The new GSFC slant column density retrieval of nitrogen...
Transcript of The new GSFC slant column density retrieval of nitrogen...
The new GSFC slant column density retrieval of nitrogen dioxide
Marchenko, S., Celarier, E., Lamsal, L., Krotkov, N., Swartz, B., Bucsela, E.
19th OMI STM, 31 Aug.- 2 Sept. 2015
The problem: ~30% high bias traceable to the OMI SCD(NO2), and the SP V3.0 solution
Before (SP v2.1) Now (planned SP V3.0)
Monthly Pacific (140W- 180W) zonal-mean NO2 columns for March 2010, appropriately adjusted for the OMI orbital times and SCIAMACHY swath.
The new GSFC DOAS retrieval
1. Adjustment of the instrumental wavelength shifts combined with iterative removal of the Ring spectral features in multiple ‘micro-windows’; 7 windows in the NO2 retrieval range (402-465 nm). 2. Iterative, sequential estimates of SCDs of the trace gases (NO2, H2O, CHOCHO) in optimized (broad) windows: e.g., 433-459 nm for CHOCHO. … attempting to ‘orthogonize’ the relevant trace-gas absorptions and optimize retrievals of the individual gas species. 3. Iterative removal of the instrumental noise; correction of the fixed spectral patterns good performance in the SAA region. --------------------------------------------------------------------------------------------------- The result: an overall reduction of the OMI SCD(NO2) by 10-45%.
Sequential SCD retrieval
Iterative wavelength adjustment and Ring-pattern removal in ‘micro-windows’
SCD(NO2): 402-465 nm, omitting 441.5-444.0 nm
SCD(CHOCHO): 433-459 nm
SCD(H2O): 440-449 nm
Instrument noise removal
OMI reflectances
OMI RS-free reflectances
SCD retrieval in
broad, optimized windows
Flexible wavelength adjustment and Ring removal: ‘micro-windows’
Ring+NO2+… (Beijing)
Ring+NO2+… (open water)
‘micro – windows’
OMI: 20 March 2005, orbit #03610
Ring H2O NO2
Diff.
Opt
. Dep
th [%
]
1
0
-1
0.001 nm wavelength error 𝟎𝟎.𝟐𝟐 × 𝟏𝟏𝟎𝟎𝟏𝟏𝟏𝟏 [molec cm-2] SCD(NO2) change (van Geffen et al. 2015, AMT 8, 1685) Our SCD(NO2) errors ~ 𝟎𝟎.𝟖𝟖 × 𝟏𝟏𝟎𝟎𝟏𝟏𝟏𝟏 [molec cm-2] I.e., we are [potentially] sensitive to Δλ > 0.002 nm errors (~1/100 OMI pix).
Flexible wavelength adjustment and Ring removal: ‘micro-windows’
1. In each ‘micro-window’, irradiance spectrum is offset by Δλ, then splined to radiance
wavelengths reflectance 2. Iterative polynomial (2nd order) smoothing of the reflectances.
3. Linear fit of the reflectance to the Ring spectrum (R1 , R2) 4. Removal of the RS : 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 = 𝑅𝑅𝑅𝑅𝑅𝑅
𝐼𝐼𝐼𝐼𝐼𝐼𝑅𝑅𝑅𝑅 [ 𝑅𝑅1∗𝑅𝑅𝑅𝑅+ 𝑅𝑅2]
5. Cost function = Standard Deviation of the RS-free reflectances.
6. Δλoptimal at min(Cost function) in each window. 7. In each window, application of Δλoptimal shifts to irradiances; final evaluation of RS amplitudes. 8. RS smoothing (running mean), then removal of the Ring patterns from the reflectances. Remember that RS amplitudes are wavelength-dependent.
