U.S. patent application number 10/895240 was filed with the patent office on 2005-03-17 for systems and methods for measuring nitrate levels.
Invention is credited to Allen, George A., Ding, Yiming, Koutrakis, Petros.
Application Number | 20050059157 10/895240 |
Document ID | / |
Family ID | 26855462 |
Filed Date | 2005-03-17 |
United States Patent
Application |
20050059157 |
Kind Code |
A1 |
Allen, George A. ; et
al. |
March 17, 2005 |
Systems and methods for measuring nitrate levels
Abstract
The systems and methods described herein relate to the
measurement of nitrate levels in a sample of gas, for example, air,
exhaust, or other sources of gas. Moreover, the systems and methods
described herein are capable of operating using short sample
collection periods, permitting rapid data collection and finely
time-resolved nitrate monitoring over a span of time. Additionally,
ambient nitrate can effectively be distinguished from other
airborne particles, such as sulfate and carbon.
Inventors: |
Allen, George A.;
(Swampscott, MA) ; Koutrakis, Petros; (Weston,
MA) ; Ding, Yiming; (Malden, MA) |
Correspondence
Address: |
FOLEY HOAG, LLP
PATENT GROUP, WORLD TRADE CENTER WEST
155 SEAPORT BLVD
BOSTON
MA
02110
US
|
Family ID: |
26855462 |
Appl. No.: |
10/895240 |
Filed: |
July 20, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10895240 |
Jul 20, 2004 |
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10267807 |
Oct 9, 2002 |
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6764857 |
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10267807 |
Oct 9, 2002 |
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09687190 |
Oct 12, 2000 |
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6503758 |
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60158861 |
Oct 12, 1999 |
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Current U.S.
Class: |
436/110 ;
422/83 |
Current CPC
Class: |
Y10T 436/173076
20150115; G01N 31/005 20130101; G01N 33/0037 20130101; G01N
2001/2223 20130101; Y02A 50/20 20180101; G01N 1/2205 20130101; Y10T
436/17 20150115; Y10T 436/176921 20150115; Y02A 50/245 20180101;
G01N 1/40 20130101 |
Class at
Publication: |
436/110 ;
422/083 |
International
Class: |
G01N 033/00 |
Claims
We claim:
1. A system for measuring nitrate levels, comprising a sample inlet
for receiving a sample of gas, a collection body coupled to said
sample inlet, a filter mounted within said body to collect
particles from said sample of gas, a heater coupled to the body to
heat the body, a gas inlet coupled to said body to provide a flow
of gas through said body, and a detector coupled to said body to
measure an NO.sub.x concentration.
2. The system of claim 1, further comprising a source of gas
coupled to said gas inlet.
3. The system of claim 2, wherein said gas is nitrogen.
4. The system of claim 2, wherein said gas is substantially free of
oxygen.
5. The system of claim 1, further comprising a catalyst, coupled to
said body and to said detector, capable of reducing NO.sub.2 to
NO.
6. The system of claim 5, wherein said catalyst comprises
molybdenum.
7. The system of claim 1, wherein said detector includes a light
sensor.
8. The system of claim 7, wherein said detector further includes an
ozone generator.
9. The system of claim 1, wherein said detector includes an
infrared sensor.
10. The system of claim 1, wherein said detector includes a
material which reversibly binds NO.
11. The system of claim 1, wherein said filter comprises quartz
fibers.
12. The system of claim 1, further comprising an extractor coupled
to said sample inlet and to said collection body to substantially
remove NO.sub.2 from the gas sample.
13. The system of claim 12, wherein the extractor comprises a
hydroxyl-bearing solvent and a base.
14. The system of claim 13, wherein the hydroxyl-bearing solvent is
glycerol and the base is an organic base.
15. The system of claim 14, wherein the organic base is an
amine.
16. The system of claim 12, further comprising a selection
platform, situated between said sample inlet and said extractor, to
substantially remove particles larger than about 2.5 microns.
17. The system of claim 16, wherein said selection platform is a
filter.
18. The system of claim 16, wherein said selection platform is an
inertial impactor.
19. The system of claim 1, further comprising a cooling system to
cool the collection body.
20. A method for measuring a level of nitrate, comprising receiving
a gas sample, collecting nitrate particles from the gas sample on a
filter, passing a stream of gas substantially free of oxygen over
the collected particles, volatilizing the collected particles by
heating to generate NO.sub.x, and measuring a level of
NO.sub.x.
