Transcript QUANTITATIVE MEASUREMENT OF INTEGRATED BAND

QUANTITATIVE MEASUREMENT OF INTEGRATED BAND INTENSITIES OF
BENZENE (C6H6) VAPOR IN THE MID-INFRARED AT 278, 298 AND 323 K
Curtis P. Rinsland
NASA Langley Research Center
Mail Stop 401A
Hampton, VA 23681-2199
U.S.A.
V. Malathy Devi, Department of Physics, The College of William and Mary,
Box 8795, Williamsburg, VA 23187-8795, U.S.A.
Thomas A. Blake, Robert L. Sams and Steven W. Sharpe
Pacific Northwest National Laboratory, P.O. Box 999, Mail Stop K8-88,
Richland, WA 99352, U.S.A.
Linda S. Chiou, Science Systems and Applications, Inc., 1 Enterprise
Parkway, Suite 200, Hampton, VA 23666 U.S.A.
Atmospheric Importance of Benzene (C6H6)
• Benzene is an aromatic hydrocarbon produced in the Earth’s
atmosphere and is found in air due to emissions from the burning of
coal and oil and also from gas stations, and from motor vehicle
exhaust
• It is used in the manufacture of plastics, detergents, pesticides, and
other chemicals and is a carcinogen with exposures that have led to
the development of and death by leukemia in humans
occupationally exposed
• The U.S. environmental protection agency (EPA) has classified it as
a group A human carcinogen
• Few Earth atmosphere remote sensing measurements have been
reported, likely due to its short atmospheric lifetime. It is destroyed
in the Earth’s atmosphere primarily by reaction with OH radicals with
an important influence on air quality and ozone production at
elevated levels
Astronomical Importance of Benzene
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The high abundances of N2 and CH4 in the atmosphere of Titan, Saturn’s largest moon, lead
to high abundances of nitrogen and carbon compounds, and its atmosphere and smog-like
haze are of particular interest because of its similarity to the atmosphere that may have
existed on Earth before life began
A tentative detection of benzene in Titan’s atmosphere was reported based on an average
over all latitudes of Infrared Space Observatory (ISO) measurements
The results were based on a prediction of the 674-cm-1 4 band Q branch assuming
spectroscopic parameters of Dang-Nhu et al. [J Mol Spectrosc. 134, 237-239, 1989].
Those results were based on tunable diode laser spectrometer measurements of 30
absolute line intensities measured at room temperature in the P branch between 657.5 and
664.5 cm-1 with an absorption path length of 41 m
A firm C6H6 detection around 70°N latitude of the 674-cm-1 4 band Q branch [26] was
reported in Titan’s stratosphere from CIRS (Composite InfraRed Spectrometer) [Coustenis
et al. Icarus 189, 35-62, 2007] Fourier spectrometer limb emission spectra recorded during
Cassini spacecraft fly-bys between July 2004 to January 2006 assuming the spectroscopic
parameters of Dang-Nhu et al. [1989]
The detection of C6H6 in Titan’s stratosphere is consistent with known chemical reactions
and model predictions and may serve as a precursor to more complex hydrocarbons, such
as amino acids
Measurements of the ν4 C6H6 band have also been reported from ISO upper atmospheric
measurements of Jupiter and disk average spectra of Saturn
Additionally, the 4 band Q branch of benzene has been measured in a proto-planetary
nebula, and benzene is likely to survive in dense parts of envelopes of carbon-rich evolved
stars surrounding interstellar molecular clouds in regions with attenuation of ultraviolet
photons
Laboratory Measurements
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Integrated band intensities have been measured at temperatures of 278, 298,
and 323 K from laboratory spectra covering 600-6500 cm-1
The spectra were recorded at the Pacific Northwest National Laboratory (PNNL)
in Richland, Washington, U.S.A.
The absorption spectra of benzene vapor were recorded with a Bruker-66V
Fourier transform spectrometer
The instrument resolution was set to 0.112 cm-1 (instrument resolution =
0.9/maximum optical path difference)
The pressure of each benzene vapor sample was measured using high
precision capacitance manometers and a minimum of nine sample pressures
were recorded at each temperature
Samples were introduced into a temperature-stabilized static cell (19.94(1) cm
pathlength) that was hard-mounted into the spectrometer
Two-hundred fifty-six interferograms were averaged for each sample spectrum
Sample pressures ranged from approximately 0.1 to 22 torr
A composite spectrum was calculated for each cell temperature from the
individual absorbance spectra recorded at that temperature
For the 5°C spectra the average type-A uncertainty is 0.40%, for the 25 °C
spectra the average type-A uncertainty is 0.38%, and for the 50°C spectra the
average type-A uncertainty is 0.54%
Infrared Spectrum of Benzene
• Although C6H6 has twenty fundamental modes covering
410 to 3063 cm-1, only four fundamentals are infrared
active because of the molecule’s high degree of
symmetry
– 4 parallel band centered at 674 cm-1
– three perpendicular bands ν12 at 3048 cm-1, 13 at 1484 cm-1,
and 14 at 1038 cm-1
– The molecule has a large number of infrared active combination,
difference, and hot bands throughout the mid-infrared
– The 4 is the most intense infrared band and the one that has
been used for infrared remote sensing of the atmospheres of
Titan, Jupiter, and in the interstellar medium and we focus on
that region here
Overview of Composite Spectra
Composite Spectrum of the 4 Band
17–20 Difference Band
14 Band
Integrated Band Intensities
• A table of measured integrated band intensities
at the 3 measurement temperatures for bands
between 615 and 6080 cm-1 with the
identification of the primary bands in each region
is reported (Herzberg notation)
• Corresponding integration limits (cm-1) and
integrated band intensity in cm molecule-1  10-19
and cm-2 atm-1 units are provided
• Measurements for each region have been
compared with previously reported results
Summary and Prospects for Improvements
• Temperatures in Titan’s atmosphere range from
170 K in the high stratosphere and 70 K at the
tropopause, much colder than the lowest
temperature in our experiment
• Based on the benzene vapor pressure curve, we
estimate it may be feasible to measure benzene
vapor in the PNNL cell cooled to ~210 K
• The complexity and high density of lines in the Q
branch region and the need for partition function
calculations covering the same temperature
range will make it difficult to create a line list for
line-by-line analysis
PNNL C6H6 composite spectra