluted column
amount is also equal to *V~smooth~*. For all other cases, the unpolluted
column is defined to be the total column (which is also the initial
column in such cases). The VCDQualityFlags data item contains a one-bit
flag that indicates whether each pixel was treated as polluted or
unpolluted.

Note that the NO~2~ fields described here are somewhat different from
those discussed in Bucsela et al. \[2006\], In that document, the
unpolluted column amount is referred to as "stratospheric" NO~2~, and
the polluted column amount is called "tropospheric" NO~2~. In the OMNO2
data product, tropospheric NO~2~ means the polluted amount plus the 5%
of the unpolluted column. Another difference is that *Bucsela et al.*
present a direct method for retrieving the complete above-ground column
amounts. In OMNO2, only the *observed* columns are retrieved. The
complete columns are obtained by adding the ghost column to the total,
tropospheric and polluted columns, as described above.

### Data Quality Assessment

The quality of the data in this Public Release is currently being
established by independent measurements in ongoing validation campaigns
from ground-, aircraft-, and satellite-based instruments. Data quality
issues are described in the Data Quality document found at 
[http://toms.gsfc.nasa.gov/omi/no2/OMNO2_data_quality.pdf[\
]{.underline}](http://toms.gsfc.nasa.gov/omi/no2/OMNO2_data_quality.pdf)An
overview of some of the validation efforts may be found in the
presentation
<http://earth.esa.int/workshops/atmos2006/participants/330/pres_330_kroon.pdf>.
Stratospheric amounts are in reasonable agreement with climatological
measurements from the Halogen Occultation Experiment (HALOE) instrument
aboard the Upper Atmosphere Research Satellite (UARS) and with model
calculations from the NASA/GSFC chemical transport model (CTM).
Tropospheric amounts are generally consistent with the GEOS-CHEM model
and indicate prominent sources near urban areas.

The data product includes estimates of uncertainties associated with the
various geophysical quantities. The uncertainty estimates have been
improved since the provisional release of the OMI data. However, these
estimates may not account for all actual sources of random or systematic
errors. We anticipate further improvements through the validation
process and in understanding the probability distributions of the
underlying data and the algorithmic sensitivity to the data.

#### Known Issues

As a result of unforeseen characteristics of the OMI CCD detector, most
retrieved data products show a varying cross-track anomaly: The average
value along any orbital track at one cross-track position may differ
significantly from that along nearby cross-track positions, resulting in
the appearance of \"stripes\" when the data are displayed on a map. The
origin of this artifact is almost certainly in the performance of the
CCD detectors themselves; a suitable correction to the measured solar
irradiances and earthshine radiances has been developed and will be
implemented in the next release of the Level 1B data (collection 3). In
the current release the striping in the retrieved data can only be
removed in a *post-hoc* fashion.

The OMNO2 algorithm has been fitted with a module, used in the third
PGE, that performs a \"de-striping\" of NO~2~ column amounts by
adjusting the mean SCD for a given cross-track position to the mean
value at all positions. The current version of the de-striping algorithm
computes the mean SCD and the mean initial AMF for each cross-track
position from 24-hours of data using measurements obtained at latitudes
between ±55°. These are used to generate a set of 60 correction
constants, one for each cross-track position, which are subtracted from
the SCD values before computation of the vertical columns. The user
should note that the slant columns (*SlantColumnAmountNO2*) actually
stored in the Level-2 file are from the original spectral fit and have
*not* been de-striped. However the all stored vertical columns, with the
exception of *ColumnAmountNO2Initial*, have been de-striped.

The de-striping procedure will almost certainly introduce an unknowable
systematic bias in the retrieved NO~2~ VCDs, which could vary from orbit
to orbit. As mentioned above, we anticipate that as the understanding of
the instrumental artifact evolves, the OMI Level 1B product will
ultimately contain consistent, calibrated radiances and irradiances,
which will lead to retrieved NO~2~ VCDs that do not show a cross-track
dependence. This will eliminate the need for *post-hoc* corrections.

