Identification and measurement of atmospheric ethane (C2H6) from a 1951 infrared solar spectrum Curtis P. Rinsland and Joel S. Levine

NASA Langley Research Center, Atmospheric Sciences Division, Hampton, Virginia 23665-5225. Received 10 September 1986. Recently, there has been considerable interest in the mea­

surement and interpretation of trace gas concentration trends. For a number of gases, such as carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and several chlorofluo-rocarbons (e.g., CF2C12 and CFC13), there is overwhelming

4522 APPLIED OPTICS / Vol. 25, No. 24 / 15 December 1986

evidence for a global increase from direct tropospheric mea­surements performed at a number of sites over the last several years (see Refs. 1–3). These changes have important implications for atmospheric chemistry, such as the decline in tropospheric levels of OH (Refs. 4-6), the possible catalyt­ic destruction of stratospheric O3 (Refs. 7-9), and for global climate change.10–12 Less direct data, such as measurements of trapped air in ice cores13.14 and analysis of CH4 and CO absorption lines in 1950-51 solar spectra,15,16 indicate that the concentrations of some trace gases have been increasing over an even longer period of time.

The trend for ethane, the most abundant nonmethane hydrocarbon in the atmosphere, has yet to be defined. (The importance of nonmethane hydrocarbons in the remote tro­posphere has been discussed recently by Ehhalt and Ru­dolph17 and Ehhalt et al.18) Blake and Rowland19 note that their measurements in the north temperate zone are often higher than those reported for the same latitude band a few years earlier,17,20 but because of the large variations in C2H6 concentration with latitude and season, Blake and Row­land19 were not sure that these differences necessarily repre­sent an upward trend in C2H6 concentrations during the 1980s. To separate the relatively small, systematic long-term trend from shorter term fluctuations in concentration, it is clearly desirable to have continuous measurements at a remote site spanning several decades. Although no such data base exists for C2H6, some important information on the long-term trend for this molecule can be inferred by compar­ing C2H6 concentrations deduced from solar absorption spectra recorded several decades ago with modern measure­ments of C2H6 for the same geographical area and season.

In this Letter, we report the identification of C2H6 absorp­tion features in the 2980-cm–1 spectral region of a solar spectrum recorded from a remote high-altitude observatory in April 1951. The observed C2H6 absorptions have been analyzed to determine the total vertical column amount and corresponding average free tropospheric mixing ratio of C2H6 above the observing site. This average free tropo­spheric mixing ratio has been compared with recent mea­surements of free tropospheric C2H6 obtained near the same geographical location.

The spectral measurements analyzed in this work were reported in an infrared solar atlas.21 The data were recorded at the Jungfraujoch Scientific Station in the Swiss Alps (46.5°N, 8.0°E, 3578-m altitude) with a coelostat and Pfund-type spectrograph and detected by a Perkin-Elmer thermo­couple together with a 13-Hz chopping system and amplifier manufactured by the same firm and connected to a Leeds & Northrup type-G Speedomax recorder. A 16- × 12-cm grat­ing mled with 7500 lines/in. and an effective slit width of 0.24 cm–1 were employed in the spectral region analyzed in this study. Additional details of the instrumentation are de­scribed in the atlas21 and elsewhere.22,23

The spectral measurements from the solar atlas were digi­tized using a Houston Instruments Hipad digitizer and a Superset model PGM-2 computer. The wavenumber scale was calibrated using accurate positions of CH4 and H2O lines,24 as described by Rinsland et al.15

