Environ. Sci. Technol. 1983, 17, 562-564
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
10
required reagent, standard cm3/min
resultant response, lO.’O(photon/s)/ unit mole fraction
57.0
10.0
51.0
8.3
47 42 36
28
7.0 5.7 4.1 2.5
16 2.1
0.9 0.01
Acknowledgments 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.
0.2 0.000 02 a Calculated from eq 2 by using m = 310 standard cm3/ min. Sample flow is (310 - reanent flow). 1
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
Environ. Sci. Technol., Vol. 17,No. 9, 1983
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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cation. The intent of the research was to describe and document a modification which can be made to many currently available instruments (but not all) such that instrument output does not need to be “corrected” (or adjusted, if this term is more acceptable) for pressure changes. The described modification is simple, reliable, and results in no permanent or major modification to the original instrument. While the modification is applied to + C2H4)ozone detector, the cona chemiluminescent (0, cept has applicability to other instruments. OHMs statement as to the “rationale” for the need to define an altitude correction factor is solely dependent on one’s frame of reference. A correction factor for pressure sensitivity is not an original concept of the authors. Chamber studies as well as analytical considerations have been used for many years to adjust instrument output for pressure effects. In the authors’ judgment, any adjustment to the basic instrument output is a correction to the instrument and its sensitivity as determined in the laboratory during standard calibration. The application of OHM’S eq 1 or 2 to the output of the chemiluminescent detector is in a practical sense a correction factor. OHMs suggestion that mole fraction (x)is not an appropriate term for concentration and that absolute units (mass/volume or molecules/volume) are more representative is again a matter of one’s frame of reference, purpose, or choice. Mole fraction is still the most frequently used concentration unit in field studies, is almost exclusively used by most instrument manufacturers, and is extensively used in OHM’S own cited literature (2-5). OHM are correct itl noting that sample pressure (or density) is required to convert mole fraction to absolute units and vice versa. They are also correct in that some applications are more suited to statement of concentration in absolute units. Concerning OHMs comment that the authors’ equation instrument output 0: c m x (1) where c = instrument constant determined by manufacturer’s design, m = sample mass flow rate, and x = species concentration (mole fraction), has no general applicability is partly correct but again depends on one’s frame of reference. The equation was introduced in context with a discussion of instrument calibration, in which a Calibration constant is determined for the instrument at a fixed pressure (generally 760 torr) and a set mass flow rate. The intent was not to suggest that eq 1 is a general equation stating conceptual or scientific relationships. As noted by OHM, eq 1is not applicable to the UV absorption instrument whose output is independent of mass flow rate. In retrospect, the authors perhaps should have pointed this out rather than assuming its implication based on the reader’s familiarity with the two types of O3 instruments. For the UV absorption instrument as well as others whose output is not a function of mass flow rate, the discussed modification is not applicable. For these instruments, reaction chamber pressure control can be used. The UV absorption instrument was introduced into the original discussions only in terms of documenting to the reader a “referencen ozone measurement for comparison against the modified instrument during the laboratory tests. In this context the authors showed an altitude correction curve (correction factor vs. pressure) for both the UV absorption and unmodified chemiluminescent O3 detector. Data were from earlier research conducted in an altitude simulation chamber (6). OHM are correct in their observation that the UV absorption curve is and should be the ideal gas law pressure density relationship. The UV absorption detector was included in the original chamber studies as a means of verifying the accuracy of
