of the SOr need be converted t o particulate S042- t o account for a large fraction of the particles measured by the authors. Furthermore, the rate of decay of SOr must therefore be considerably slower than estimated in the paper. The SO2 to SOi2- conversion by catalytic oxidation in solution (Junge and Ryan, 1958; Scott and Hobbs, 1967) is likely to be greater at high than at low relative humidities. The results of the paper seem t o confirm this quite well. As a final note it is pointed out that in stack plumes as well as in many urban atmospheres, the mass of sulfur in the form of sulfur dioxide is much larger than the mass of sulfur in
SIR: With regard to the correspondence concerning our recent article, we would like t o point out that the term SO2 decaj’ was used rather than oxidation to cover several possible mechanisms of SO2 removal from the plume. It is quite possible, of course, that SO2may be absorbed o r adsorbed o n particulate matter, or it may go into solution in the presence of sufficient moisture, and/or it may certainly undergo heterogeneous o r homogeneous oxidation processes. The question is, what is the probability that these potential SOs loss mechanisms will affect the numbers of particles larger than the size range measured and thus influence the particulate/SO? ratio? One of the more probable mechanisms of loss would be the catalyzed oxidation of SO, at one of the already established particulate surfaces. In our studies, particle counts were recorded for sizes greater than 0.3 or 0.5 p diam. There are large numbers of particles in the size range below these values which were not counted, and which would also present surfaces for SO2 reactions. Since it has been shown that further oxidation ceases at a p H of about 3, which would be achieved quite soon on a small particle surface in the absence of a neutralizing agent, we feel that this mechanism would not exert a major influence o n the particulate/S02 ratio. The effect of humidity on the particle count was, of course, acknowledged in the paper. Inasmuch as there would be a distribution of SOr over all
particulate sulfate. Nonetheless, sulfate comprises a large fraction of the particle mass. Literature Cited Junge, C. E., Ryan, T. G., Quart. J. Roy. Meteorol. SOC., 84,46-56 (1 958). Scott, W. D., Hobbs, P. V., J . Atmos. Sci., 24, 54-7 (1967).
James P. Friend Department of Meteorology and Oceanography New York Unicersity Unirersity Heights Bronx, N . Y . 10453
sizes of particulate matter and the reaction most probably occurs at the surface of existing particles, the probability of the formation of new particles, and more important perhaps, the formation and growth of new particles t o sizes larger than 0.3 and 0.5 p which would have been counted would be of much less significance. It should be noted that plume age was a maximum of 4 hr. It has been shown, of course, that gas-phase photochemical oxidation of SO2 may produce sulfuric acid mist, although the rate of reaction is on the order of 0.1 z,’hr. If one considers that even this rate requires sunlight and that these studies were conducted just after sunrise in plumes formed and transported at night, then it seems likely that contributions to the particle population of a size large enough t o be counted from this source would be of minor significance. As previously pointed out, the technique is certainly not definitive, however we feel that the effort was not without merit, particularly in view of the corroborative nature of the results with data obtained by widely different techniques and the contributions it may have t o future development of more precise procedures.
N. Thomas Stephens Air Pollution Research Laboratory Department of Cicil Engineering Virginia Polyteclinic Institute and State Unicersity, Blacksburg, Va.
Volume 6, Number 2, February 1972
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