21. The method of claim 20, further comprising substantially
removing NO.sub.2 prior to collecting nitrate particles.
22. The method of claim 21, wherein substantially removing NO.sub.2
includes passing the received sample over a hydroxyl-bearing
solvent and a base.
23. The method of claim 21, wherein substantially removing NO.sub.2
includes passing the received sample over a hydroxyl-bearing
solvent and an organic base.
24. The method of claim 21, wherein substantially removing NO.sub.2
includes passing the received sample over a mixture comprising
glycerol and triethanolamine.
25. The method of claim 20, further comprising removing particles
larger than about 2.5 microns from the received gas sample.
26. The method of claim 25, wherein substantially removing
particles larger than about 2.5 microns includes passing the
received sample through an inertial impactor.
27. The method of claim 25, wherein substantially removing
particles larger than about 2.5 microns includes passing the
received sample through a filter.
28. The method of claim 20, wherein passing a stream of gas
includes passing a stream of nitrogen over the collected
particles.
29. The method of claim 20, further comprising reducing generated
NO.sub.2 to NO using a metal catalyst.
30. The method of claim 29, wherein reducing generated NO.sub.2
includes contacting said NO.sub.2 with a molybdenum catalyst.
31. The method of claim 20, wherein measuring a level of NO.sub.x
includes reacting NO with ozone.
32. The method of claim 20, wherein measuring a level of NO.sub.x
includes detecting infrared absorption.
33. The method of claim 20, wherein measuring a level of NO.sub.x
includes adsorbing NO.sub.x on a conductive material.
34. The method of claim 20, wherein collecting nitrate particles
comprises collecting nitrate particles on a filter comprising
quartz fibers.
35. The method of claim 20, wherein volatilizing the collected
particles includes rapidly heating the collected particles to at
least 300.degree. C.
36. A system for measuring nitrate levels, comprising a sample
inlet to receive a sample of gas, an extractor coupled to said
sample inlet to substantially remove NO.sub.2 from the gas sample,
a collection body coupled to said sample inlet, an inertial
impactor mounted within said body to collect particles from the gas
sample, a current source coupled to the inertial impactor to heat
the inertial impactor and generate NO.sub.x, and a detector coupled
to said catalyst to measure an NO.sub.x concentration.
37. A method for measuring a level of nitrate, comprising receiving
a gas sample, substantially removing NO.sub.2 from the gas sample,
collecting nitrate particles from the gas sample with an inertial
impactor, passing a stream of gas substantially free of oxygen over
the collected particles, volatilizing the collected particles by
heating to generate NO.sub.x, and measuring a level of NO,
generated by the heated particles.
38. A system for measuring nitrate levels, comprising means for
receiving a sample of gas, support means coupled to said means for
receiving, means for collecting particles coupled to said support
means, means, coupled to said support means, for heating the
support means to generate NO.sub.x, means, coupled to said support
means, for flowing a stream of gas through said support means, and
means for measuring an NO.sub.x concentration coupled to said
support means.
39. The system of claim 38, further comprising means for
substantially removing NO.sub.2 from the sample of gas, coupled to
said means for receiving and said support means.
40. The system of claim 38, further comprising means for reducing
NO.sub.2 to NO, coupled to said support means and to said means for
measuring.
41. A method of manufacturing a nitrate measurement apparatus,
comprising providing a sample inlet for receiving a sample of gas,
coupling a collection body to said sample inlet, disposing a filter
within said body, coupling a heater to the body, coupling a gas
inlet to said body, and coupling an NO.sub.x detector to said
body.
42. The method of claim 41, further comprising disposing an
NO.sub.2 extractor between said sample inlet and said collection
body.
43. The method of claim 41, further comprising disposing a catalyst
capable of reducing NO.sub.2 to NO between said collection body and
said NO.sub.x detector.
Description
[0001] This application is based on U.S. Provisional Application
No. 60/158861, filed Oct. 12, 1999, the specification of which is
hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] Particulates are tiny clumps of soot, dirt, and various
chemicals that have been linked to a wide variety of health
problems--asthma, and higher rates of disease affecting the
cardiovascular system or lungs. Since 1987, EPA standards have
governed all particulates under 10 micrometers in diameter. This
category of particulate matter is called PM10. Recently, however,
studies have suggested that the most dangerous particles are
actually the smaller ones, which penetrate deeper in the lungs'
aereoles. Thus, new regulations will build in a separate standard
for particles less than 2.5 micrometers in diameter--PM2.5.
