&EFA
United States
Environmental Protection
Agency
,
Environmental Sciences Research 2
Laboratory "
Research Triangle Park NC 21T\ 1 "~f ± \~
Research and Development
EPA-600/S3-80-093 June 1981
Project Summary
Application of Fourier
Transform Spectroscopy to
Air Pollution Problems
John W. Spence
In this study, in which Fourier Trans-
form Spectroscopy has been applied
to air pollution problems, there are
two phases. In the first phase, the
results of investigations of the nature
of the information that can be
retrieved from spectra obtained with
Fourier Transform Spectroscopy are
presented. It is shown that nonlinear,
least-squares analysis of the spectra is
capable of retrieving types of informa-
tion beyond the reach of conventional
methods and with improved precision
and accuracy. In the second phase of
this work, Fourier Transform Infrared
Spectroscopy has been employed to
study quantitatively the kinetics and
reaction mechanisms of several
chemical species: peroxynitric acid,
hypochlorous acid, and dimethylni-
trosamine. Rate constants related to
the formation and decay of these
species and infrared extinction data
necessary for the quantitative anal-
yses of these compounds are deter-
mined.
This Project Summary was develop-
ed by EPA's Environmental Sciences
Research Laboratory. Research Tri-
angle Park, NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report,
ordering information at back).
Introduction
Infrared Spectroscopy is used routine-
ly in many laboratories to aid in the
identification of the composition of
samples and to estimate, quantitatively,
the amounts of the individual
components of the sample. During the
past 30 years there have been continual
improvements in the design of infrared
spectrometers, and these have led to
increased versatility in manipulating
the spectra produced. Fourier
Transform Spectrometers (FTS) are a
new class of spectrometers, and their
use has been widespread in the past
decade. They allow extended spectral
regions to be observed in short periods
of time, and the collected data can be
stored on magnetic tape for future use,
as well as displayed on paper or on oscil-
loscopes. The digital nature of the data
allows mathematical operations such
as addition, multiplication, and/or inte-
gration to be carried out on the spectra.
These allow the data to be processed
quickly and more accurately than the
spectra obtained from earlier types of
instruments where the only record con-
sisted of the analog spectrum on a paper
chart. Despite these increases in the
speed of data manipulation, the typical
methods used to analyze the spectra are
essentially those used before the advent
of sophisticated digital displays.
Phase One of the Study
A Digilab FTS was used to obtain
spectra of ground-level ambient air
samples for path lengths ranging from
10 m to 1 km with high spectral
resolution (0.1 cm"1) between 500 and
-------
3500 cm 1 Solar spectra in the same
spectral region were also obtained.
These spectra were searched for the
presence of absorption features of a
number of gases. In addition to the well-
known lines of gases such as H20, CO,
COz, N20, CH4, and O3, absorption due
to CCbFa near 1160 cm"1 was observed
in solar spectra. A mean tropospheric
abundance of about 0.34 ppb was esti-
mated. To aid in this search a computer
program was written to allow synthetic
spectra to be calculated by using the
compilation of the line parameters of
atmospheric gases prepared by the Air
Force Geophysical Laboratories. This
program was used to calculate spectra
to match the absorption observed in
experimental spectra of ground level air
samples of path lengths of approxi-
mately 10, 100, and 1000 m, and solar
spectra corresponding to both high and
low sun elevations. These synthetic
spectra showed generally good agree-
ment with the observed spectra al-
though some deficiencies in the AFGL
line compilation were observed. An
atlas of computer-generated infrared
transmission spectra of the atmosphere
for low and high humidity for a path
length of 3 km at 296 K and 1013 mb
between 700 and 3000 cm"1 was also
prepared.
Additional spectra of ground-level air
samples and solar spectra were ob-
tained and explorations of the. best
methods of analyzing these spectra to
retrieve accurate estimates of the
abundances of atmospheric gases were
continued. These methods included
spectral ratioing of the air spectra to
remove the features of each individual
absorbing gas one at a time and by using
observer judgment to estimate when
the best removal had been obtained. It
was found that this method allowed the
abundances of gases in ground-level air
samples to be estimated with precisions
of a few percent. However, the method
could not be used to analyze solar
spectra because the atmospheric ab-
sorbing path is inhomogeneous.
The next method attempted was to
remove the absorption features of the
individual gases from the air spectrum
by ratioing the air spectrum with
spectrum calculated from the AFGL line
listing. Some success was achieved in
analyzing a CO spectrum, although the
precision with which the amount of CO
in the sample could be estimated was
again of the order of a few percent. This
technique was used to retrieve the
amounts of CO,N2O, andC02ina 171 m
path of ground-level air. Theseamounts
were obtained with estimated
precisions of a few percent. Portions of a
solar spectrum analyzed in this way
allowed the CCI2F2 feature near 1160
cm"1 to be revealed clearly.
