1. 18th OMI Science Team Meeting
Next-generation OMI SO2 Retrieval Algorithm based on Principal Component Analysis
Can Li1,2, Joanna Joiner2, Nick Krotkov2, P. K. Bhartia2
1ESSIC, University of Maryland College Park
2NASA GSFC
18th OMI Science Team Meeting
KNMI, De Bilt, The Netherlands
March 13, 2014

2. Outline Background and Motivation Methodology (Framework)
Application to OMI
Results (Planetary Boundary Layer SO2)
Results (Volcanic SO2)
Data Continuity: Comparison of OMI and OMPS
Next Steps and Conclusions

3. Background and Motivation
Operational OMI SO2 (Sept – Feb. 2008)
Motivation: Band Residual Difference (BRD) algorithm fast and sensitive, but large noise and artifacts (only 3 pairs of wavelengths)
Objective: develop an innovative approach to utilize the full spectral content from OMI while maintaining computational efficiency

4. Basis – Spectral fitting algorithms
First look at the DOAS Equation:
Measured sun-normalized radiances
Rayleigh and Mie scattering, surface reflectance etc.
Various gas absorbers (O3, SO2 etc.)
The Ring effect
Plus additional measurement artifacts terms (e.g., wavelength shift, stray light, etc.) and/or radiance data correction schemes
Utilization of the full spectral content, but some terms are difficult to model (e.g., RRS)

5. Methodology (Framework): PCA
Instead of explicit modeling of ozone, RRS, and other instrumental features, we use a data-driven approach based on principal component analysis (PCA) with spectral fitting
Measured N-value spectrum
SO2 column amount
PCs from SO2-free regions, (O3 absorption, surface reflectance, RRS, measurement artifacts etc.) other than SO2 absorption
Pre-calculated SO2 Jacobians (assuming O3 profiles, albedo, etc.)
Fitting of the right hand side to the spectrum on the left hand side -> SO2 column amount and coefficients of PCs
(See Guanter et al., 2012; Joiner et al., 2013; Li et al., 2013)

6. Application to OMI
Spectral window: nm – avoid stray light at shorter wavelengths
Each row (scan position) processed individually – different characteristics between different rows of the 2-D CCD
Each swath processed individually – account for orbit-varying dark current
# of PCs determined dynamically – exclude SO2-related PCs and avoid overfitting by checking the correlation between PCs and SO2 Jacobians
1st step: Simple Jacobians similar to those used in operational BRD algorithm for straight-forward comparison
Step 1 Ps
See Li et al., [GRL, 2013] for details

7. Principle Components and Residuals
Example PCs from entire row # 11, Orbit 10990
(Var.% )
PC #1: Mean spectrum
(a-c) First few PCs
Blue line: scaled reference Ring spectrum
(Var.% )
PC #2: O3 absorption
(Var.% )
PC #3: Surface reflectance (also Ring signature)
(Var.% 5.32E-5)
PCs #4 and #5: likely measurement artifacts, noise (>99.99% variance explained)
(Var.% 4.79E-5)
(d) Least squares fitting residuals for a pixel near Hawaii
Smaller residuals with SO2 Jacobians fitted

8. Results: noise and artifact reduction
August, 2006
OMI operational BRD
PCA algorithm reduces retrieval noise by a factor of two as compared with the BRD algorithm
SO2 Jacobians for PCA algorithm calculated with the same assumptions as in the BRD algorithm

9. Results: Boundary layer pollution SO2
PCA
Operational BRD
eastern U.S., August 2006
PCA algorithm reveals major SO2 point sources (circles), with much reduced noise and artifacts.

10. Ex. Sudbury, Canada (~220 kt in 2006)
Ex. analyzed with pixel averaging (super sampling) reveals details of emission sources [e.g., Fioletov et al., 2011]
PCA, 2006
BRD, 2006 only
BRD,
Largely hidden by artifacts
One year’s worth of PCA retrievals yield results similar to that from 3-5 years worth of BRD data.
Global survey shows that PCA SO2 removes most artifacts in BRD data without significantly altering signals from real sources (Fioletov and McLinden, personal communication)

11. Volcanic SO2: Kasatochi eruption August 7-8, 2008
For volcanic SO2, nonlinearity due to saturation at shorter wavelengths
Iteration of SO2 Jacobians (pre-calculated assuming loadings of DU)
Shift of spectral fitting window to longer wavelengths
PCA closest to estimated released SO2 mass of ~2200 kt based on observed decay of SO2 [Krotkov et al., 2010]

12. Transport of the plume August 10, 2008 August 11, 2008 August 12, 2008
Ln(SO2)

13. Comparison with OMPS
Our algorithm eliminates the need for explicit instrument-specific radiance correction schemes
Test on OMPS: minimal changes to algorithm
biggest is the use of OMPS slit function for Jacobians
spectral window, etc., same as in OMI
Reduces the chance of introducing artifacts/biases between different instruments

14. OMI and OMPS comparison
OMPS and OMI PCA SO2 retrievals show good agreement despite somewhat different sampling
October, 2013

15. OMI and OMPS comparison
OMPS, Jan. 2013
OMI, Jan. 2013
Both OMI and OMPS PCA SO2 retrievals show enhanced SO2 loading over northern China in January 2013, when severe pollution attracted media and public attention.

16. OMI and OMPS comparison
OMI and OMPS PCA SO2 data show similar seasonal patterns and SO2 signals over eastern India (several coal-fired power plants built in recent years) [Lu et al., 2013].

17. Next Steps
Expanded table for SO2 Jacobians to more accurately account for measurement conditions (e.g., O3 amount, reflectivity, geometries)
Addition of scattering weight to output to allow convenient adjustment of SO2 column amount based on user-provided profile
Inclusion of error estimates
Operational implementation, public release
>1 year processed and currently under evaluation
Initial release for boundary layer pollution this year
Improved Jacobians and volcanic data to follow soon
On 12 CPUs, 1-2 days to process a year of OMI data

18. Conclusions
Significant improvements in retrieval quality – PCA algorithm uses full spectral content from OMI and similar instruments offering increased temporal resolution and source detection
Computation efficiency – over an order of magnitude faster than comparable spectral fitting algorithms; increasingly important given the greater data volumes expected from future missions (e.g., TROPOMI, TEMPO)
Maximal data continuity between instruments – no need to develop instrument-specific radiance data correction schemes
Flexibility – fitting window can be easily adjusted to optimize sensitivity under different conditions

19. Backups

20. Results: Daily boundary layer SO2
August 13, 2006
August 14, 2006
August 15, 2006
August 16, 2006