Figure 1 compares (a) laboratory, (b) simulated, and (c) Jungfraujoch solar spectra in the 2970-2990-cm–1 region. The laboratory spectrum was recorded at 0.06-cm–1 resolu­tion and room temperature and has been degraded to match the resolution of the Jungfraujoch solar spectrum. The evenly spaced features in the laboratory spectrum are the intense, unresolved Q branches of the v7 band of C2H6, a number of which have been identified recently in both high resolution aircraft and ground-based solar absorption spec-

tra.25 Spectrum (b) is a simulation of atmospheric absorp­tion by CH4 and H 2 0 lines for the observing conditions and resolution of the Jungfraujoch measurements. The line pa­rameters were taken from the 1982 Air Force Geophysics Laboratory (AFGL) compilation,24 except for corrections to some of the CH4 line intensities.26 (Numerous weak absorp­tion lines of atmospheric 0 3 have also been identified in the ground-based high-resolution solar spectra studied by Cof­fey et al.25; these features are poorly reproduced by the spectroscopic parameters in the 1982 AFGL compilation.24) The pressure-temperature and the CH4 and H2O mixing ratio profiles were assumed to be the values deduced for the same day, 15 Apr. 1951.15 The 1951 Jungfraujoch solar spectrum (c) shows several of the C2H6 Q branches, the best isolated of which have been marked by arrows in the figure. These features are weak in the solar spectrum with a peak absorption of ~ 5 % .

In Table I, the positions and identifications reported by Migeotte et al.27 for the three marked features in Fig. 1 are compared to values deduced for the C2H6 Q branches in this work. Our revised positions (estimated accuracy ±0.01 cm–1) are from measurements on the C2H6 laboratory spec­trum; the assignments for the Q-type subbranches are taken from the work of Pine and Lafferty.28 The comparison of high-resolution ground-based solar, simulated atmospheric,

Fig. 1. Comparison of (a) laboratory, (b) simulated, and (c) the 15 Apr. 1951 Jungfraujoch solar spectra. The vertical scaling of the three spectra is the same; the upper two have been offset vertically for clarity. The laboratory spectrum (a) was recorded at room temperature with 0.21 Torr of natural C2H6 in a 5.125-cm absorption path; the resolution has been degraded to match the resolution of the solar spectrum. Spectrum (b) is a line-by-line simulation with H2O and CH4 lines for conditions corresponding to those of spectrum (c). The solar zenith angle of spectrum (c) varies slowly with frequency; its value is ~67.8° at 2985 cm–1. The arrows indicate Q-type sub-

branches of C2H6 detectable in the solar spectrum.

and C2H6 laboratory spectra presented in Fig. 2 of Coffey et al.25 clearly shows that the PQ 3 C2H6 subbranch is the only significant atmospheric absorption feature near 2976.8 cm–1. The pQ1 subbranch at 2983.4 cm– 1 is calculated to be nearly free of interference in the Jungfraujoch spectrum; a H2O line at 2983.314 cm– 1 may also absorb weakly in atmospheric spectra recorded from lower altitude sites or in especially wet conditions at the Jungfraujoch. A weak solar line occurs as a shoulder of pQ1 near 2983.65 cm–1 . High sun spectra record­ed with a balloon-borne interferometer at a resolution simi­lar to the Jungfraujoch measurements29 do not show signifi­cant solar absorption near 2986.52 cm–1; the Ti I solar line attributed to this feature by Migeotte et al.27 is not reported in a recent listing of solar neutral iron-group identifica­tions.30

The accuracy of the solar atlas positions for sharp lines between 2.8 and 4.2 µm is estimated to be 0.1 cm– 1 (Ref. 27); considering this estimate, the weakness of the 1951 C2H6 atmospheric absorption, and inaccuracies in the positions arising from blending with neighboring features (for exam­ple, the RQ0 subbranch appears as a shoulder of a strong H2O line), there is satisfactory agreement between the solar atlas positions and the corresponding C2H6 laboratory values.

The best isolated of the three C2H6 features in the 1951 Jungfraujoch solar spectrum is the pQ1 subbranch. An equivalent width of 0.0099 ± 0.0025 cm– 1 for this feature has been estimated by overlaying spectra (b) and (c) of Fig. 1 and measuring the additional absorption present in the atmo­spheric spectrum (c) in the region of this C2H6 Q branch.