An attempt was made to increase the
precision of the abundance estimates by
converting the observer signals to
absorbances and comparing these latter
with the corresponding absorbances of
the individual gases as calculated from
the line parameters. This method of
analyzing a ground-level air spectrum
enabled the abundances of N2O and CO
to be estimated with precision of the
order of 1 %. It was found, however, that
the abundance estimates depended on
the estimated position of the back-
ground signal in the absence of the
absorber and on the spectral region
analyzed. The position of the back-
ground was based on the judgment of
the observer. Other differences could be
due to the use of incorrect line param-
eter values to calculate the synthetic
spectra.
The ground-level air spectrum was
reanalyzed by using a nonlinear, least-
squares regression method. In this
method the position of the background
and the spectral resolution are esti-
mated, in addition to the abundances of
the absorbing gases in the sample. It
was shown that the abundances esti-
mated in this manner showed a preci-
sion somewhat better than that
obtained by linear regression analysis of
the absorbances and that there was less
dependence of the absorber amounts on
the spectral region analyzed. This im-
provement was due primarily to the
removal of the observer from the
process of determining the background
and the spectral resolution of the
observed spectrum.
The problems associated with the
accurate estimation of the abundances
of absorbing atmospheric gases from
spectra of air samples have been ex-
plored in greater depth.
In these analyses, empirical models
or models based on theoretical consid-
erations are used to describe the
spectra. The values of the adjustable
parameters in these models, their
asymptotic standard deviations, and
their asymptotic correlation matrices
are estimated by curve fitting the experi-
mental spectral data.
Estimates of the "nuisance" param-
eters, such as the position of the back-
ground in the absence of the absorber
and the spectral resolution, can be
obtained in addition to other informa-
tion. Some examples of the types of
information that can be retrieved
include the following:
• The retrieval of the individual
absorbing gas amounts in a mixed
sample together with its tempera-
ture and pressure. This requires a
knowledge of the individual line
parameters or a suitable collection
of reference spectra of the absorb-
ing gases.
• The determination of the mixing
ratios of absorbing atmospheric
gases by analyzing solar spectra
collected at ground level.
• The retrieval of the individual line
positions, intensities, and widths
from spectra of groups of Voight-
or Lorentz-shaped lines.
• The simultaneous analysis of
entire rotation-vibration bands to
retrieve the upper and lower state
molecular constants, the band
intensity parameters, and the line-
broadening parameters. ,
The accuracy of the results depends
on the correctness of the models used.
The precision of the parameter
estimates depends on the quality of the
spectrum analyzed. In addition to ana-
lyzing spectra, the techniques can be
used to predict the precision of the
parameter estimates for a given
experimental design. Thus, quantitative
criteria for choosing between different
designs can be established.
The methods can be applied to other
types of experimental data. However, if
the data sets are large and many param-
eters are to be retrieved, extensive
computer time is required.
Phase Two of the Study
The photochemical experiments were
carried out using a Digilab FTS-20
(model 496 interferometer) coupled to a
photolysis cell. The large, evacuable cell
was constructed of 30.5 cm diameter
pyrex tubing, 6.3 m long (445 I volume).
It housed a modified White optical
system which had a base path of 5.3 m
and provided a choice of pathlengths
which were multiples of eight-times the
base length. Most of these experiments^^
were carried out at 170 m (32 traversalsl^B
which gave adequate absorption for the^^
compounds studied here in the ppm
-------
concentration range. One cell endplate
was equipped with KBr windows to
allow the passage of the analytical IR
beam. A liquid-helium-cooled Cu:Ge
detector was employed in most experi-
ments (450-2400 cm"1 range), while a
liquid-nitrogen-cooled ln:Sb detector
was used when an expanded range of
spectral response was desired (to A <
4000 cm"1). Potential problems created
by varying background absorption from
H2O and C02 in the air path between the
transfer optics, Nernst glower IR source,
interferometer, cell, and detector, were
substantially lowered by enclosing the
entire optical system in a plastic (acrylic)
housing which was purged continuous-
ly with nitrogen gas. Open dishes of
solid NaOH and KOH were placed in the
housing to keep C02 levels low.
The large photochemical cell was
illuminated by black-light fluorescent
lamps which surrounded it. An outer
reflective shield of aluminum encom-
passed both the lamps and the cell. The
cell lighting was designed to mimic
ground-level solar radiation, both in
distribution and intensity, within the
photochemically important region 300-
450 nm. Room air was circulated in the
> space between the lamps and the cell to
help stabilize the cell temperature.
Thermocouples and a thermometer
placed inside of the cell were used to
determine the actual cell temperature
during an experiment. The temperature
of the cell could be varied from experi-
ment to experiment within a small
range (15-27°C) by regulation of the
room temperature.
