MAXDOAS formaldehyde slant column measurements during CINDI: intercomparison and analysis improvement G. Pinardi, M. Van Roozendael, N. Abuhassan, C. Adams,

G. Pinardi, M. Van Roozendael, N. Abuhassan, C. Adams, A. Cede, K. Clémer, C. Fayt, U. Frieß, M. Gil, J. Herman, C. Hermans, F. Hendrick, H. Irie, A. Merlaud, M. Navarro Comas, E. Peters, A.J.M. Piters, O. Puentedura, A. Richter, A. Schönhardt,

R. Shaiganfar, E. Spinei, K. Strong, H. Takashima, M. Vrekoussis, T. Wagner, F.

Wittrock, S. Yilmaz

Overview

HCHO measurements during CINDI and DSCD intercomparison

Sensitivity study Analysis improvement Error budget Conclusions

NDACC/NORS meeting – July 2012

HCHO Measurements Overview HCHO measurements during CINDI:

Instruments Measurement Period

Detector characteristics

Total Integration Time UV Resolution (nm)

BIRA From 13/6 to 22/7 2048×512 pixels (-30°C)

60s ~0.4

INTARASASII

From 7/7 to 24/7 1024x256 pixels(-40°C)

50s ~0.39

Bremen From 8/6 to 21/7 2048×512 pixels (-35°C)

40s 0.4

HeidelbergInstrument2

From 17/6 to 24/7 2048×256 pixels (-30°C)

60s 0.5

JAMSTEC From 8/6 to 24/7 uncooled CCD, 3648 pixels

spectra average for 5 min 0.7

NASA From 22/6 to 20/7 uncooled CCD, 2048x14 pixels

16s 0.6

WSU From 21/6 to 5/7 2048x512 pixels(-70°C)

45s 0.83

Toronto From 30/6 to 4/7 2048x512 pixels(-72°C)

~2min 0.2-0.8

Mainz From 21/6 to 10/7 Stabilized CCD 2048 pixels

(4°C)

60s 0.6

All the instruments pointing in the same direction, with common set of elevation angles


Parameter Specification Fitting interval 336.5-359 nm Wavelength calibration Calibration based on reference solar atlas (Chance and

Kurucz, 2010) Cross sections

HCHO Meller and Moortgat (2000), 293°K O3 Bogumil et al. (2003), 223° and 243°K, I0-corrected NO2 Vandaele et al. (1996), 220°K, I0-corrected BrO Fleischmann et al. (2004), 223°K O4 Hermans et al. (2003)

(http://spectrolab.aeronomie.be/o2.htm) Ring effect Chance and Spurr (1997) Closure term Polynomial of order 3

(corresponding to 4 coefficients) Intensity offset Linear correction Wavelength adjustment All spectra shifted and stretched

against reference spectrum


DSCD Comparison HCHO common DOAS retrieval settings:

336.5-359

BIRA-IASB instrument, 30/6/2009, ~14h30 UT, 4° elevation angle (Daily ref. ~11h40 )

JAMSTEC and NASA: more scatter and larger gaussian distribution linear arrays detectors and not CCD and not cooled

TORONTO: slight under-estimation BUT only 5 days of measurement (and instrumental problems)HEIDELBERG and MAINZ: slight under-estimation (Mainz, at 20m on the tower: possibly affecting the comparison )

DSCD Comparison

HCHO DSCD comparisons of every instrument vs reference: scatter plots and histograms of abs. difference (example at 4° elevation)

HC

HO

DS

CD

[1

0x15

mo

lec/

cm²]


Creation of a reference dataset: BIRA, Bremen, INTA

DSCD Comparison Overview of scatter plot statistical results (each instrument vs

reference for all off-axis elevation)

1.15

0.85

within 15%


In order to evaluate the sensitivity of HCHO results to possible changes in the retrieval settings, sensitivity tests are performed on BIRA data of 4 July 2009.

