655D73O01
ASSESSMENT OF THE ANALYTICAL METHODOLOGY
AVAILABLE FOR THE DETERMINATION OF '
NITROGEN DIOXIDE IN AMBIENT AIR .
Revised Draft
March 24, 1973
Prepared by Thomas R. Hauser with the assistance of
J. Finklea, J. Clements, andR. Thompson
'Quality Assurance and Environmental Monitoring Laboratory
National Environmental Research Center
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
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fND/YQ
im/Ai i
r.»rv
NOTICE
This document is a prnlivninnry draft. H.
hnr- not bcnii formally released by TOT'A
nnd should not ,".t thi.i i:t-n;.:^ be consf.n.u'd
to represent Agency policy. It is bcin^r
circnlatod for comment on its technical
accuracy nnd policy implications.
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INTllOnUCTlON
In October 1072, the Environmental Protection (EPA) published
a position paper concerning nitrogen dioxide measurement in ambient
air. (1) In that paper it was stated that the current Federal Reference
Method (FRM) for the analysis of NC>2 in ambient air seems to possess
inherent deficiencies which may have caused an overestimation of the
extent of the NO2 problem in various Air Quality Control Regions (AQCR)
throughout the United States. In part, the paper contained a plan to
gather additional air quality data to support the AQCR reevaluation study
and to determine, if possible, whether or not a consistent relationship
exists in data obtained from a simultaneous field application of the FRM
and other available analytical techniques. The results of the recent
studies indicate more than ever that the FRM should not be used to
collect data to measure achievement of the National Air Quality Standard
(NAQS) for N02.
STATUS IN 1968
In 1968, the year in which the Chattanooga Study was initiated,
there were three analytical methods recognized by most air pollution
•chemists as being applicable to the analysis of NO in the ambient air.
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continuous Saltznian) . The G-S manual method was only applicable 1.0 u
sampling period of 30 minuter* or ler-;s and analysis of 1110 samples was;
to be completed within one hour after sampling. The J-H manual method
was applicable to a*24-hour or possibly longer sampling period and the
analysis period could.be extended to at least two weeks if necessary.
Hence, the method of choice for a network operation was the J-H method.
The reasons that a manual method was chosen over the continuous method
in the Chattanooga Study were a simple matter of economics, manpower,
and timing.' In addition, the continuous Saltzman method was employed
in other Chattanooga studies by the PI-IS and US Army.
In 10G8, the NASN was using the J-H procedure to analyze. NC° in
ambient air throughout the Nation. When this method was initially
applied to the Chattanooga Study, sampling problems immediately arose.
For some yet unexplained reason1, excessive foaming occurred in the
bubbler, coupled with the clogging of the air flow regulating device
(a syringe needle) . Therefore, the J-H method was slightly modified
to adapt it to the unique sampling problem present in Chattanooga in
1968. The chemistry involved in the method was not changed in any
manner arid only slight modifications were made in the sampling procedure.
The air quality standard and the reference method for the analysis
of N02 were promulgated by the Administrator of KPA in April 1971. The
reference method selected was the J-1'I procedure as used by the NASN.
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At that time it. was thought that the continuous Snltzrnan method would give
*•)
accurate data above l.!"!fi iig/nr' and probably couJd be used to measure
relatively high ambient levels of NO,,. As indicated in the; Air Quality
ij
Criteria for NO2 in 1071, research was still very much needed to develop
and thoroughly evaluate sensitive, reliable, and practical methods for
determining MO, N02, and NOX.
