(3) Williamson, W. B.; Gandhi, H. S.; Heyde, M. E.; Zawacki, G. A. SAE (Society of Automotive Engineers), 1979, Paper 790942. (4) Jackson, H. R.; McArthur, D. P.; Simpson, H. D. SAE (Society of Automotive Engineers), 1973, Paper 730568. (5) Gandhi, H. S.; Stepien, H. K.; Shelef, M. SAE (Society of Automotive Engineers). 1975. Paaer 750177. (6) Shelef, M.;Otto, K.; Otto,”. C. Adu. Catal. 1978,27, 311. ( 7 ) Gandhi, H. S.; Piken, A. G.; Shelef,M.; Delosh, R. G. SAE (Society of Automotive Engineers), 1976, Paper 760201. (8) Hegedus, L. L.; Summers, J. C. J . Catal. 1977,48, 345. (9) Hegedus, L. L.; Summers, J. C.; Schlatter, J. C.; Baron, K. J . Catal. 1979,56, 321. (10) Kobylinski, T. P.; Taylor, B. W. J . Catai. 1974,33, 376. (11) Ashmead, D. R.; Campbell, J. S.; Davies, P.; Farmery, K. SAE (Society of Automotive Engineers), 1974, Paper 74029. (12) Taylor, K. C. In “The Catalytic Chemistry of Nitrogen Oxides”; Klimisch, R. L., Larson, J. G., Eds.; Plenum: New York, 1975; p 173. (13) Gandhi, H. S.; Yao, H. C.; Stepien, H. K.; Shelef, M. SAE (Society of Automotive Engineers), 1978, Paper 780606. (14) Summers, J. C.; Baron, K. J . Catal. 1979,57, 380. (15) Katzer, J. R. In ref 12, p 133. (16) (a) Spearot, J. A,; Caracciolo, F. SAE (Society of Automotive Engineers), 1977, Paper 770637. (b) Caracciolo, F.; Spearot, J. A.
SAE (Society of Automotive Engineers), 1979, Paper 790941. (17) Gagliardi, J. C.; Smith, C. S.; Weaver, E. E. A P I D i u . Refining Proc. 1972,52, 989. (18) Wotring, U’. T.; Meguerian, G.H.; Gandhi. H. S.; McCuiston. F. D.; Piken, A. G. SAE (Society of Automotive Engineers), 1978, Paper 780608. (19) Acres, G. J. K.: Cooper. B. .J.; Shutt, E.; Malerbi, B. W. A d c . C h e m . Ser. 1975, N o . 145. (20) Shelef, M.; Dalla Betta, R. A,; Larson, J. A.; Otto, K.; Yao, H. C. Presented at the 74th National Meeting of the AIChE, New Orleans, March 1973. (21) McArthur, D. P. In ref 12, p 263. (22) Williams, F. Id.;Baron, K. J . Catal. 1975, 40, 108. ( 2 3 ) Otto, K.; Dalla Betta, R. A,; Yao, H. C. APCA J . 1974, 24, 596. (24) Shelef, M.; Gandhi, H. S. I n d . Eng. C h e m . Prod. Res. Deu. 1972, 11, 393. (25) Schlatter, J. C.; Taylor, K. C. J . Catal. 1977,49, 42. (26) Williamson,W. B.; Gandhi, H. S. Ford Engineering and Research Staff, unpublished data, 1979. Received for reuiew June 25, 1979. Accepted December 26, 1979. Presented at t h e S i x t h N o r t h American Meeting of t h e Catalysis Society, Chicago, Ill., M a r c h 1979.
NO, (= NO -tNO2) Monitor Based on an H-Atom Direct Chemiluminescence Method Arthur Fontijn’”, Hermann N. Volltrauer, and William R. Frenchu AeroChem Research Laboratories, Inc., P.O. Box 12, Princeton, N.J. 08540
,A monitoring method has been developed for measurement of NO, based o n t h e chemiluminescence reaction system of
monitoring. This direct chemiluminescence monitoring method is based On Reactions lL4 (’”’)’
NO and NO2 with H atoms. This method eliminates t h e need for, and errors connected with, tly NO2 to NO converters that are required when t h e NO/O3 chemiluminescence method is applied to NO, measurement. Feasibility studies have been performed and a first prototype constructed for measuring motor vehicle emissions. T h i s unit has a linear response to NO, over a concentration range from 6 to a t least 4000 p p m (v/v), independent of t h e [NO]/[N02] ratio. Interferences by 0 2 and ethylene were encountered a n d experimental arrangements are described t h a t reduce these to acceptable levels in t h e prototype. T h e prototype performance characteristics represent a trade-off between sensitivity and interference levels; prospects for a n ambient air NO, monitor are discussed.
