SIR: The Crabtree and Mayrsohn letter does not build my confidence in the reliability of the methods employed for the collection and storage of the hydrocarbons obtained during the LARPP study. My concern remains in the possible alteration of the alkene content of the bag samples as a result of inadvertent ozone retention during sampling, generation during collection and storage, and the subsequent reaction between alkenes and ozone during storage prior to analysis. There is no question that the composition of a dilute mixture of light hydrocarbons in air is reasonably stable in Tedlar bags in the absence of ozone, as Crabtree and Mayrsohn state. This has never been a disputed point. My concern lies with other factors which are not documented and which can alter the alkene content in bag samples of urban air during storage. In particular, the procedures employed by the ARB (and/or the Environmental Protection Agency) in the LARPP study must be documented and published in the scientific literature so that we may all share the evident confidence of Crabtree and Mayrsohn in their methods. Specifically, the following information is needed: (1)How effective is the ozone removal method which was employed as the bag samples were collected in the LARPP study? (2) How much ozone is regenerated during the transfer of the clear plastic bag samples from the aircraft to the van and during subsequent storage of the bags in the warehouse and laboratory prior to analysis? (3) How was the estimate of h 1 < 0.004 min-’ determined in the storage area? (4) Was NO, present and a t what levels when, as Crabtree and Mayrsohn state, mixtures of alkenes were observed to remain unaltered in storage in Tedlar bags in repeated analyses performed in the ARB laboratories. ( 5 ) What are the specific times logged in the ARB data record books a t which each of the reported LARPP hydrocarbon bag samples was analyzed? (6) Have test mixtures of known NO,, 0 3 , alkane, and alkene composition been subjected to the actual analytical procedures employed in the LARPP program (including ozone removal, transfer in sunlight, storage in the warehouse, etc.)? If so, what are these results? Answers to these questions should ultimately supplement the present data base in the LARPP data archives which are now available to the scientific community. The point which Crabtree and Mayrsohn wish to make with regard to the influx of air masses of different age, that is, of different hydrocarbon and NO, composition, into the 400-ft level of the LARPP Operation # 33, is not a t all clear from a complete analysis of the available analytical data for this operation. One must certainly look a t more than the data from one elevation if he is to form any meaningful conclusion on this matter. I have summarized in Figure 1 all of the data from the several elevations for some of the key contaminants followed in Operation #33. High carbon monoxide and acetylene are reasonably good markers for an “aged” air sample, while high alkene levels give strong indication that the air mass is of reasonably recent origin. Thus, the concentration of the highly reactive butenes is rapidly attenuated in air containing 0 3 and HO radicals, while the much less reactive acetylene and carbon monoxide (also a major product of hydrocarbon oxidation) tend to accumulate as the air mass ages. The time variation of the CO, the butenes (trans-2-butene plus isobutene), isopentane, and acetylene and the associated concentration gradients show that the dominant source of each of these pollutants is a t ground level. Note in Figure 1that a t any given time period, there is no significant preferential accumulation of acetylene over the butenes in the air masses a t ground level, 200, 400, 600, or 800-ft elevations above ground level which would give evidence of the presence of air masses of very different ages. Furthermore, there is a good correlation between the variation of concentrations of CO, CiH?, iso-C4Hl,l, and 98
Environmental Science & Technology
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.c
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12
+ I
\
TIME ( P S T I
Ground
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( c ) Carbon Monoxide
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a a
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Figure 1. Time dependence of concentrations of several key pollutants observed during Operation #33 of LARPP program at several elevations Circles, ground level: triangles, 200 ft: closed circles, 400 ft: closed squares, 600 ft; open squares, 800-ft elevation above ground level
the butenes with time; the fine structure which one sees in the butene concentration vs. time data is reflected as well in the acetylene curves. Of course, the ratio of butenes to acetylene shows a steady decrease with time, presumably as a result of the large difference in reactivity of the two hydrocarbons for 0 3 and the HO radical; it is this change in the alkene to acetylene ratio with time which I have utilized to estimate the average hydroxyl radical concentrations in the Los Angeles atmosphere ( I ) . The time dependence of the isopentane concentration between 0800 and 1000 h appears to be of somewhat different structure than those for the other pollutants shown. This may suggest additional sources of this hydrocarbon as Crabtree and Mayrsohn suggest, or it may merely reflect the high uncertainty in the analyses of these very small quantities of this hydrocarbon. It is evident, however, that its major sources are also a t ground level. As I have reported previously, the time dependence of the ratio of the isopentane to acetylene a t a given elevation is consistent. within large error limits, with a similar origin of the two compounds and the theoretical expectations based upon the small differences in the HO-reactivity with CIH2 and iso-C1H1,, (1-3). I cannot agree with Crabtree and Mayrsohn that, “The influx of pollutants of different hydrocarbon and NO, composition into the 400-ft level is at experimental fact exemplified by the LARPP data for Operation #33.” The complete data set simply does not support this conclusion within the un-
certainty of the analytical data. There can be no question that the air mass monitored receives a continuing supply of pollutants from the ground level throughout the day and that the mixture is diluted as the inversion height rises, but obviously mixing is very rapid during this time period. I t is the continuous addition of new pollutants, however, which leads t o an underestimation in my very approximate method of calculation of the [HO], as I have pointed out previously ( I ) . I certainly make no claim of high accuracy for the estimations of [HO] derived from the LARPP data ( I ) . However, I feel that the use of more sophisticated and realistic methods of data treatment, which may be readily applied in principle, must await the documentation of the bag collect,ion and storage procedures. We must be assured that the hydrocarbon concentration data are representative of the true atmospheric concentrations present a t the collection sites, or suitable corrections for the inadvertent attenuation of the alkenes
must be made before any such time-consuming calculations are justified. I encourage Crabtree and Mayrsohn to publish their findings or to perform the necessary experiments if these have not yet been made; the scientific community may then assess properly the significance of the LARPP hydrocarbon data and ultimately gain new insight into the role of hydrocarbons in photochemical smog formation.
Literature Cited (1) Calvert, J. G., Enuiron. Sci. Technol., 10,256 (19’76). (2) Ludlum, K. H., Bailey, B. S., ibid.,p 1162. (3) Calvert, J. G., ibid.,p 1163.
Jack G. Calvert Chemistry Department The Ohio State University Columbus, Ohio 43210
Correction In the article, “Partitioning of Mercury and Polychlorinated Biphenyl of Oil, Water, and Suspended Sediment”, by Gary S.Sayler and Rita R. Colwell [Enuiron. Sci. Technol., 10, 1142-45 (1976)], the financial support notation a t the end of the article on page 1145 should read: Work supported by Sea Grant Project No. 04-5-15811, National Oceanic and Atmospheric Administration, Washington, D.C., and Environmental Protection Agency, Grant No. R-803300-01-0.
Volume 11, Number 1, January 1977
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