RECENT ACTIVITIES IN N02 METHODOLOGY
The reference method for the determination of NO2 in ambient air,
as published in the Federal Register, has been critically reviewed both
within EPA and by outside.scientists (2,3) , and several serious drawbacks
have been pointed out. The two main difficulties observed in the reference
method are its variable collection efficiency and a positive interference
resulting from NO in the atmosphere. These limitations confute the
continued use of the J-I-I procedure as the Federal Reference Method. The
present Air Quality Standard for NOo is based upon an annual average
pollutant concentration and both continuous and short-term integrated
methods (e.g. , 24-hour bubbler methods) can be used to demonstrate
achievement of the annual standard. However, if an air quality standard
bdsed on hourly exposure is adopted, then a continuous monitoring method
will be needed to measure compliance and the 24-hour bubbler methods,
which are cheaper and easier to operate, can be used to identify problem
areas as well as satisfy some implementation plan needs. To answer these
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and ol.her questions, considerable effort :is buiup, expended at NEIIC-JTJT
to totally evaluate tlic: currcal .stale of NO., measurement technology. (1)
1. 24-Ilour Bubbler Method:.-;
• • i
In view of the drawbacks in the reference method, a consider-
able effort is being made to develop an acceptable NO,, 24-hour
.bubbler method using well established color forming reactions
and colorimetric measurement techniques as the analytical tools
of choice. Therefore, the major effort: of our work has been
devoted to developing improved collection systems.
Many absorbing systems have been looked at, both here
at EPA NERC-RTP and by outside laboratories. The systems
seemingly giving the best results are based on adding small
amounts of sodium arsenite, as suggested by Christie, el ajU (4)
to an 0.1 N sodium hydroxide solution; an aqueous solution of
triethanol amine (TEA); and a TEA system that included
guaiacol (o-methoxyphenol) and sodium-metabisulfite referred
to as TGS. When the present difficulties in the FRM were--
recognized, it became immediately necessary to collect d.ata to
assist in reclassification of the AQCRs. Hence, limited investiga-
tions into some of the analytical parameters of these methods were
initiated. .Time did not permit the very thorough investigations
and experimentation needed to establish a new reference method.
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These systems were preliminarily evaluated in Uu: laboratory
for collection efficiency, precision, accuracy, and some major
intcrfercnts; in the Held, they were placed side by side
with the current FRM.
All in all, four systems were evaluated both in the laboratory
and in the field. These were two arsenite methods, one TEA
method, and one TGS method. The two arsenite methods differ
only in the type of gas-dispersing device used. One method
used an upturned frit (referred to as ASFB); the other used a
straight glass tube drawn to a small diameter at the end immersed
in the absorbing reagent (referred to as Arsenite Orifice. Bubbler
or ASOB) . The TEA method uses an upturned frit while the TGS
method uses an orifice. It should be noted that an orifice is
preferred over a frit because of economic, safety, and calibration
considerations.
All four methods performed well in the lab and collection
efficiencies were of the order of 90% for both TEA and TGS., 85%
for the arsenite frit, and 82% for the arsenite with orifice. These
collection efficiencies were constant for the four methods from at
0 O
least 20 jig/m° to over 700 ;jg/m N02. No lower levels have
been tested at this time.
Preliminary in-house investigations indicate that NO causes
a positive interference in the ASFB method. When the NO: N02
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-li-
ratio is 2: 1 and NOo levels arc 3f',B ,i)j';/"i :ind higher, the inlor-
fercnce is aboi.il: 20-25-I;. Conceivably, such interferences could
result in an ovcrcstimalion of NO^ IcvtiJs in certain air quality
regions, for example, Los Angeles. The NO interference
effects at 04 jug/m and lower NO? concentrations are still under
.investigation.
All four methods were placed in a field test along with the
current reference method. Additionally, the ASFB method has
been used in the NASN network for the entire 1972 calendar
year. The ASFB, A-SOB, and the TEA method performed well
during the field test; however, an abnormally high and varied
background blank value was found with the TGS method, but
tin's problem now seems to be corrected. A correlation of 0.92
was obtained between the two arsenite methods, and this is
only slightly lower than the repeatability of the methods them-
selves. A correlation among the TEA and the two arsenite
methods was about 0.85. These correlations were made without
including any collection coefficients.
In a field study conducted at the Durham CAM10 station,
the ASFB method was found to have a precision of R.lpo and
4.0!b relative standard deviation (RSI))* and a precision of
*RSD - An Analytical Chemistry term and is the standard deviation
expressed as a fraction of the mean, S/X.