H
H +NO
H
+
+ HNO-
H2
(1)
(2)
+hv
(3)
+ NO
(4)
T h e emission from Reaction 3 is a series of bands between 628 and 800 nm (9,12,13).Reaction 3 leads to t h e expression for t h e chemiluminescence light intensity:
I
a
lHl[NOl
(5)
Reaction 1 is a very fast process ( 1 4 , I 5 ) , h 1 = 3 X 1O1O L mol-’ s-1, and hence conversion of NO2 to NO in t h e reactor is essentially instantaneous and the equation governing t h e monitoring method is thus:
I
+
Present address, Department of Chemistry, Queen Mary College, Mile End Road, London, E l 4NS, England. Environmental Science & Technology
+M
+ OH HNO + M
NO
H + NO-HNO
Following the initial development ( 1 ) of a NO analyzer, based on t h e N 0 / 0 3 chemiluminescent reaction, this method also gained rapid acceptance for NO, = NO NO2 monitors when Sigsby e t al. (2) demonstrated t h e practicality of converting NO2 to NO prior to passing pollutant gases through t h e reaction chamber. Both thermal/catalytic and chemical converters are in use ( 3 ) ,t h e latter only for ambient monitoring because of their low capacity. While such converters have generally performed well, errors have been observed in source monitoring due to reduction of NO, to N2 and oxidation of other N-containing compounds (“3, H C N ) to KO ( 4 - 6 ) ; problems have also been encountered in ambient monitoring (7, 8). To avoid such complications it is desirable to have a method t h a t does not require a separate converter. In t h e work described here such a method is established for (mobile) source
324
+ NOn+
a
[HI [NO,]
(6)
Much higher concentrations of NO, and other potentially interfering compounds are present in source monitoring than in ambient air. Thus, operating conditions of monitors usually represent a trade-off between sensitivity and levels of interference. Sensitivity, linearity, and interferences are the factors to investigate in a monitor feasibility and development study such as reported here. In t h e present case, negative interference due to consumption of H atoms could be calculated a priori because the rate coefficients for t h e pertinent H-atom reactions are available ( 1 5 ) . Such calculations for t h e most reactive ( 1 5 ) compounds present in engine exhausts (see below) showed ( 1 6 ) that it was desirable to use sample volume flow rates much lower t h a n the reagent (H/H2) volume flow rates. For a 1 : l O O ratio, a n operating pressure of 0.8 Torr ( a t 300 K), and a reaction time a t 5 X lop3s, t h e calculated fractional decrease in [HI and hence chemiluminescence intensity
0013-936X/80/0914-0324$01.00/0
@ 1980 American Chemical Society
Table 1. Calculated Fractional H-Atom Consumption by Some Species Present in Mobile Source Exhausts species
assumed concn. % (,v/v)
NOXC
0.3
C2H4
1
rate coeff, a L mol-‘ s-1
3 X 10”
1 x 108 ~2
NOd 0 2
0.3 20
mol-?
