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Near-IR absorption of water vapor: Pressure dependence of line strengths and an upper limit for continuum absorption
Journal of Molecular Spectroscopy 232 (2005) 223–230 www.elsevier.com/locate/jms
Near-IR absorption of water vapor: Pressure dependence of line strengths and an upper limit for continuum absorption M. Aldener a,b, S.S. Brown a,b, H. Stark a,b, J.S. Daniel a, A.R. Ravishankara a,b,c,* a
b
NOAA Aeronomy Laboratory, 325 Broadway, Boulder, CO 80305, USA Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, USA c Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA Received 23 September 2004; in revised form 6 April 2005 Available online 13 June 2005
Abstract Water vapor absorption cross-sections in the near-infrared region (10 500–10 800 cm1) were measured using cavity ringdown spectroscopy. Linestrengths were measured for several absorption lines around 10 604 cm1 (943 nm) between 500 and 850 Torr of N2 and found to be independent of pressure. Our measured linestrengths of these individual lines agree well with values from databases such as HITRAN and the ESA-WVR, which are currently used for atmospheric calculations, but the integrated strength over the entire measured spectral region is slightly larger than that contained in these databases. Water vapor pressure-broadening coefficients due to nitrogen were also estimated from these measurements. The absorption due to water vapor continuum was determined to be less than (9.2 ± 0.2) · 1027 cm2 molecule1 at 11 500 cm1. This measured upper limit, though larger than the estimated values from continuum models, would not contribute significantly to the calculated radiation absorption in this wavelength region. 2005 Elsevier Inc. All rights reserved. Keywords: Water vapor; Linestrength; Continuum; Near-IR; Cavity ringdown
1. Introduction Water vapor is one of the most important absorbers of incoming solar radiation in the atmosphere. It possesses relatively weak cross-sections through much of the visible (where the solar actinic flux is largest) and near-infrared regions. However, because of its large atmospheric abundance, water vapor absorbs a significant amount of energy and thus impacts the EarthÕs radiative budget. Quantification of solar radiation absorption by water vapor in the atmosphere is essential for radiative transfer calculations. Accurate radiative transfer models require databases that include all spec*
Corresponding author. Fax: +1 303 497 5822. E-mail address: [email protected] (A.R. Ravishankara). 0022-2852/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2005.04.011
tral features as well as their line shapes and dependence on pressure and temperature. This is especially true in the near-IR (between 7700 and 13 300 cm1) region, where water vapor is the most prominent tropospheric absorber. The water vapor continuum absorption varies slowly with wavelength and underlies the structured absorption bands. The continuum is an absorption whose cross-section depends on the pressure of both the buffer gas (foreign-broadening) and the water vapor (self-broadening). This absorption is usually described as a non-Lorentzian contribution in the far-wings of a pressure-broadened absorption line, a contribution due to collision-induced transitions, or a combination of these effects [1–3]. It has also been suggested that it could be attributed to absorption of bound H2O complexes [4]. It is of interest to characterize the continuum more carefully to better
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understand this absorption and to more accurately calculate the total energy absorbed by water vapor in the atmosphere. The water vapor continuum has mainly been studied in the mid-infrared region, at wavelengths longer than 3 lm, where its absorption is up to two orders of magnitude stronger than in the near-IR. Yet, because of much larger solar flux and the smaller degree of saturation in the water vapor bands at wavelengths shorter than 1.2 lm, near-IR continuum absorption could be an important contributor. Any extra near-IR absorption in addition to that by absorption lines, as given in currently available databases, would be of interest for the radiation budget. The water vapor continuum is currently considered in most radiative transfer calculations but has so far mainly been validated by measurements in the far-IR, with only a few field studies at shorter wavelengths [5,6]. Cross-sections for water vapor absorption in the near-IR are also needed for LIDAR measurements of water as well as other retrievals of atmospheric abundances by remote sensing. Cavity ringdown spectroscopy (CRDS) [7,8] is a sensitive technique for measuring small absorptions. This method is based on the observation of the first-order decay of light intensity from an optical cavity in the presence and absence of an absorbing (or scattering) species. The difference between these single-exponential decay rate coefficients leads to an absolute measure of the absorption coefficient, a, at that wavelength. Measurements of the decay rate coefficients, rather than the absolute intensities of the light, make the CRDS technique relatively insensitive to fluctuations in the light source intensity. This is an important advantage for the current study since it allows the use of lasers with relatively poor pulse-to-pulse energy stability (see below) for high-sensitivity absorption measurements. The tech-
nique has been widely used in many different studies and wavelength regions because of its inherent sensitivity, wavelength accessibility, and absolute absorption measurement capability. We have used CRDS in the near-IR region around 11 000 cm1 (900 nm) to study water vapor absorption, the dependence of individual linestrengths on pressure, and the magnitude of the continuum. To the best of our knowledge, this is the first constraint on the water vapor continuum in the near-IR derived from laboratory measurements.
2. Experiments The CRDS technique was used to detect water absorption in the near-IR region from 10 470 to 11 495 cm1 (870–955 nm). The apparatus was similar to the one described previously for measurement of vibrational overtone intensities in HNO3 and H2O2 [9]. Fig. 1 shows a schematic of the experimental apparatus. A pulsed dye laser (10 Hz, FWHM 0.07 ± 0.02 cm1 at 10 640 cm1 assuming a Lorentzian lineshape), pumped by the second harmonic of a Nd:Yag (532 nm) laser was used as the near-IR light source. Two different laser dyes, LDS 925 and LDS 867, were used to cover the region of 10 470–11 495 cm1; the bandwidth of the CRDS mirrors covered a similar wavelength range. The output of the dye laser was coupled directly into the optical cavity, which consisted of two high-reflectivity 100 cm radius of curvature mirrors separated by 95.3 ± 1 cm (L1 in Fig. 1). The ringdown time in vacuum, s0, was measured to be longer than 212 ls at 10 695 cm1 suggesting a mirror reflectivity R > 99.9985%. Small gas flows were introduced adjacent to the mirrors to keep them clean, and the absorber was
Fig. 1. A schematic of the experimental setup.