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1.!)% and n 5.1% RSD in two sets of 10 sample:-; on two different .
days when sample roLurn and analysis occurred \vJthin a few
hours. Corresponding data obtained in the: laboratory using an
• , i
NO2 permeation tube (calibrated by intermittent weighing) to.
spike scrubbed air shows that a set of 5 samples gave an RSD
. of 3. 0-6 for ASFB and 2 . 3% using the ASOB . The RSD for two sets
of 10 samples taken in Durham on two different days with TEA was
8.3% and 11.0%; in the laboratory one set of 5 samples gave an RSD
of 3.5%.
The TGS method lias been tested in the laboratory and field.
The RSD at the bench was found to be 3.0% while two sets of
*
field samples gave an RSD of 2.0 and 5.6%. Difficulties in blank
coloration which were noted during initial field testing of the TGS
method have been corrected, and the method still offers promise
as a new 24-hour bubbler method.
2. Continuous Methods
a. Chemiluminescence Method: The chemiluminescence
method is based on the measurement of the chemiluminescence
that occurs when NO in an air sample reacts with ozone
(introduced) by u.v. excitation of oxygen in a gas stream) .
Therefore, to measure NC^ in an air sample, the NO, must
first be reduced by a suitable converting device to NO.
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Consr.quc.ntly, depending on whclhciv the air sUoam is
passed through the converter or not, the cucvnihiiiunoscence
instrument is capable of measuring NO directly or NO plus
•
any N00 reduced to NO (termed NO,.) . The NO, is then
^ '-^ /*
measured by subtracting corresponding NO data from NO
Jv.
data.
The chemiluminescence equipment used to gather data
for the AQCR reelassification study was beset with many
operating problems. The shortcomings and down time of
these instruments have been recognized, and suggestions
for their correction and development of a more rugged,
field-reliable instrument have been given to the instrument
manufacturers. The chemiluminescence instrument still
i
shows technical promise as a good acceptable method to
monitor NO, in the air. It is felt that the malfunctions in
chemiluminescent instrumentation during the AQCR study
were the results of poorly manufactured equipment rather
than a breakdown in analytical methodology. Hence, addi-
tional field testing with chemiluminescence instruments is
needed before they can be firmly recommended for use as
an acceptable NO? monitor.
b. Continuous Saltzraan: The continuous Saltzman involves
drawing sampled air through an absorber at an accurately
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determined flow rr.Lo counter- current 1.o a controlled flow
of absorbing, reagent. A.ny MO., pvuserit in the air roods
fj
.to form a deeply eolored nxo dye. The colored solution
formed is passed to a colorimeter Where the intensity is
measured and recorded continuously. The continuous
Saltzman method lias not been completely evaluated, but
probably suffers less from variable collection efficiency and
NO than does the current Federal Reference Method. However,
a negative ozone interference occurs when the ozone to NO9
£j
ratio in ambient air is above 1.4.
METHODOLOGY RECOMMENDATION
*
1. Current Reference Method for NO2 (See Table 1)
The current reference method for the analysis of NO in_
ambient air should be abolished. As published, the refercrence
method has very serious deficiencies that warrant its removal
from further use for the monitoring of NO2 • Perhaps the most
serious drawback and the one responsible for the very erratic
results at certain locations is that the collection efficiency of
the absorbing reagent is concentration dependent. There is
also a high degree of variation in the collection efficiency at a
given level of N0? and several investigators have listed values
that vary 20% or more for the same NO2 level. Additionally, the
method as published indicates a collection efficiency of 35"-,.
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TABLE 1
" SUMMARY OF METHODS Er-iPLOYED TO MONITOR NITROGEN DIOXIDE IN AHBIEKl AIR
MsasureniSn u
Method
Federal Reference (FRM) .
Griess-Sal tzran
Griess-Sal tzman
Arsenite
Cnemi] urm'nescence
Tn'ethanol Arnine (TEA)
Triet'-r.r.olarr.ine-
Guiacol-Sulfite (TGS)
Statu
Type of
Samnl ing
24-hour bubbler
30-min. bubbler
Continuous
24-hour bubbler
Continuous
24-hour bubbler
24-hour bubbler
s Sun":T:?,rv
Availabi !i ty I .'hen
Press nt Standard Set
Yes
Yes '
Yes
No
V
No
No
T'5^
So
Remarks
•Variable collection efficiency and nitric
oxide interferences rake method i~practicc
•Impractical for rcsasuring achieverrorit of
standard because of 30-min. sa-pling tire t
be followed by immediate analysis.
