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CORRESPONDENCE Comment on “Laboratory Evaluation of an Airborne Ozone Instrument That Compensates for Altitude/Sensitivlty Effects” SIR: The article (1) by G. L. Gregory, C. H. Hudgins, and R. A. Edahl, Jr. (GHE), addresses a commonly encountered difficulty with chemiluminescence (CL) analyzers: their sensitivity to sample pressure. We read this article with interest because we have recently studied the general behavior of CL analyzers (2). We would like to point out some inconsistencies in the paper, as well as make some suggestions. Figure 1of GHE depicts an “altitude correction factor” for the CL instrument as well as for the reference UV absorption instrument. This correction factor converts the instrument reading to relative concentration in mole fraction, parts per million, etc. Referring to Figure 1, GHE state “In all cases, the uncertainties in obtaining the true relationship between pressure and sensitivity result in additional error in the measured parameter as well as require additional data taking and reduction”. Figure l b applies to the UV instrument which, according to Beer’s law, responds to absolute concentration. Thus, the “true relationship” to which GHE refer is simply the ideal gas law, P V = nRT, which does not really require the verification shown in Figure l b , nor is its accuracy or applicability limited by the experimental measurements which Figure l b portrays. Since the appropriate correction factor is easily found from the ideal gas law and the ambient where temperature and pressure [for Figure l b it is Pref/P Prefis the pressure corresponding to unit relative response (about 705 torr)], and since both temperature and pressure must be known or measured in any atmospheric situation where the data are to be useful, we question GHE’s rationale in defining an altitude correction factor in the first place. More interesting in this light, however, is Figure l a for the CL instrument, whose points differ from those of Figure l b by less than the error bars. Thus, the CL analyzer with the manufacturer’s operating parameters also responds linearly with absolute concentration, and its output can be converted to mole fraction (if desired) by using the ideal gas law. However, since chemical reaction rates and many biological processes are proportional to absolute concentration rather than mole fraction, even if mole fraction is measured by the instrument, use of the data for many quantitative applications will require conversion from mole fraction back to absolute concentration anyway. Indeed, the National Air Quality Standards are written in terms of absolute concentration. In modifying the CL instrument for constant molefraction response with altitude, GHE base their approach on eq 1which states that the “output response of an airquality instrument is governed by an equation of the type output cx cmx”. This assumption, which is clearly of no general applicability, has led to several errors in the paper. On the basis of the paper’s context, when GHE say “x =
species concentration”, they refer to relative concentration (mole fraction, or mixing ratio), not to concentration per se in absolute units such as molecules, moles, or weight per unit volume. The UV reference instrument is itself an example of an air-quality instrument to which this equation does not apply, since UV absorption measures absolute rather than relative concentration, and its steady-state response is independent of mass flow into the instrument. We discourage the use of this equation either as a definition or as a statement of fact, particularly with regard to CL analyzers. Since our own work (2) has not been published, we will base our remarks largely upon equations in the paper ”Optimization of the Operating Parameters of Chemiluminescent Nitric Oxide Detectors” (3). Equation 9 of this paper gives the correct response equation of a CL analyzer operating in a plug-flow mode. This equation gives the total light emission rate (photons/s) as
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where fNo = mass flow of NO into the reactor in molecules/s, [MI is the total gas density in the reactor in molecules/cm3,TNO is the chemical lifetime of NO and -7raa the residence time of NO in the reactor, and the four rate constants are defined by the chemical mechanism which produces and quenches the chemiluminescence. This equation is equally applicable to the ozone/ethylene CL analyzer used by GHE, with the appropriate change in the rate constants, since the underlying CL mechanism is the same as in the NO/ozone system (4).The two lifetimes are defined by Steffenson and Stedman (3) in terms of the rate constants and analyzer flow rates. In their own experimental approach, GHE have ignored the chamber pressure term [MI. Thus, although they have succeeded in maintaining response approximately constant over a small pressure range, their approach could be improved upon. In their article, GHE define m as the “sample mass flow rate”. However, closer inspection reveals that m is in fact the total flow rate, sample plus reagent, through the analyzer. I t is convenient to rewrite eq 1 at this point. Defining the ratio of reagent to sample total molecular flows as 2, and observing molecular conservation [m = sample flow + reagent flow = (1 + 2) (sample flow)], we rewrite eq 1 of Steffenson and Stedman as
Here p is the total sample gas density in molecules/cm3, and the quantity [ O J / p is equivalent to mole fraction as expressed by GHEs symbol x. From eq 2 we may define
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ABSOLUTE CONCENTRATION
’/-
MOLE FRACTION
MOLE FRACTION
Y . 0
200
GHE‘r RANGE
I
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SAMPLE PRESSURE. TORR
Flgure 1. Simulated response of the unmodlfied CL analyzer by using eq 1 or 2. The upper curve gives the response per unit ozone concentration in units of 1O-g (photons/s)/(molecules/cm3) as a function of sample pressure. The lower curve gives the mole-fraction response (photons/s)/(unit mole fraction) and has been normalized to the first curve at 760 torr. The concentration response is relatively constant over the range reported by GHE.