Assuming an integrated intensity of 3.3 ± 0.4 × 10–19

cm–1/molecule cm– 2 at 296 K measured for this C2H6 feature from laboratory spectra similar to the one shown in Fig. 1,31

linear absorption by C2H6 molecules in the solar spectrum, and a vertical C2H6 distribution (constant volume mixing ratio in the troposphere and scale height of 3.9 km in the stratosphere) based on the IR measurements of Coffey et al.,25 a total vertical column amount of 9.7 × 1015 C 2 H 6 molecules cm– 2 corresponding to an average free tropospher-ic mixing ratio of 0.9 ppbv (estimated accurate to ±30%) is estimated for C2H6 in April 1951 above the Jungfraujoch observatory. The sources of error are (1) uncertainty in the laboratory intensity measurements, (2) uncertainty in the vertical distribution of C2H6 in the atmosphere, and (3) uncertainty in the equivalent width caused by the weakness of the C2H6 absorption in the Jungfraujoch solar spectrum and by the minor blending with adjacent atmospheric and solar absorption features. Similar analyses from equivalent widths of the PQ3 and RQ0 subbranches produce values that are consistent within their uncertainties with the results from pQ1 . To our knowledge, this is the earliest measure­ment of the concentration of atmospheric C2H6. It repre­sents a bench mark for comparison with more recent data on atmospheric C2H6 obtained during the same season and in the same geographical region.

Recent aircraft measurements indicate that the vertical tropospheric distributions of C2H6 and CO are very similar.17

Table I. Comparison of Positions and Identifications of the Features Marked in Fig. 1

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In that study, simultaneous tropospheric vertical profiles of C2H6, CO, and a number of light hydrocarbons (C2-C5) were obtained over southern France (43 °N) in June 1979 and over the Eifel mountains in Germany (52°N) in October and November 1979. Both C2H6 and CO exhibited enhanced concentrations within the planetary boundary layer (up to 2 km), ~2-3 ppbv for C2H6, and ~200–300 ppbv for CO, and then showed a slow decline in mixing ratio up to ~12 km, from ~2.0 to ~1.0 ppbv for C2H6 and from ~150 to ~100 ppbv for CO. Near 6 km, the mean altitude of tropospheric molecules above the Jungfraujoch observatory, the C2H6 mixing ratio was ~1.2 ppbv, 1.3 times larger than the value inferred from the April 1951 Jungfraujoch spectrum. Addi­tional aircraft measurements of C2H6 obtained over western Europe in 1981 and 1982 show evidence for a seasonal cycle with minimum concentrations in the summer and a factor of ~ 4 higher concentrations in winter.32 The March 1981 mea­surements from this study indicate a free tropospheric C2H6 mixing ratio of ~2.0 ppbv, 2.2 times larger than the value we have inferred from the April 1951 Jungfraujoch spectrum.

The comparisons between the 1951 and 1979-82 measure­ments cited above are suggestive of a long-term increase in the free tropospheric concentration of C2H6 over western Europe. However, the uncertainty in the 1951 measurement is rather large (±30%) and only a single solar spectrum was available for analysis. Also, the number of modern free tropospheric measurements in the same geographical region is rather limited. Ca. 1970-76 solar spectra show absorption by the C2H6 v7 band Q branches33-34 and by the v9 band RQ0 subbranch at 822.32 cm"1.35 We plan to analyze C2H6 fea­tures in such spectra and in modern high-resolution solar spectra to obtain additional information on the long-term trend and the seasonal and short-term variability of atmo­spheric C2H6. Accurate C2H6 spectroscopic parameters (in­dividual line positions, intensities, and air-broadened half-widths) would be useful for the analysis of the ethane Q branches in these atmospheric spectra. If the concentration of C2H6 is increasing, it may contribute to future global warming through increased absorption by the moderately strong v9 band, which occurs in the window region at 12 µm.

The authors thank G. A. Harvey of NASA Langley and V. Malathy Devi of the College of William and Mary for their assistance in obtaining the C2H6 laboratory spectra.

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