Spectral parameters for the region of HCOOH and its measurement in the infrared tropospheric spectrum

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. D15, PAGES 18,661-18,666, AUGUST 20, 1999 Spectral parameters for the region of HCOOH and its measurement in the infrared tropospheric spectrum A. Pertin Laboratoire de Photophysique Mo] cu]aire, Universit Paris-Sud, Orsay, France C. P. Rinsland NASA Langley Research Center, Hampton, Virginia A. Goldman Department of Physics, University of Denver, Denver, Colorado Abstract. New spectral parameters (line positions and intensities) have been generated for the ya band of HCOOH, which agree well with cm -1 resolution infrared laboratory spectra. The line positions were derived from previously published spectroscopiconstants [Bumgarner et al., 1988]. New relative intensities were derived from the analysis of A type and B type transitions. The new line parameters allow improved quantitative analysis of atmospheric spectra from the 1105 cm - Q branch, provided that the previously neglected HDO strong tropospheric absorption in this region is taken into account. 1. Introduction Formic acid (HCOOH) is ubiquitous in the troposphere with measurements reported since the early 1980s, by sample collection chemical methods [e.g., Dawson et al., 1980; Dawson and Farmer, 1988; Helas et al., 1992] and by long-path infrared absorption spectroscopy [Hanst et al., 1982; Goldman et al., 1984; Rinsland and Goldman, 1992; Worden et al., 1997; Yokelson et al., 1996, 1997]. For spectroscopic quantification, HCOOH line parameters for the strong v6 band are included in 1996 HITRAN frothman et al., 1988], which originate from the early work of Goldman and Gillis [1984]. Room-temperature spectral cros sections at ~0.125 cm-1 resolution at several pressures are available in the commercial database by Hanst and Hanst [1993]. The v6 band Q branch at cm- 1 is the strongest IR feature located in an atmospheric window. The band is in Coriolis resonance with the nearby ys band at cm - [Bumgarner et al., 1988]. The Y3 band, which was studied by Weber et al. [1987] is quite strong, but at 5.6 pm, it is less favorable for IR atmospheric remote sensing investigations. The Q branch feature at 1105 cm- was noted by Hanst et al. [1982] in long-path spectra of Los Angeles smog. Later, Goldman et al. [1984] reported evidence for the same spectral feature in the upper troposphere based on a comparison of laboratory and balloon-borne Copyright 1999 by the American Geophysical Union. Paper number 1999JD /99/1999JD ,661 solar occultation spectra recorded at 0.02-cm - resolution. Rinsland and Goldman [1992] observed the cm- feature in 0.5 and 1.5 km horizontal path spectra recorded at cm- 1 resolution from Kitt Peak at 2.09 km altitude in the desert southwest United States. They attributed the observed feature to HCOOH but noted that the measured formic acid abundance was typically 3 times higher than values measured with a condensation method at rural sites in the same region [Dawson and Farmer, Worden et al. [19971 analyzed 0.07-cm - spectral resolution airborne emission spectra of wildfires and reported measurements of HCOOH based on the 1105 cm - spectral feature. Quantification of the HCOOH amount was based on the laboratory absorption cross sections [Hanst and Hanst, 1993]. This same absorption cros section database has been used to quantify emission factors from controlled biomass fires [e.g., Yokelson et al., 1997]. For the y line parameters 1984 work, a listing of Hamiltonian constants and assigned lines for the v band of HCOOH was provided (E. Weinberger, private communication, 1983). The HamiltonJan used is the Watson S form for p2 and p4 terms and the Watson A form for P terms. It was assumed that the y6 band is predominantly type A but also includes type B and C transitions as well as electric-dipole forbidden Q branch transitions. Because the calculation did not handle for- bidden transitions, the intensity of the forbidden transitions was given to the type A transitions. Line parameters were generated from 1000 to 1200 cm-, T- 296 K, $ cm-2/atm. A, B, and C type transitions were assigned relative strengths of 79%, 6%, and 15%,