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absent in this region. The path length inside the ringdown cavity where water was present was 73.6 ± 0.5 cm (L2). This absorption path length was calibrated by measurement of the absorption due to a known concentration of O3 in the Chappius bands [10,11]. Light leaking out of the cavity through the back mirror was detected with a near-IR sensitive photomultiplier tube (PMT); the signal from the PMT was collected by a fast A/D card in the computer used to control the laser. To obtain the ringdown time constant, a weighted linear least-square fit was made to the logarithm of individual decay traces. The background level (measured PMT signal prior to the laser pulse) was subtracted from the signal before fitting. The average of several (typically 30–50) ringdown times at every wavelength gave the measured data-points. A small fraction of the laser beam was split off prior to the cavity and used for wavelength calibration with a commercial wavelength meter. For simulations of spectra, taking into account the response function, it is of importance to characterize both the spectral shape and the width of the laser emission. The laser lineshape was inferred to be Lorentzianlike from low-pressure measurements, where the linewidths were mainly Doppler-limited, of various absorption lines of O2 and H2O using either CRDS or photoacoustic spectroscopy. The reason for the Lorentzian lineshape of the dye laser output is unknown. Modeling of the lines in the water vapor spectrum using the parameters in the HITRAN database [12], and a Lorentzian laser lineshape best reproduced the measured water lines. We used a pulsed laser system for these studies, both because of its broad tunability over the range of interest and because of the simplicity of coupling a pulsed laser to an optical cavity. The disadvantage of the pulsed CRDS system was in the measurement of discrete water vapor lines with finite widths that were comparable to the linewidth of the dye laser itself. CRDS temporal profiles become non-exponential and harder to interpret if the linewidth of the laser is comparable to, or wider than, the spectral width of the transition. The measured absorption can be underestimated if the laser linewidth is large [13] relative to the width of the absorption line. This effect is pronounced at large values of the absorption coefficient, a, and affects the ringdown profile primarily at longer times. The measured temporal profile under such conditions is a convolution of absorbances at different wavelengths encompassed by the laser output. Each wavelength gives a different loss-rate in the cavity because it interrogates different parts of the absorption feature. Thus a multi-exponential ring down decay is observed. A single exponential fit to this profile tends to overestimate the ring down time and thus underestimate the absorbance at line center, although for small absorbances the integrated linestrength
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remains unaffected by this artifact [13]. To minimize these effects, absorption lines were only measured at pressures greater than 500 Torr where the absorption linewidths were >0.12 cm1 (FWHM), larger than the laser linewidth. Only the early part (less than two times the ringdown time constant or P90% of the initial intensity) of the ringdown signal was used for the analysis to further suppress the above complication. Also, small water vapor concentrations (less than 5 · 1014 molecule/cm3) were used so that the extinction was small. Simulations of the CRDS profiles show that at 500 Torr and with our laser characteristics, laser line width effects should be negligible [14] if the absorption coefficient is below 107 cm1; such a low absorbance was always maintained in this study. Water vapor samples were entrained in a small flow of ultra-high-purity (UHP) nitrogen (from US Welding, with a specified water content less than 1 ppm) that passed through a temperature-controlled bubbler containing distilled water. A part of this flow, controlled by a needle valve, was then added to a larger flow of UHP N2 (1–4 slm) to reduce the water concentration when needed. When larger water concentrations were used, the flow from the bubbler was directly combined with the main gas flow. The water concentration in the flow was further reduced by dilution with another calibrated flow after the cell where water concentration was measured using Lyman-a absorption (UV1) to a level sufficiently small for the sensitive CRDS measurements. The pressures in the CRDS cell and the water bubbler were measured using absolute capacitance manometers. The temperature was measured with a thermocouple and all gas flows (except water) were measured using calibrated electronic mass flowmeters. All the reported measurements were carried out in N2. Water vapor concentration in the cell was quantified via UV absorption. Depending on the water concentration, different absorption cells, UV1 or UV2 (as shown in Fig. 1), were used in series with the CRDS cell. For lower H2O concentrations used in the pressure-dependent spectral measurements the Lyman-a line at 121.6 nm (r = (1.59 ± 0.10) · 1017 cm2 molecule1 [15,16]) was used in the UV1 absorption cell. Light from an H2-doped microwave discharge lamp was passed through an absorption cell (50 cm, MgF2 windows), the 121.6 nm light isolated by a nitrogen purged 0.3 m vacuum UV monochromator, and detected with a solar blind PMT. To correct for drifts in the intensity of the light from the discharge, the 121.6 nm light coming out through the back of the lamp was isolated using a 1 cm cell filled with 600 Torr of oxygen and detected by a PMT. Oxygen acted as an efficient filter for the light from the discharge since it has a transmission ‘‘window’’ at 121.6 nm [17] absorbing most other emitted light from the discharge. For large concentrations of water, the
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weaker absorption at the Hg-line at 184.9 nm (r = (7.1 ± 0.1) · 1020 cm2 molecule1 [17,18]) was used. The light from an Hg-pen-lamp was detected by a UV phototube (160–200 nm) through a 100 cm absorption cell, UV2. Neither photo-dissociation nor losses of water due to condensation on the walls of the flow system occurred to any measurable extent. Possible photo-dissociation was checked by turning off the hydrogen/helium discharge with the water absorption signal measured in the CRDS cell. Condensation on the walls was checked by reversing the gas flow direction through the apparatus. All measurements were carried out at room temperature, 295 ± 2 K, which is very close to the reference temperature (296 K) for HITRAN. This small difference in temperature would give a correction for the linestrengths in the database much smaller than 1%, which is well within our error limits and was not taken into account in this work.
3. Data analysis and results 3.1. Pressure-dependent absorption The measured water vapor spectrum at 760 Torr total pressure of N2 is shown in Fig. 2 together with a simulated spectrum calculated using the 2000 version of the HITRAN database (updated from the web February 2002). The spectra were obtained using low water concentrations to avoid saturation effects. About 150 lines were seen in the 10 580–10 820 cm1 region; this spectrum is a part of the 3m polyad. In this region, more than 30% of the lines overlapped and were not resolved. Essentially all lines observed are documented in the HITRAN database and no strong lines from the database are missing in our spectra. The spectral region was scanned with a step
size smaller than 0.03 cm1 and integrated to obtain the total bandstrength in this region. The integrated crosssection between 10 580 and 10 820 cm1 is (1.49 ± 0.13) · 1020 cm2 molecule1 cm1. This value is 9 and 2% larger than the values derived from HITRAN and ESA-WVR [19,20], respectively. Our larger linestrengths than those obtained from HITRAN database is consistent with the suggestion of Belmiloud et al [21]. These authors proposed a 6% larger cross-section for the entire 3m polyad. To retrieve linestrengths of individual lines, the lines were scanned at a step size of <0.03 cm1. A Lorentzian line shape, which matched the observed lineshape well, was fitted to the observed absorption line. Then the integrated area under this Lorentzian line was calculated. There was almost always some residual water vapor absorption in the background CRDS signal (i.e., in the absence of a flow through the water bubbler) especially in the regions of stronger water lines. This is attributed to a small water content in the UHP nitrogen. Nitrogen was passed through a trap at 195 K used to remove water. In some cases, nitrogen gas was directly taken from the top of liquid nitrogen. Yet, small amounts of water residual always remained (<0.4 ppmv). This background water absorption was integrated, in the same manner in which the signal was analyzed, and subtracted from all measured lines. The background water vapor concentration was also measured by the UV absorption cell, so that the CRDS signal was referenced to the relative change in water vapor concentration upon addition of the flow through the water bubbler rather than the absolute concentration of water vapor. In most cases, the background was a negligibly small fraction of the total signal, although in a few limited cases it reached 10% of the measured absorption. This observed net absorption was converted to cross-sections using the water concentration as measured via 121.6 nm absorp-
Fig. 2. Water vapor spectrum measured at 760 Torr along with that obtained by simulation (see text) shown on top of the graph.
