# Odin-OSIRIS O3 and Aerosols: Saskmart Processing Algorithm

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Profiles of O3 and aerosols are retrieved from measured OSIRIS limb radiances using the Saskmart algorithm. The algorithm employs the following key features

1. Generates retrieval vectors using ratios of radiances. This makes the algorithm insensitive to errors in absolute calibration.
2. Generation of altitude dependent weighting functions for each retrieval vector. Allows combination of different spectral regions
4. Inversion using a MART algorithm. Converges to consistent, physically meaningful solutions with minimal knowledge of initial conditions or a-priori.

# Algorithm Overview

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The SASKMART algorithm applies a Multiplicative Algebraic Reconstruction Technique coupled with the Sasktran V2  radiative transfer forward model to invert the OSIRIS line of sight radiances and produce height profiles of ozone and aerosols.  The OSIRIS line-of-sight radiances typically scan from 7 km to 80 km altitude at approximately 2 km intervals with ~1 km height resolution although the presence of tropospheric cloud raises the lowest useful altitude.

The MART algorithm combines ratios of the measured radiances at specific wavelengths and altitudes to produce a set of retrieval vectors ($$y_{kj}$$) which are the entities used by the algorithm to derive the desired height profiles ($$x_{i,k}$$). The choice of retrieval vectors is an important part of the algorithm. They have been selected so they are almost always positive throughout the fitting region, a condition which helps the algorithm avoid the undesirable oscillations common in many other techniques.  The O3 section defines 8 retrieval vectors using 11 wavelengths providing good sensitivity across the entire altitude range.  The aerosol section defines 1 retrieval vector. The retrieval vector definitions also bestows the algorithm with almost no sensitivity to errors in instrument absolute calibration.

The MART algorithm iteratively converges the selected height profile using a mulitplicative correction factor for each iteration

$$x^{n+1}_i = \alpha_{i}x^{n}_i$$

where

$$\alpha_{i} = \sum_k \alpha_{ik}W_{ki}$$

and

$$\alpha_{ik}=\sum_{j}\left( \frac{y_{kj}^{obs}}{y_{kj}^{mod}}W_{ji}\right)$$

# O3 Retrieval Vectors

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## O3 Retrieval Vectors

The version 5.0.xx retrieval algorithms use 11 unique wavelengths to generate 8 ozone retrieval vectors. 7 vector are in the Hartley Huggins region and 1 is in the Chappuis region. The large selection of retrieval vectors allows the algorithm to have good sensitivity across the entire alitude range.

Spectral Region Retrieval Vector
Hartley Huggins $$y_{j,k} = \ln\left( \frac{I(j,\lambda_{ref})}{I(j,\lambda_{abs})}\right)$$
Chappuis $$y_{j,k} = \ln\left(\frac{\sqrt{I(j,\lambda_{ref1})I(j,\lambda_{ref2})}}{I(j,\lambda_{abs})}\right)$$

 $$\lambda_{ref}$$ nm $$\lambda_{abs}$$ nm 292.43 350.31 302.17 350.31 306.06 350.31 309.58 350.31 315.82 350.31 322.00 350.31 331.09 350.31

 $$\lambda_{ref1}$$ nm $$\lambda_{abs}$$ nm $$\lambda_{ref2}$$ nm 543.84 602.39 678.85

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SASKMART is the program we use at the Odin-OSIRIS processing centre in Saskatoon to retrieve profiles of ozone (O3) and aerosols from the line of sight radiances measured by the OSIRIS spectrograph.  The program was first deployed in October 2009 and has been used since that time to produce version 5.0.xx Level 2 products. The program uses the Sasktran V2 radiative transfer model as a forward model and a Multiplicative Algebraic Reconstruction Technique (MART) as the inversion scheme. Later versions of the code, 5.0.7 onwards, employ an error analysis scheme similar to those found in Optimal Estimation techniques. The algorithm does not require a-priori information in the weighted statistical sense used by optimal estimation techniques but does need initial guesses which we take from existing climatologies for NO2, aerosols and O3 and ECWMF for atmopsheric density and temperature.  In our experience the final solution is insensitive to the initial guess.