•Used in CAM? stations and Chatty-coca stjc:
•Measurement of lev; levels of NO- in arr.b-ie..
ai r are suspsct.
•Oxidant and other interferences pay be a
problem.
•Relatively stable collection efficiency c1,^
wide range of NO? concentrations.
•Interferences may be possible but are not y
quantitated.
•Appears relatable to continuous Saltz-an
observations.
•P.eclassi fication of AQCRs based o" this r.s:
•Under critical evaluation in QAi;;U-v
•Avoids drawbacks of v:si chemical rethocs.
•Additional field testing is new ij-.-iderv/ay.
•Sr.ort-tenn air quality standard ~sy r2?uire
continuous method.
•Provided back-up data for AQCR recl?.ss"'fic-r
•Constant high collection efficiency.
•Shows promise and is under critic?.1 e'/^.l'.'-t
-•/ ns^-'-'i -
L/' '• l,'« • L-' •>- •
• Co.-istcnt hioh collocticn efficiency.
•Snov/s proinise and is under critical evaluo"
by QAEM* ]
^vel j^tions are beino conducted by .the Measurement Standardization Branch of the Quality Assurance a.id cnviro.'v.
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The latest v.'ork by several groups in II PA indicates that tin's
value only lio'k's for concentration levels between about 120
o
to 180 ,ug/i;r. . This range is somewhat lower than that reported
by Blacker and Brief.(2) They found a 56% collection effi-
ciency at 183 ,ug/m3 and the 35% at about 400 ,un/m3. In the
work at EPA, NERC-RTP, several investigators found a
logarithmic-type relationship between collection efficiency
and HO2 concentration. Collection efficiencies above 80% were
o
found, at concentrations below 5 ug/nr3 and only 15% were obtained
o
at levels greater than 800 ,ug/m . In addition, variations from
•3
25 to over 40% were found in the concentration range 120 to 180 ,uq/nr.
• Since the l^ concentration levels vary considerably in a
given 24-hour period, a 24-hour collection method should have
a constant collection efficiency over the entire concentration
••
range expected. This requirement is met by the arsenite method
as well as other recently available methods but not the current
Federal Reference Method.
Another major drawback reported for the reference method
is that NO interferes. The extent of the interference is not
completely known; however, since NO levels vary greatly.in
'the ambient air of various cities, this effect will vary from
site to site.
There may be other serious drawbacks to the method that
have not yet been determined. However, the severity of the
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tv/o above mentioned problems arid the fact that, the collection
efficiency problem cannot be resolved, no further effort should
be spent on looking for other sources of errors.
•
During the past year, the reference method lias been, run
si de-by-sidc with the four other bubbler methods discussed
earlier in this paper. In general, the current reference method
gives values approximately 2 to 2.5 times higher than the other
four methods at the great majority of sampling sites.
Therefore, in summary, the reference method is totally
unacceptable because of its variable collection efficiency, and
positive NO interference which probably causes the high results
obtained during field sampling and analysis.
%
2. Candidate Methods to Replace the Reference Method
At the present time and still subject to further examination,
it is recommended that thre'e methods be adopted as candidate
methods to replace the current reference method for the analysis
of NOp in ambient air if the current Air Quality Standard is
maintained. These are the arsenite method using an orifice
bubbler (ASOB), the chemiluminescence method, and the continuous
Saltznian method. This would give EPA the added advantage of
.having another continuous monitoring method in the event the
chemiluminescent method fails.
Before any candidate method can be promulgated as a new
reference method, it should be thoroughly evaluated and
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collaborati vely tested. Only in tin's way can all the require-
ments for establishing a new reference method be met thereby
ensuring to the best of our ability that future problems with
. j
analytical methodology are avoided. To complete all the
in-house evaluations and collaborative testing needed will
require about one year. Therefore, we should be able to
promulgate a new reference methoci(s) by March 1974 if the
results of current investigations warrant it. The reasons why
this length of time is required is clearly evident in the
t
following paragraphs.