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tion. We also used a Voigt lineshape to calculate the integrated absorptions. These values agreed, within the error of the fits, with those calculated using the Lorentzian shape. Typically the integrated areas differed by less than 0.2% as shown in Fig. 3 for the line centered at 10 600.85 cm1. The two fits were visually indistinguishable. The procedure of using a Lorentzian shape in calculating linestrengths is insensitive to any nonLorentzian behavior, such as increased absorption in the wings of the line (super-Lorentzian). Absorptions in the wings would be a feature that could be attributed to a continuum absorption that should appear in the residuals of the fit. There was no systematic spectral
pattern in the residuals of a Lorentzian fit to the observed line that could be attributed to different lineshapes (Fig. 3). These measurements were carried out as a function of N2 pressure between 500 and 850 Torr. The resulting integrated line strengths for the 10 610.74 cm1 line are plotted as a function of pressure in Fig. 4. The largest contributor to the scatter in these measurements is due to the uncertainty in the determination of the water vapor concentration. The total error in every measurement is usually about 10%. The linewidth increases linearly with pressure, as shown in Fig. 5.
Fig. 3. Lorentzian (solid line) and Voigt (dashed line) fits to a measured (solid points) water vapor absorption line at 10 600.8 cm1. The Voigt fit and data has been shifted, by 2 · 108 cm1, for clarity. The residuals are shown in the upper panel.
Fig. 5. Measured linewidths (circles) at 10 610.74 cm1 plotted against the total pressure. A linear least-square fit to the data made with a fixed offset (see text) is shown as the solid line. A simulation, of the Lorentzian pressure broadening is displayed as a dashed line.
Fig. 4. Integrated linestrengths (filled circles) for the water vapor absorption line at 10 610.74 cm1 measured at different pressures, along with those obtained from HITRAN (dashed line) and ESA (dotted line) databases. A linear fit to all measured data is represented with the dash-dot line. The uncertainties in the individually measured linestrengths are mainly due to the uncertainty (drift in the UV light source intensity and accuracy of water vapor cross-section) in the water concentration measurements. The averaged value in every pressure interval is shown as triangles.
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The measured integrated linestrengths are shown in Fig. 4 and Table 1. Clearly, the measured linestrengths do not change with pressure and agree well with values reported in previous studies. The independence of the integrated linestrength shows that these transitions do not increase in intensity due to the presence of colliding gases. Further, the integrated cross-section is the same in the troposphere irrespective of pressure. The baselines underneath most individual lines were not constant but showed a small wavelength dependence due to either nearby lines or changes in the reflectivity of the mirrors with wavelength. The sloping baseline could be corrected for by measuring the background ringdown time constants. However, since these often showed some influence from the residual water, the baseline was obtained by fitting a straight line to the residuals of the Lorentzian fit. This procedure would not affect the analysis of a super-Lorentzian lineshape since such a feature should be symmetric around the line center. After this subtraction of the baseline the data was again fit to a Lorentzian. This procedure led to an integrated crosssection that at most differed 1.5% from that not taking into account the baseline change. Our measured linestrengths (see Table 1) agree well with the values from the ESA-WVR and HITRAN databases. The measured line profiles also provide information on the pressure broadening of water vapor absorption lines in nitrogen. The measured halfwidths (HWHM) for the line at 10 610.74 cm1 are plotted, as circles, against the total pressure in Fig. 5. These widths are the Lorentzian part of a Voigt fit to the measured absorption lines, with the Doppler width kept constant at its calculated value. The nitrogen pressure-broadening coefficients ðcN2 Þ for the water lines were taken as the slope of a linear fit to the half widths as a function of pressure. As long as the pressure broadening is the
dominant component to the observed width, the slope of this plot gives the pressure-broadening coefficient without the need to correct the linewidths for the convolution with the laser line width. If the laser lineshape is a Lorentzian, which is approximately true, then the plot will be linear to zero pressure with an intercept equal to the laser linewidth. The intercept of the solid line in Fig. 5 is 0.020 ± 0.002 cm1. This compares to an approximately Lorentzian laser linewidth of (FWHM) 0.040 ± 0.004 cm1. Fixing the laser linewidth at the measured width reduces the slope of the line by approximately 20%. The difference between these slopes gives an approximate measure of the uncertainty in the derived pressure-broadening coefficient. This uncertainty is likely due to the uncertainty in both the laser linewidth and shape. To get the air-broadening coefficients (cAIR), the resulting slope was multiplied with a factor of 0.90 [22] to account for broadening by O2, and these results are presented in Table 2. The pressure-broadening part of the linewidths for H2O was calculated, with cAIR taken from the HITRAN database and divided by 0.90, to compare with the measured linewidths and is shown in Fig. 5 together with the measured data. Table 2 Air-broadening coefficients (HWHM) for the water lines studied in this experiment compared to values from the HITRAN and ESA-WV databases Wavenumber (cm1)
Air-broadening halfwidths (cAIR cm1 atm1) This work
HITRAN
ESA-WV
10 600.85 10 610.74 10 583.59
0.112(22) 0.109(22) 0.093(19)
0.093 0.094 0.082
0.0951 0.0961 0.0849
The uncertainties are due to systematic shifts from fitting the data with or without a fixed offset (see text).
Table 1 Measured and literature values for the linestrengths Wavenumber (cm1)
Pressure (Torr)
Linestrength (1022 cm2 molecules1) Measured
Mean value
Hitran
ESAWVR
10 600.85
500.7 ± 0.8 601.2 ± 0.7 827.7 ± 19.8
1.950 ± 0.072 1.966 ± 0.075 1.922 ± 0.053
1.940 ± 0.037
1.93
2.09
10 610.74
500.7 ± 0.5 615.9 ± 14.7 827.8 ± 0.6
1.027 ± 0.042 1.011 ± 0.030 1.023 ± 0.031
1.019 ± 0.019
1.01
1.09
10 583.59
500.5 ± 0.8 601.3 ± 0.5 822.7 ± 26.5
0.694 ± 0.036 0.731 ± 0.035 0.709 ± 0.036
0.712 ± 0.020
0.657
0.719
The HITRAN value is from the 2000 version and ESA is from the European Space Agency. The measured linestrengths are the weighted averages of all the data for the lines measured, in the respective pressure regime or, in the case of the mean values, for all pressures. The error bars are the uncertainties in these averages. The error bars for the pressure shows the pressure interval (one standard deviation) over which the measured linestrength values were averaged.