The ozone data product has had considerbale validation work and is in excellent agreement with other instrumentation over the entire stratosphere. Greater variability occurs in the upper troposphere, part of which can be explained by natural variability.    The aerosol product has had less validation but is still an excellent product.  The aerosol product currently fits the radiances with an assumed aerosol particle size distribution which undoubtedly results in systematic errors associated with the incorrect choice of phase matrix. None-the-less we are confident the aerosol product does reflect true variations of radiance in the atmosphere due to the prescence of aerosols in the atmosphere.  We are working on new algorithms to improve the data products but we truly believe the Saskmart version 5.0.xx family will continue to be a high quality product for years to come.

# Initial Conditions

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## Initial Conditions

1. The background atmosphere is approximated using the ECMWF analysis for both presssure and temperature. The profiles are interpolated to the median location o fthe OSIRIS scan.
2. The initial estmate of NO2 is derived from interpolation of the the Pratmo climatology.
3. The initial aerosol estimate is a typical log normal sulphate  profile provided by Deschler.
4. The initial ozone is derived from an interpolation of the Labow climatology.

# Version 5.0x Series

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The version 5.0.x processing series is the first version of  Odin-OSIRIS O3 and aerosol data products that we store at the OSIRIS data processing centre. The version 5.0.x series employ the same internal algorithm and produce the same profiles. Our latest version 5.0 product, 5.0.7, now includes a complete error analysis, quality assurance and the complete source code is available (1.3 GB) via our subversion (SVN) source code control server. You will need username and password info to access the server, and Windows XP or higher along with Visual Studio 2008 to build the code.

# Mart Retrievals Version 5.07 Summary

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Version 5.07 Mart_Retrievals was first implemented on October 11, 2011 and has been used to process all data collected during the mission, 2001-10-27 to current date. The code produces identical results to the earlier versions (5.06) but has been extended to include a full error analysis and has been subjected to a quality insurance and inspection before being released to the public.

## Retrieval Code

LEVEL 2 Specifications description
Build Executable Name mart_retrievals507r.exe

SVN Repository Version Tag

Repos_MartRetrievals/tags/V507_with_externaldependencies

Reference Version

5.07

Control Loop Process

Retrieve NO2;

Retrieve Albedo;

Retrieve Stratospheric Aerosol;

Retrieve Albedo;

Retrieve Stratospheric Aerosol;

Retrieve O3;

Error Estimate

Yes.

2.0

Data Availablility 2001-10-27 to Present

## Ozone

O3

Properties

Quantity

Concentration ( molecules/cm3)  is the primary retrieval parameter. VMR is  calculated from the concentration and stored in the files.

Cross Sections

SCIAMACHY Bogumil Version 1. The cross-section is convolved with an instrument point spread function that changes with wavelength across the instrument but is fixed for the duration of the mission

Base Algorithm

Degenstein et al., ACP, 2009. Algorithm uses a combination of Hartley-Huggins and Chappuis wavelengths to get O3 profile from cloud tops to 65 km. The error analysis follows Bourassa et al 2012

Modifications

Tangent Altitude Smoothing Vector.  The version 5.0x series uses a tangent altitude smoothing vector that is too coarse (the omission occurred during porting of code) but has minimal impact.

## Aerosol

Aerosol Properties

Quantity

The technique calculates aerosol extinction at 750 nm (km-1).  The technique requires knowledge of the scattering phase matrix and errors in this knowledge generate systematic errors.

Microphysical Assumptions

Composition

H2SO4-H2O (75%-25%)

Particle Size

Constant Altitude Profile

Single Mode Lognormal

Mode Width 1.6

Base Algorithm

Bourassa et al., JGR, 2007

Modifications

Bourassa et al., ACP, 2012

## NO2

NO2 Properties

Quantity

Concentration (cm3) is calculated and stored on-site but is not distributed. The NO2 DOAS algorithm produces the standard operational NO2 product.

Cross Sections

GOME (Burrows et al., 1998)

Base Algorithm

Bourassa et al., AMT, 2011

Modifications

Measurement Vector

2x 4-let

1x 8-let

Tangent Altitude Smoothing Vector

0.5, 0.3, 0.2

## Albedo And Cloud Tops

Albedo Properties

Quantity

Effective Lambertian Surface Albedo at 675 nm.