1
x
IAHI = A / , b ref
%
3 X lo-’
14, 75 15
2
x
10-1
s-1
10’0
4 x 10’0
x 10-4 a x io-* 2
15 17
These loss processes involve both bimolecular reactions such as 1, rate coefficients for which are expressed here in L mol-’ s-l, and termolecular processes such as Reaction 2 , which followed by the faster Reaction 4 destroys two H atoms per “catalytic” cycle; the rate coefficients given for the latter processes are expressed in L’ mol-2s-1 and are those for removal of two H atoms. Calculated for sample to reagent volume flow rate ratio = 1 X lo-’: P = 0.8 Torr; T = 300 K; f = 5 X s; and [H]/[H2] = 1 X lo-*. CTaken as NO2, which represents an upper limit as NO does not react with H in a bimolecular process. d Since NO’ IS essentially instantaneously converted to NO. the [NO] here represents the total [NO,]. The chemiluminescent Reaction 3. though phenomenologically written as a two-body process, is included in Reaction 2 and hence in the k~ value given. Reaction 3 is thus actually a three-body process; however, the light emission is independent of [MI at the pressures of interest here, since the emitter quenching is also proportional to [MI ( 9 ) . a
(cf. Equation 6) are given in T a b l e I and may he seen to be negligible. T h e approach followed in this work was to first establish feasibility and (approximate operating conditions using a conveniently adaptable rack-mounted apparatus and then to construct a prot1:)type instrument, suitable for field use. and optimize its performance. In the course of this feasibility work, major positive interferences h y 0 2 a n d ethylene were e n countered due to some light emission processes, apparently not previously reported. involving these compounds. These interferences were reduced to acceptable levels in t h e prototype by increasing reaction time to decrease the [C?H,] levels in t h e observation zone and hy designing a “wall-less” reactor t o sharply reduce heterogeneous H/O2 chemiluminescence. These cures involved deviations from t h e ideal conditions of Table I, a n d t h e prototype performance characteristics consequently represent a trade-off between positive and negative interferences, as well as sensitivity.
tui
Expcrini
T h e rack-mounted apparatus was essrntially the flow tube device described h y Fontijn and Ellison (181, equipped with a variety of light filters. Only one I’MT was used. Because the H/NO, emissions are in t h e red and near-111 p a r t of the spectrum, a trialkali P M T (Centronics P4283TIR) was selected. T h e “reagent” H atoms were initially produced by passing Hs through a 2450-MHz microwave discharge in a l.?-cm o.d. Vycor tuhe; a hot (-1900 “ C as determined by a n optical pyrometer) tungsten wire was suhsequently used. Previous users of‘ such hot wire sources [see, e.g.. Trainor et al. (lY)]seem to have invariably used water cooling for such sources. Since this is less practical for a field instrument. part of t h e investigations with t h e rack-mounted apparatus concerned testing a number of air-cooled designs, culminating in t h e design used in t h e prototype instrument. A schematic of t h e prototype NO, analyzer is shown in Figure 1. Its primary difference from previous chemiluminescence monitors and t h e rack-mounted apparatus, just discussed, is t h e split reaction tube or wall-less reactor. I t consists of two 2.2-cm i d . lengths of reaction tuhing separated hy a n -1-cm gap, which constitutes t h e observation zone. These tubes are contained in a 1-1, stainless steel sphere to provide vacuum. T h e upstream section of t h e reaction tuhe is constructed of Pyrex and has a side a r m through which the H/H? enters t h e reactor; the downstream tuhe is made of aluminum. T h e Pyrex tube is coated on t h e inside with phosphoric acid ( t o prevent H-atom recombination) and is painted hlack on the outside. Sweeper gas can he added in t h e upstream section o f t h e reactor; this gas flows from t h e outer part of t h e sphere through the gap and t h u s can he used to prevent the bulk of t h e reacting mixture from reaching t h e walls of t h e sphere. T h e sample inlet nozzle is situated 8 ern upstream f’rom t h e gap. A 5-cm diameter window on one side of t h e sphere permits the P M T (Centronics P4283TIR) to view t h e gap. which thus represents the observed reaction zone. .4conhination of two light filters is used. a Ditric 7415SP interference type “cut-on” and a Corning ’L-(iO “cut-off’ filter. which results in a band pass of -640 -73-0 nm. I\t room t e m perature the dark current contribution of t h e PMT to t h e hackground signal was found to be negligihle, and hence t h e €’h“ \yas not coolrd. T h e thermal 13-atom source. shown in Figure 1 , is contained i n a 20 cm long, 5 cm o.d. Pyrex rnvelope, which is cooled by SAMPLE