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3.2. Water vapor continuum The minimum detectable absorbance (amin) for CRDS can be written as [23] pffiffiffi RL 2 rðs0 Þ . ð1Þ amin ¼ c s20 Here RL is the ratio of the geometrical length of the optical cavity to the absorber path length (i.e., L1/L2 in Fig. 1), c is the speed of light, s0 is the ringdown time for an empty cavity, and r(s0) is the uncertainty in s0. The inverse proportionality between amin and s20 demonstrate that mirror reflectivity determines detection sensitivity, even if r (s0) depends linearly on s0. An upper limit to the water vapor absorption continuum between the intense absorption bands can be estimated from Eq. (1). At 11 500 cm1, a region where there is little direct influence from adjacent bands (about 300 cm1 from the closest band) the ringdown time constant and its standard deviation were s0 = 12.12 and 0.011 ls, respectively; this yields amin = 4.6 · 109 cm1. The ringdown time is relatively short at this wavelength because it is on the edge of the mirror reflectivity curve, thus decreasing the sensitivity. No absorption of water was seen at any pressures of bath gas (up to 805 Torr total pressure) or water concentrations (up to 15.4 Torr partial pressure of water) demonstrating that the absorption from the water continuum at this wavelength must be less than (9.2 ± 0.2) · 1027 cm2 molecule1. This number represents the sum of extinctions due to Rayleigh scattering and water absorption at this wavelength since CRDS measures the total extinction. Attempts were made to measure and distinguish the self-broadened continuum from the foreign-broadened contributions. A large amount of water was added to the flow at low pressures of N2. Extinctions were measured at a constant total pressure of approximately 40 Torr, which would lower the foreign-broadened part by more than a factor of 20 compared to the measurements at 800 Torr. The partial pressure of water vapor varied up to 14 Torr. The detection sensitivity at this wavelength was 3.1 · 109 cm1. The difference in extinction with and without water in the flow was not measurable, indicating that the sum of the continuum absorption and the scattering cross-section for water vapor is less than 6.9 · 1027 cm2 molecule1 at 11 495 cm1 at these conditions. This would give an upper limit for the self-broadened continuum absorption for water vapor of 3.7 · 1025 cm2 molecule1 atm1, about 40 times higher than predicted at this wavelength. Rayleigh scattering has not been taken into account in the above calculations and would only matter if the difference in scattering cross section for water vapor and nitrogen are significantly different. Rayleigh scattering cross-sections for nitrogen, obtained by extrapolation from measured scattering cross-sections between
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15 385 and 17 860 cm1 (560 and 650 nm) by Naus and Ubachs [24], is 7.2 · 1028 cm2 molecule1 at this wavelength. If the water vapor scattering cross-section is negligibly small, the upper limit for the continuum absorption would be 10% higher than the values stated above. However, the Rayleigh scattering cross-section of water is likely close to that of nitrogen since the index of refraction for nitrogen and water vapor are very similar; therefore scattering from water and nitrogen should cancel out in our measurements. We were not able to calculate the Rayleigh scattering cross-section for water vapor since the depolarization factors [25] needed for water are, to the best of our knowledge, not available in the literature.
4. Conclusion Our upper limit for the water vapor continuum absorption at 11 500 cm1 is measured to be (9.2 ± 0.2) · 1027 cm2 molecule1 and is consistent with the 30–40 times lower values reported by Ma and Tipping [1] and Clough et al. [26] at the total pressures of nitrogen and water concentrations used here. Our value is an upper limit since it is derived from our detection sensitivity and also does not take into account the Rayleigh scattering from water. The effect of absorption in this spectral ‘‘window,’’ between vibrational overtone bands of water, on the total EarthsÕ radiation field can then be estimated. If we take a column of precipitable water of 45 mm, our measured upper limit for the continuum will give a optical depth of 1.4 · 103. The spectral window between the water vapor absorption bands, where the continuum has a minimum, is about 20 nm wide and this together with a spectral radiance of 0.8 W m2 nm1 gives a total estimated absorption of less than 0.023 W m2. This simple calculation is sufficient since such a small contribution will not add significantly to the modeled atmospheric absorption in this wavelength region. The linestrengths were observed to be independent of pressure, indicating that the Lorentzian lineshape (impact theory) provides an adequate description of the lineshape in the near-wing region. Our measurements provide cross-sections in the near-IR for atmospheric calculations. Although the continuum in the transmission window between bands appears to be unlikely to play a role in the atmospheric radiation budget, the absorption closer to or within the bands may still be significant. It is difficult to draw any conclusion from these measurements regarding the continuum underlying the absorption peaks since it is only of the order of 1% relative to the total linestrength, well within the measurement precision. However, the invariance of the integrated line strengths with pressure, as determined from the Lorentzian fitting procedure, to a precision of 2% over the range
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500–800 Torr of N2 (see Table 1 and Fig. 4) implies that any foreign-induced continuum absorption above this 2% must arise from an additional, collisionally induced absorption (as suggested by Mlawer et al. [3]) rather than from intensity borrowing from the total linestrength. The latter effect, if it were strong, would appear as a reduction in the linestrengths with pressure. Finally, our measured air-broadening coefficients agree with values from databases although the broad laser bandwidth in this experiment does not allow for a detailed study of linewidths. Our measurements support the larger cross-sections of ESA-WVR.
Acknowledgments This work was funded by NOAAÕs climate and global change research program.
