2756 Discussions

gas phase via chlorine-aldehyde reaction. Enuir. Sri. Technol. 10,82-f@.

Hendry C. D. and Kenley R. A. (1977) Generation of peroxy radicals from peroxy nitrates (RQNO,). Decomposition of peroxyacyl nitrates. J. Am. Chem. Sot. 99, 3198-3199.

Kravetz T. M., Martin S. W. and Mendenhall G. D. (1980) Synthesis of peroxyacetyl and peroxyaroyl nitrates. Complexation of peroxyacetyl nitrate with benzene. Enuir. Sci. Technol. 14, 1262-1264.

Louw R., Sluis G. J. and Vermeeren H. P. W. (1975) Facile preparation of peroxyacyl nitrates. J. Am. Chem. Sot. 97, 439&4397.

Lonneman W. A., Bufalini J. J. and Namie G. R. (1982) Envir. Sri. Te~hnol. (submitted).

Stephens E. R. (1969) The formation, reactions and properties of peroxyacyl nitrates (PANS) in photochemical air pollu- tion. Adti. Enair. Sci. Terhnol. 1, 119-146.

AUTHORS’ REPLY

Drs Lonneman and Bufalini’s comments upon our article raise some very interesting points, and we are pleased to respond to them.

Ourarticledescribes a method for preparation ofstandards of peroxyacetyl nitrate for the calibration of the GC-EC detector used for the atmospheric analyses of PAN. The usefulness of such a method depends upon:

(1) Does it require advanced synthetic experience to pre- pare the PAN?

(2) Does it require any precautions to avoid risk of explosion?

(3) Does it require equipment not available in most reasonable advanced laboratories?

(4a) Can it be excluded that the PAN standards do not contain components interfering in the calibration of the PAN standard, and is the accuracy and the reproducibility of the calibration procedure of the PAN standards satisfactorily?

(4b) Can it be excluded that the PAN standards do not contain components interfering in the calibration of the GC-EC detector, and is the accuracy and the precision of the calibration procedure of the GC-EC detector satisfactorily?

(5) Can the stock solution of PAN and the purified and calibrated solutions of PAN be stored for long periods?

(6a) Can it be used for calibration of GC-ECD systems located a remote field site location?

(6b) Can it be used for intercalibration between different laboratories? In the following we will discuss the questions in relation to the comments of Drs Lonneman and Bufalini:

(1) The synthesis of the peracetic acid and the PAN solutions is a series of rather simple chemical operations. Everybody having some experience with synthetic chemistry should be able to do these operations. Regarding the point on the 21 h used for the synthesis of the peracetic acid, this can hardly represent a serious objection. It requires about I h work on the first day, and on the second day the solution is poured into a bottle and put into the refrigerator. Finally, whether one will consider 307, hydrogen peroxide, aceticacid anhydride, sulphuric acid and nitric acid as exotic chemicals is a matter of taste.

(2) Regarding the solution methods for the preparation of PAN used by others (Hendry and Kenley, 1977; Kravetz et nL, 1980), we prefer working with 8-9 7; peracetic acid instead of 40% peracetic acid, whenever it is possible. As to the comparison between purification of the PAN with HPLC technique used by us and GC technique used by Stephens (1969) it is obviously more safe to work with PAN in adiluted solution than with pure liquid PAN.

(3) The procedure described by us requires a HPLC equipment with a u.v.-absorbance detector, an ion- chromatograph and some glass equipment. The determi-

nation of nitrite {and maybe nitrate) after the hydrolysis of PAN is possible using the modified Saltzmann method instead of using ion-chromatography. Most laboratories working with air pollution have the equipment needed.

(4) Kravetz et al. (1980) observed a new peak in the gas chromatogram ofa stored PAN solution in n-pentane, despite no decomposition of PAN was observed. Apparently, the new compound formed was 2-pentyl nitrate. A possible hypothesis is, that the PAN solution contain a component able to transform n-pentane to 2-pentyl nitrate. If so, thiscomponent perhaps also produce nitrite and/or nitrate when doing the hydrolysis of PAN. As far as this problem is unresolved, we consider, one should purify the PAN solution by HPLC before doing any calibration of the PAN standards and the GC-EC detector. This is also the case, if one uses the infra-red technique for the calibration of the PAN standard, as the i.r. spectrum of this component apparently present in the PAN solution is unknown.

Whether one should choose the hydrolysis-to-nitrite method or the infra-red technique for the calibration of the PAN standards depends on the equipment available. Our control experiments show that the efficiency of the hydrolysis of PAN is better than 99 %. This means that the hydrolysis of PAN is quantitative. Besides this, Sandalls er al. (1974) tested both methods and preferred to use the hydrolysis-to-nitrite method. However, if it is possible to use both methods, there does not seem to be any strong reasons for preferring one for the other. If we would use the infra-red technique, we had to determine the extinction coefficients of PAN in a n-heptane solution. Using the vapour phase extinction coeIIicients determined by Stephens (1964) would require highly sensitive infra-red equipment, as the concentration of PAN in the measuring cell had to be low in order to avoid any liquid n- heptane in the cell.

Another possibility is that one can synthesize PAN by the aldehyde-NOz-chlorine reaction (Gay et al., 1976) and use this gas mixture for the calibration of the GC- EC detector without performing any determination of the PAN content, but assuming the yield of PAN is a 95 %. This suggestion is in our opinion a most unsatisfactorily solution for calibration purposes.

(5) The PAN content in purified samples did not show any observable changes after 3 months storage at - 15 “C. Besides this the same stock sotution of PAN has been used for more than one year for the preparation of purified PAN standards. So the solution phase is a useful way to store PAN for longer periods.