the chamber test. OHM are also correct in observing that within the indicated error bars of the chamber measurements, the correction curve for the unmodified chemiluminescent O3 is the same as for the UV absorption instrument. Their implication is to use the ideal gas law and measurement pressure to correct for pressure/sensitivity effects. While probably acceptable (15% accuracy) for the particular chemiluminescent instrument illustrated in the paper, this will generally not be the case for an instrument whose output is mass flow rate dependent. As stated in the paper, sample mass flow rate normally decreases with pressure for two reasons: (1) density effect and (2) decreased pumping efficiency at lower pressures. In the case of the subject chemiluminescent detector (unmodified), the instrument pump and orifice controlled sample flow are well designed, providing essentially choked flow for the orifice to at least 400 torr. One last general comment before addressing specific points concerning the validity of the reported instrument modification. As originally stated by the authors, one has several choices for addressing the pressure/sensitivity issue, all of which are acceptable and viable techniques. One can use empirical data (authors’ figure 1) and strive to reduce errors associated with the empirical data. Governing equations such as OHM’S eq 1 and 2 or the ideal gas law can be used, in which case such parameters as pressure and flow rates must be measured or determined a priori. This technique is most suitable where the various parameters can be simultaneously measured and data computer reduced on board the sampling platform. One can modify the instrument to maintain constant reaction chamber pressure, but as pointed out by both OHM and the authors, such modifications require careful consideration of the sample inlet to ensure the representativeness of the sample. Lastly, in the case of an instrument whose output is mass flow rate sensitive, the flow rates can be maintained constant by modifications on the exhaust side of the instrument. In our laboratory all techniques have been used, with the choice being a function of the instrument type, anticipated pressure range, and the availability of other data and/or real-time data processing capabilities. In terms of addressing OHMs specific comments on the applicability of the reported modifications, the authors reproduce the two equations on which OHM base their analyses: Itot
=
and
Equation 2 is OHMs eq 1from Steffenson and Steadman (2) where the subscripts for NO are replaced by O3 in order to reflect the ozone detector rather than the NO detector discussed in ref 2. As discussed in the reference, the photon output of a chemiluminescent detector is dependent on four terms: (1)to,, the mass flow of the O3 sample into the reaction chamber; (2) K,/(K, + Kz),a term that accounts for the fact that only a portion of the O3 + C2H4encounters results in an excited state (chemilumiEnviron. Sci. Technol., Vol. 17,No. 9, 1983 563
Environ. Sci. Technoi. 1983, 17, 564-565
nescent state); (3) K 3 / ( K 3+ K4[M]),a term which further decreases the photon count due to quenching; (4)the exponential term which accounts for the potential that some of the O3 sample can exit the reaction chamber unreacted with ethylene (C,H,). Equation 3 is identical with OHM’s eq 2 where the mole fraction notation (x)is used instead of [03l/P. As correctly stated by OHM, the authors’ instrument modification is basically an application of the ideal gas law (pV = nRT) and focuses on maintaining m and hence the first term constant with respect to pressure. An early analysis of eq 3 by the authors showed that for the subject detector, this was the only significant term that varies with pressure. OHMS analyses agree with the authors’ with the exception of the exponential term. This term requires further analysis in order to clarify the disagreement. OHM state that with the modified instrument and at 400 torr, only 74% of the O3 reacts in the reaction chamber, and hence, the instrument output should decrease (i.e., modification is not valid). The authors’ data show the modification is valid to at least 350 torr (lowest pressure tested). While OHM’s calculations are correct (verified by the authors), their error is in the method of determining the reaction chamber volume (V) used in the calculation of the exponential term. They assumed that the manufacturer’s recommendation of an CzH4 flow rate of 55 standard cm3/min was chosen to maximize photon output (IbJand, as such, determined the reaction chamber volume to be 1.5 cm3 from a derivative = 0 type analysis. While the calculation is correct (verified by the authors), the assumption is not. The reaction chamber volume as stated by the manufacturer (verified by physical inspection) is = 16 cm3. The end result in terms of the exponential term of eq 3 is that the exponent is approximately 10 times larger than calculated by OHM (i.e., at 400 torr the exponent is -10.2 as compared to -0.91). Thus, at 400 torr, 99.996% of O3has reacted compared to OHMS