2 18,662 PERRIN ET AL.' HCOOH 6 REGION IN THE TROPOSPHERIC SPECTRUM respectively. This gives the 28 cm-2/atm intensity in the Q branch region used by Goldman et al. [1984]. Comparisons of a University of Denver (DU) laboratory spectrum (1.6 torr in a 10-cm cell at room temperature, 0.05 cm -1 resolution) and a calculated spectrum gave good agreement in the Q branch region. Agreement throughout the rest of the band is somewhat poorer for three reasons: (1) perturbations of u6 line positions (some exceeding 0.1 cm -1) are ignored because they have not been analyzed; (2) no effort has been made to account for a significant part of the intensity (perhaps 10 or 20%); and (3) the value of the Lorentz half width is unknown. Because of these short- A significant improvement was achieved in modeling the laboratory spectra, which is applicable to atmospheric studies of HCOOH. 2. Line Parameters Calculations For the line positions, the upper state parameters are from Bumgarner et al. [1988], who adopted the ground state parameters of Willemot et al. [1980]. The HamiltonJan matrix is the same as used by Bumgarner et al. [1988], that is, a Watsoh's A type with I r reduction for the vibrational diagonal operator, and /f/ 6, 8- sjz as the vibrational nondiagonal operator for the Coriocomings, it is not surprising that the observed and cal- lis term. The constants used here are shown below in culated spectra agree in some regions and disagree in others. On the basis of the good match between the observed and calculated Q branch intensities, HCOOH u6 line parameters were generated in HITRAN format [Goldman and Gillis, 1984] but, inadvertently, were not Table 1, converted to cm - units. It was found, however, that while the published constants have sufficient number of significant digits to reproduce the infrared transitions and the microwave transitions within u and us, they do not reproduce completely the laser saturaincluded in the HITRAN database prior to the 1996 edi- tion data results. Our calculations were performed for tion. These line parameters include all lines stronger J<_ 79, K <_ 39 and E<_ 4000cm -, and E <_ cm. than 1% of the strongest line. The intensities of all lines on the file sum to 1.757x10 -? cm- /(molecule The line intensities were calculated at T- 296 K, for cm -2) at 296 K. It should be noted that the calculated which Zrot , Zvib , so that Ztot(296 Q branch intensity is approximately twice as large as K) - 4 x ,930.9, including the nuclear spin that used by Goldman et al. [1984] The air-broadened degeneracy of (2I(1H)+ 1) 2-4. These values agree half width on the HITRAN database is 0.1 cm- /atm with the direct summation results reported by Vander at 296 K, with temperature dependence of n , for Auwera, [1992] Zvib (296 K) and Zrot (296 K) all lines. The self-broadened halfwidth is not given, but - 4 x The initial total intensity was normalized to the HITRAN value of 1.757x10 -? 0.4 cm- /atm was used in the laboratory simulations. More detailed comparisons with tropospheric spectra cm- /(molecule cm -2) (accurate to J:12%)assuming showed that while the spectral modeling of laboratory I I o/l a I for the B type to A type transitions. HCOOH is quite good in the Q branch region near 1105 Spectral fittings in several subintervals of the band cm - a large discrepancy was observed in the simula- provided (see next section) a new estimate for the B tion of the atmospheric spectrum in this region. It was type to A type intensities and the total band intensity subsequently found (by C.P. Rinsland) that the cur- as lpo/p. l and 'band-- SHITRAN )< ISorent HITRAN compilation does not include the HDO topic abundance of C'i o has been incorporated lines reported in the literature for this interval [Flaud by using Zi o - Ztot/Ci o Statistics of the calculated lines is shown in Table 2. et al., 1986; Toth, 1993] and that taking into account these lines explains well the observed features. Thus, It should be noted that only the most abundant isoin the lower troposphere spectrum, most of the absorp- tope of HCOOH is represented in HITRAN. A recent tion in the 1105 cm- feature is due to HDO, with only study by Ong et al. [1999] provides the first highsmall part due to HCOOH. In the upper