Peroxynitric Acid Formation
A kinetic study was made of the time
dependence of the photolysis of dilute
mixtures of CI2, H2, NO2, and NO in syn-
thetic air. The major products of the
reaction, CINO2, HCI, HONO2, HO2NO2,
O3, N205, H2O2 with smaller amounts of
MONO, CINO, and CIONO were
identified and followed in situ using
long-path, infrared FTS. The possible
mechanisms for HO2NO2 formation and
decay in this system were considered. It
was concluded that H02NO2 levels are
controlled largely by the reactions: HO2
+ NO2 (+M) - H02N02 (+M); 2H02 -
H202 + 02; H02N02(+Wall) - products.
From a kinetic treatment of the H02NO2
data for the dark decay, estimates of the
equilibrium constant for the system,
| H02N02i=;H02 + N02, were derived in
experiments at several temperatures
(28.4 - 20.3°C). A kinetic scheme con-
sisting of 65 elementary reactions is
proposed to rationalize the rates of the
many different products of the irradi-
ated CI2, NO2, NO, H2, air mixtures.
Computer simulations incorporating the
present kinetic information on HO2N02
may rise to the ppb level in the sunlight-
irradiated, NOx-RH-polluted tropo-
sphere (25°C).
Study of HOCI and Its
Absolute Integrated Infrared
Band Intensities
The long-path, infrared FTS was
employed in the kinetic study of the
products of the photolysis of dilute CI2,
O3, H2 mixtures in excess O2 and N2 in
experiments at 25 ± 3°C and 700 Torr
total pressure. The initial rates of forma-
tion of the products of the reaction, HCI,
HOCI, and H202, and 03 loss were
studied as a function of the ratio of
reactants, [H2]/[O3], over the range of
0.10 x 103 to 6.8 x 103. The match of
these experimental data with computer-
generated rate data employing a rather
complete reaction set was used to test
the mechanism and refine some of the
rate constant estimates. From these
data, the absolute extinction coeffi-
cients for the three fundamental bands
of HOCI were derived. The integrated
band intensities for the 1/1, i/2, i/3 ab-
sorption regions were estimated to be
2.3x102, 3.0 x 1O2, and 4.3 x 101 cm"2
atm"1, respectively.
Formation of Nitrosamines
and Nitramines
The kinetics of the reactions of the
(CH3)2N radical with NO, N02, and 02
were studied using FTIR spectroscopy.
The photolysis of (CH3)2N-N=O and
(CH3)2N-N=N-N(CH3)2 in dilute mixtures
proved to be excellent homogeneous
sources of the CH3N radical. The prod-
ucts of the (CH3)2N radical reactions in
NO, NOa, 02 mixtures were identified as
primarily (CH3)2N-N=O, (CH3)2N-N02,
CH2=N-CH3 with small amounts of
HONO2, H2CO and traces of HCN. In
these studies a large reactivity differ-
ence for (CH3)2N radicals with O2 and
NO or NO2 was observed; the rate
constants for (CH3)2N with NO and N02
are(6.8±0.3)x105and(2.6±0.2)x106
times that for the reaction of (CH3)2N
with O2. This suggests that (CH3)2N-
N=O and (CH3)2N-N02 may form in the
atmosphere at significant rates even
when the NO and NO2 concentrations
are in the 10 pphm range. The dimethyl-
nitramine, CH3N-NO2, is not photo-
chemically active in the lower
atmosphere and would therefore likely
be a major product of dimethylamine
photooxidation in an NOx-polluted
troposphere.
Recommendations
In the first phase of this work we have
developed a new approach to the
retrieval of information from infrared
spectra of gases and it has been applied
to the analysis of many types of spectra.
It has been demonstrated that more
information, with higher precision can
be retrieved than has previously been
recognized The applications of
nonlinear, least-squares techniques
investigated under this grant have been
exploratory in nature, and it has not
been possible to explore all the possible
uses which have occurred to us.
Provided suitable computer programs
are available large bodies of data can be
analyzed rapidly and accurately and this
method should prove useful to many
other workers provided descriptions of
the programs and their use are made
available.
In the second phase of this work we
have demonstrated the quantitative
nature of the results which may be
obtained using long-path, FTS in the
study of complex chemical reactions of
importance in atmospheric transforma-
tions. It is clear that important kinetic
and mechanistic information can be
obtained for many of the metastable
compounds which are expected to be
generated in the polluted atmosphere. It
is recommended that the Environ-
mental Protection Agency utilize the
Fourier transform infrared spectro-
scopic methods for the unambiguous
detection and analysis of metastable
complex species which are deemed to
be of significance in the control of air
pollution.
t US GOVERNMENT (WINTINS OFFICE: 1M1-757-012/7138
-------
This Project Summary was authored by John W. Spence who was also the EPA
Project Officer (see below).
The complete report, entitled "Applications of Fourier Transform Spectroscopy
to Air Pollution Problems," was authored by John H. Shaw and Jack G.
Calvert who are with the Ohio State University Research Foundation,
Columbus, OH 43212.
This report (Order No. PB 81-120 792; Cost: $12.50, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park. NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
PS 0000329
------- |