Discussions 2523

the same term. Historical records such as those of hearth- taxes may allow a better estimation of the spatial distribution of pollution sources and sophistication of the model.

Despite the fact that recalculation and improved spatial distributions are likely to increase the predicted pollutant concentrations they will not be high enough to agree with the earliest measured annual means for London which were almost 400pgrn-” in the 1930s. However it is important to remember that the single box model averages not only temporally, but also spatially throughout the whole box. London’s air pollution monitoring network is densest near the centre of the metropolis, so unless the measurements from the stations are weighted carefully they are more typical of the higher pollution levels experienced by the central part of the city.

One has to acknowledge the uncertainties with the quanti- tative values obtained from the model and even more the difficulties of verification against the measured values which extend over such a short period. It is important to look at qualitative agreements between the model and contemporary observations. The decline of pollution after 1890 is a more startling feature of the model results, than the rise of pollutants in earlier centuries. In London there was a contemporaneous decline in the incidence of fog and an increase in sunshine. (Mossman, 1897; Bernstein, 1975). The earliest years ofdeposit gaugemeasurements (from 1914) also

show a decline in smoke levels (Shaw and Owens, 1925). One contemporary explanation agrees with the suggestion from the single box model, that emission sources were more widely distributed.

“Nay, it is not improbable that the amount of smoke rising from the entire metropolis is greater than it was fifty Yeats ago. The area of production is however now vastly greater also, and there is less density of smoke contamination.. . .” (Ewart, 1902).

University of East Anglia School of Environmental Sciences Norwich, NR4 7TJ

PE--TER BRIMLUCOMBE

REFERENCES

Bernstein H. T. (1975) The mysterious disappearance of Edwardian London Fog. The London Journal 1,189-206.

Ewart W. (1902) Report on the counties of London and Middlesex, in The Climate and Baths of Great Britain (London).

Mossman R. C. (1897) The non-instrumental meteorology of London, Q. JI R. met. Sot. 23, 287-298.

Shaw N. and Owens J. S. (1925) ?‘he Smoke Problem of Great Cities. Constable.

THE REACTION OF SULFUR DIOXIDE WITH OZONE IN WATER AND ITS POSSIBLE

ATMOSPHERIC SIGNIFICANCE* The study of Drs. Erickson, Yates, Clark and McEwen points out once again (see Penkett, 1972; Penkett and Garland, 1974), the relevance of ozone to the oxidation of atmospheric SO1 in droplets. Extrapolating their laboratory-obtained data to typical atmospheric concentrations (those used by Penkett, 1972), the authors found that the rate of oxidation of S(IV) by 0, may be as high as 32% h- ’ (which is almost the same as that found by Penkett (1972) if one adjusts for differences in HSO; concentration). This result is important, for it indicates that the reaction constitutes a very important path of the oxidation of atmospheric SO1.

The validity of the authors’ findings depends on the validity of their determination of the oxidation rate constants for S(IV) species, which was the major thrust of their work. I concur in their using the premise that not only HSO; but also SO1 (aq) and SO:- might be oxidized by O3 to SO:- ; and I wonder why they did not take H&O; into consideration as well. HS,O; has been shown to be in aqueous equilibrium with the other S(IV) species (Falk and Giguere, 1958).

On the basis of their analysis for pH = 3 (which is characteristic for atmospheric droplets), the authors con- cluded that

i.e. that most of the oxidation will occur via the SOi- ion rather than via the HSO; ion as was concluded by Penkett (1972) and by Penkett and Garland (1974). But their calcu- lation of the rate constants was derived using the assumption that there is evolved an equilibrium among the three S(IV) species. The authors themselves admit (p. 816) that “it is

l Erickson R. E., Yates L. M., Clark R. L. and McEwen D. (1977) Atmospheric Environment 11, 813-818.

questionable whether equilibrium is achieved”. However, they contend that even without such an equilibrium the buld of the oxidation will occur via SOf - rather than via HSO;. asserting that KsOl- will necessarily be higher under a& condition of disequ\librium than it is at equilibrium.

The truth of this assertion is not self-evident. Moreover, even assuming that the ‘equilibrium” value of Ks,;- is indeed the lower limit of I&,:-, there remains the question of how large K,,;- might actually be. And, correspondingly, how high is the actual rate of SO, oxidation (which is almost linearly dependent on Kq): 60% h-l? 2%min-‘? An oxidation rate significantly higher than that associated with the alleged lower limit of Kso:- could disrupt the good agreement between the authors’ oxidation rate and Penkett’s which was obtained despite their disagreement on the kinetics. Perhaps analysis of Penkett’s (1972) data using the authors’ method and vice versa would help resolve the uncertainties I have mentioned.

Finally, I should like to note that Barrie (1975) and Espenson and Taube (1965) also reported on the oxidation of SOi- by 0,, although they did not calculate rate constants. Barrie (1975) points out that his rate of oxidation is much smaller than the one reported by Penkett (1972).

REFERENCES

Barrie L. A. (1975) An experimental investigation of the absorption of SO2 by cloud and rain drops containing heavy metals, Ph.D. Thesis Im Eigenverlag des Instituts Frankfurt am Main.

Espenson J. M. and Taube J. H. (1965) Inorg. Chem. J. 704. Falk M. and Giguere P. A. (1958) Can. J. Chem. 36, 1121. Penkett S. A. (1974) Nature: Phys. Sci. 240, 105. Penkett S. A. and Garland J. A. (1974) Tellus 26, 284.

Rutgers University JOHNNY FREIBERG Department of Environmental Science P.O. Box 231 New Brunswick, N.J. 08903, U.S.A.