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[8] A. OÕKeefe, D.A.G. Deacon, Rev. Sci. Instrum. 59 (12) (1988) 2544–2551. [9] S.S. Brown, R.W. Wilson, A.R. Ravishankara, J. Phys. Chem. A 104 (21) (2000) 4976–4983. [10] S.S. Brown, A.R. Ravishankara, H. Stark, J. Phys. Chem. 104 (30) (2000) 7044–7052. [11] J.B. Burkholder, R.K. Talukdar, Geophys. Res. Lett. (USA) 21 (7) (1994) 581–584. [12] L.S. Rothman, C.P. Rinsland, A. Goldman, S.T. Massie, D.P. Edwards, J.-M. Flaud, A. Perrin, C. Camy-Peyret, V. Dana, J.Y. Mandin, J. Schroeder, A. McCann, R.R. Gamache, R.B. Watson, K. Yoshino, K.V. Chance, K.W. Jucks, L.R. Brown, V. Nemtchinov, R. Varanasi, J. Quant. Spectrosc. Radiat. Transf. 60 (5) (1998) 665–710. [13] P. Zalicki, R.N. Zare, J. Chem. Phys. 102 (7) (1995) 2708–2717. [14] S.M. Newman, I.C. Lane, A.J. Orr-Ewing, D.A. Newnham, J. Ballard, J. Chem. Phys. 110 (22) (1999) 10749–10757. [15] R.K. Vatsa, H.-R. Volpp, Chem. Phys. Lett. 340 (2001) 289–295. [16] D. Kley, J. Atmos. Chem. 2 (2) (1984) 203–310. [17] E.J. Dunlea, Atmospheric reaction of Electronically Excited Atomic and molecular oxygen, in: CIRES, The University of Colorado at Boulder, Boulder, 2002, p. 220. [18] C.A. Cantrell, A. Zimmer, G.S. Tyndall, Geophys. Res. Lett. 24 (17) (1997) 2195–2198. [19] R. Schermaul, R.C.M. Learner, D.A. Newnham, J. Ballard, N.F. Zobov, D. Belmiloud, J. Tennyson, J. Mol. Spectrosc. 208 (2001) 43–50. [20] R. Schermaul, R.C.M. Learner, D.A. Newnham, R.G. Williams, J. Ballard, N.F. Zobov, D. Belmiloud, J. Tennyson, J. Mol. Spectrosc. 208 (2001) 32–42. [21] D. Belmiloud, R. Schermaul, K.M. Smith, N.F. Zobov, J.W. Brault, R.C.M. Learner, D.A. Newnham, J. Tennyson, Geophys. Res. Lett. 27 (2) (2000) 3703–3706. [22] B.E. Grossmann, E.V. Browell, J. Mol. Spectrosc. 138 (2) (1989) 562–595. [23] S.S. Brown, H. Stark, A.R. Ravishankara, Appl. Phys. B 75 (2–3) (2002) 173–182. [24] H. Naus, W. Ubachs, Opt. Lett. 25 (5) (2000) 347–349. [25] R. Penndorf, J. Opt. Soc. Am. 47 (2) (1957) 176–182. [26] S.A. Clough, M.W. Shephard, E.J. Mlawer, J.S. Delamere, M.J. Iacono, K. Cady-Pereira, S. Boukabara, P.D. Brown, J. Quant. Spectosc. Rad. Trans. 91 (2005) 233–244.
Near-IR absorption of water vapor: Pressure dependence of line strengths and an upper limit for continuum absorption M. Aldener a,b, S.S. Brown a,b, H. Stark a,b, J.S. Daniel a, A.R. Ravishankara a,b,c,* a
b
NOAA Aeronomy Laboratory, 325 Broadway, Boulder, CO 80305, USA Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, USA c Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA Received 23 September 2004; in revised form 6 April 2005 Available online 13 June 2005
Abstract Water vapor absorption cross-sections in the near-infrared region (10 500–10 800 cm1) were measured using cavity ringdown spectroscopy. Linestrengths were measured for several absorption lines around 10 604 cm1 (943 nm) between 500 and 850 Torr of N2 and found to be independent of pressure. Our measured linestrengths of these individual lines agree well with values from databases such as HITRAN and the ESA-WVR, which are currently used for atmospheric calculations, but the integrated strength over the entire measured spectral region is slightly larger than that contained in these databases. Water vapor pressure-broadening coefficients due to nitrogen were also estimated from these measurements. The absorption due to water vapor continuum was determined to be less than (9.2 ± 0.2) · 1027 cm2 molecule1 at 11 500 cm1. This measured upper limit, though larger than the estimated values from continuum models, would not contribute significantly to the calculated radiation absorption in this wavelength region. 2005 Elsevier Inc. All rights reserved. Keywords: Water vapor; Linestrength; Continuum; Near-IR; Cavity ringdown
1. Introduction Water vapor is one of the most important absorbers of incoming solar radiation in the atmosphere. It possesses relatively weak cross-sections through much of the visible (where the solar actinic flux is largest) and near-infrared regions. However, because of its large atmospheric abundance, water vapor absorbs a significant amount of energy and thus impacts the EarthÕs radiative budget. Quantification of solar radiation absorption by water vapor in the atmosphere is essential for radiative transfer calculations. Accurate radiative transfer models require databases that include all spec*
Corresponding author. Fax: +1 303 497 5822. E-mail address: [email protected] (A.R. Ravishankara). 0022-2852/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2005.04.011
tral features as well as their line shapes and dependence on pressure and temperature. This is especially true in the near-IR (between 7700 and 13 300 cm1) region, where water vapor is the most prominent tropospheric absorber. The water vapor continuum absorption varies slowly with wavelength and underlies the structured absorption bands. The continuum is an absorption whose cross-section depends on the pressure of both the buffer gas (foreign-broadening) and the water vapor (self-broadening). This absorption is usually described as a non-Lorentzian contribution in the far-wings of a pressure-broadened absorption line, a contribution due to collision-induced transitions, or a combination of these effects [1–3]. It has also been suggested that it could be attributed to absorption of bound H2O complexes [4]. It is of interest to characterize the continuum more carefully to better
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understand this absorption and to more accurately calculate the total energy absorbed by water vapor in the atmosphere. The water vapor continuum has mainly been studied in the mid-infrared region, at wavelengths longer than 3 lm, where its absorption is up to two orders of magnitude stronger than in the near-IR. Yet, because of much larger solar flux and the smaller degree of saturation in the water vapor bands at wavelengths shorter than 1.2 lm, near-IR continuum absorption could be an important contributor. Any extra near-IR absorption in addition to that by absorption lines, as given in currently available databases, would be of interest for the radiation budget. The water vapor continuum is currently considered in most radiative transfer calculations but has so far mainly been validated by measurements in the far-IR, with only a few field studies at shorter wavelengths [5,6]. Cross-sections for water vapor absorption in the near-IR are also needed for LIDAR measurements of water as well as other retrievals of atmospheric abundances by remote sensing. Cavity ringdown spectroscopy (CRDS) [7,8] is a sensitive technique for measuring small absorptions. This method is based on the observation of the first-order decay of light intensity from an optical cavity in the presence and absence of an absorbing (or scattering) species. The difference between these single-exponential decay rate coefficients leads to an absolute measure of the absorption coefficient, a, at that wavelength. Measurements of the decay rate coefficients, rather than the absolute intensities of the light, make the CRDS technique relatively insensitive to fluctuations in the light source intensity. This is an important advantage for the current study since it allows the use of lasers with relatively poor pulse-to-pulse energy stability (see below) for high-sensitivity absorption measurements. The tech-
nique has been widely used in many different studies and wavelength regions because of its inherent sensitivity, wavelength accessibility, and absolute absorption measurement capability. We have used CRDS in the near-IR region around 11 000 cm1 (900 nm) to study water vapor absorption, the dependence of individual linestrengths on pressure, and the magnitude of the continuum. To the best of our knowledge, this is the first constraint on the water vapor continuum in the near-IR derived from laboratory measurements.
