CORRESPONDENCE
SIR: The article “A New Method of Generation of Gases a t Parts per Million Levels for Preparation of Standard Gases” (ES&T 1980 1 4 , 4 1 3 ) describes a simple method for generation of test gases a t low levels. I t should be pointed out that, for three out of the five systems described (NO2, SOz, and HzS), the solute is subject to oxidative degradation and it is unlikely that a constant concentration of the desired gas will be maintained in the effluent stream over a long period of time. This is, of course, what Figure 7 shows when NazS03 is used as a solute: the SO2 concentration decays rapidly with time. In this context, the authors consider the greater oxidizability of S032-,as opposed to HS03-, to be responsible. Yet, the legend associated with the figure states that both the NaHS03 and Na2S03 solutions were maintained a t p H 5 . If the buffer employed had had sufficient capacity to maintain a constant pH, it would have mattered little whether Na2S03 or NaHS03 was put in solution insofar as the relative distribution of S032-/HS03- is concerned. The S032-/HS03- ratio is uniquely governed by the final p H of the solution and not by the form of the salt used; S0a2- is reportedly protonated with a rate constant of 1011L mol-1 s-1 ( I ) . There are other problems associated with NO, generation by this technique. Nitrous acid is sufficiently stable in the vapor phase to constitute an appreciable fraction of what is being measured as NO,. In our studies with vapor- and aerosol-phase nitrous acid, we encounter a very similar problem; HNOz is measured totally as NO2 by the Saltzman procedure and it decomposes to NO and NO2 in the chemiluminescence-type analyzers. The reliability of the data presented in Figure 4 is, therefore, open to question. The method is, however, attractive for situations where chemical instability, either for the solute in the solution or for the gas in the effluent stream, is not a problem. If fresh solution is pumped in with a peristaltic pump a t an adequate rate, constant solution composition and thus constant gas composition in the effluent stream can be assured. Oxidative degradation may, of course, be eliminated by using N2 instead of air.
hand, NaHS03 and NazS for SO3 and H2S are not stable, so that it is a problem that the decrease of solute due to oxidative degradation occurs in the long-time gas generation over 1day. We tried to resolve this problem and now we have succeeded in stabilizing the concentration of solute for 1 week by the addition of 1mM EDTA to the solution. Trace metal ions in the solution which work as catalysts of the oxidation are masked by EDTA, so that the solute is kept stable for a long time, and it is possible to generate SO2 and H2S of constant concentrations. It is also effective for the stability of solute to lower the temperature of the solution, although the temperature was kept a t 25 “C in this experiment. The Saltzman and chemiluminescence methods have generally been used for the measurement of NO,. However, if nitrous acid is sufficiently stable in the vapor phase, the problem described by Dr. Dasqupta could occur in both methods. In this experiment, we did not count nitrous acid vapor in the generated gas because of its unstability, so that we do not know how much nitrous acid vapor exists in the gas prepared by our technique. We should try to check its existence. It is a good suggestion to cycle fresh solution in the gas generator or to use Nz instead of air in order to generate gas for a long period by preventing the oxidation of solute. We have already tried to test these points. We attached an extra supply tank containing the gas-generating solution to the gas generator, and it was successful in generating continuously NO, a t 0.86 ppm for 1week within a variation of a few percent. Moreover, we also observed that the use of N2 instead of air prevented the oxidation of NO generated to NO2.
Yoshikazu Hashimoto Shigeru Tanaka Department of Applied Chemistry Keio University Hiyoshi 3-14-1, Kohoku-Ku Yokohama 223, Japan
Literature Cited (1) Erickson, R. E.; Yates, L. M. Atmos. Enuiron. 1977,11,813.
Purnendu K. Dasgupta California Primate Research Center University of California Davis, California 95616
SIR: As for the question of Figure 7 that shows the relationship between SO2 concentration generated and the stability of solute in the gas-generating solution, we agree with Dr. Dasgupta’s opinion. I t is true that the chemical form of sulfite in the solution depends on the p H of the solution as he pointed out. In this case, NaHS03 and NazS03 solutions were maintained a t p H 5 , so that most of the sulfite in both solutions could not be S0s2- but HS03-. Therefore, it cannot be theoretically considered that the difference of stability of solute in the solutions is caused by the form of salt used. However, we really observed that SO2 concentration from N a ~ S 0 3solution decreased gradually with time while that from NaHS03 solution was stable, as shown in Figure 7. So our conclusion is that we had better use NaHS03 for SO2 generation, although the reason is not well understood yet. In this method, the stability of solute in the solution is a very important factor for generating the gas of stable concentration for a long period. The generating solutions such as KCN, and NH4C1for HCN and NH3 are generally stable. On the other 600
Environmental Science & Technology
SIR: A recent article ( I ) describes computer simulations of photochemical production of ozone in the vicinity of Norfolk, VA. Data for comparison are taken from the southeastern Virginia urban plume study of the summer of 1977, which was instituted to determine whether the urban plume of this area would be a suitable test site for the evaluation of remote sensors. Data for one day are cited, a day on which there were “substantial ozone concentrations downwind of Norfolk in the afternoon, presumably due to photochemical aging of the urban plume.” Having no information about initial concentrations (Le., 8 a.m. concentrations) of nonmethane hydrocarbons, the authors proceeded to use their model to estimate what concentration would be consistent with the observed afternoon ozone, assuming the model and all other input assumptions were correct. They note that the resulting initial NMHC concentration, 250-350 ppbC, is “within the expected range” and therefore suggests that the kinetic mechanism used in the model is consistent with reasonable simulations. I want to point out one aspect of this simulation that the authors have not seen fit to mention. If their simulation is indeed a correct representation of the atmospheric events of August 4, 1977, then it follows that the ozone observed downwind of Norfolk in the afternoon was definitely not due to photochemical aging of the urban plume of the Norfolk area. I t could have been caused only by reactions of contam0013-936X/81/0915-0600$01,25/0 @ 1981 American Chemical Society