AIR SWEEFfR
II
U
SPLIT REACT1
I METERINQ
VALVE
SOLENOID
VALVE
SONIC
ELECTRONICS
I
ORIFICE
U PUMP
Figure 1. Schematic of prototype NO, analyzer
Volume
14, Number 3, M a r c h 1980 325
L
I
660
680
I 700
1
I
740
760
I 720
1 780
A , nm
Figure 2. H/NO spectrum at 4.6 Torr. [H2] = 2.1 X mol L-’; [NO] = 3.2 X mol L-’; v = 720 cm s-’. H atoms produced by a mi-
crowave discharge C O N C E N T R A T I O N , ppm ( V / V ) N O ~
a n air fan. T h e active element is a n -10 cm long, 0.35 cm diameter helical filament made of 0.025 cm diameter tungsten wire. T h e filament, which has a room temperature resistance of -0.7 12, draws -8 A a t 40 V. Relatively large size tubing (1.6 cm a d . Pyrex), coated with phosphoric acid, is used between this source and t h e reactor to minimize pressure drop. T h e general reagent gas flow system, cf. Figure 1,was similar for the prototype a n d t h e rack-mounted apparatus. Hz flows through traps to remove 0 2 (Engelhard Deoxo catalytic hydrogen purifier) and, H 2 0 (activated alumina). Except where noted, in thermal H-atom source experiments ultrahigh purity and prepurified grades of H2were used, which gave indistinguishable results. T o reduce background radiation in experiments with the microwave discharge, prepurified grade 10?i H2 in He was substituted. In the rack-mounted apparatus, a n exponential dilution flask (18)was used for introduction of t h e “sample” (NO, a n d potential interferant gases in N:! carrier gas).’However,H 2 0 was introduced directly (18)into the flow tube using a saturator through which 75% of the total N P carrier flow passed; t h e degree of saturation was checked by trapping t h e H e 0 in a n activated alumina t r a p and weighing. With the prototype apparatus, in addition to t h e exponential dilution flask, a dynamic dilution system was used; some checks were made using a NO/NO, monitor, based on t h e NO/O:j reaction, which yielded indistinguishable results. N2 was used as t h e sample carrier gas in experiments with t h e prototype.
Results Rack-Mounted Apparatus. T o aid in the selection of light filters for the P M T , spectra of the H/NO emission were taken, e.g., Figure 2. I t may be seen from these t h a t maximum sensitivity can be anticipated by measuring a t X 2660 nm. T h e 762-nm peak appeared unsuitable for inclusion in t h e wavelength band to be observed by t h e P M T , since it is subject to interference by t h e peak intensity of 0 2 (lZg+)atmospheric band emission in t h e presence of 0 2 ; this emission is apparently due to the homogeneous H/OZ reaction system ( 2 0 , Z l ) . An available cut-off filter, transparent to X 2 600 nm, was the only light filter used for all these preliminary experiments, except those on 0 2 interference, for which additional filters were used to remove the 02(lZg+)emission, as discussed below. Sample volume flows 1%of t h e reagent flows were used. Flow conditions were established by throttling t h e p u m p a t various reagent volume flows and observing t h e chemiluminescence intensities. Using t h e microwave discharge, a reaction zone pressure of 5.4 T o r r a t 13.2 m L (STP) s-l and a n average gas velocity of 250 cm s-l were t h u s selected for maximum H N O emission intensity. For t h e thermal source, 326 Environmental Science & Technology
Figure 3. Response vs. NO, concentration: (-) average of three NO tests; (- - -) individual injections of 50% NO-50% NO? mixtures: (0) initial injection points: ( 0 )points from exponential dilution
0.8 Torr at 6 m L (STP) s-l and a n average gas velocity of 760 cm s-l were similarly chosen. T h e distance from t h e sample inlet to the viewing area of the P M T was 1.5 cm. Experiments were performed, using t h e microwave discharge source, to check the linearity of response for NO, NO2, and 50% NO-50% NO2 mixtures. Identical results were obtained. showing linear response from 4 to -30 000 p p m of NO,. Hence, the instrument response is independent of t h e NO/NO, ratio. T h e limit-of-sensitivity here is taken as two times the smallest signal observable. A typical result is shown in Figure 3. T h e solid line represents t h e average of a series of three N O tests and the dashed line pertains to the NO/N02 mixture. T h e magnitude of t h e deviations between t h e solid and dashed lines is comparable to that found in the