[0003] While PM10 contains a lot of wind-blown soil, PM2.5 is
derived mainly from burning fossil fuels. PM2.5 typically contains
a mixture of elemental carbon, organic carbon, sulfate and nitrate
particles, and acid droplets. It is unlikely that all components of
PM2.5 contribute equally to the observed health effects, yet the
present lack of sufficient data quantifying the individual
components prevents the EPA from separately regulating these
components. Because regulating PM2.5 collectively is not a
cost-effective solution, the agency is under great scientific,
industrial, and political pressure to specifically identify sources
of the observed particle health-effects. Thus, interest in
measuring the individual components of PM2.5 has increased
dramatically over the last few years.
[0004] A number of methods are known for measuring atmospheric
nitrate levels. Koutrakis et al., Environ. Sci. Technol. 22:1463,
1988 disclose an integrated sampling method (Harvard/EPA Annular
Denuder System (HEADS)) which is designed to measure various
atmospheric components including particulate nitrate. The method
provides a non-quantitative conversion of particulate nitrate to
nitric acid vapor by collection of atmospheric fine particles on a
Teflon filter, with a sodium carbonate-coated filter downstream to
collect nitric acid vapor produced by volatization of ammonium
nitrate and by the reaction of ammonium nitrate with acidic sulfate
particles.
[0005] Wendt et al., "Continuous monitoring of atmospheric nitric
oxide and nitrogen dioxide by chemiluminescence" in Methods of Air
Sampling and Analysis, editor, J. P. Lodge Jr., Lewis Publishers,
Chelsea, Mich., pp 415-421 (1989), disclose a continuous
chemiluminescent NO.sub.x detection method. Yamamoto et al., Anal.
Chem., 1994, 66, 362-367, describe a nitrate analysis method
relying on chemiluminescent NO.sub.x detection. NO.sub.x generally
refers to NO.sub.2 and NO taken together.
[0006] Brauer et al., Environ. Sci. Technol. 24:1521, 1990 disclose
a method for the continuous measurement of nitrous acid and nitric
acid vapors which does not distinguish between the two species.
Klockow et al., Atmospheric Environment, 1989, 23, 1131-1138,
disclose thermodenuder systems for the discontinuous measurement of
nitric acid vapor and ammonium nitrate. Buhr et al., Atmospheric
Environment, 1995, 29, 2609-2624, teach a denuder for sampling
nitric acid, nitrate, and sulfate. Wolfson et al., U.S. Pat. No.
5,854,077, present a continuous differential nitrate measurement
method.
[0007] Many of these and other existing methods for nitrate
measurement require labor-intensive, manual collection of 24-hour
integrated samples and laboratory analysis of the collected
components. Not only are such samples expensive to collect, but the
lengthy collection period prevents the detection of cycles and
patterns which occur over the course of a day. Convenient
techniques which offer improved temporal resolution and are capable
of unifying the collection and analysis processes are badly needed
now to reveal these daily patterns, both for epidemiological
research and for regulatory monitoring.
SUMMARY OF THE INVENTION
[0008] The systems and methods described herein relate to the
measurement of nitrate in gas samples by collection and analyzing
samples by a technique which permits a short cycling time. Thus, in
one aspect, the invention provides a system for measuring nitrate
levels having a sample inlet for receiving a sample of gas, a
collection body coupled to said sample inlet, a filter mounted
within said body to collect particles from said sample of gas, a
heater coupled to the body to heat the body, a gas inlet coupled to
said body to provide a flow of gas through said body, and a
detector coupled to said body to measure an NO.sub.x
concentration.
[0009] In a certain embodiment, the system further comprises a
source of gas coupled to said gas inlet. The gas may be nitrogen or
another gas which is substantially free of oxygen.
[0010] In another embodiment, the system also includes a catalyst,
coupled to said body and to said detector, capable of reducing
NO.sub.2 to NO. The catalyst may comprise molybdenum, carbon, or
ferrous sulfate.
[0011] In certain embodiments, the detector included in the system
has a light sensor, and may further include an ozone generator, for
example, for the detection of the chemiluminescent oxidation of NO.