the concentration response, Ibt/[O,], and the mole-fraction response, Itot/x. In Figure 1we use eq 2 to simulate the data of GHE for the unmodified instrument (GHE’s Figure la). The range of their data is shown, and both the concentration and mole-fraction responses are shown. Note that the concentration response is relatively linear over their range, as they found, but drops off precipitously below 300 torr. To simulate their data, we assumed [MI = p ; that is, the chamber operates at sample pressure. The initial flow at 760 torr sample pressure was m = 310 standard cm3/min. The initial inlet total-molecular-flow ratio 2 = 55/(310 55). Their volume flow Q through the instrument was assumed constant and is given by m/[M]. The reagent flow was assumed constant at 55 standard cm3/min, and the sample flow was calculated from molecular conservation in the chamber. These are choked-flow assumptions for the two flow restrictors. We have recently determined (5) the relevant rate constants to be kl + k2 = 1.9 X cm3molecules-l~-~, k 4 / k 3= 2.8 X 1020molecules cm-,, and kl = 3.8 X cm3 molecules-l s-l. Thus, in eq 2, only the chamber volume is unknown. However, obtaining the derivative of eq 2 with respect to 2 allows one to calculate the optimum reagent flow, stated by the manufacturer to be 55 standard cm3/min. Using the value of 2 = 55/(310 - 5 9 , we calculate an effective chamber volume of 1.56 cm3. This value checks well with the following calculations. In Figure 2 we simulate the response to the modified instrument, with reagent flow of 110 standard cm3/min. We maintain m constant with decreasing sample pressure, and since reagent molecular flow is constant, sample flow and 2 remain constant as well. The predicted response, although relatively constant over the pressure range reported by GHE, initially rises slightly with decreasing pressure and then falls steeply. The rising portion is controlled by the presence of [MI in the denominator of the first term, while the eventual decrease at lower pressures is caused by the exponential term. GHE remark that the modified instrument with the manufacturer’s reagent flow of 55 standard cm3/min only detected 60-70% of the chamber ozone at pressures below 500 torr. They mention
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SAMPLE PRESSURE, TORR Figure 2. Simulated response of the modifled instrument, as in Figure 1. The upper curve is the response per unit concentration and may be compared quantitatlveiy with Figure 1. The response to unit mole fraction is normalizedto this curve at 760 torr and is relatively constant over the range of GHE.
an “analysis” which indicated that some ozone left the reaction chamber unreacted at the higher flow rates employed at lower sample pressures. They say that ”Increasing the C2H, from 55 to 110 standard cm3/min allowed complete reaction to occur”. Two misconceptions are present here. First, evaluation of the exponential term in eq 1or 2 indicates that at 400 torr sample pressure and with 110 standard cm3/min reagent flow only 74% of the ozone reacts within the chamber, rather than the 100% they claim. However, the 2 derivative of eq 2 indicates that at 400 torr the optimum reagent flow should be 103 standard cm3/min, in very good agreement with the value of 110 the authors used. Thus, the authors chose the optimum reagent flow (probably empirically) but for the wrong reason. Another common misconception about CL analyzers is that maximum response will occur when all sample molecules react in the chamber. This is not necessarily the case, as 2 in the first term contributes to the reagent optimization. Increasing the reagent flow dilutes the sample in the chamber, and optimum 2 may occur at significantly less than 100% reaction of the sample molecules. Needless to say, we are unconvinced by GHE’s rationalization of the decrease in ozone ratio in their Figure 7 at low pressures, since our application of eq 2 indicates this decrease is to be expected. Incidentally, our own Figure 2 shows the best agreement with GHE’s data (constant response) when a chamber volume of 1.5 cm3 is used, in agreement with the above calculation. Our calculation of optimum reagent flow of 103 standard cm3/min also used the assumed chamber volume of 1.5 cm3, In summary, we do not argue with the fact that GHE did succeed in modifying a commercial instrument that responded approximately linearly with concentration to one that responded approximately linearly with mole fraction under conditions of changing sample pressure. We questioned above their rationale for the modification, but one advantage of it is that the absolute sensitivity (minimum detectable concentration or relative noise level as distinct from response) will improve with increasing altitude. GHE state that both the UV and the modified CL method will decrease in sensitivity with increasing altitude, due to decreasing concentrations. They are correct for the Environ. Sci. Technol., Vol. 17,
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quenching half-pressure (k4/k3)is much lower, allowing [MI independence in the preexponential term to be more easily attained. We have discussed this situation in more detail in ref 2, including methods of achieving altitudeinvariant response without servoregulation of flows or pressures.