individual NO runs from which t h e solid line was obtained a n d appears to represent injected sample size error and read-out uncertainty. Since t h e concentration coordinate is calculated for exponential dilutions, errors in pressure and flow-rate readings will affect the slopes. For this reason the slope of t h e line drawn through t h e initial injection points (circles) is more reliable than that from the dilutions. T h e limit-of-sensitivity in these experiments is determined by t h e noise in t h e background signal. T h e occurrence of such a background (Le., no sample present) “hydrogen glow” has also been noted by Hislop and Wayne (211, but its nature has not yet been elucidated. Since t h e microwave discharge experiments showed t h a t 10% H2 in H e mixtures caused roughly a factor of 10 decrease in t h e background over t h a t observed with undiluted Hz, essentially without affecting signal intensity, such mixtures were used. No such reduction in background was realized with the thermal source for which substituting 10% H2 in H e caused slight, I 5 0 % , decreases in signal levels. Tests for potential interference were made by adding t h e potential interferant a t two extreme conditions: (i) with 3000-ppm NO, injections and (ii) with no NO,. T h e first tests optimized chances for synergistic effects, t h e second for discovering positive interference and influence on t h e background. Because of t h e potential for interference by t h e product O H of Reaction 1,NO2 was used as the principal NO, for each interferant. Frequent checks with NO yielded indistinguishable results. T h e interference gases checked and t h e concentrations used were, in (v/v): H20 (2.3%);0 2 (20%); hydrocarbons (l%), taken as ethylene, toluene, and isopentane; CO (7%); COn (14%); nitrogen compounds (200 ppm), taken as “3, H C N , and C H ~ N H LThese . gases and concentrations were selected in consultation with F. M. Black and
.J. E. Sigsby of t h e E P A as representing a n extreme case of those present in mobile source exhausts. For the gases checked a t concentrations up to 200 p p m , co-injection with the N O , samples was used. T h e larger concentrations used for the other interferants were ohtained by substituting the gas injected for t h e required fraction of t h e N2 dilution flask flow a n d c o n N:,flow through this flask. Isopentane and parison to toluene were tested by producing a l0CTi mixture in N?in a stirred predilution flask. These samples then were co-injected with NO, and compared to equivalent co-injected samples of pure N2 a n d NO,. T h e criteria used for "no interference" were that, in the presence of 3000 pprn of NO,, t h e differences measured had to be within the scatter of the d a t a of a series of NO, injections (15%),while in t h e absence of NO,, less t h a n t h e equivalent signal of 2 p p m of NO, (hackground noise level) had to be observed. Positive interference, Le., interfering emissions. was found with the hydrocarbons a n d 0 2 (see helow). while t h e other compounds tested did not interfere. Of the hydrocarbons. the strongest interference was found for ethylene; using the microwave H-atom source, for either 200 ppm of ethylene or 1%isopentane, t h e signal levels were on the order of 10'~ of t h a t of 200 p p m of NO,. which was taken to he unacceptable. Toluene interference was less t h a n for isopentane and was not definitely estahlished. Experiments with the thermal H-source suggested considerably lower interference levels, hut subsequent work, using t h e prototype instrument. suggested this to he, a t least in part, due to t h e inefficient operation of t h e thermal source a t this stage of its development. 0 2 interference was investigated first with t h e Ditric 741t5SP"cut-on" filter (this filter has 4 2 5 transmission at 743 n m , 20% at 750 nm. and 16 at 760 nm. while the transmission from 600 to 730 n m is 275%') in combination with a X 1 6 8 0 - n m transmission filter. LVith this filter combination. suitable for removing nearly all 762-nm 0 2 radiation i 2 0 , 2 1 ) without affecting t h e bulk of t h e remainder of t h e H S O radiation, 20%'0 2 caused a positive interference (tested in t h e absence of added NO,) equivalent to -800 ppm of N O , ; the interference intensity was approximately proportional to [ O l ] . These levels are clearly unacceptable. T h e experiments were therefore repeated using a 692-nm, 13.4-nm fwhm interference filter, behind a honeycomb-type collimator ( I8 1%which only reduced t h e 20% 0 2 level t,o -100 ppm of