In another embodiment, the detector includes an infrared sensor. In
yet another embodiment, the detector includes a material which
reversibly binds NO.
[0012] In one embodiment, the filter comprises quartz fibers.
[0013] In yet another embodiment, the system includes an extractor
coupled to the sample inlet and to the collection body to
substantially remove NO.sub.2 from the gas sample. The extractor
may comprise a hydroxyl-bearing solvent and a base, e.g., glycerol
and an organic base, e.g., an amine, such as triethanolamine.
[0014] In yet another embodiment, the system also includes a
selection platform, situated between the sample inlet and the
extractor, to substantially remove particles larger than about 2.5
microns. The selection platform may be a filter, an inertial
impactor, or any other suitable device.
[0015] In one embodiment, the system further includes a cooling
system to cool the collection body.
[0016] In yet another aspect, the invention relates to a method for
measuring a level of nitrate by receiving a gas sample, collecting
nitrate particles from the gas sample on a filter, passing a stream
of gas substantially free of oxygen over the collected particles,
volatilizing the collected particles by heating to generate
NO.sub.x, and measuring a level of NO.sub.x.
[0017] In one embodiment, the method further includes substantially
removing NO.sub.2 prior to collecting nitrate particles, e.g., by
passing the received sample over a hydroxyl-bearing solvent and a
base, e.g., an organic base such as triethanolamine.
[0018] In another embodiment, the method further includes removing
particles larger than about 2.5 microns from the received gas
sample, e.g., by passing the received sample through an inertial
impactor or by passing the received sample through a filter.
[0019] In one embodiment of the method, passing a stream of gas
includes passing a stream of nitrogen over the collected
particles.
[0020] In yet another embodiment, the method further comprises
reducing generated NO.sub.2 to NO using a metal catalyst, e.g., by
contacting the NO.sub.2 with a molybdenum catalyst.
[0021] In certain embodiments, measuring a level of NO.sub.x
includes reacting NO with ozone. In yet another embodiment,
measuring a level of NO.sub.x includes detecting infrared
absorption. In certain other embodiments, measuring a level of
NO.sub.x includes adsorbing NO.sub.x on a conductive material.
[0022] In one embodiment, collecting nitrate particles comprises
collecting nitrate particles on a filter comprising quartz
fibers.
[0023] In another embodiment, volatilizing the collected particles
includes rapidly heating the collected particles to at least
300.degree. C.
[0024] In yet another aspect, the invention provides a system for
measuring nitrate levels, including a'sample inlet to receive a
sample of gas, an extractor coupled to said sample inlet to
substantially remove NO.sub.2 from the gas sample, a collection
body coupled to said sample inlet, an inertial impactor mounted
within said body to collect particles from the gas sample, a
current source coupled to the inertial impactor to heat the
inertial impactor and generate NO.sub.x, and a detector coupled to
said catalyst to measure an NO.sub.x concentration.
[0025] In yet another aspect, the invention relates to a method for
measuring a level of nitrate by receiving a gas sample,
substantially removing NO.sub.2 from the gas sample, collecting
nitrate particles from the gas sample with an inertial impactor,
passing a stream of gas substantially free of oxygen over the
collected particles, volatilizing the collected particles by
heating to generate NO.sub.x, and measuring a level of NO.sub.x
generated by the heated particles.
[0026] In yet another aspect, the invention provides a system for
measuring nitrate levels having means for receiving a sample of
gas, support means coupled to the means for receiving, means for
collecting particles coupled to the support means, means coupled to
the support means, for heating the support means to generate
NO.sub.x, means, coupled to the support means, for flowing a stream
of gas through the support means, and means for measuring an
NO.sub.x concentration coupled to the support means.
[0027] In one embodiment, such a system also includes means for
substantially removing NO.sub.2 from the sample of gas, coupled to
said means for receiving and said support means.
[0028] In another embodiment, such a system further includes means
for reducing NO.sub.2 to NO, coupled to the support means and to
the means for measuring.
[0029] In yet another aspect, the invention relates to a method of
manufacturing a nitrate measurement apparatus by providing a sample
inlet for receiving a sample of gas, coupling a collection body to
the sample inlet, disposing a filter within the body, coupling a
heater to the body, coupling a gas inlet to the body, and coupling
an NO.sub.x detector to the body.