Sensitivity study

Tested parameters: Degree of polynomial and Ring effect The O4 absorption cross-section

DOAS fitting interval (and minimizing the impact of BrO)

Different absorption cross-sections Calibration and slit function


Optimizations that lead to new recommended analysis settings

Error budget (random and systematic contributions)


Only the 5th order case leads to geophysical consistent results

Sensitivity study While testing the polynomial degree, we found very large

differences in the DSCD behavior during the day

Why and which one is « the best »?

investigate the consistency of VCD estimates

2 VCD estimations: geometrical approximation and from direct conversion of the zenith-sky observations using appropriate AMFs


Large sensitivity of HCHO ΔdDSCD to changes in the Ring cross-section, especially for 3rd order polynomial.

HCHO DSCD changes (dDSCDs) are linearly related to changes in the Ring fit coefficients.

Sensitivity study Why?

- Baseline: Chance and Spurr (1997)

- Case A: Wagner et al. (2009)

- Case B: from SCIATRAN RTM in a Rayleigh atmosphere

- Case C: Principal Component Analysis of a range of SCIATRAN calculations in an ozone containing atm. (Vountas et al. (1998))


Root-mean-square of HCHO VCD differences obtained using two alternative methods for the calculation of VCD.

Optimal stability (corresponding to smallest HCHO VCD differences), for the different sets of Ring xs, is obtained for cases using a polynomial of degree 5.

Sensitivity study Test the stability of the retrievals:

Various combinations of polynomials (degree 3, 4 and 5) and different Ring cross-sections


Sensitivity study

Greenblatt xs is needed to retrieve coherent BrO! (in all tested windows)

Misfit due to O4 correlate both to HCHO and BrO

O4 cross-section: Hermans et al. (2003) and Greenblatt et al. (1990)


Change in retrieved HCHO and BrO DSCDs when exchanging the Hermans et al. (2003) O4 absorption cross-section by the Greenblatt et al. (1990) data set, expressed as a function of the O4 DSCD values.

Sensitivity study O4 cross-section

a misfit to the O4 absorption (larger in this case using the Hermans et al. data set) activates a correlation between HCHO and BrO DSCDs

Sensitivity study

Correlation matrix of the absorption xs in 336.5-359 nm

Overall correlation (RMS of non-diagonal elements of the matrix)

Start wavelength

DOAS fitting interval: search for an optimized wavelength window by minimizing the correlations between the xs of the different absorbers

smaller correlations for fitting intervals starting at short wavelengths; the 333-358 nm wavelength range presents a local minimum


large instabilities in 333-358nm region with respect to the Ring effect interference to be used with care!

Sensitivity study DOAS fitting interval: test of the theoretical results on real

data


Sensitivity study

Others tests (xs uncertainties, slit fct and calibration): are discussed as part of the systematic uncertainties

New recommended settings:

1. 5th degree polynomial, in order to minimize the interference with the Ring effect

2. O4 Greenblatt cross-section, that leads to more consistent BrO DSCD

3. Fitting window: a new candidate (333-358nm) is highlighted from the theoretical study, but leads to instabilities related to the Ring effect when applied to real data use with care!

Error budget DSCD random error comparison:

When normalizing wrt the integration time, 2 group of instruments show up: Scientific instruments with large cooled detectors miniDOAS like devices


Summary assessment of the error budget on optimized HCHO dDSCD (systematic and random contributions)

Error budget

Scientific instruments: dominated by the systematic contribution (around 20% and slightly increasing with SZA)

miniDOAS like instruments: both contributions are similar

Typical dDSCD (4°elev) = 3.8x1016 molec/cm²

Conclusion

Very good comparisons of HCHO DSCD retrieved by 9 groups using harmonized retrieval settings. The scatter plot slopes are close to one, within ~15%.

Sensitivity study has been performed leading to new recommended DOAS settings (a 5th order polynomial and the O4 Greenblatt cross-section are needed in order to reduce interference and misfits with Ring and BrO).

An error budget has been obtained for HCHO DSCD, with total errors around 20-30% (8-15x1015 molec/cm²). Larger systematic contributions: Ring effect and HCHO and O3 xs uncertainties.

The paper is ready to be sent to co-authors for last checks and then submitted!


Documents

MAXDOAS formaldehyde slant column measurements during CINDI: intercomparison and analysis improvement G. Pinardi, M. Van Roozendael, N. Abuhassan, C. Adams,