Table I. Parameters for Maintaining Constant Mole-Fraction Response through Constant Chamber Pressurea chamber pressure, torr 760 600 500 400
300 200 100
required reagent, standard cm3/min 57.0
51.0 47 42 36
28 16
resultant response, lO.’O(photon/s)/ unit mole fraction
Acknowledgments
10.0 8.3 7.0 5.7 4.1 2.5 0.9 0.01
The work upon which this comment is based was supported, in part, by the U.S. Environmental Protection Agency Office of Research and Development Grant R807733. The views expressed in this comment have not been subjected to the Agency’s required peer and administrative review and therefore do not necessarily reflect the views of the Agency and no official endorsement should be inferred.
10 2.1 1 0.2 0.000 02 a Calculated from eq 2 by using m = 310 standard cm3/ min. Sample flow is (310 - reanent flow).
Registry No. Ozone, 10028-15-6.
UV instrument, but they are too modest for the modified CL instrument. Since it responds to mole fraction over a limited pressure range, its sensitivity will not drop off with increasing altitude in this range, an obvious advantage of the modification. However, in concluding, we suggest an alternate modification of the commercial instrument that will ensure complete rather than approximate linearity with sample mole fraction under changing sample pressure. In this modification, chamber pressure [MI is maintained constant rather than total flow rate m, by variation of sample inlet flow rather than total sample flow. This may be done either with the mass flow sensor which GHE employed or, more simply, with a pressure gauge in the chamber. If total sample inlet flow is regulated to maintain constant chamber pressure, then reagent flow as well as 2 will remain constant. The total molecular flow will remain or can be made constant since the pump will always see the same upstream pressure. The only difficulty will be in choosing a bellows control valve that is unreactive with ozone. In order for this modification to work, the chamber pressure must be reduced somewhat below the lowest ambient pressures to be encountered, so that gases will flow into the chamber rather than out of it. Lowering [MI does involve some loss in response. The responses predicted by eq 2 along with the calculated optimum reagent flows are shown in Table I for several chamber pressures. For instance, if the chamber pressure is reduced to 300 torr, the response will fall by slightly more than a factor of 2, but the instrument will be completely linear in mole fraction as long as chamber pressure can be maintained constant. Both the instrument manufacturer and GHE have succeeded in finding the correct values of 2 under their different operating conditions. However, the instrument itself would be capable of higher response and would be more versatile in adapting to changing sample pressure if the chamber volume were made 1-2 orders of magnitude larger. Large chambers with reflective walls are commonly employed in CL analyzers, and modification of a commercial instrument in this manner has been reported (6). Another approach with very much the same effects is the substitution of the much more reactive NO for ethylene as the reagent (7-9), requiring a red-sensitive phototube to observe the chemiluminescence. Since chamber volume and ozone rate constant appear in the numerator of the exponent in eq 2, increasing the rate constant has the same effect as increasing the chamber volume. With either modification, total sample flow can be increased to give higher response. However, with NO as the reagent, the 562
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Literature Cited (1) Gregory, G. L.; Hudgins, C. H.; Edahl, R. A., Jr. Environ. Sci. Technol. 1983, 17, 100. (2) Mehrabzadeh, A. A.; O’Brien, R. J.; Hard, T. M . Anal. Chem., in press. ( 3 ) Steffenson, D. M.; Stedman, D. H. Anal. Chem. 1974,46, 1704. (4) Pitts, J. N., Jr.; Finlayson, B. J.; Akimoto, J.; Kummer, W. A.; Steer, R. P . Adv. Chem. Ser. 1972, No. 113 246. (5) Mehrabzadeh, A. A.; Hard, T. M.; O’Brien, R. J., unpublished results. (6) Delany, A. C.; Dickerson, R. R.; Melchior, F. L., Jr.; Wartburg, A. F. Rev. Sci. Instrum. 1982, 53, 1899. (7) Stedman, D. H.; Daby, E. E.; Stuhl, F.; Niki, H. J . Air Pollut. Control Assoc. 1972, 22, 260. (8) Pearson, R., Jr.; Stedman, D. Atmos. Technol. 1980,11,51. (9) Lenschow, D. J.; Delany, A. C.; Stankov, B. B.; Stedman, D. H. Boundary-Layer Meteorol. 1980, 19, 249.
Robert J. O’Brlen,” Thomas M. Hard Ahmad A. Mehrabzadeh Chemistry Department and Environmental Sciences Doctoral Program Portland State University Portland, Oregon 97207
SIR: The authors acknowledge the comments of 0’Brien, Hard, and Mehrabzadeh (OHM) concerning our paper (1). OHM have brought out several important points that should not go unnoticed to those who are concerned with pressure sensitivity of air-quality instrumentation. However, some of these points require additional clarification as they apply to the subject paper. But, first, before addressing specific points, some comment on general “inconsistencies” observed by OHM are appropriate. The intent of the research was not to develop an “improved” instrument in terms of lower direction limit, higher sensitivity, or quicker response. As correctly implied by OHM, such an effort should focus on basic concepts and design of selected components to maximize signal and minimize noise and losses. Such studies are being done by numerous researchers resulting in improved prototype instruments maximized for a particular appli-
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