calculation of only 74%. On the basis of the 16 cm3 volume, even at 200 torr, 96% of O3 will have reacted, which suggests the modification is valid to at least that pressure level. Tests (run since the initial paper was submitted for publication) show that the modification is valid to at least 230 torr. At 230 torr and a concentration of =122 ppb O3 (reference technique), the modified instrument reading was 115 ppb, well within our laboratory accuracy for such tests. Thus the authors’ statements in the original paper showing verification of the modification of 350 torr (now 230 torr) and OHMS analysis using eq 3 are in agreement. The only term “overlooked” by the authors’ modification is the quenching term, which both the authors and OHM agree is small. One additional error in OHM’s analysis is their interpretation of the flow rates for the modified instrument. The quantity m in eq 1and eq 3 is the true sample flow rate of the sample into the reaction chamber and is 310 standard cm3/min for the modified instrument. The two flows controlled by the instrument modifications are the reagent CzH4flow at 110 standard cm3/min and the exhaust flow at exit from the chamber at 420 standard cm3/min (310 110). This has little effect on the above discussions but needs clarifying in the original text. In summary, the authors would like to commend OHM for their comments on the original paper. Their analyses were theoretically correct and complete based on the data available to them. While the deficiencies pointed out are not applicable to the subject instrument and modifciations, they surface an important point in that governing equations and basic concepts must be carefully reviewed prior to embarking upon instrument modifications in order that
+
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one is sure that the modifications are directed toward the proper governing and dominating terms. Registry No. Ozone, 10028-15-6.
Literature Cited (1) Gregory, G.L.; Hudgins, C. H.; Edahl, R. A., Jr. Environ. Sci. Technol. 1983, 17,100-103. (2) Steffenson, D. M.; Steadman, D. H. Anal. Chen. 1974,46, 1704-1709. (3) Delany, A. C.; Dickerson, R. R.; Melchior, F. L.; Wartburg, A. F. Rev. Sci. Instrum. 1982,53, 1899-1902. (4) Steadman, D. H.; Daby, E. E.; Stuhl, F.; Niki, H. J.Air, Pollut. Control Assoc. 1972,22, 260-263. (5) Lenschow, D. H.; Delany, A. C.; Stankov, B. B.; Stedman, D. H. Boundary-Layer Meteorol. 1980, 19, 249-265. (6) White, J. R.; Strong, R.; Tommerdahl “Altitude Characteristics of Selected Air Quality Analyzers”; NASA CR159165, 1979.
Gerald L. Gregory”
Atmospheric Sciences Division
Charles H. Hudglns, Robert A. Edahl, Jr. National Aeronautics and Space Administration Langley Research Center Hampton, Virginia 23665
Comment on “Automobile Traffic and Cancer. An Update on Blumer’s Report” S I R From a friend of the deceased Dr. Max Blumer I received the critical comments from Polissar and Warner concerning our study “Polycyclic Aromatic Hydrocarbons in Soils of a Mountain Valley: Correlation with Highway Traffic and Cancer Incidence”. We had found that the percentage of cancer deaths among all people living in houses situated along a main road was 9 times higher than among the inhabitanb of a quarter without through traffic. The content of carcinogenic hydrocarbons in soil tests along the road was 10 times higher than in the outlying section (3). In contrast to this, Polissar and Warner found no significant associations between traffic volume and cancer risk in Seattle concerning lung cancer and skin cancer (1). Polissar and Warner have not objected to the statement of A. A. Kubly that Seattle possibly was not favored by nature and traffic patterns for producing clearly defined results and that they could not supply any data on length of exposure of the inhabitants. In their study they did not point out the percentage of lung cancer cases on busy streets and traffic-free streets, because they did not know the total number of inhabitants residing on these streets. The results are based only on the hypothesis that their “lung cancer cases would have been exposed to a higher volume of traffic than the skin cancer controls”. This hypothesis is not proven. On the contrary it is known from many reports that hydrocarbons and even automobile exhaust gases produce also skin cancer in animal experiments (4). So the applied statistical method of Mantel and Haenszel (9) is not applicable to the Seattle study. Polissar and Warner have argued furthermore that air pollution would cause predominantly lung cancer whereas the cancer deaths along the Swiss automobile road involved all types of cancer. However, a number of studies lead to
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