2. Experiments The CRDS technique was used to detect water absorption in the near-IR region from 10 470 to 11 495 cm1 (870–955 nm). The apparatus was similar to the one described previously for measurement of vibrational overtone intensities in HNO3 and H2O2 [9]. Fig. 1 shows a schematic of the experimental apparatus. A pulsed dye laser (10 Hz, FWHM 0.07 ± 0.02 cm1 at 10 640 cm1 assuming a Lorentzian lineshape), pumped by the second harmonic of a Nd:Yag (532 nm) laser was used as the near-IR light source. Two different laser dyes, LDS 925 and LDS 867, were used to cover the region of 10 470–11 495 cm1; the bandwidth of the CRDS mirrors covered a similar wavelength range. The output of the dye laser was coupled directly into the optical cavity, which consisted of two high-reflectivity 100 cm radius of curvature mirrors separated by 95.3 ± 1 cm (L1 in Fig. 1). The ringdown time in vacuum, s0, was measured to be longer than 212 ls at 10 695 cm1 suggesting a mirror reflectivity R > 99.9985%. Small gas flows were introduced adjacent to the mirrors to keep them clean, and the absorber was
Fig. 1. A schematic of the experimental setup.
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absent in this region. The path length inside the ringdown cavity where water was present was 73.6 ± 0.5 cm (L2). This absorption path length was calibrated by measurement of the absorption due to a known concentration of O3 in the Chappius bands [10,11]. Light leaking out of the cavity through the back mirror was detected with a near-IR sensitive photomultiplier tube (PMT); the signal from the PMT was collected by a fast A/D card in the computer used to control the laser. To obtain the ringdown time constant, a weighted linear least-square fit was made to the logarithm of individual decay traces. The background level (measured PMT signal prior to the laser pulse) was subtracted from the signal before fitting. The average of several (typically 30–50) ringdown times at every wavelength gave the measured data-points. A small fraction of the laser beam was split off prior to the cavity and used for wavelength calibration with a commercial wavelength meter. For simulations of spectra, taking into account the response function, it is of importance to characterize both the spectral shape and the width of the laser emission. The laser lineshape was inferred to be Lorentzianlike from low-pressure measurements, where the linewidths were mainly Doppler-limited, of various absorption lines of O2 and H2O using either CRDS or photoacoustic spectroscopy. The reason for the Lorentzian lineshape of the dye laser output is unknown. Modeling of the lines in the water vapor spectrum using the parameters in the HITRAN database [12], and a Lorentzian laser lineshape best reproduced the measured water lines. We used a pulsed laser system for these studies, both because of its broad tunability over the range of interest and because of the simplicity of coupling a pulsed laser to an optical cavity. The disadvantage of the pulsed CRDS system was in the measurement of discrete water vapor lines with finite widths that were comparable to the linewidth of the dye laser itself. CRDS temporal profiles become non-exponential and harder to interpret if the linewidth of the laser is comparable to, or wider than, the spectral width of the transition. The measured absorption can be underestimated if the laser linewidth is large [13] relative to the width of the absorption line. This effect is pronounced at large values of the absorption coefficient, a, and affects the ringdown profile primarily at longer times. The measured temporal profile under such conditions is a convolution of absorbances at different wavelengths encompassed by the laser output. Each wavelength gives a different loss-rate in the cavity because it interrogates different parts of the absorption feature. Thus a multi-exponential ring down decay is observed. A single exponential fit to this profile tends to overestimate the ring down time and thus underestimate the absorbance at line center, although for small absorbances the integrated linestrength
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remains unaffected by this artifact [13]. To minimize these effects, absorption lines were only measured at pressures greater than 500 Torr where the absorption linewidths were >0.12 cm1 (FWHM), larger than the laser linewidth. Only the early part (less than two times the ringdown time constant or P90% of the initial intensity) of the ringdown signal was used for the analysis to further suppress the above complication. Also, small water vapor concentrations (less than 5 · 1014 molecule/cm3) were used so that the extinction was small. Simulations of the CRDS profiles show that at 500 Torr and with our laser characteristics, laser line width effects should be negligible [14] if the absorption coefficient is below 107 cm1; such a low absorbance was always maintained in this study. Water vapor samples were entrained in a small flow of ultra-high-purity (UHP) nitrogen (from US Welding, with a specified water content less than 1 ppm) that passed through a temperature-controlled bubbler containing distilled water. A part of this flow, controlled by a needle valve, was then added to a larger flow of UHP N2 (1–4 slm) to reduce the water concentration when needed. When larger water concentrations were used, the flow from the bubbler was directly combined with the main gas flow. The water concentration in the flow was further reduced by dilution with another calibrated flow after the cell where water concentration was measured using Lyman-a absorption (UV1) to a level sufficiently small for the sensitive CRDS measurements. The pressures in the CRDS cell and the water bubbler were measured using absolute capacitance manometers. The temperature was measured with a thermocouple and all gas flows (except water) were measured using calibrated electronic mass flowmeters. All the reported measurements were carried out in N2. Water vapor concentration in the cell was quantified via UV absorption. Depending on the water concentration, different absorption cells, UV1 or UV2 (as shown in Fig. 1), were used in series with the CRDS cell. For lower H2O concentrations used in the pressure-dependent spectral measurements the Lyman-a line at 121.6 nm (r = (1.59 ± 0.10) · 1017 cm2 molecule1 [15,16]) was used in the UV1 absorption cell. Light from an H2-doped microwave discharge lamp was passed through an absorption cell (50 cm, MgF2 windows), the 121.6 nm light isolated by a nitrogen purged 0.3 m vacuum UV monochromator, and detected with a solar blind PMT. To correct for drifts in the intensity of the light from the discharge, the 121.6 nm light coming out through the back of the lamp was isolated using a 1 cm cell filled with 600 Torr of oxygen and detected by a PMT. Oxygen acted as an efficient filter for the light from the discharge since it has a transmission ‘‘window’’ at 121.6 nm [17] absorbing most other emitted light from the discharge. For large concentrations of water, the
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weaker absorption at the Hg-line at 184.9 nm (r = (7.1 ± 0.1) · 1020 cm2 molecule1 [17,18]) was used. The light from an Hg-pen-lamp was detected by a UV phototube (160–200 nm) through a 100 cm absorption cell, UV2. Neither photo-dissociation nor losses of water due to condensation on the walls of the flow system occurred to any measurable extent. Possible photo-dissociation was checked by turning off the hydrogen/helium discharge with the water absorption signal measured in the CRDS cell. Condensation on the walls was checked by reversing the gas flow direction through the apparatus. All measurements were carried out at room temperature, 295 ± 2 K, which is very close to the reference temperature (296 K) for HITRAN. This small difference in temperature would give a correction for the linestrengths in the database much smaller than 1%, which is well within our error limits and was not taken into account in this work.