Base Algorithm

5 point linear fit

No spectral dependence

Cloud Tops

## Level 1 Products

LEVEL 1 Specifications Properties

Osiris Level 1 Services

Version 2.42. Now accounts for small timing discrepancies in STW-UTC conversion table

Attitude files

Includes correction for satellite position due to STW interpolation issues.

Spectrograph Files

Standard processing

Osiris Absolute Calibration

Varies per calibration trends until September 2008 then fixed at 2008 levels.  Minor impact.

Osiris Dark Current

Tracks both band-gap energy and exponent magnitude. Minimal impact. Band gap energy starts to slightly oscillate after September 2010. Minor impact

Function of wavelength only.  No variation with time. Might be important during eclipse season in June,July,August. Investigations suggest that varying the point spread function with instrument temperature may create changes in results on the order of 2%

RSAS/Attitude analysis

No corrections for any possible systematic twisting of satellite frame during eclipse season. We know that the OSIRIS instrument may have a pointing issue of the order of 100-300 meters altitude during the peak satellite eclipse season (July/August of each year).

No pre-filtering of radiation hits in radiance profile. Post analysis quality assurance detects most outlier scans due to radiation hits and removes them from the release

ECMWF

We use 28 pressure levels on a 24 hour grid from ECMWF T106 grid point analyses from 2001 until May 2011. From May 2011 to present we use ECMWF Interim on a 6 hour grid.

ECMWF pressure and temperature products are linearly interpolated  in time, latitude, longitude and height. We assume the Earth is a perfect oblate-spheroid and extrapolate ECMWF data (which follows the true shape of Earth) to the oblate spheroid surface. Pressure and temperature products are joined to MSIS90 from top of ECMWF (~55-65 km) to 100 km.

## Source Code Control: subversion dependencies

Module Subversion Tag

Repos_MartRetrievals and all dependent software

Repos_MartRetreivals/tags/V507_with_externaldependencies

# Mart Retrievals Version 5.06 Summary

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Version 5.06 Mart_Retrievals was first implemented around April 21, 2011 and was used to process data collected after 2008-08-31. The code was written to address a few logistical issues in the 5.01 code but uses an identical algorithm and produces identical results (to about 6 decimal places).  The code uses an upgraded Level 1 product which eliminated a small timing jitter in the system which should tighten up the spread of results but should not introduce systematic biases.

## Retrieval Code

LEVEL 2 Specifications description Changed
Build Executable Name mart_retrievals503r.exe

SVN Repository Version Tag

n/a

Reference Version

5.06

Control Loop Process

Retrieve NO2;

Retrieve Albedo;

Retrieve Stratospheric Aerosol;

Retrieve Albedo;

Retrieve Stratospheric Aerosol;

Retrieve O3;

N

Error Estimate

Not available in operational data products.

N

2.0

N

Data Availablility 2008-08-31 to Present

## Ozone

O3

Properties Changed

Quantity

Concentration ( molecules/cm3)  is the primary retrieval parameter. VMR is  calculated from the concentration and stored in the files.

N

Cross Sections

SCIAMACHY Bogumil Version 1. The cross-section is convolved with an instrument point spread function that changes with wavelength across the instrument but is fixed for the duration of the mission

N

Base Algorithm

Degenstein et al., ACP, 2008. Algorithm uses a combination of Hartley-Huggins and Chappuis wavelengths to get O3 profile from cloud tops to 65 km.

Modifications

Tangent Altitude Smoothing Vector.  The version 5.0x series uses a tangent altitude smoothing vector that is too coarse (the omission occurred during porting of code) but has minimal impact.

## Aerosol

Aerosol Properties Changed

Quantity

The technique calculates aerosol extinction at 750 nm (km-1).  The technique requires knowledge of the scattering phase matrix and errors in this knowledge generate systematic errors.

