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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                         -7-


 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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                          -10-


 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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                          -11-


 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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                         -12-

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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                         -13-



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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                       -14-

 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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                               -15-



          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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                              -16-


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

-------
                   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;
-------
                              -17-



consistent agreement on the level of nitrogen 
-------
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,
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