(6) Purified and calibrated PAN standards have been made at Rise and used for the calibration of the GC-EC detector applied for measurements of PAN in Giiteborg without any troubles (Nielsen et al., 1981). The distance between the two locations is 230 km. We do not see any serious problems in making the purified and calibrated solution PAN standard in the laboratory, transporting it cold in a small thermal box to a remote field site location and using this solution for the calibration of the GC-EC detector as described in our article.

Despite the objections of Drs Lonneman and Bufalini, we still consider the procedure described by us as a convenient way to solve the problems connected with the calibration of the GC-EC detector used for the m~surements of PAN. Whether someone should choose this procedure, that of Lonneman and Bufalini described in their comments, or another method, must be left to the individual judgement.

Chemistry Department Ris) National Laboratory DK-4000 Roskilde, Denmark

TORBEN NIELSEN

Air Pollution Laboratory National Agency of

Environmental Protection Ris# Nationai Laboratory DK-4000 Roskilde, Denmark

ANNE MARIA $~ANSEN ERLING LUND THOMSEN

Discussions 2157

REFERENCES

Gay B. W., Jr., Noonan B. W., Bufalini J. J. and Hanst P. L. (1976) Photochemical synthesis of peroxyacyl nitrates in gas phase chlorine-aldehyde reaction. En&. Sci. Technol. 10, 82-85.

Hendry C. D. and Kenley R. A. (1977) Generation of peroxy radicals from peroxy nitrates (RO,NO,). Decomposition of peroxyacyl nitrates. J. Am. Chem. Sot. 99, 3198-3199.

Kravetz T. M., Martin S. W. and Mendenhall G. D. (1980) Synthesis of peroxyacetyl and peroxyaroyl nitrates. Complexation of peroxyacetyl nitrate with benzene. Enuir. sci. Technol. 11, 194.-195.

Nielsen T., Samuelsson U., Grennfelt P. and Thomsen E. L. (1981) Peroxyacetyl nitrate in long-range transported polluted air. Nature, Lond. 293, 553-5.55.

Sandalts F. J., Penkett S. A. and Jones B. M. R. (1974) Preparation of peroxyacetyl nitrate (PAN) and its determi- nation in the atmosphere. AERE Report R 7807, HMSO, London.

Stephens E. R. (1964) Absorptivities for infra-red determi- nation of peroxyacyl nitrates. Analyt. Chem. 36, 928-929.

Stephens E. R. (1969)The formation, reactions and properties of peroxyacyl nitrates (PANS) in photochemical air pollu- tion. Adv. Envir. Sci. Technoi. I, 119146.

NITRIC ACID MEASUREMENT METHODS: AN INTERCOMPARISON*

In their excellent publication comparing methods for atmos- pheric nitric acid measurement, the authors referred to our previous publications in this area (Appel et al., 1981; Appel et al., 1980a,b). These were based on sampling done at three California sites, Pittsburg, Lennox and Claremont, the latter being the same location used for their study. The principal difference in our observations from those of Spicer et al. appears to be in the comparison of HNO, by the denuder difference (DDE) and infra-red (i.r.) methods. At Claremont we observed, on average, 20% higher HNO, by DDE and inferred from this that loss of large-particle nitrate in our denuder (e.g. by impaction) was probably significant during the period sampled. The regression equations derived by Spicer et al. for the Brookhaven National Labs DDE and the University of California i.r. data indicate relatively little

difference between methods except at low HNO, levels, where the i.r. results were higher.

Future applications of the DDE method can minimize large-particle nitrate loss by excluding large particles with a cyclone in front of both samplers. The major uncertainty in this method will continue to be the possibility of loss of small- particle nitrate by volatilization followed by diffusion to the denuder walls. Our control studies (Appel et al., 1981) as well as those of Forrest et al. (1982) indicate such losses to be insignificant with NH&NO3 particles under laboratory con- ditions. However, m~surements of the losses of atmospheric particulate nitrate have not yet been reported. Techniques permitting determination of the nitrate on the walls of the denuder as a function of distance along the tubes should permit useful conclusions. Pending such measurements, the use of the DDE technique as a reference method must be viewed with caution.

Acknowledgement-I wish to thank Dr. Forrest for a preprint of his publication.

Air and Industrial Hygiene Laboratory

B. R. APPEL

California Dept. of Health Services 2151 Berkeley Way Berkeley, CA 94704, U.S. A.

REFERENCES

Appel B. R., Tokiwa Y. and Haik M. (1981) Sampling of nitrates in ambient air. Atmospheric ~~v~ro~~ent IS, 283-289.

Appel B. R., Tokiwa Y., Hoffer E. M., Kothny E. L., Haik M. and Wesolowski J. J. (1980a) Evaluation and development of procedures for determination of sulfuric acid, total particle-phase acidity and nitric acid in ambient air-phase II. Final Report to the California Air Resources Board. NTIS PB81-203499.

Appel B. R., Wall S. M., Tokiwa Y. and Haik M. (1980b) Simultaneous nitric acid, particulate nitrate and acidity measurements inambient air. Atmospheric Environment 14, 549-554.

Forrest J., Spandau D. J., Tanner R. L. and Newman L. (1982) Determination of atmospheric nitrate and nitric acid employing a diffusion denuder with a filter pack. Atmospheric ~~viroff~e~t 16, 1473-1485.

* Spicer C. W., Howes J. E., Jr., Bishop T. A., Arnold L. H. and Stevens R. K. (1982) Nitric acid measurement methods: an intercompa~~n. At~ospherjc Environ~nt 16, 1487-l 500.