NO, equivalent. However, the H/On signal was found to he unstable and poorly reproducible-changes on t h e order of ,5Wt on various days were not uncommon-while the H/NO, response was very stable a n d reproducible. Such behavior is typically what may be anticipated for heterogeneous (wall) reaction. It was therefore decided to construct t h e wall-less reactor of' Figure 1. With this react~ora decrease in the On/NO, signal ratio by two orders of magnitude in t h e fi90-nm region was achieved. which reduced t h e 0 2 interference prohlem to acceptable proportions, and further confirms t h e H/Op wall chemiluminescence hypothesis. Prototype Instrument. T h e results with the rack-mounted apparatus t h u s ;howed t h a t t h e problems to be concerned with for a field instrument are positive interference by On and CgH4, which are also t h e primary sources for negative interferences as discussed in t h e introductory statement. LVhile isopentane represents a problem: it exhibits less interference a n d , by arriving a t operating conditions which reduce C2H.r interference to acceptable levels, hydrocarhon interference may, in general, be assumed to have also been reduced to acceptable levels. Many tests were performed in which reactor pressure, sample flow rate, sweeper gas (air) flow rate, H:: flow rate, and thermal H-atom source filament voltage were varied, and t h e effects of these parameters on positive interference, negative interference, (limit-of-) sensitivity, and linearity were determined. While in t h e work with t h e rack-mounted a p -
paratus no negative interferences had been encountered, to minimize positive interference and maintain adequate sensitivity in t h e prototype instrument, reaction conditions selected led to negative interference due to H-atom consumption. Since the work with the rack-mounted apparatus did not involve a split reaction,tuhe (Figure l), t h e sweeper gas flow rate represented an additional variable to he investigated here. Details o f these tests would make the present paper undesirahly lengthy; they are reported in ref 16 and summarized in Figures 4-1 1. included in the microfilm edition of this journal (see paragraph at end of paper). Here we will give t h e results at the operating conditions finally selected and discuss a few salient points. The selected operating conditions are: sample flow rate, 0.25 mL, i S T P ) s-I; sweeper air flow rate, 0.4 m L (STP) s-I: H:! flow rate. 12 mI, (STP)s - ] ; reactor pressure, 2.5 Torr; sample nozzle-to-obselvation zone distance, 8 cm; H-atom source tungsten filament voltage and current, 40 \.' and 8 A. Under these conditions the performance of t h e instrument is t h e following: limit-of'-seiisitivity.2 ppm of NO, with a 3-s electronic time constant and 6 ppm of NO, for a 0.3-s time constant: positive interferences for 1%CnH i and 20% Or, 6 p p m of equivalent KO, each: negative interferences for t h e same respectively; negative quantities of C r H j and 0 2 , 4 and 34, interference for 13%C o n , less t h a n 3 7 . T h e response of t h e instrument has heen found to he linear to within f2? from 6 ppm to at least 4000 p p m of' NO,. T h e fraction of H in t h e H2 entering the reactor was estimated via titration (11, 23) with NO2 to he in t h e range of 10-20%. Hecause the reaction zone in t h e wall-less reactor is not sharply defined (though the sweeper gas flow probably serves to essentially confine this zone to t h e volume between the two tubes): the exact reaction time is not known. Reaction time to t h e observation zone at t h e 8-cm sample nozzle-to-observation zone distance is 8 X lo-:{s. T h i s was found to be sufficient t o achieve t h e relatively low positive CzH4 interference level given ahove. Compared to experiments where this distance was 0.5 cm. t h e concentration of a n d positive interference from CnH4 were sharply reduced, b u t [HI was not a p preciably affected, as the linearity of response to NO, was not affected. However, this linearity was not maintained a t a p preciably lower flows of t h e sweeper gas, demonstrating t h e necessity to use it for maintaining a reasonably well-defined flow pattern. T h e 40-\.' filament voltage was selected conservatively; 50 \.' could probably also have been used safely and would have increased [HI and hence sensitivity somewhat; however, beyond -60 \.' the filament began to evaporate and coat the walls (particularly the thermal source envelope) with tungsten. This coating resulted in a sharp decrease in sensitivity due to Hatom wall recombination.