N

Microphysical Assumptions

Composition

H2SO4-H2O (75%-25%)

N

Particle Size

Constant Altitude Profile

Single Mode Lognormal

Mode Width 1.6

Base Algorithm

Bourassa et al., JGR, 2007

N

Modifications

Bourassa et al., ACP, 2011

N

## NO2

NO2 Properties Changed

Quantity

Concentration (cm3) is calculated and stored on-site but is not distributed. The NO2 DOAS algorithm produces the standard operational NO2 product.

N

Cross Sections

GOME (Burrows et al., 1998)

N

Base Algorithm

Bourassa et al., AMT, 2011

N

Modifications

Measurement Vector

2x 4-let

1x 8-let

Tangent Altitude Smoothing Vector

0.5, 0.3, 0.2

N

## Albedo And Cloud Tops

Albedo Properties Changed

Quantity

Effective Lambertian Surface Albedo at 675 nm.

N

Base Algorithm

5 point linear fit

No spectral dependence

N

Cloud Tops

## Level 1 Products

LEVEL 1 Specifications Properties Changed

Osiris Level 1 Services

2.40

Properly accounts for small timing discrepancies in STW-UTC conversion table

Y

Attitude files

Includes correction for satellite position due to STW interpolation issues.

Y

Spectrograph Files

Standard processing

N

Osiris Absolute Calibration

Varies until September 2008 then fixed at 2008 levels.  Minor impact

N

Osiris Dark Current

Tracks both band-gap energy and exponent magnitude. Minimal impact.

N

Function of wavelength only.  No variation with time. Might be important during eclipse season in June,July,August.

RSAS analysis

No corrections for any possible systematic twisting of satellite frame during eclipse season (typically < 300 m altitude error)

ECMWF

Interpolation of 00:00 UTC ECMWF pressure and temperature products in time, latitude, longitude and height to scan, extrapolated to oblate-spheroid ground and joined with MSIS90 from top of ECMWF (55-65 km) to 100 km.

## Source Code Control: subversion dependencies

Module Subversion Tag

Repos_MartRetrievals

Repos_MartRetreivals/tags/MartRetrievals_506

Osiris Level 1 services

Repos_OSIRIS/tags/MartRetrievals_506

skclimatology

Repos_skclimatology/tags/MartRetrievals_506

skopticalproperties

Repos_skopticalproperties/tags/MartRetrievals_506

Base Code

Repos_BaseCode/tags/MartRetrievals_506

ARG Standard Libraries

Repos_Libs/tags/MartRetrievals_506

ARG OSIRIS Libraries

# Mart Retrievals Version 5.01 Summary

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Version 5.01 Mart_Retrievals was first implemented on 2009-10-28 and was used to process data collected from 2001-10-30 to  2008-08-30 (orbits 3800 to 41001).

## Retrieval Code

LEVEL 2 Specifications description Changed
Build Executable Name mart_retrievals503r.exe

SVN Repository Version Tag

MartRetrievals_503

Reference Version

5.01

Control Loop Process

Retrieve NO2;

Retrieve Albedo;

Retrieve Stratospheric Aerosol;

Retrieve Albedo;

Retrieve Stratospheric Aerosol;

Retrieve O3;

N

Error Estimate

Not available in operational data products.

N

2.0

N

Data Availablility 2001-11-03 to 2008-08-31

## Ozone

O3

Properties Changed

Quantity

Concentration ( molecules/cm3)  is the primary retrieval parameter. VMR is  calculated from the concentration and stored in the files.

N

Cross Sections

SCIAMACHY Bogumil Version 1. The cross-section is convolved with an instrument point spread function that changes with wavelength across the instrument but is fixed for the duration of the mission

N

Base Algorithm

Degenstein et al., ACP, 2008. Algorithm uses a combination of Hartley-Huggins and Chappuis wavelengths to get O3 profile from cloud tops to 65 km.

Modifications

Tangent Altitude Smoothing Vector.  The version 5.0x series uses a tangent altitude smoothing vector that is too coarse (the omission occurred during porting of code) but has minimal impact.

## Aerosol

Aerosol Properties Changed

Quantity

The technique calculates aerosol extinction at 750 nm (km-1).  The technique requires knowledge of the scattering phase matrix and errors in this knowledge generate systematic errors.