Before making specific recommendations with respect to
a reference method.for N0?, there are several general aspects
v £•
of reference methodology which need to be brought into focus.
The term reference method is widely, and often loosely, used
and seems to mean different'things to different people. For
the benefit of this paper, an EPA Reference Method is a method
which has been thoroughly evaluated; has its accuracy, pre-
cision and limitations established; and represents the best
state-or-the-art for measuring the subject pollutant as it
is found in the environment. The term environment specifically
includes emission sources of pollutants.
The only purpose a reference method should serve if to
provide for .the best measurement of a pollutant in an environ-
ment and questions of cost, field worthiness and convenience
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should not. be weighted heavily. This ir.c^ns that there is
no requirement that the reference method be convenient for
field use and this important point deserves special emphasis.
*
It is recognized that reliable field methods are necessary
for gathering data on which far-reaching decisions are made
and for this reason the rclatability, or equivalency, of a
field method to a reference method is extremely important;
therefore, a vigorous program in equivalency determination
is essential. For these reasons, the naming of a new
reference method for N02 must be delayed until March 1974.
Detailed analytical procedures for the three tentative
candidate methods to replace the FRM have been prepared and
%
will be (have been) published in the FedcraJ Register.
a. The ASOB method as well as TGS, TEA, chemiluminescence
and continuous Saltzuian" methods are currently being
thoroughly evaluated in the laboratory and field tested
by scientists at NERC-RTP. The ASOB and TGS methods are
wet bubbler techniques using a restricted orifice rather
than a frit to disperse the sampled air in the collecting
solution. The frit that is currently being used in the
reference method, the TEA method, and the ASFB method
has several drawbacks. One of the major drawbacks is
that it.is very fragile and as much as a 25% loss has
been encountered in one sampling period. The replacement
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of the broken frits is both time consuming and co.stly
(^ $7 per frit.). In addition, because tiie frits are
fragile they pose the danger of cutting the workers
installing them and much, care must be taken. This
also causes delay. Also, the frits are easily clogged
and therefore, the system needs a prefilter which may
have some effect on overall collection of NC^. The
clogging of frits also causes calibration problems..
Therefore, the elimination of the frits would be a
benefit to the system. This, however, does not
preclude that the TEA method is no longer being given
equal consideration as a tentative reference method.
L
b. Chemiluminescence Method: As stated earlier, even
though the chemiluminescence equipment currently
available has certain r-ecognized operating problems,
it still has considerable technical merit and should
be considered a valid candidate method pending clari-
fication of these problems.
At the present time EPA has in final draft stages
guidelines for the determination of equivalent and .
reference methods. Any chemiluminescence instrument
meeting the performance specifications as given in
the "equivalency document" will be acceptable as a
candidate to replace the current reference method.
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c. Continuous Salt/man: The continuous Saltzman liioihod
has been used nationwide to accumulate the largest
amount of-N00 and NOV data thus far. It is thouoht to
(.<-.•
yield precise and accurate results to within the +_ 10 to
15% expected of field data at the 188 jjg/m3 level of N02.
The.method still needs further evaluation especially at
lower levels of NC^ in ambient air as well as collabora- ' •
tive testing. Recent investigations have shown a
negative interference in the method caused by ozone -when
the 03 to NO2 ratio exceeds 1.4.
IMPLICATIONS OF RECOMMENDATIONS FOR STANDARD SETTING
Sinxe the current Federal Reference Method has proved inadequate
and since the present national primary ambient air quality standard
is largely based upon the Chattanooga School Study that utilized this
*
discredited method, what changes are necessary in the standard?
