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