SCILOV-10Validation of SCIAMACHY limb

operational NO2 product

F. Azam, K. Weigel, Ralf Bauer, A. Rozanov, M. Weber, H. Bovensmann and J. P. Burrows

ESA/ESRIN, Frascati, Italy27-02-2014

Contents:

SCIAMACHY ESA vs IUP (datasets)

Validation Strategy

Validations:

ESA/DLR - IUP Inter-comparisons

Validation with occultation instruments

Validation with limb instrument OSIRIS

Conclusion/Outlook

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ESA /DLR vs IUP NO2: main retrieval differences

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ESA/DLR IUPProcessors: Speed Optimized Precision optimized

Pre-processing no yes auxiliary spectral fits for each tangent height independently, (improves quality of spectra)

Spectral range 420-470 nm 420-450 nm

See for details; Rozanov et al, Atmos. Meas. Tech., 4, 1319–1359, 2011

Regularization Optimal regularization weak statistical parameter using L- curve regularization method (smoothness constrain)

* L-curve: too strong regularization with deteriorating vertical resolution and large smoothing errors

Validation StrategySCIAMACHY limb coverage:

Profiles/day:1500 profiles for Aug 2002 - Apr 2012: above 4.5 million (a single measurement is performed for four azimuths of each limb state)

Data versions: ESA/DLR NO2 version 5.02 and IUP version 3.1

Sub-sampling:ESA SCIAMACHY Sub-sampling (allows for faster computation):

Distance between two profiles is set larger than 5000 km

A profile is not allowed in the same 5° latitude band as any of 26 profiles before

Each latitude band is limited to 20% more profiles than the average

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Validation StrategySubsampling results in 3% of the entire datasets well distributed over all latitudes, longitudes and time

ESA – IUP Collocation criteria: time = 0.001 h, distance = 60 km

Reason: although same measurements but the determination of the horizontal positions of the tangent point is different for both datasets.

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Statistics

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Du

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et a

l. (2

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9)

Mean relative difference

Standard deviation of the bias corrected difference

Standard error of the mean (too small to be visible on plots)

Results will be shown as profiles, annual cycles and time series comparisons for 30° latitude bins.

ESA/DLR v5.02 – IUP v3.1

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ESA/DLR vs IUP Profile comparisons

Tropics

Near global

NH mid lat.

SH mid lat.

NH high lat.

SH high lat.

meanrelative

differences

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Annual cycle

90S to 60S60N to 90N

Annual cycles comparisons for high lat. show large differences in winter month

(low NO2 concentrations)

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Time series30S to 30N

60N to 90N 90S to 60S

ESA/DLR v5.02 – Occultation instruments

Occultation instruments and their NO2 datasets:

ACE-FTS: version 3.0 (2002-2010)

HALOE: version 19 (2002-2005)

SAGE II: version 6.2 (2002-2005)

Note: SAGE II sunrise (SR) and sunset (SS) events are separately compared sunrise events suffer from instrumental problem (Bauer et al., 2012). The sunset events have a better quality but are mainly restricted to the northern mid and high latitudes

ESA – Occultation instruemnts Collocation criteria:

time = 6 h, distance = 1000 km

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Comparisons with occultation instruments

To Consider:

1)- Strong diurnal variation of NO2: pronounced at sunrise and sunset, SZA ≥ 90 largely effected

diurnal effect error

usually larger for occultation instruments below 25 km (e.g. Bauer et al., 2012McLinden et al., 2006)

a)- Changing illumination conditions (SZA) along the line of sight

b)- Different instruments measure at different SZA

Photochemical correction needed

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Photochemical Correction Application:

Look up table with precalculated diurnal cycles provided by Chris McLinden, model from the University of California (McLinden et al, 2000, Prather 1992)

2 km vertical resolution and 2.5° latitude grid size.

For each collocation pair, matching geolocations and SZAs read in look- up-table

Calculation of scaling factors by dividing profile from look up table at SCIAMACHY SZA by the profile corresponding to the SZA of the other instrument

Scaling factors applied to the NO2 profile of the other instrument

Estimated errors introduced by this photochemical correction ~ 20% Bracher et al. 2005

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To Consider:

2)- Avoid comparison of profiles at different vortex conditions and air masses Strong horizontal gradients at the edge of the polar vortex

Modified potential vorticity (MPV) calculated from ECMWF-Interim

Profiles polewards of 35° latitude are excluded, if:

- the MPV is > 30 and < 40 PVU (vortex edge)

- their MPV differs by more than 3 PVU

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ESA/DLR vs Occultation profile comparisons

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ESA/DLR vs Occultation profile comparisons

ESA/DLR v5.02 – OSIRIS

OSIRIS version 3.0 is used.

OSIRIS coverage is near global with the exception of the winter hemispheres

ESA –OSIRIS Collocation criteria:

time = 12 h, distance = 1000 km (OSIRIS performs measurements in a sun-synchronous orbit with an equator crossing time of the ascending node at 18:00 local solar time)

Note: Based on OSIRIS comparisons with other instruments, the precision of OSIRIS NO2 measurements is observed to be16% for 15–25 km and 6% between 25 and 35 kmReference: (http://osirus.usask.ca/?q=node/245)

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6

ESA/DLR vs OSIRIS profile comparisons

Tropics

Near global

NH mid lat.

SH mid lat.

NH high lat.

SH high lat.

meanrelative

differences

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Time series30S to 30N

60N to 90N 90S to 60S

Summary time series plots

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Tropics

Time series

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NH high lat.

Time series

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SH high lat.

Time series

ConclusionsESA-IUPLarge differences observed in the high latitude winter, elsewhere agreement within a few percentsRetrieval differences (regularization) probable cause of the differences

ESA-Occultation

25–35 km:-

ACE-FTS ~10% , HALOE ~15% and SAGE II ~20% on the average

20 -25 km:- differences with the instruments may approach 50%.

Below 20 km and above 40 km: large biases observed

Diurnal effect error a potential dominant source of differences below 25

ESA-OSIRIS

Good agreement within the precision limit of OSIRIS

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Outlook/Recommendations

For ESA/DLR limb NO2, retireval should be precision optimized, studies on the choice of photochemical correction should be part of future validation activities

Extra Slides

NH mid.lat

Time series

SH mid lat.

Time series

ResultsProfile comparisons: mean relative differences plots with the standard deviation of the bias corrected differences for 20-35 km

Annual cycles:annual cycle vs altitude plots as monthly mean absolute amounts, monthy mean percental difference and the monthly mean percental differences for selected altitudes (20, 24, 27 and 31 km)

Time Series:compared for 20–35 km on a monthly grid. For the selected altitudes (20, 24, 27 and 31 km), comparisons carried out on 30 days running averages if more than 10 collocations are found

Photochemical correction effect: Mean latitude-altitude cross section (monthly averages in 10° bins)

Mean latitude-altitude cross section (monthly averages in 10° bins)

MPV criteria effect: