INTRODUCTION

1. Knowing [CO] and [HCOOH] , one can deduce the amount of reacted HCHO. WHAT ABOUT THE CONSISTENCY OF ABSORPTION COEFFICIENTS USED WITH DIFFERENT REMOTE SENSING INSTRUMENTS ?Aline Gratien (1), Bénédicte Picquet-Varrault (1), Jean-François Doussin (1), Matthew S. Johnson (2), Claus J. Nielsen (3), Johannes Orphal (1) and Jean-Marie Flaud (1)(1) Laboratoire Inter-universitaire des Systèmes Atmosphériques, Créteil, France (2) Copenhagen Center for Atmospheric Research , University of Copenhagen, Denmark(3) Centre for Theoretical and Computational Chemistry, Department of Chemistry, University of Oslo, Norway HCHO + Br → HCO + HBr HCO + Br2 → HCOBr + Br CO + HBr + Br → HCO + Br → CO + HBr → HCHO + Br HCO + HBr → (CHO)2 HCO + HCO → HCHO + CO → 2 CO + H2 HCHO + HO2 → HOO-CH2O → HO-CH2-OO HO-CH2-OOH+O2 HO-CH2-OO + HO2 → → HCOOH + O2 + H2O 2 HO-CH2-OO → HO-CH2-OH + HCOOH +O2 2 HO-CH2-O +O2 → HCOOH + HO2 HO-CH2-O +O2 → HCO + O2 → CO + HO2 agreement < 5%disagreement > 5% INTRODUCTION Optical measurements of atmospheric minor constituents are performed using spectrometers working in the UV-visible, infrared and microwave spectral ranges. The combined use of nadir viewing UV-visible and thermal infrared spectrometers (such as GOME2 and IASI onboard MetOP) will provide an important improvement of vertical trace gas concentration profiles. The analysis and interpretation of the corresponding atmospheric spectra require good knowledge of the molecular parameters of the species of interest as well as of the interfering species. In particular meaningful comparisons of profiles retrieved by various instruments using different spectral domains require that the spectral parameters are consistent in these spectral domains. To illustrate these points we will focuse on formaldehyde.Formaldehyde (HCHO) is the most abundant carbonyl compound in the atmosphere and plays a key role in atmospheric photochemistry in particular for the production of HOx species. Consequently the aim of the study performed at LISA was to intercalibrate formaldehyde spectra in the infrared and ultraviolet regions. This work has been financed by the French National Program for Atmospheric Chemistry (PN LEFE CHAT). EXPERIMENTAL SECTION The experiments were performed by acquiring simultaneously UV and IR spectra using a common optical cell Complementary experiments in Oslo • Experiments were performed in an evacuable Pyrex photoreactor (6 meters length, volume 977 L) at atmospheric pressure and room temperature. • The reactor contains two multiple-reflection optical systems interfaced to a Fourier-transform infrared (IR) spectrometer and to an UV-visible absorption spectrometer. • The UV and IR optical path lengths were set to 72 m and 12 m, respectively. LISA, Créteil, France Experiments were performed in a 250 L electropolished stainless steel smog chamber equipped with a White type multiple reflection mirror system ( IRTF Bruker) with a 120 m optical path length in Oslo to obtain infrared cross sections and to identify potential artefacts. Gas-phase formaldehyde was generated by sublimating paraformaldehyde in a glass vacuum system and introduced in the reactor by flushing with pure nitrogen.By this method, the concentration of formaldehyde (few mbars) in the reactor is not precisely known. UV and IR absorption spectra of formaldehyde were simultaneously recorded and the absorbances A were obtained by calculating A = ln(I0/I) INTERCALIBRATION IR/UV IBIUV : ultraviolet integrated band intensities with IBIIR: infrared integrated band intensities with According to the Beer-Lambert law : slope = IBIIR/IBIUV UV spectrum (Résolution : 0,18 nm) IR spectrum (Résolution : 