Algorithm improvements for Dutch OMI NO2 retrievals (towards v3.0)Folkert Boersma, Jos van Geffen, Bram Maasakkers, Henk Eskes, Jason Williams, and Pepijn Veefkind

1. Spectral fitting (lead: Jos van Geffen)

OMI NO2 slant columns higher than GOME-2 & SCIAMACHY(similar to findings N. Krotkov)A difference of about 0.5x1015 molec/cm2 is expected due to difference in observation time

OMI GOME-2 SCIAMACHYfit window [nm] 405 – 465 425 – 450 426.5 – 451.5secondary trace gases O3, H2O O3, H2O, O2-O2 O3, H2O, O2-O2

pseudo-absorbers Ring Ring Ringdegree of polynomial 5 3 2slant column processing * KNMI / NASA BIRA-IASB BIRA-IASB

*) Data assimilation, separation of tropospheric and stratospheric column, and conversion to total columns for all at KNMI.

So what else is different?

A. Improve solar reference spectrum

Update of high-resolution solar spectrum: - better representation of OMI ITF (measured)- better Ring-spectrum and I0-corrected reference spectra

New solar spectrum improves effective λ-calibration With old parameterized ITF

RMS of fit: -17%NO2 slant columns: -6%

Other reference spectra otherwise identical to OMNO2A

B. Improve other reference spectra

NO2: still Vandaele et al. [1998] but with new ITFO3: Bogumil et al. [2000] instead of obscure WMO-1975H2O: HITRAN-2012 intead of HITRAN-2004O2-O2: Now included!LQW: Now included! Stronger amplitude in new NO2 cross section!

RMS of fit: -30%NO2 slant columns: -11%

Including liquid water

Strong feature for clear water pixel !

Fit results for cloudy pixels are not affected much when including liquid water absorption.

Including liquid water absorption in the NO2 fit is a good idea.

Including LQW and O2-O2 improves the fit

Lowest RMS when including both LQW and O2-O2

Including LQW and O2-O2 reduces NO2 SCDs by 0.3 1015 molec.cm-2

More realistic ozone columns

C. Improve λ-calibration window to map I to I0

Identify λ-calibration window that minimizes RMS/NO2 fit error

Best results for 409-428 nmPreviously: 408-423 nm

RMS of fit: -32%

NO2 slant columns: -13%

Summary of the spectral fitting improvements

Reduction of 1.0-1.3 x 1015

• Statistical NO2 fitting error reduces from 1.3 to 1.0 x 1015

• Good consistency with QDOAS (thanks Isabelle De Smedt)

• Difference between 425-450 nm and OMI window 0.2-0.4 x 1015

2. A new framework for DOMINO retrievals

The 3-D Tracer Model is at the heart of DOMINO retrieval for:• data assimilation to estimate stratospheric NO2 columns• a priori NO2 profile shapes input to AMF• temperature corrections

DOMINO v1 and v2: based on TM4 with 3° × 2°DOMINO v3: based on TM5 on 1° × 1°

Motivation for re-coupling:• TM4 no longer supported/developed• TM5 v3.0 is a benchmarked (Huijnen et

al., 2010) model • Allows 1° × 1°• Employs up-to-date emission inventories

more representative for the OMI-era

Bram Maasakkers, M.Sc

2. Re-coupling worked (3° × 2°)

TM5-basedTM4-based

White areas: negatives

With new TM5: 27% fewer negatives

20-30 Oct 2004

2. Re-coupling worked (3° × 2°)

TM4-based

Fewer negatives in TM5-based retrieval

Lower emissions in TM5

20-30 Oct 2004

2. Updating to TM5 at 1° × 1°

TM4-based

We improve the retrieval by using TM5 a priori profiles at 1o×1o instead of 3o×2o

Better resolved a priori profile shapes lead to a better understanding of pollution gradients observed from space

Madrid Madrid

2. Updating to TM5 at 1° × 1°

TM4-based

Average tropospheric NO2 2°×3°: 0.31 1015 molec./cm2

1°×1°: 0.31 1015 molec./cm2

20-30 Oct 2004

2. Updating to TM5 at 1° × 1°

TM4-based

Sharper contrast between urban and rural profile shapes will resolve some of the previously found underestimations over hotspot regions (Ma et al., 2013; Shaiganfar et al., 2011)

3x higher surface NO2 for Barcelona in 1x1 vs. 3x2

Lower AMFs over hotspots

2. DOMINO with TM5 at 1° × 1° vs. at 3° × 2°

TM4-based

Some of the urban-rural contrast is indeed a consequence of the coarse a priori profiles

Barcelona

+22%

3. Systematic error in O2-O2 cloud pressures

• Retrieved O2-O2 slant column depends on atmospheric temperature profile

• LUT for O2-O2 based on fixed AFGL mid-latitude summer T-profile

• O2-O2 slant columns require T-correction since‘true’ T-profile may differ strongly from mls-

profile

Acknowledgment: Johan de Haan

3. Corrections to O2-O2 cloud pressures

TM4-based

3. Impact on NO2 retrievals

TM4-based

Higher clouds more screening lower AMFs

3. Conclusions

• Improved NO2 slant columns are smaller by 1.0-1.3 1015 molec.cm-2 with 30% lower fitting residuals

• Coupled DOMINO to TM5 v3: 27% fewer negatives• Improved resolution for a priori profile leads to increases over

hotspots (+20%) and stronger contrast urban-rural• Temperature-correction for cloud pressures relevant over polluted

areas

(TROP)OMI NO2 team from 2014 onwards:

Differences between TM4 and TM5

Emissions TM4: POET 1997

Integrate over pressure, not altitude Express number density O2 as a function of pressure and temperature

Retrieved slant column will depend on 1/T, but LUT slant columns do not…