N

Microphysical Assumptions

Composition

H2SO4-H2O (75%-25%)

N

Particle Size

Constant Altitude Profile

Single Mode Lognormal

Mode Width 1.6

Base Algorithm

Bourassa et al., JGR, 2007

N

Modifications

Bourassa et al., ACP, 2011

N

## NO2

NO2 Properties Changed

Quantity

Concentration (cm3) is calculated and stored on-site but is not distributed. The NO2 DOAS algorithm produces the standard operational NO2 product.

N

Cross Sections

GOME (Burrows et al., 1998)

N

Base Algorithm

Bourassa et al., AMT, 2011

N

Modifications

Measurement Vector

2x 4-let

1x 8-let

Tangent Altitude Smoothing Vector

0.5, 0.3, 0.2

N

## Albedo And Cloud Tops

Albedo Properties Changed

Quantity

Effective Lambertian Surface Albedo at 675 nm.

N

Base Algorithm

5 point linear fit

No spectral dependence

N

Cloud Tops

## Level 1 Products

LEVEL 1 Specifications Properties Changed

Osiris Level 1 Services

2.02

N

Attitude files

Excludes correction for satellite position jitter due to STW interpolation issues.

Spectrograph Files

Standard processing

N

Osiris Absolute Calibration

Follows variation until September 2008

N

Osiris Dark Current

Tracks both band-gap energy and exponent magnitude. Minimal impact.

N

Function of wavelength only.  No variation with time. Might be important during eclipse season in June,July,August.

RSAS analysis

No corrections for any possible systematic twisting of satellite frame during eclipse season (typically < 300 m altitude error)

ECMWF

Interpolation of 00:00 UTC ECMWF pressure and temperature products in time, latitude, longitude and height to scan, extrapolated to oblate-spheroid ground and joined with MSIS90 from top of ECMWF (55-65 km) to 100 km.

## Source Code Control: subversion dependencies

Module Subversion Tag

Repos_MartRetrievals

Repos_OSIRIS, revision 151, 2009-10-28

Osiris Level 1 services

Repos_OSIRIS, revision 151, 2009-10-28

skclimatology

skopticalproperties

Base Code

Repos_BaseCode, revision 72,  2009-09-25

ARG Standard Libraries

n/a

ARG OSIRIS Libraries

n/a

# Aerosol Version 6.0x Series

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The version 5 aerosol algorithm requires an assumed particles size distribution which introduces errors into the retrieved extinction. Version 6 couples the retrieval of extinction and a particle size parameter for more accurate retrievals. The main differences between versions 5 and 6 include:

• Incorporation of the 1.53 µm infrared imager channel for a second measurement vector.
• A lognormal size distribution with a mode width is still assumed, however both mode radius and number density are retrieved.
• Parameter space is searched using the Levenberg-Marquardt algorithm.

# Retrieval Vectors

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The aerosol retrieval vector has been modified slightly for version 6. In version 5 the spectral ratio of two wavelengths was used to decrease sensitivity to neutral atmosphere density fluctuations. However, the normalization wavelength of 470 nm is not completely insensitive to aerosol, particularly small particles, and this normalization can cause unwanted non-linearities in the particle size retrieval. Because of this the 470 nm normalization is not used the Version 6 retrieval and the measurement vector is

# Version 6 Algorithm Description

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For the Version 6 retrieval a unimodal lognormal distribution is assumed with a mode width of 1.6 and the mode radius and extinction are retrieved using aerosol measurement vectors at 750 nm and 1.53 µm. The SASKTRAN model is initialized with the albedo and aerosol profile retrieved using the Version 5 algorithm, as it provides a good estimate of extinction with particle sizes typical of background conditions. At each iteration the mode radius and extinction are then updated at each tangent altitude using the Levenberg-Marquardt algorithm.

To improve computation time the entire aerosol profile is shifted and the forward difference method is used to compute the jacobian element at each altitude. This neglects the effects of altitude coupling, however this affect is relatively week at these longer wavelengths, and convergence is typically achieved in 10 iterations. The convergence limits are given in Table 1, and convergence is said to be satisfied if any of the limits are met for all retrieved altitudes.

Table 1: Convergence Limits

 Jacobian Norm Step SIze Total Residual Iterations Limit 10-5 0.01% 10-4 20