[0030] In one embodiment, the method further comprises disposing an
NO.sub.2 extractor between said sample inlet and said collection
body.
[0031] In another embodiment, the invention further comprises
disposing a catalyst capable of reducing NO.sub.2 to NO between
said collection body and said NO.sub.x detector.
BRIEF DESCRIPTION OF THE FIGURES
[0032] The following figures depict certain illustrative
embodiments of the invention in which like reference numerals refer
to like elements. These depicted embodiments are to be understood
as illustrative of the invention and not as limiting in any
way.
[0033] FIG. 1 depicts a system for measuring nitrate levels as
described herein.
[0034] FIG. 2 illustrates the accuracy of a method for measuring
nitrate levels as described herein.
[0035] FIG. 3 shows the effect of atmospheric conditions on the
method described herein.
[0036] FIG. 4 demonstrates a method of distinguishing between
nitrate and NO.sub.2 in a sample of gas using the systems and
methods described herein.
[0037] FIGS. 5A and B present nitrate measurement results obtained
over a 72-hour period.
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
[0038] The description below pertains to several illustrative
embodiments of the invention. Although many variations of the
invention may be envisioned by one skilled in the art, such
variations and improvements are intended to fall within the compass
of this disclosure. Thus, the scope of the invention is not to be
limited in any way by the disclosure below.
[0039] The systems and methods disclosed herein are useful for
measuring nitrate levels, for example, in the atmosphere, and may
be capable of performing sample collection and analysis within
about ten minutes. Thus, variability of nitrate levels can be
determined over relatively short intervals, e.g., for use in
epidemiological studies, regulatory monitoring, or other research.
Furthermore, the system can be assembled or manufactured using
convenient, commercially available components.
[0040] An exemplary system 100 for measuring nitrate levels is
depicted in FIG. 1. The system 100 includes a sample inlet 105, an
extractor 110, a collection body 115, a filter 120, a heater 125, a
cooling system 130, a catalyst 135, a detector 140, a gas inlet
145, and a gas source 150. Other components, such as a control
system, a data acquisition and recording system, or a second
independent heater may optionally be included. Variations on the
depicted system which are capable of functioning as described
herein will be apparent to those of ordinary skill in the art and
are intended to be encompassed by this disclosure.
[0041] A sample of gas, such as a sample of air or exhaust, may be
received by the system using sample inlet 105. The sample of gas
may be forced into the system 100, for example, by passing an
exhaust stream through the system 100. Alternatively, the sample of
gas may be drawn into the system 100 by a vacuum, e.g., by
providing a vacuum beyond the detector 140, or by utilizing the
Bernoulli effect, e.g., by passing a stream of gas rapidly past the
inlet 105, e.g., using the gas inlet 145. The sample inlet 105 may
include a selection platform for removing particles larger than
about 2.5 microns, such as an inertial impactor or a filter, as is
well known in the art. The sample of gas may then pass into the
extractor 110 to remove contaminant gases. The extractor 110 may be
a denuder, such as the honeycomb denuder described in U.S. Pat. No.
5,302,191 or an annular denuder, another diffusion denuder, or any
other system known in the art for removing gases from a sample of
gas. For example, the extractor 110 may include an acidic material,
such as citric acid or sulfiric acid, to trap basic compounds, such
as ammonia. In one embodiment, the extractor 110 is selected to
remove at least 50%, or at least 90%, or even at least 95% of the
gaseous NO.sub.2 from the sample of gas, as gaseous NO.sub.2 may
introduce error into the nitrate measurement. Such an extractor may
include a hydroxyl-bearing solvent, such as ethylene glycol,
propylene glycol, glycerol, benzyl alcohol, or another hydroxylic
solvent, and a base, including an inorganic base, such as a metal
carbonate, bicarbonate, hydroxide, or phosphate, e.g., sodium
hydroxide or potassium carbonate, and/or an organic base, such as
an amine, e.g., 1,8-bis(dimethylamino)-naphthalene,
diazabicyclooctane, diazabicyclononane, triethanolamine,
diethanolamine, N,N-dimethyl-2-hydroxymethylaniline, or another
organic base. In certain embodiments, the hydroxyl-bearing solvent
and the organic base are selected to have low vapor pressures at
atmospheric pressure, e.g., less than 50 Torr, or less than 10
Torr. Other systems for removing NO.sub.2 or other selected
contaminants are known in the art, and may be used alone or in any
combination to remove any such compounds from the sample of
gas.