0,08 cm-1) IR versus UV integrated optical depths of HCHO. IBIIR/IBIUV= 0.246  0.006 (2600-3100 cm-1) IBIIR/IBIUV= 0.113  0.003 (1660-1820 cm-1) Examples of formaldehyde spectra acquired simultaneously QUANTIFYING HCHO IN THE CELL USING TITRATION BY BROMINE ATOMS COMPARISON WITH THE IBI FROM LITTERATURE Beer-Lambert law : The infrared cross sections of formaldehyde were determined by quantifying HCHO in the cell using titration by bromine atoms. • IBIIR in 10-17 cm/molecule Infrared Integrated Band Intensities • IBIUV in 10-16 cm/molecule IBI (2600-3100 cm-1) = (2.9  0.1).10-17 cm/molecule IBI (1660-1820 cm-1) = (1.31  0.04).10-17 cm/molecule Titration by bromine atoms first photolysis Second photolysis The UV absorption cross sections of the formaldehyde were then derived from these ratios IBIIR/IBIUV and from the infrared cross sections. • There is an excellent consistency between the infrared data except for the one published by Hisatsune and Eggers,1955 for the two bands (2600-3100 cm-1 and 1660-1820 cm-1). • The UV absorption coefficients of Meller and Moortgat, 2000 are in good agreement with this study. • This study shows that the Cantrell et al. ,1990 and Rogers,1990 cross-sections are underestimated by about 20%. • The UV absorption coefficients of Meller and Moortgat, 2000 are in very good agreement with independent sets of IR cross-sections except Hisatsune and Eggers,1955 . IBI (300-360 nm) = (1.17  0.07).10-16 cm/molecule • CONCLUSION • This study present the first simultaneous measurement of the IR and UV cross sections of the formaldehyde and the improved cross sections calibrated using quantitative conversion to CO and HCOOH. This will help in experiments using spectroscopic data, including remote sensing and laboratory kinetics. • The IR cross sections are in agreement with the whole of the data IR exceptHitsatsune and Eggers,1955 and the UV cross section confirms the databases JPL 2006 and IUPAC 2004 which recommend to use the cross sections of Meller and Moortgat, 2000. ATMOSPHERIC IMPLICATIONS • The underestimation of the UV cross-sections of Cantrell et al., 1990 has important consequences on atmospheric chemistry. • The data of Cantrell et al., 1990 are recommended to be used in the HITRAN (Orphal and Chance, 2003). Their use may lead to an overestimation of the formaldehyde concentration in atmospheric measurements. • Another important consequence is the underestimation of the atmospheric photolysis rates of formaldehyde in photochemical models, which frequently use the cross-sections published by Cantrell, 1990. Cantrell, C. A., Davidson, J. A., McDaniel, A. H., Shetter, R. E. and Calvert, J. G. (1990) J. Phys. Chem., 94, 3902-3908. Gratien, A.; Picquet-Varrault, B.; Orphal, J.; Perraudin, E.; Doussin, J. F.; Flaud, J.-M. (2007),Journal of Geophysical Research, 112, D05305 Gratien, A., E. Nilsson, L. Bache-Andreassen, , J-F. Doussin, M. S.  Johnson, . J.  Nielsen, Y. Stenstrøm and B. Picquet-Varrault. J. Phys. Chem, soumis Herndon, S. C., Nelson, D. D. J., Li, Y. and Zahniser, M. S. (2005) J. Quant. Radiat. Transfer, 90, 207-216. Hisatsune, C. and Eggers, D. F. (1955) J. Chem. Phys., 23, 487-492. Klotz, B., Barnes, I. and Imamura, T. (2004). Phys. Chem. Chem. Phys., 6, 1725-1734. Meller, R. and Moortgat, G. K. (2000) J. Geophys. Res., 105, 7089-7101. Nakanaga, T., Kondo, S. and Saëki, S. (1982) J. Chem. Phys., 76, 3860-3865. Orphal, J. and Chance, K. (2003) J. Quant. Radiat. Transfer, 82, 491-504. Rogers, J. D. (1990) J. Phys. Chem., 94, 4011-4015. Sharpe, S. W., Johnson, T. J., Sams, R. L., Chu, P. M., Rhoderick, G. C. and Johnson, P. A. (2004) Appl. Spect., 58, 1452-1461. Contact: picquet@lisa.univ-paris12.fr