troposphere, resolution analysis of the ua band of H COOH centered at cm -. the contribution of HDO is negligible compared to that of HCOOH. The high abundance of water vapor emissions from fires [e.g., Worden et al., 1997] emphasizes 3. Experimental the importance of accurate quantification of the over- Details lapping HDO lines for quantitative atmospheric studies from both controlled and wild fires. The more recent spectroscopic constants and line po- The laboratory spectra were recorded at cm - resolution in the cm- range at room temperature (+23øC) with a modified Bornera model DA3.002 sitions (but not intensities) for both u and us published Fourier transform spectrometer. Several sets of specby Bumgarner et al. [1988] have not been applied yet for updating the HCOOH line parameters. Line potra were recorded with HCOOH pressure of 0.2 and 0.4 torr in a 10 cm cell. The estimated accuracy of the pressitions and intensities for the pure rotation spectrum sure measurement is ~8%. The measured positions of of formic acid are available from the work of Vander unblended lines have an absolute accuracy of ~ Auwera [1992]. In the present study, the spectroscopic cm -1, on the basis of the cm -1 N20 lines constants of Bumgarner et al. [1988] and cm -1 resolution laboratory spectra obtained at DU were comlisted by Maki and Wells [1991]. Figure 1 shows a portion of the spectral least squares bined to update the u6 region HCOOH line parameters. fittings made to the laboratory spectra in the u6 HCOOH

3 PERRIN ET AL.: HCOOH u6 REGION IN THE TROPOSPHERIC SPECTRUM 18,663 Table 1. Spectroscopic Parameters for the v6 and vs Bands of HCOOH Ground State a ve = 1 u vs = 1 u A E+00 ½ E E+00 B E E E-01 C E E E+00 A j E E E-06 A JK E E E-05 A K E E E E E E-07 K E E E-06 H j E E E- 11 HjK E H c J E E E-08 H E E E-08 ha E E hs c E-09 hk E E E E E+04 s = Parameters are in units of cm -. afrom Willernot et al. [1980]. UFrom Burngarner et al. [1988]. ½Read E+00 as x 10 ø. band. The figure shows fittings in the v6 Q branch region, the main spectral feature used for atmospheric quantification of HCOOH, and demonstrates the improvement obtained with the newly generated HCOOH line parameters over the 1996 HITRAN line parameters. Similar improvements were obtained all across the band, showing that many laboratory lines that were not accounted for are now modeled well with the A type and B type analysis mentioned above. However, this limited set of new laboratory spectra could not provide improved absolute intensities for this band. 4. Atmospheric Absorption in the 1105 ½m -1 Region The bottom plot of Figure 2 presents a solar absorption spectrum in the region of the 6 band Q branch of HCOOH. The spectrum was recorded with the Fourier transform spectrometer located in the National Solar Observatory facility on Kitt Peak in southern Arizona (31.9øN, 111.6øW, 2.09 km altitude). The spectral resolution was cm - (maximum optical path difference of cm). Above the measured spectrum are displayed molecule-by-molecule simulations of the absorptions by H20, HDO, 03, and HCOOH, the four most important absorbers in the interval. Owing to tropospheric pressure broadening, the improvements in modeling the individual line structure at high resolution, as demonstrated in Figure 1, are averaged considerably in Figure 2 and are not as significant for quantiffcation purposes as for low-pressure spectra. The calculated absorption for each molecule has been offset vertically for clarity. Weaker absorptions by several other species are ignored. The line parameters for H20 and 03 were taken from the 1996 HITRAN compilation [Rothman et al., 1998]. Line positions and intensities for HDI O lines on the 1996 HITRAN database were replaced by values calculated by J.-M. Flaud (private Table 2. Bands Summary for the ya and us of HCOOH Transition # Lines Vmin //max ES Stain Smax A Type Bands (v"=0)-(v =l) E E E-20 (v"=0)-(v =l) E E E-22 B Type Bands (v"=0)-(v =l) E E E-21 (v"=0)-(v/,=l) E E E-22 The total number of lines of H 2C O O1H is 18,000. Total band intensity for t/ is 1 Sv6=l.746E-17. Total band intensity for t/s is Sva=5.434E-20. KNote: t/ values are in units of cm-. S values are in units of cm- /(molecule cm -2) at 296.