Fortunately, the emission of nitrogen dioxide from the point source
of pollution in Chattanooga, the Volunteer Army Ammunition Plant,
were similar for a three-year period. Furthermore, the Division of
Abatement of the National Air Pollution Control Administration
conducted an extensive aerometric monitoring program in Chattanooga
'between December 1967 and November 1968.(5)
Continuous analyzers using the Griess-Saltzman method were
employed in this program. The Griess-Saltzman method is quite suitable
for the measurement of high nitrogen dioxide levels in the absence of
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interfcring pollutants, such as photochemical oxidonts. Therefore,
aerometric data from the Division of Abatement study'inay be used to
estimate community air pollution exposures for the years covered by
the previously published'Chattanooga health studies.
The Saltzrnan data base can be used to evaluate the adverse
health effects of repeated exposures to short-term elevations in
nitrogen dioxide, but the method does not appear ideal for monitoring
low levels of nitrogen dioxide in ambient air that usually result
from area emission sources. The problem then involves relating more
»
suitable analytical methods for nitrogen dioxide in en-blent air to
the Griess-Saltzman method upon which the health intelligence for .
the primary standard now rests.
Field testing of nitrogen dioxide measurement methods, though
incomplete, shows that the arsem'te bubbler method correlates fairly
well with simultaneous Sal tzman measurements at the same s.ite (Table 2).
On the other hand, the two methods did not always agree on the exact
level of nitrogen dioxide present. The Saltzman method often gave
significantly higher values than the arsem'te method; however, the
lower readings given by the arsem'te bubbler method are most likely
the result of procedural difficulties at field sampling sites.
. T,he chemiluininescence continuous monitoring method also correlated
well with the continuous Saltzman method (Table 3). This correlation
was better than -that observed for the arsem'te and Sal tzman methods.
Furthermore, the two continuous monitoring methods showed a more
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RQ.ATio;isiiii' nrrHr.ri'i Tiir AKSLIDTI: ruinm.rR AMD THE SAI.TZMAN
CONTINUOUS Kf.TilOOS FOR tfO.'HTOiUiir, I.'i'lW.rj'i DIOXiDi; Ifi AI-iiiir.Tr AIR
(OUTLIEUS RiinOYQj)
Monitoring ."
Station
Chicago CAMP
Cincinnati CAMP
Denver CAMP
Durham CAMP*
Was hi nqton.D. C.
CAMP'
St. Louis CAMP
St. Louis //902
Chattanooga #632
Chattanooga //633
California //841
California //832
Number
of
."Paired
Observations
• 19 .
7
0
JO
11
23
.... 80
51
30
36
40
AVG N0;> Love!:, (nrj/r.i-O i
Snltzman
Continuous
104
66
76
24
78'
.82
. 48
49
57
m
93
Arsenitc
Bubbler
Uncorrcctcd
102
57
75
52
75
70
--.
-•-
*
-. •
--
Arsenite
[JubMor
Corrncted
for 85% C.F..
122
79
90
62.
90
84
36
35
38 •'
57
46
Ratio of
Sal tziiiiin Tl
to Arscriito
Corrected
0.9
0.8
0.8
0.4
0.9
1.0
1.3.
1.4
1.5
1.9
2,0
Cor re 11
r/i-i r- •' •
000 i i ii
0.8L
0.9?
0.8L
0.5J;
0.3"
O.H/,
0.7!
0.7u
0 . 6 6
O.G7
0.71
* Using -Saltzman values above
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^SIill' i/TWa'N TilC Cliri-iIl.UMl.Ni.'.'-.fil'KT AND T!!l- SAI.T7..viAI!
CONTINUOUS i-oioos FOR ;-;o;;iio;
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consistent agreement on the level of nitrogen
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1. Mauser, T. R. and C. M, Shy, Env. Sci. Tech., 6., 090 (1972).
2. Blacker, J. M..and R. S. Brief, Chemospherc, ]_, 4:43-46 (1972).
3. lleuss, J. M., G. J. flebel , and J. M. Colucci, J. Air Poll.
Control Assn., ?!_, 535-548 (1971).
4. Christie, A. A., R. G. Lidzey, and D. W. F. Radford, Analyst,
9&, 519-524 (1970).
5. Helms, G. T., J. H. Southerland, K. R. Woodard, I. J. Hindawi,
D. H. Coventry and C. H. Robson. National Air Pollution Control
Administration Publication APTD-0583.
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