[0042] The sample of gas may then pass into the collection body 115
and through filter 120. The filter 120 may then trap nitrate
particles, in addition to other particles of similar size, e.g.,
about 2.5 microns or less, while allowing gaseous compounds to pass
through. The collection body may be composed of any material
capable of withstanding sufficient heat to perform the method as
described herein, such as metal, ceramic, glass, quartz, or other
heat-resistant material. For example, the collection body may be
composed of steel, molybdenum, or an alloy comprising either
material. The filter may be composed of any suitable material,
e.g., quartz fibers, glass fibers, metal, or other material capable
of withstanding temperatures sufficient to volatilize the trapped
particles. A stream of gas substantially free of oxygen, e.g.,
including less than about 5% or less than about 1% oxygen, such as
nitrogen gas, helium, or argon, may then be passed over the trapped
particles. This procedure helps to reduce unwanted oxidation of
ammonia or other low oxidation state nitrogen-containing compounds,
such as ammonium sulfate, during heating. The gas may be introduced
using gas inlet 145 from gas source 150.
[0043] The heater 125 may then heat the filter 120 or the
collection body 115 to volatilize the trapped particles. The heater
may perform this function by any means known in the art. For
example, the heater 125 may generate heat itself, such as with a
heating element, e.g., a nichrome wire or a heat lamp, used to heat
the collection body or filter, or it may apply current to the
filter 120 or the collection body 115 to heat that element by
resistance, or it may heat the sample by any other means known in
the art. In addition to heating the collection body 115 and/or the
filter 120, the heater 125 or a second heater may heat all or a
portion of the path between the collection body 115 and the
catalyst 135. Upon volatilization, nitrate may be converted to
species such as HNO.sub.3, NO.sub.2, and NO which are carried by
the stream of gas to the catalyst 135.
[0044] In certain embodiments, for example, wherein an extractor
110 is not used to remove NO.sub.2 from the gas sample, a portion
of the gaseous NO.sub.2 in the sample of gas may be adsorbed by
material on the filter, such as soot or other particulate matter,
rather than passing through the filter. Upon heating such NO.sub.2
may be desorbed at a temperature below that at which nitrate begins
to substantially volatilize. In such embodiments, it may be
advantageous to heat the collection body 115 and filter 120
gradually in order to release this unwanted NO.sub.2 prior to
detection and measurement of the NO.sub.x species liberated by
volatilization of nitrate. NO.sub.x, as used herein, refers
generally to NO and NO.sub.2. By this method, more accurate nitrate
determinations may be measured. Rapid heating, however, may permit
more rapid cycling between collection and analysis phases.
Similarly, cooling system 130 may cool the collection body 115
and/or filter 120 by any means, such as by passing an unheated
fluid, e.g., air or water, over the exterior surface of the
apparatus, to further enable more rapid cycling between collection
and analysis phases. Thus, the speed of heating may be adjusted to
balance cycling time with measurement accuracy, depending on the
needs of a particular situation and the relative importance of
accounting for NO.sub.2 in the sample of gas.
[0045] The catalyst 135 may be any material, such as a molybdenum
or carbon converter, ferrous sulfate, or any other material capable
of reducing NO.sub.2 to NO, as is known in the art. In embodiments
where a detector 140 is used which is capable of simultaneously
detecting NO.sub.2 and NO, a catalyst 135 need not be included in
the system, and the stream of gas may flow directly from the
collection body 115 and filter 120 to the detector 140.
[0046] The detector 140 may be any component capable of detecting
the amount of NO, in the stream of gas. A number of methods are
known for detecting NO.sub.x in flowing gas streams. Perhaps the
most well known and widely used process involves instruments using
the chemiluminescent reaction of nitric oxide (NO) and ozone. The
process operates by the reaction of ozone and nitric oxide within a
reaction chamber having a transmissive window, allowing light
produced by the chemiluminescent reaction to be monitored by a
detector. Typical components using this process may be found in
U.S. Pat. Nos. 3,967,933 to Etess et al.; 4,236,895 to Stahl;
4,257,777 to Dymond; 4,315,753 to Bruckenstein et al.; 4,657,744 to
Howard; 4,765,961 to Schiff; and 4,822,564 to Howard. The use of a
chemiluminescent nitrogen oxide monitoring device in controlling a
nitrogen oxide removal unit placed on the outlet of a boiler is
shown in U.S. Pat. No. 4,188,190 to Muraki et al. Because these
systems are typically not capable of detecting NO.sub.2 in the gas
stream, a catalyst 135 may be employed in conjunction with such a
detector.