Di.scctssion T h e operating conditions arrived a t above have resulted in a prototype instrument t h a t is adequate for mobile source monitoring requirements. Actually, t h e interferant levels investigated are too high for present day vehicles; a n ethylene level of 0.1? rather t h a n 1% would have been more realistic. Similarly, in typical measurements, t h e 0 2 content of t h e sample varies only over a limited range, not t h e 20% variation investigated in the present work. Thus, for future instruments improved sensitivity could be obtained a t t h e expense of t h e interference levels encountered for a given amount of interferan t . I t may be possible to improve sensitivity to t h e point t h a t a n H/iYO, instrument could he used for ambient air monitoring. Factors to he considered here are that, of the two major interferants encountered in t h e present work, one, 0 2 , is present a t a constant concentration in ambient air and t h e Volume 14, Number 3, March 1980
327
other, hydrocarbons, a t a much lower concentration than used in t h e present experiments ( 10 I p p m as compared to lo4 ppm). Ambient air sample flow rates comparable to the reagent flow rates could be used, probably resulting in a t least a n order of magnitude improvement in limit-of-sensitivity. While this would result in a n increased H consumption by Or, such consumption would be constant and therefore not of overriding importance. Since t h e variation in [HI decrease would essentially be eliminated in ambient air monitoring, the observation zone could be extended to include a larger portion of t h e reaction zone; this combined with moving t h e P M T nearer to t h e light emitting volume should result in close to a n order of magnitude improvement. A point that could be investigated here would be t h e noise in t h e H/On heterogeneous reaction signal. At t h e constant [O,] of ambient monitoring, this problem could well be less severe t h a n in t h e present work a n d might, for example, allow t h e use of a n integrating sphere or a spiral flow reactor (24),which is the most efficient method known for obtaining high sensitivity in chemiluminescence monitoring. Coupled with these changes t h e inherent sensitivity of t h e method could be improved further by using a more sensitive PMT and increasing [HI and hence I , cf. Equation 6. A number of means for t h e latter a p pear available: (i) operating a t a somewhat higher filament voltage t h a n used in t h e present prototype, (ii) further improvements in t h e thermal source design, and (iii) reconsidering t h e use of a n electrical discharge method for H production. T h e latter appears reasonable, even for a source monitor, now that t h e original reason for selecting a thermal source for t h e prototype instrument no longer appears valid. T h e a b sence of positive interference from ethylene, when using this source on the rack-mounted apparatus, was probably mainly d u e to t h e inefficient H-atom production from t h e thermal source in its early stages of development, as such interference was only encountered with t h e prototype instrument. LVe previously demonstrated t h a t electrical discharge methods for atom production can be practical in field instruments (18, 22). An alternative approach to a NO, chemiluminescence a n alyzer, which deserves further attention, is the use of 0 atoms, based on t h e cycle:
+ NO2 NO + 0 2 0 + N O + M + NO2 + M 0 + N O + NO2 + hv 0
Acknoii,ledgment
We are grateful to F. M. Black and J. E. Sigsby of E P A for helpful discussions, particularly with regard to mobile source monitoring requirements. L i t e r a t u r e Cited (1) Fontijn, A , . Sabadell, A . .J., Ronco, R. ,J., Anal. C h e m . , 42, 575 (1970). ( 2 ) Sigsby, ,J. E., Black, F. M., Bellar, T. A,; Klosterman. D. L., E n -
L'iron. Sci. Technol., 7, 51 (1973). ( 3 ) Fontijn, A., " h d e r n Fluorescence Spectroscopy". Vol. 1,h'ehry,
E. L., Ed., Plenum Press, New York, 1976. p 159. (4) Black, F. M.,EPA/NERC, private communications to authors. (5) Matthew, R. D., Sawyer, R. F., Schefer, R. W., Enciron. Sci. T e c h n o . , 11, 1092 (1977). (6) Siewert, R. M., C'ombust. Flame, 25, 273 (1975). ( 7 ) LViner, A. M., Peters. J. W., Smith, J. P., Pitts, J. N., Jr., Enciron. Sci. T e c h n o / . , 8, 1118 (1974). (8).Joshi, S. B., Bufalini, J. J., Encircin. Sei. Technol., 12, 597
(1978). (9) Clyne, M.A. A , , Thrush. B. A., Discuss. Faradaj, Soc., 33, 139 (1962). (10) Clyne, hl. A. A,: Thrush, B. A,. Trans. Farada? Sot., 57, 1305 (1961). i l l ) Clgne, M. A. A , , Thrush, B. A , , Trans. Faraday Soc., 57, 2176 (1961). (12) Pearse. R. LV.B., Gaydon. A. B., "The Identification ofhfolecular Spectra". 3rd ed., Chapman and Hall. London, 1963, p 173, (13) Ishiwata. T., Akimoto. H., Tanaka, I.. Chem. P h p . Lett., 21, 322 i 19731. (14) Phillips, L. F., Schiff, H. I., J . C h e m . P h y s , , 37, 1 (15) Jones. R'. E., MacKnight, S. D., Teng. C., C h c m . R e [ ' . ,73,107 (1973).