3. Data analysis and results 3.1. Pressure-dependent absorption The measured water vapor spectrum at 760 Torr total pressure of N2 is shown in Fig. 2 together with a simulated spectrum calculated using the 2000 version of the HITRAN database (updated from the web February 2002). The spectra were obtained using low water concentrations to avoid saturation effects. About 150 lines were seen in the 10 580–10 820 cm1 region; this spectrum is a part of the 3m polyad. In this region, more than 30% of the lines overlapped and were not resolved. Essentially all lines observed are documented in the HITRAN database and no strong lines from the database are missing in our spectra. The spectral region was scanned with a step
size smaller than 0.03 cm1 and integrated to obtain the total bandstrength in this region. The integrated crosssection between 10 580 and 10 820 cm1 is (1.49 ± 0.13) · 1020 cm2 molecule1 cm1. This value is 9 and 2% larger than the values derived from HITRAN and ESA-WVR [19,20], respectively. Our larger linestrengths than those obtained from HITRAN database is consistent with the suggestion of Belmiloud et al [21]. These authors proposed a 6% larger cross-section for the entire 3m polyad. To retrieve linestrengths of individual lines, the lines were scanned at a step size of <0.03 cm1. A Lorentzian line shape, which matched the observed lineshape well, was fitted to the observed absorption line. Then the integrated area under this Lorentzian line was calculated. There was almost always some residual water vapor absorption in the background CRDS signal (i.e., in the absence of a flow through the water bubbler) especially in the regions of stronger water lines. This is attributed to a small water content in the UHP nitrogen. Nitrogen was passed through a trap at 195 K used to remove water. In some cases, nitrogen gas was directly taken from the top of liquid nitrogen. Yet, small amounts of water residual always remained (<0.4 ppmv). This background water absorption was integrated, in the same manner in which the signal was analyzed, and subtracted from all measured lines. The background water vapor concentration was also measured by the UV absorption cell, so that the CRDS signal was referenced to the relative change in water vapor concentration upon addition of the flow through the water bubbler rather than the absolute concentration of water vapor. In most cases, the background was a negligibly small fraction of the total signal, although in a few limited cases it reached 10% of the measured absorption. This observed net absorption was converted to cross-sections using the water concentration as measured via 121.6 nm absorp-
Fig. 2. Water vapor spectrum measured at 760 Torr along with that obtained by simulation (see text) shown on top of the graph.
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tion. We also used a Voigt lineshape to calculate the integrated absorptions. These values agreed, within the error of the fits, with those calculated using the Lorentzian shape. Typically the integrated areas differed by less than 0.2% as shown in Fig. 3 for the line centered at 10 600.85 cm1. The two fits were visually indistinguishable. The procedure of using a Lorentzian shape in calculating linestrengths is insensitive to any nonLorentzian behavior, such as increased absorption in the wings of the line (super-Lorentzian). Absorptions in the wings would be a feature that could be attributed to a continuum absorption that should appear in the residuals of the fit. There was no systematic spectral
pattern in the residuals of a Lorentzian fit to the observed line that could be attributed to different lineshapes (Fig. 3). These measurements were carried out as a function of N2 pressure between 500 and 850 Torr. The resulting integrated line strengths for the 10 610.74 cm1 line are plotted as a function of pressure in Fig. 4. The largest contributor to the scatter in these measurements is due to the uncertainty in the determination of the water vapor concentration. The total error in every measurement is usually about 10%. The linewidth increases linearly with pressure, as shown in Fig. 5.
Fig. 3. Lorentzian (solid line) and Voigt (dashed line) fits to a measured (solid points) water vapor absorption line at 10 600.8 cm1. The Voigt fit and data has been shifted, by 2 · 108 cm1, for clarity. The residuals are shown in the upper panel.
Fig. 5. Measured linewidths (circles) at 10 610.74 cm1 plotted against the total pressure. A linear least-square fit to the data made with a fixed offset (see text) is shown as the solid line. A simulation, of the Lorentzian pressure broadening is displayed as a dashed line.
Fig. 4. Integrated linestrengths (filled circles) for the water vapor absorption line at 10 610.74 cm1 measured at different pressures, along with those obtained from HITRAN (dashed line) and ESA (dotted line) databases. A linear fit to all measured data is represented with the dash-dot line. The uncertainties in the individually measured linestrengths are mainly due to the uncertainty (drift in the UV light source intensity and accuracy of water vapor cross-section) in the water concentration measurements. The averaged value in every pressure interval is shown as triangles.
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The measured integrated linestrengths are shown in Fig. 4 and Table 1. Clearly, the measured linestrengths do not change with pressure and agree well with values reported in previous studies. The independence of the integrated linestrength shows that these transitions do not increase in intensity due to the presence of colliding gases. Further, the integrated cross-section is the same in the troposphere irrespective of pressure. The baselines underneath most individual lines were not constant but showed a small wavelength dependence due to either nearby lines or changes in the reflectivity of the mirrors with wavelength. The sloping baseline could be corrected for by measuring the background ringdown time constants. However, since these often showed some influence from the residual water, the baseline was obtained by fitting a straight line to the residuals of the Lorentzian fit. This procedure would not affect the analysis of a super-Lorentzian lineshape since such a feature should be symmetric around the line center. After this subtraction of the baseline the data was again fit to a Lorentzian. This procedure led to an integrated crosssection that at most differed 1.5% from that not taking into account the baseline change. Our measured linestrengths (see Table 1) agree well with the values from the ESA-WVR and HITRAN databases. The measured line profiles also provide information on the pressure broadening of water vapor absorption lines in nitrogen. The measured halfwidths (HWHM) for the line at 10 610.74 cm1 are plotted, as circles, against the total pressure in Fig. 5. These widths are the Lorentzian part of a Voigt fit to the measured absorption lines, with the Doppler width kept constant at its calculated value. The nitrogen pressure-broadening coefficients ðcN2 Þ for the water lines were taken as the slope of a linear fit to the half widths as a function of pressure. As long as the pressure broadening is the
dominant component to the observed width, the slope of this plot gives the pressure-broadening coefficient without the need to correct the linewidths for the convolution with the laser line width. If the laser lineshape is a Lorentzian, which is approximately true, then the plot will be linear to zero pressure with an intercept equal to the laser linewidth. The intercept of the solid line in Fig. 5 is 0.020 ± 0.002 cm1. This compares to an approximately Lorentzian laser linewidth of (FWHM) 0.040 ± 0.004 cm1. Fixing the laser linewidth at the measured width reduces the slope of the line by approximately 20%. The difference between these slopes gives an approximate measure of the uncertainty in the derived pressure-broadening coefficient. This uncertainty is likely due to the uncertainty in both the laser linewidth and shape. To get the air-broadening coefficients (cAIR), the resulting slope was multiplied with a factor of 0.90 [22] to account for broadening by O2, and these results are presented in Table 2. The pressure-broadening part of the linewidths for H2O was calculated, with cAIR taken from the HITRAN database and divided by 0.90, to compare with the measured linewidths and is shown in Fig. 5 together with the measured data. Table 2 Air-broadening coefficients (HWHM) for the water lines studied in this experiment compared to values from the HITRAN and ESA-WV databases Wavenumber (cm1)
Air-broadening halfwidths (cAIR cm1 atm1) This work
HITRAN
ESA-WV
10 600.85 10 610.74 10 583.59
0.112(22) 0.109(22) 0.093(19)
0.093 0.094 0.082
0.0951 0.0961 0.0849
The uncertainties are due to systematic shifts from fitting the data with or without a fixed offset (see text).
