Chemistry: analytical and immunological testing – Nitrogen containing – Oxides of nitrogen
Reexamination Certificate
2000-06-26
2002-02-12
Snay, Jeffrey (Department: 1743)
Chemistry: analytical and immunological testing
Nitrogen containing
Oxides of nitrogen
C436S172000, C436S905000, C422S052000, C422S093000
Reexamination Certificate
active
06346419
ABSTRACT:
The invention described herein may be manufactured, used, and licensed by the U.S. Government for governmental purposes without the payment of any royalties thereon.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a system and a method for measuring a quantity of NO
2
in a gas sample. The invention relates more specifically to a system and a method which employ ultraviolet light to effect the photolytic dissociation of NO
2
to NO.
2. Description of Related Art
Improved understanding of human-induced and natural atmospheric chemistry requires a sensitive and specific measurement of NO
2
, a molecule which is a key species in atmospheric ozone formation and loss processes. An ideal measurement of NO
2
would be inexpensive and simple to operate, while providing quality data at high time resolution.
Most conventional commercially available instruments used for measuring NO
2
in the atmosphere employ hot metal catalysts for NO
2
conversion. These conventional devices, however, are not specific for NO
2
.
For example, one commercially available converter design is based on the reduction of NO
2
to NO on a heated substrate (i.e., thermal decomposition), typically molybdenum oxide or, less of ten, ferrous sulfate. These surface-based converters are not specific for NO
2
, as they also efficiently reduce other atmospheric nitrogen-containing compounds to a detectable form. (Fehsenfeld, F. C., et al., Intercomparison of NO
2
measurement techniques, Journal of Geophysical Research, 95, 3579-3597, 1990; Fehsenfeld, F. C., et al., Ground-based intercomparison of nitric acid measurement techniques, Journal of Geophysical Research, 103, 3343-3353, 1998.) Use of these converters can result in a gross overestimate of ambient NO
2
.
Another technique, the photolytic dissociation of NO
2
with UV light, followed by chemiluminescence detection of the product NO, has been employed for ambient measurements of NO
2
for over two decades. (Kley, D., et al., Chemiluminescence detector for NO and NO
2
, Atmospheric Technology, 12, 63-69, 1980.) This broadband photolysis technique has provided field measurement data used to evaluate and improve the current understanding of tropospheric and stratospheric ozone chemistry, radiative transfer, and sources and fate of reactive nitrogen compounds. The photolysis-chemiluminescence (P-CL) technique has been compared to other NO
2
measurement techniques on the ground (Mihelcic, D., et al., An improved method of measuring tropospheric NO
2
and RO
2
by matrix isolation and electron spin resonance,
Journal of Atmospheric Chemistry
, 3, 341-361, 1985.; Fehsenfeld et al., 1990) and aboard aircraft (Del Negro, L. A., et al., Comparison of modeled and observed values of NO
2
and J
NO2
during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) mission,
Journal of Geophysical Research
, 104, 26, 687-26, 703, 1999), and been shown to provide useful data over a wide range of concentrations, ambient environments, and integration times.
NO
2
is photodissociated at ultraviolet (UV) wavelengths below about 420 nm in a first-order process,
NO
2
+h&ngr;→NO+O (1)
with the rate constant for photolysis given by j (units of s
−1
), which is the wavelength-integrated product of the photon flux (photons cm
−2
s
−1
), the weakly temperature-dependent NO
2
absorption cross-section (cm
2
molecule
−1
), and the quantum yield for photodissociation (molecules photon
−1
)(DeMore, W. B., et al., Chemical Kinetics and Photochemical Data for use in Stratospheric Modeling, NASA Jet Propulsion Laboratory, Pasadena, Calif., 1997). In air, the O atom formed in (1) reacts rapidly with molecular oxygen to form O
3
:
O+O
2
→O
3
(2)
which can then react with NO to re-form NO
2
:
NO+O
3
→NO
2
+O
2
(3)
with the second-order rate constant for (3) given by k (cm
3
molecule-'s
−1
). During the daytime in the atmosphere, a photostationary state (characterized by zero net concentration change occurring over time) is established via these coupled reactions (Leighton, P. A.,
Photochemistry of Air Pollution
, Academic Press, New York, 1961). Under daytime conditions, a new photostationary state will be established within 1-2 minutes of a perturbation to j or to the concentrations of the chemical species listed above.