Flexible wavelength adjustment and Ring removal: ‘micro-windows’
The Ring line-filling scales for two fields of view (rows). RS[1] - the first micro-window, 402-410 nm; RS[7] - the last micro-window, 451-465 nm. 20 March 2005, orbit #03610
off-nadir FOV near-nadir FOV
The Ring-pattern amplitudes: ‘micro-windows’
Orbital exposures: 200—599; 600—999; 1000—1399. The ±1σ bars characterize the spread within the orbital blocks. 20 March 2005, orbit #03610
the farthest off-nadir FOV the closest to nadir FOV
The wavelength corrections: micro-windows
RMS of spectral residuals
Single-window (402-465 nm)
shift-and-squeeze
‘Micro-windows’
20 March 2005, orbit #03610
Spectral cross-talk
Cross-correlation: NO2 absorption and the Ring line-filling
Ring NO2 H2O CHO CHO
Ring -0.034 0.005 -0.040
NO2 -0.389 -0.089
H2O -0.117
CHO CHO
At the OMI 0.63 nm spectral resolution:
Spectral cross-talk
At the OMI 0.63 nm spectral resolution:
1. NO2 absorptions are typically ~5-7 weaker than RS. CHOCHO is ~5 weaker than NO2. H2O is ~comparable to RS.
2. Wavelength errors may increase the correlation between NO2 and RS.
3. ~30% RS-amplitude error may lead to ~10% SCD(NO2) bias.
4. One should consider: the relative strengths of the trace-gas absorptions and RS
patterns; the wavelength dependence of the RS and trace-gas spectra; the spectral resolution.
RS may influence the NO2 retrievals. Sequential NO2 may affect the H2O and CHOCHO retrievals. SCD retrieval
The new SCD(O4) retrieval
1. Temperature-dependent O4 cross-sections from Thalman & Volkamer (2013) – 5 temperatures btw. 203 - 293 K. 2. Removal of the O3, NO2 and H2O absorptions prior to the O4 retrieval. See also Yang’s et al. poster.
A few more things to consider: striping; saturation, affecting up to ~0.3% of SCD(NO2) in some cases (spring and fall seasons in the Sun-glint areas); Solar reference spectrum (fixed vs. dynamic): potentially small impact on NO2 @ OMI resolution and S/N.
20 %
σ=5.0% (as is) σ=3.3% (single-orbit de-striping)
20 March 2005, orbit #03610
NO2 diurnal cycle
Spring-2005 NO2
diurnal rates
(Dirksen et al. 2011)
Radiances: o#03597 (19 March, 2005), iTime=803
SATURATION SATURATION
* Rows #18-38 are marked as potentially saturated
Grey line: long-term (5 years) Solar variability (OMI resolution) Black line: short-term (27-day) Solar changes (OMI resolution) Marchenko, S. & DeLand, M., 2014, Astrophys. J., 789, 117
The presented approach practically eliminates the ~30% OMI SCD(NO2) bias. The new GSCF and improved KNMI SCD(NO2) * retrievals agree to within ~ 5-10% The new GSFC SCD(NO2) should be available by ~Dec. 2015. You may assess Y2005 results at GSFC-TLCF: /omi/live/dd/70003/OMNO2SCD/2005/ See more details and results in: • Marchenko, S.V., Krotkov, N.A., Lamsal, L.N., Celarier, E.A., W. H. Swartz, W.H., Bucsela, E.J.,
2015, "Revising the slant-column density retrieval of nitrogen dioxide observed by the Ozone Monitoring Instrument", JGR-Atmospheres, 120, Issue 11, pp. 5670-5692.
* Improved KNMI NO2 data courtesy Jos van Geffen et al.
Backup
Left panel: Bremen (courtesy Andreas Richter) and Goddard SCD(NO2) retrievals for different cloud conditions: OMI orbit #03610 from 20 March 2005. Right panel: the (Goddard / Bremen) ratios (black line) and (Goddard - Bremen) differences (orange line).
Spectral cross-talk: typical absorption spectra @ OMI resolution
Slant Column Density calculation
OMI radiances
Averaged OMI
irradiances
1st Trip
Slant columns
(1st approx.)
2nd Trip
Fixed-pattern
residuals in reflectance
s Slant
columns (Final
approx.)
Slant column
uncertainties
SCD calculation
1st Trip
1st Pass
2nd Pass
Noise removal
2nd Trip
1st Pass
2nd Pass
Noise removal
Calculation of fixed-pattern residuals
1st Pass
𝜆𝜆 offset
RRS signal removal
Slant column estimation
Spike removal
2nd Pass
Low-pass filt.
Slant Column estimation
Correlation Check
SCD Uncertainties