(16) Fontijn. A,, Volltrauer, H. N., Frenchu. \V.R., "Mobile Source NO, Monitor. Hydrogen Atom Direct Chemiluminescence Method". Final Report. AeroChem TP-379a. EPA-600/2-79-120, ,July 1979. (17) iVong, CV., Davis, D. D.. I n t . J . Chem. K i n e t . , 6,401 (1971). (18) Fontijn, A,, Ellison, R., Enciron. Sci. Technol., 9, 1157 (1973). 119) Trainor, D \ V , Ham, D 0 , Kaufman. F , J C h e m P h j s , 58, 4599 (1973) (20) Giachardi, D. J.. Harris, G. \V.!Wayne, R. P., Chem. Phys. Lett., 32,586 (1975). (21) Hislop. .J. R., \Vagne, R. P., J . Chem. Soc., Faraday Trans. 2, 73,
nm -. i
i
~
n
( 2 2 ) Black, F. M ,High, L. E., Fontijn, A,. Environ. Sci. Technol., 11,
+
,597 (1977).
(8)
T h i s approach has been used in two previous feasibility studies, Le., those by Snyder and Wooten (2,5)and Black and Sigsby (26). Snyder and Wooten's work was unsuccessful because the electrical discharge source used for 0 atoms itself produced fluctuating quantities of NO (due to t h e difficulty of removing all NPfrom standard 0 2 supplies), which resulted in apparently unsolvable background radiation problems. In the work of Black and Sigsby this problem was solved by using thermal decomposition of 0 3 for the 0 - a t o m production. They obtained only a limited linear response range, u p to 50 ppm of NO,, inadequate for the high NO, concentrations encountered in source monitoring. This upper limit appears to be due to the low 0 - a t o m concentration achieved and the high reactivity of 0 atoms toward other species present in mobile source exhaust. By using small sample volume flow rates relative to t h e reagent flow rate, as was done in t h e present Hatom work, it might, however, be possible to extend t h e use of their method to t h e higher [NO,] (3000 p p m ) needed for exhaust monitoring. Their (26) signal-to-noise ratio suggested
328 Environmental Science & Technology
t h a t extension of t h e lower limit to 1 ppb, quite adequate for ambient monitoring conditions, is feasible.
( 2 3 ) Westenberg, A. A., deHaas. N.. J . C h e m . Phys., 43, 1550
(1965).
N., Fontijn, A . , C'ombust. Flame, submitted for publication. (25) Snyder, A. D., Wooten, G. W.."Feasibility Study for the Development of a Multifunctional Emission Detectoi for NO, CO and SOe", Monsanto Research Corp., Contract CPA 22-69-8, Final Report, October 1969. (26) Black, F. M.. Sigsby, J. E., Enciron. Sci. Technol., 8, 149 (1974). (24) Volltrauer, H.
Receiced for recieu, August 27, 1979. Accepted December 28, 1979. T h i s project has been funded at least in part u i t h f u n d s from t h e l ' . S . Eni'ironmental Protection Agency under Contract 'Vo. 680%-2 744. Supplementary Material Available: Results of preliminary parametric experiments with t h e prototype instrument (Figures 4-11) ( 4 pages) will appear following these pages in t h e microfilm edition of this volume of t h e journal. Photocopies of t h e supplementary material from this paper or microfiche (105 X 148 m m , 24X reduction, negatives) m a y be obtained from Business Operations, Rooks and Journal Dicision, American Chemical Society, 1155 16th S t . , N . W . , Washington, D.C. 20036. Full bibliographic citation (journal, title of article, a u t h o r ) and p r e p a y m e n t , check or money order for 85.50for photocopy (S7,OOforeign)or $3.00 f o r microfiche ($4.00 foreign), are required.