4 , 18,664 PERRIN ET AL.: HCOOH u6 REGION IN THE TROPOSPHERIC SPECTRUM HCOOH CELL CM TORR DU CM- 1 HITRAN LINES [ [ I [ I [ [ ] ' I ' ' ' I [ [ [ I 1.0., i, - I i, [ ] I [ [ i ] I t [ ] [ I t t i [ I -.2 J I I I I I I i I i I I I i I I i I I I I , WAVENUMBER (cm - ) HCOOH CELL CM TORR DU CM LINES. I 1.0. ', I i I I i I i i I I I 1104, WAVENuMBER (cm - ) Figure 1. Spectraleast squares fitting in the cm - interval of the v6 Q branch of HCOOH from laboratory spectra obtained at DU. The measured spectrum was recorded at 23øC with 0.4 torr of HCOOH in a 10 cm cell at cm - resolution. The transmittance scale is expanded for clearer presentation of the spectral features. The line parameters used for the fitting are the 1996 HITRAN lines (which originate from Goldman and Gillis [1984]) and the newly derived lines in the present work in the top and bottom parts of the figure, respectively.

5 PERRIN ET AL.. HCOOH u6 REGION IN THE TROPOSPHERIC SPECTRUM 18, H HDO 1.0 HCOOH I KITT PEAK i i i t Wavenumber (½m ) Figure 2. Simulations of the absorption by H20, HDO, O3, and HCOOH in the cm-1 region of the '6 Q branch of HCOOH, and a solar spectrum recorded from Kitt Peak. The spectra are normalized to the highest value in the interval and offset vertically for clarity. The measured spectrum was recorded on June 25, 1996, at a solar astronomical zenith angle of ø. communication, 1998) based on the work of Flaud et al. [1986]. A constant air-broadening coefficient of 0.08 was selected arbitrarily. Quantification of the contributions of the two components to the total absorption cm-1 arm-1 at 296 K and a T -ø'68 temperature depen- will require accurate spectroscopic parameters for both. dence were assumed for all HD160 lines. The positions It should be noted that the missing HDO lines will be and intensities measured for the strongest lines by Flaud less important in the upper troposphere because of the et al. [1986] for HD160 are similar to the more recent reduced amount of water vapor at high altitudes; but measurements by Toth [1993, 1999a,b]. The newer mea- all previously published spectroscopic quantification of surements show that the stronger line is overlapped by HCOOH need to be revised. lines 1 order of magnitude weaker at and It is also of interest that there should be a weak cm -1 (R.A. Toth, private communication, absorption (less than 3% in Figure 2) by HCFC ). A comprehensive list of positions, intensities, (CHCIF2) in the high wavenumber wings of the HCOOH and air-broadening coefficients for HD160 tra. nsitions 1105 cm- 1 Q branch. This was confirmed by comparbetween 500 and 2650 cm -1 is in preparation (R.A. ing 0.03 cm-1 resolution crossections of HCFC22 [e.g., Toth, private communication, 1999). Massie et al., 1991; Clerbaux et al., 1993] in the 1106 The spectral features observed in the solar spectrum cm-1 region with those in the 809 and 829 cm-1 regions are reproduced satisfactorily in the simulation, though the depths are not exactly matched. Mixing ratios from (the latter are routinely observable in atmospheric spectra). a reference set of the profiles were assumed with no effort made to adjust them to achieve a best fit to the observations. The HCOOH spectral parameters from the current work were assumed with a mixing ratio of 0.5 Summary and Conclusions In this paper, we have reported a new analysis of ppbv (10- per unit volume) between the surface and both atmospheric and laboratory spectra of HCOOH 12 km altitude. A HCOOH mixing ratio of zero was with a focus on the,6 band Q branch at 1105 cm -1. assumed above. The calculations show that the feature This feature is the most favorable one for quantifying near cm-1 is likely to be due to a combination HCOOH atmospheric amounts from tropospheric and of the absorption by HD160 and HCOOH. The HCOOH feature is predicted to be broader than observed in the solar spectrum, though it should be recalled that the air-broadening coefficient of 0.1 cm -1 atm-1 at 296 K lower stratospheric infrared spectra. We have generated a new set of HCOOH spectral parameters which yield improved predictions of the absorption in the Q branch region. These parameters have been verified by com-