The air mass factors are computed in PGE-B using a look-up table from
radiative transfer calculations. The calculations are performed on a
6-dimensional grid of pressures, solar zenith angles, viewing zenith
angles, relative azimuth angles, albedos and surface pressures, and the
algorithm interpolates between grid points. A finer grid is planned for
implementation in the next version of the algorithm. Use of the current
coarse grid can cause errors in the polluted AMF, and hence the
retrieved polluted and tropospheric column amounts, as large as10%
relative to the fine grid.

An OMI instrument anomaly required that a fixed OMI solar irradiance
measurement be used in PGE-A from February 2006 through July 2006. A
daily solar spectrum is normally used in the retrieval, but a reference
solar spectrum is being developed for future implementation in the
algorithm.

The user is referred to the **Release Notes** for additional information
and cautions pertaining to the use and interpretation of the data:

http://disc.sci.gsfc.nasa.gov/Aura/OMI/documents/v003/OMNO2_release_notes_v003.doc

### Product Description

A single OMNO2 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 the
\~2600 km-wide OMI ground swath. Fig. 2 shows that due to optical
aberrations and the asymmetric alignment between the instrument's
optical axis and the spacecraft axes, the ground pixels are not
symmetrically aligned with respect to the orbital plane.

![](media/image3.jpeg){width="5.2347222222222225in"
height="3.372916666666667in"}

***Figure 2**: The position of 60 ground pixels for an OMI measurement
in the tropics. Note that the x*

*and y axes cover different distances on the Earth's surface. The arrow
shows the satellite trajectory*

The latitude and longitude provided with each ground pixel represent the
location of the center of each pixel on the ground. Fig. 3 gives the
size of the ground pixels, as a function of pixel number in the across
track direction. Although the pixels also increase in the flight
direction when going off nadir, this effect is much smaller and may be
neglected for most applications.

![](media/image4.jpeg){width="5.1305555555555555in"
height="3.4590277777777776in"}

***Figure 3**: The size of the OMI ground pixels, as a function of the
across track pixel number*

The OMNO2 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.gsfc.nasa.gov/Aura/tools.shtml>,

A single OMNO2 file contains information retrieved from each OMI pixel
over the sun-lit portion of one Aura orbit. The data are ordered in
time. The information provided in these files includes latitude,
longitude, a set of viewing angles (solar zenith and solar azimuth;
viewing zenith and viewing azimuth), column amounts of NO~2~,
intermediate quantities used in the calculation, such as AMFs and
coefficients of the smooth NO~2~ field, and a large number of ancillary
parameters that provide information to assess data quality. Among the
latter are several data quality flag fields (ground pixel quality,
measurement quality, fit quality, AMF quality, and unpolluted field
quality). These should be examined to assess the quality of individual
measurements, particularly for inter-comparison studies. A two-bit
summary flag is also provided to indicate the overall quality of the
data for a given pixel. These two bits are the least-significant bits of
the vcdQualityFlag data item in a Level 2 file. The following table
gives the values of the summary flag.

  ----------- -----------------------------------------------------------
  **Flag      **Meaning**
  Value**     

  0           Best quality data.

  1           (not used)

  2           Poor quality data (uses values that came with warnings)

  3           Data should not be used.
  ----------- -----------------------------------------------------------

The simplest way to use the summary flag is to use the least significant
bit only: If vcdQualityFlag is read as an integer, then the data should
be used only if that integer is even.

For a complete list of fields and ancillary data contained in the OMNO2
file, please read the [OMNO2 data product
specification](http://disc.gsfc.nasa.gov/Aura/OMI/OMNO2_data_product_specification.pdf).
Users who are not interested in the detailed information provided in
OMNO2 dataset, may wish to use one of the several gridded products that
are routinely produced using the OMNO2 dataset. A Level 3 (gridded,
using weighted-averages of columns within each grid cell) product is
available in text or HDF format. A level 3G data set is also available.
This is a gridded data set where each grid cell contains a "stack" of
Level 2 data for every OMI field-of-view that fell within that cell.
Only a subset of the full Level 2 data are included, but enough data is
present in the Level 3G data, that the user could easily find the
corresponding data in the OMNO2 files. Full OMNO2 data, as well as
subsets of these data over many ground stations and along Aura
validation aircraft flights paths are available to those investigators
who are associated with the various Aura science teams through the Aura
Validation Data Center (AVDC) website, <http://avdc.gsfc.nasa.gov/>.
Bojan Bojkov Bojan.Bojkov@avdc.gsfc.nasa.gov is the point of contact at
the AVDC.