Significant changes to concentrations of these coupled species can therefore occur during measurement, a result of perturbing the j value when ambient air is sampled into an instrument (Butcher, S. S., et al., Effect of inlet residence time on analysis of atmospheric nitrogen oxides and ozone,
Analytical Chemistry
, 43, 1890-1892, 1971; Bollinger, M. J., Chemiluminescent measurements of the oxides of nitrogen in the clean troposphere and atmospheric chemistry implications, Doctoral thesis, University of Colorado, Boulder, Boulder, 1982; Ridley, B. A., et al., NO and NO
2
in the troposphere: technique and measurements in regions of a folded tropopause,
Journal of Geophysical Research
, 93, 15, 813-15, 830, 1988; Parrish, D. D., et al., Systematic variations in the concentration of NOx (NO plus NO
2
) at Niwot Ridge, Colorado,
Journal of Geophysical Research
, 95, 1817-1836, 1990). This occurs despite the minimal surface loss on most materials exhibited by these species. If total instrument sample residence times, from inlet tip to detector, are greater than a second or so, non-negligible bias in the derived concentrations of NO, NO
2
, and O
3
can result from reactions (1) through (3) occurring during sampling.
The presence of other ambient oxidants (e.g., HO
2
or RO
2
species), or the occurrence of surface-induced oxidation of NO (Ridley et al., 1988), act to increase this bias. Data reduction procedures have been developed to account for reactions (1) through (3) during sampling and are relevant to all NO, NO
2
, and O
3
measurements except open-path designs [Kley et al., 1980; Bollinger, 1982; Ridley et al., 1988; Parrish et al., 1990). These procedures were developed assuming pseudo-first-order conditions, i.e., that ozone is in large excess relative to NO and NO
2
, and that peroxy radical concentrations are negligibly small. These assumptions do not necessarily apply in many urban areas and in power plant plumes, as indicated in the NO
2
Data Reduction section, below.
Reaction (1) is exploited in the P-CL measurement to photodissociate NO
2
to NO, and the resulting product NO is measured as an increase in chemiluminescence signal above that from ambient NO (Kley et al., 1980). Ambient NO
2
concentrations are derived from the difference between two signals, both of which can be large and vary quite rapidly under changing atmospheric conditions. Efficient conversion of NO
2
to NO serves to maximize that difference and improve instrumental sensitivity for NO
2
.
In sampled ambient air, the effective conversion fraction (CF) of NO
2
is given by (Bollinger, 1982),
CF=[j&tgr;/(j&tgr;+k[Ox]&tgr;)]*[1−exp(−j&tgr;−k[Ox]&tgr;)] (4)
where j is the wavelength-integrated product of the NO
2
absorption cross-section, the light source flux, and the quantum yield for photodissociation; &tgr; is the sample residence time in the photolysis cell (Kley et al., 1980). The light source flux, and thus j, is determined by the choice of lamp, reflector and filter optics, and cell geometry. Here k[Ox] denotes the rate coefficient and concentration of any oxidant that reacts with NO to produce NO
2
in the cell.
Examination of (4) shows that increasing j without increasing r is the most effective way of maximizing instrumental sensitivity to NO
2
. This is illustrated graphically in
FIG. 3
, which shows CF calculated from (4) at 298 K as a function of cell residence time and ambient ozone concentration for j values ranging over a factor of nine. Higher j values confer the additional benefit of decreasing CF dependen
Ryerson Thomas B.
Williams Eric J.
Snay Jeffrey
Stevens Davis Miller & Mosher L.L.P.
The United States of America as represented by the Department of
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