6 18,666 PERRIN ET AL.. HCOOH ve REGION IN THE TROPOSPHERIC SPECTRUM parisons of simulations with new cm-1 resolution laboratory spectra. We also have found that HDO lines A tentative identification of the 1105 cm - ve band Q branch in high-resolution balloon-borne solar absorption spectra, Geophys. Res. Left., 11, , overlapping the HCOOH Q branch are missing in the Maki, A.G., and J.S. Wells, Wavenumber calibration tables 1996 HITRAN database [Rothman et al., 1998]. Our from heterodyne frequency measurements, NIST Spec. calculations show that the missing HDO lines absorb Pub. 8œ1, U.S. Dep. of Commerce, Washington, DC, significantly over long atmospheric paths, and hence need to be included in retrievals of HCOOH amounts Massie, S.T., A. Goldman, A.H. McDaniel, C.A. Cantrell, from infrared tropospheric spectra. J.A. Davidson, R.E. Shetter, and J.G. Calvert, NCAR Tech. Note TN-358-I-STR, Natl. Cent. for Atmos. Res., Boulder, Colo., February Ong, P.P., K.L. Gob, and H.H. Teo, Analysis of high- Acknowledgments. The research at the University of resolution FTIR spectrum of the band of H acooh, J. Mol. Spectrosc., 19, , Acknowledgment is made to the National Center for Atmo- Rinsland, C.P. and A. Goldman, Infrared spectroscopic measpheric Research, which is sponsored by the National Sci- surements of tropospheric trace gases, Appl. Opt., 31, ence Foundation, for computer time used in this project , Denver was supported in part by NSF and in part by NASA. We thank Michael Dulick of the U.S. National Solar Obser- Rothman, L.S., et al., The HITRAN molecular spectroscopic vatory (NSO) for recording the infrared solar spectra. The database and HAWKS (HITRAN Atmospheric Work Sta- National Solar Observatory is operated by the Association of tion)' 1996 edition, J. Quant. Spectrosc. Radiat. Trans- Universities for Research in Astronomy, Inc. (AURA) under [er, 60, , a cooperative agreement with the National Science Founda- Toth, R.A., HD eo, HD so, and HD 70 transition frequention. The NSO observations analyzed in this study were cies and strengths in the bands, J. Mol. Spectrosc., 16œ, funded by NASA's Upper Atmosphere Research Program , References Toth, R.A., HDO and DaO low pressure, long path spectra in the 600 to 3100 cm - region, I, HDO line positions and strengths, J. Mol. Spectrosc., 195, 73-97, 1999a. Bumgarner, R.E., J.-l. Choe, S.G. Kukolich, and R.J. Butchez, Toth, R.A., Air-and-Na broadening parameters of HDO and High-resolution spectroscopy of the ve and vs bands of DaO; 709 to 1935 cm -, J. Mol. Spectrosc., in press, formic acid, J. Mol. Spectrosc., 13œ, , Clerbaux, C., R. Colin, P.C. Simon, and C. Granier, Infrared 1999b. Vander Auwera, J., High-resolution investigation of the farcross sections and global warming potentials of 10 alternative hydrohalocarbons, J. Geophys. 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Ward, Open-path Fourier transform infrared studies of large-scale laboratory biomass fires, J. Geophys. Res., 101, 21,067-21,080, Yokelson, R.J., R. Susott, D.E. Ward, J. Reardon, and D.W.T. Griffith, Emissions from smoldering combustion of biomass measured by open-path Fourier transform infrared spectroscopy, J. Geophys. Res., 10œ, 18,865-18,877, A. Goldman, Department of Physics, University of Denver, Denver, CO ( goldman@du.edu). A. Perrin, Laboratoire de Photophysique Mol culaire, Bat. 210, Universit Paris-Sud, Orsay, France. C. P. Rinsland, Atmospheric Sciences Division, NASA Langley Research Center, Hampton, VA (Received March 3, 1999; revised May 18, 1999; accepted May 21, 1999.)