**Reporting Problems and Requesting Information**

The following is a summary of links to current documentation for OMNO2:

   [http://toms.gsfc.nasa.gov/omi/no2/OMNO2_readme.pdf[\
]{.underline}](http://toms.gsfc.nasa.gov/omi/no2/OMNO2_readme.pdf)  
[http://toms.gsfc.nasa.gov/omi/no2/OMNO2_release_notes.pdf[\
]{.underline}](http://toms.gsfc.nasa.gov/omi/no2/OMNO2_release_notes.pdf)  
[http://toms.gsfc.nasa.gov/omi/no2/OMNO2_data_quality.pdf[\
]{.underline}](http://toms.gsfc.nasa.gov/omi/no2/OMNO2_data_quality.pdf)  
[http://toms.gsfc.nasa.gov/omi/no2/OMNO2_data_product_specification.pdf[\
\
]{.underline}](http://toms.gsfc.nasa.gov/omi/no2/OMNO2_data_product_specification.pdf)

To report problems, or pose questions and comments related to the OMNO2
algorithm, data quality, and file structure, please send electronic mail
to <omno2@ltpmail.gsfc.nasa.gov>. Additional questions may be directed
to the principal points of contact for OMNO2: James F. Gleason
<gleason@redwind.gsfc.nasa.gov>, and J. Pepijn Veefkind
veefkind@knmi.nl.

# **Acknowledgements**

***K. Folkert Boersma^4^, Ellen Brinksma^4^, Eric J.Bucsela^2^, Edward
A. Celarier^1^, James F. Gleason^3^, Pieternel Levelt^4^, J. Pepijn
Veefkind^4^, Mark O. Wenig^2^***

***\[1\] SGT, Inc., Greenbelt, MD;***

***\[2\] GEST Program, Univ of Maryland Baltimore County, Baltimore,
MD;***

***\[3\] Atmospheric Chemistry & Dynamics branch, NASA Goddard Space
Flight Center, Greenbelt, MD;***

***\[4\] Royal Dutch Meteorological Institute (KNMI), de Bilt, The
Netherlands***

### References

Boersma, K. F., E. J. Bucsela, E. J. Brinksma, J. F. Gleason, NO~2~,
*OMI-EOS Algorithm Theoretical Basis Document: Trace Gas Algorithms:
NO2*, **4**, 12-35, 2001.

Bucsela, E., E. Celarier, M. Wenig, J. Gleason, P. Veefkind, K.F.
Boersma, E. Brinksma, Algorithm for NO~2~ vertical column retrieval from
the Ozone Monitoring Instrument, *IEEE Transactions on Geoscience and
Remote Sensing,* **44**, 1245-1258, 2006.

Burrows, J.P., A. Richter, A. Dehn, B. Deters, S. Himmelmann, S. Voigt,
and J. Orphal, Atmospheric remote sensing reference data from GOME - 2.
Temperature-dependent absorption cross-sections of O~3~ in the 231-794
nm range, *J. Quant. Spectrosc. Radiat. Transfer* **61**, 509-517, 1999.

Chance, K. V., and R. J. D. Spurr, Ring effect studies: Rayleigh
scattering, including molecular parameters for rotational Raman
scattering, and the Fraunhofer spectrum, *Applied Optics*, **36**, 5224,
1997.

Palmer, P.I., D.J. Jacob, K. Chance, R.V. Martin, R.J.D. Spurr, T.P.
Kurosu, I. Bey, R. Yantosca, A. Fiore, Air mass factor formulation for
spectroscopic measurements from satellites: Application to formaldehyde
retrievals from the Global Ozone Monitoring Experiment, *J. Geophys.
Res*., **106**, 14539, 2001.

Vandaele A.C., C. Hermans, P.C. Simon, M. Carleer, R. Colin, S. Fally,
M.F. Mérienne, A. Jenouvrier, and B. Coquart, Measurements of the NO~2~
absorption cross-section from 42000 cm-1 to 10000 cm-1 (238-1000 nm) at
220 K and 294 K, *J. Quant. Spectrosc. Radiat. Transfer* **59**,
171-184, 1998.