Table 1 Measured and literature values for the linestrengths Wavenumber (cm1)
Pressure (Torr)
Linestrength (1022 cm2 molecules1) Measured
Mean value
Hitran
ESAWVR
10 600.85
500.7 ± 0.8 601.2 ± 0.7 827.7 ± 19.8
1.950 ± 0.072 1.966 ± 0.075 1.922 ± 0.053
1.940 ± 0.037
1.93
2.09
10 610.74
500.7 ± 0.5 615.9 ± 14.7 827.8 ± 0.6
1.027 ± 0.042 1.011 ± 0.030 1.023 ± 0.031
1.019 ± 0.019
1.01
1.09
10 583.59
500.5 ± 0.8 601.3 ± 0.5 822.7 ± 26.5
0.694 ± 0.036 0.731 ± 0.035 0.709 ± 0.036
0.712 ± 0.020
0.657
0.719
The HITRAN value is from the 2000 version and ESA is from the European Space Agency. The measured linestrengths are the weighted averages of all the data for the lines measured, in the respective pressure regime or, in the case of the mean values, for all pressures. The error bars are the uncertainties in these averages. The error bars for the pressure shows the pressure interval (one standard deviation) over which the measured linestrength values were averaged.
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3.2. Water vapor continuum The minimum detectable absorbance (amin) for CRDS can be written as [23] pffiffiffi RL 2 rðs0 Þ . ð1Þ amin ¼ c s20 Here RL is the ratio of the geometrical length of the optical cavity to the absorber path length (i.e., L1/L2 in Fig. 1), c is the speed of light, s0 is the ringdown time for an empty cavity, and r(s0) is the uncertainty in s0. The inverse proportionality between amin and s20 demonstrate that mirror reflectivity determines detection sensitivity, even if r (s0) depends linearly on s0. An upper limit to the water vapor absorption continuum between the intense absorption bands can be estimated from Eq. (1). At 11 500 cm1, a region where there is little direct influence from adjacent bands (about 300 cm1 from the closest band) the ringdown time constant and its standard deviation were s0 = 12.12 and 0.011 ls, respectively; this yields amin = 4.6 · 109 cm1. The ringdown time is relatively short at this wavelength because it is on the edge of the mirror reflectivity curve, thus decreasing the sensitivity. No absorption of water was seen at any pressures of bath gas (up to 805 Torr total pressure) or water concentrations (up to 15.4 Torr partial pressure of water) demonstrating that the absorption from the water continuum at this wavelength must be less than (9.2 ± 0.2) · 1027 cm2 molecule1. This number represents the sum of extinctions due to Rayleigh scattering and water absorption at this wavelength since CRDS measures the total extinction. Attempts were made to measure and distinguish the self-broadened continuum from the foreign-broadened contributions. A large amount of water was added to the flow at low pressures of N2. Extinctions were measured at a constant total pressure of approximately 40 Torr, which would lower the foreign-broadened part by more than a factor of 20 compared to the measurements at 800 Torr. The partial pressure of water vapor varied up to 14 Torr. The detection sensitivity at this wavelength was 3.1 · 109 cm1. The difference in extinction with and without water in the flow was not measurable, indicating that the sum of the continuum absorption and the scattering cross-section for water vapor is less than 6.9 · 1027 cm2 molecule1 at 11 495 cm1 at these conditions. This would give an upper limit for the self-broadened continuum absorption for water vapor of 3.7 · 1025 cm2 molecule1 atm1, about 40 times higher than predicted at this wavelength. Rayleigh scattering has not been taken into account in the above calculations and would only matter if the difference in scattering cross section for water vapor and nitrogen are significantly different. Rayleigh scattering cross-sections for nitrogen, obtained by extrapolation from measured scattering cross-sections between
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15 385 and 17 860 cm1 (560 and 650 nm) by Naus and Ubachs [24], is 7.2 · 1028 cm2 molecule1 at this wavelength. If the water vapor scattering cross-section is negligibly small, the upper limit for the continuum absorption would be 10% higher than the values stated above. However, the Rayleigh scattering cross-section of water is likely close to that of nitrogen since the index of refraction for nitrogen and water vapor are very similar; therefore scattering from water and nitrogen should cancel out in our measurements. We were not able to calculate the Rayleigh scattering cross-section for water vapor since the depolarization factors [25] needed for water are, to the best of our knowledge, not available in the literature.
4. Conclusion Our upper limit for the water vapor continuum absorption at 11 500 cm1 is measured to be (9.2 ± 0.2) · 1027 cm2 molecule1 and is consistent with the 30–40 times lower values reported by Ma and Tipping [1] and Clough et al. [26] at the total pressures of nitrogen and water concentrations used here. Our value is an upper limit since it is derived from our detection sensitivity and also does not take into account the Rayleigh scattering from water. The effect of absorption in this spectral ‘‘window,’’ between vibrational overtone bands of water, on the total EarthsÕ radiation field can then be estimated. If we take a column of precipitable water of 45 mm, our measured upper limit for the continuum will give a optical depth of 1.4 · 103. The spectral window between the water vapor absorption bands, where the continuum has a minimum, is about 20 nm wide and this together with a spectral radiance of 0.8 W m2 nm1 gives a total estimated absorption of less than 0.023 W m2. This simple calculation is sufficient since such a small contribution will not add significantly to the modeled atmospheric absorption in this wavelength region. The linestrengths were observed to be independent of pressure, indicating that the Lorentzian lineshape (impact theory) provides an adequate description of the lineshape in the near-wing region. Our measurements provide cross-sections in the near-IR for atmospheric calculations. Although the continuum in the transmission window between bands appears to be unlikely to play a role in the atmospheric radiation budget, the absorption closer to or within the bands may still be significant. It is difficult to draw any conclusion from these measurements regarding the continuum underlying the absorption peaks since it is only of the order of 1% relative to the total linestrength, well within the measurement precision. However, the invariance of the integrated line strengths with pressure, as determined from the Lorentzian fitting procedure, to a precision of 2% over the range
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500–800 Torr of N2 (see Table 1 and Fig. 4) implies that any foreign-induced continuum absorption above this 2% must arise from an additional, collisionally induced absorption (as suggested by Mlawer et al. [3]) rather than from intensity borrowing from the total linestrength. The latter effect, if it were strong, would appear as a reduction in the linestrengths with pressure. Finally, our measured air-broadening coefficients agree with values from databases although the broad laser bandwidth in this experiment does not allow for a detailed study of linewidths. Our measurements support the larger cross-sections of ESA-WVR.
Acknowledgments This work was funded by NOAAÕs climate and global change research program.
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