[0047] Another procedure involves the use of an infrared beam,
detector, and a comparator chamber. In U.S. Pat. No. 4,647,777 to
Meyer, a beam of infrared light is passed through a gas sample and
into a selective infrared detector. The beam is split and one
portion passes through a chamber containing a fluid that absorbs
the spectral wavelengths of the selected gas. The two beams are
compared and the difference between the two beams gives an
indication of the amount of a selected gas in the sample.
[0048] A semiconductor NO, sensor is described in U.S. Pat. No.
5,863,503. The resistance of this sensor is altered by the
absorption of NO and NO.sub.2. Such a detector 140 may thus
simultaneously measure NO and NO.sub.2 levels, and therefore may
function accurately in the absence of a catalyst 135.
[0049] One of the above detectors, or any other detector capable of
measuring NO or NO.sub.2 concentrations, may be employed as
detector 140. In the case of a detector which is capable of
detection NO.sub.2 but not NO, it may be advantageous to oxidize NO
in the stream of gas to NO.sub.2, for example, using an ozone
generator or other source of oxidant. The detector 140 may include
or may be coupled to a processor, plotter, or other recording
apparatus for displaying, recording, or storing data collected by
the detector 140.
[0050] In certain embodiments, the filter 120 may be replaced by an
inertial impactor, which is also known to be useful for collecting
particulate matter from a stream of gas. In order to volatilize the
collected sample, the inertial impactor may be heated directly, or
indirectly, as described above for a filter embodiment. Otherwise,
the system is analogous to the system described above. Thus, in one
embodiment, an inertial impactor is used in a system as described
above which uses an NO.sub.2 extractor, such as a diffusion
denuder, as described above.
[0051] A system 100 as described above may be manufactured by
coupling a sample inlet to a collection body, disposing a filter in
said collection body, coupling said collection body to an NO.sub.x
detector, and coupling a gas inlet to said body. In certain
embodiments, the method may further include coupling an extractor,
such as an NO.sub.2 extractor as described above, between said
sample inlet and said filter. When the detector employed does not
adequately detect NO.sub.2, a catalyst may be disposed between said
detector and said collection body to reduce NO.sub.2 to NO.
Alternatively, if the detector employed does not adequately detect
NO, an oxidizer may be disposed between said detector and said
collection body. The components included in such a system may be
any of the components set forth above or components that function
equivalently or analogously.
[0052] The following examples are provided solely to further
illustrate the nature and advantages of one embodiment of the
present invention and are not intended to limit the scope of the
invention in any way.
[0053] Exemplification
[0054] A system as described above and depicted in FIG. 1 was
tested to determine the accuracy and utility of the measurements
recorded thereby.
[0055] FIG. 2 shows that as nitrate concentration in the gas sample
increases, instrument response increases in turn. Furthermore, the
very linear fit indicates that the instrument provides a linear
response and should measure nitrate levels accurately over a broad
range of concentrations.
[0056] FIG. 3 shows that the introduction of species, such as water
(relative humidity (RH) saturated) or ammonia, into the sample of
gas does not significantly affect the nitrate level readings of the
instrument. In all cases, the peak area is relatively similar.
[0057] FIG. 4 illustrates how NO.sub.2 in the gas sample as
collected and NO.sub.2 released from volatilization of nitrate
particles can be distinguished using the present method, even in
the absence of an extractor.
[0058] FIGS. 5A and 5B present data collected from atmospheric air
samples over three-day periods. Considerable variation can be seen
within a given 24-hour period, and these variations can be
elucidated because of the relatively short collection-analysis
cycles possible using the systems and methods disclosed above.
[0059] All articles, patents, and other references set forth above
are hereby incorporated by reference. While the invention has been
disclosed in connection with the embodiments shown and described in
detail, various equivalents, modifications, and improvements will
be apparent to one of ordinary skill in the art from the above
description. Such equivalents, modifications, and improvements are
intended to be encompassed by the following claims.
* * * * *