1. MAXDOAS formaldehyde slant column measurements during CINDI: intercomparison and analysis improvement 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

2. Overview  HCHO measurements during CINDI and DSCD intercomparison  Sensitivity study  Analysis improvement  Error budget  Conclusions NDACC/NORS meeting – July 2012

3. HCHO Measurements Overview  HCHO measurements during CINDI: InstrumentsMeasurement Period Detector characteristics Total Integration TimeUV Resolution (nm) BIRA From 13/6 to 22/72048×512 pixels (-30°C) 60s~0.4 INTA RASASII From 7/7 to 24/71024x256 pixels (-40°C) 50s~0.39 Bremen From 8/6 to 21/72048×512 pixels (-35°C) 40s0.4 Heidelberg Instrument2 From 17/6 to 24/72048×256 pixels (-30°C) 60s0.5 JAMSTEC From 8/6 to 24/7uncooled CCD, 3648 pixels spectra average for 5 min0.7 NASA From 22/6 to 20/7uncooled CCD, 2048x14 pixels 16s0.6 WSU From 21/6 to 5/72048x512 pixels (-70°C) 45s0.83 Toronto From 30/6 to 4/72048x512 pixels (-72°C) ~2min0.2-0.8 Mainz From 21/6 to 10/7Stabilized CCD 2048 pixels (4°C) 60s0.6 All the instruments pointing in the same direction, with common set of elevation angles NDACC/NORS meeting – July 2012

4. DSCD Comparison  HCHO common DOAS retrieval settings: 336.5-359 BIRA-IASB instrument, 30/6/2009, ~14h30 UT, 4° elevation angle (Daily ref. ~11h40 )

5. 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) HCHO DSCD [10x 15 molec/cm²] NDACC/NORS meeting – July 2012  Creation of a reference dataset: BIRA, Bremen, INTA

6. DSCD Comparison  Overview of scatter plot statistical results (each instrument vs reference for all off-axis elevation) 1.15 0.85 within 15% NDACC/NORS meeting – July 2012

7.  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 O 4 absorption cross-section  DOAS fitting interval (and minimizing the impact of BrO)  Different absorption cross-sections  Calibration and slit function NDACC/NORS meeting – July 2012 Optimizations that lead to new recommended analysis settings Error budget (random and systematic contributions)

8. NDACC/NORS meeting – July 2012 Only the 5 th 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

9. NDACC/NORS meeting – July 2012  Large sensitivity of HCHO ΔdDSCD to changes in the Ring cross- section, especially for 3 rd 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))

10. NDACC/NORS meeting – July 2012 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

11. NDACC/NORS meeting – July 2012 Sensitivity study  Greenblatt xs is needed to retrieve coherent BrO! (in all tested windows)  Misfit due to O 4 correlate both to HCHO and BrO  O 4 cross-section: Hermans et al. (2003) and Greenblatt et al. (1990)

12. NDACC/NORS meeting – July 2012 Change in retrieved HCHO and BrO DSCDs when exchanging the Hermans et al. (2003) O 4 absorption cross-section by the Greenblatt et al. (1990) data set, expressed as a function of the O 4 DSCD values. Sensitivity study  O 4 cross-section  a misfit to the O 4 absorption (larger in this case using the Hermans et al. data set) activates a correlation between HCHO and BrO DSCDs

13. 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

14. NDACC/NORS meeting – July 2012  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

15. NDACC/NORS meeting – July 2012 Sensitivity study  Others tests (xs uncertainties, slit fct and calibration): are discussed as part of the systematic uncertainties  New recommended settings: 1.5 th degree polynomial, in order to minimize the interference with the Ring effect 2.O 4 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!

16. 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

17. NDACC/NORS meeting – July 2012  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) = 3.8x10 16 molec/cm²

18. 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 5 th order polynomial and the O 4 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% (8-15x10 15 molec/cm²). Larger systematic contributions: Ring effect and HCHO and O 3 xs uncertainties.  The paper is ready to be sent to co-authors for last checks and then submitted! NDACC/NORS meeting – July 2012