206 Discussrons

EFFECTS OF ATMOSPHERIC AEROSOLS ON SOLAR RADIATION*

The qualitative conclusions are basically what might be expected. The prime dependence of turbidity on air mass origin is illustrated in other studies. For example, maximum hourly direct radiation at Kew is more closely asso- ciated with westerlies than with easterlies: and relatively. the maxima occur most frequently with northerly (dry) air masses (Lawrence. 1972). The authors’ tindings that the turbrdy in Britain was greatest in southeast England is consistent with these results and presumably refect the aerosol content of easterly (continental) air masses.

Evidence suggests that in the London region, the increased maximum intensity of direct radiation in winter in the 1960’s was, at least partly, the result of local effects of the application of the Clean Air Acts (Lawrence. 1971a,b; 1972). Certainly, local effects were found to be important and in assessing the value of the Clean Air Acts, a study of solar radiation and air mass sources is more relevant than the results referred to by the authors. The downward trend of radiation intensity at Kew in summer is consistent with the more general trends else- where but this trend also may be a local effect resulting from photochemically produced aerosols and. as sug- gested by the authors, deserves further attention.

There may be conflicting evidence for supporting the claim that man’s activity may have gradually increased turbidity on a global scale but the evidence quoted. of I I remote /rig/r-lroel sites and some instrumentally doubtful data, suggests that the conflict is a little one-sided. Variations in total solar radiation at high levels of the atmos- phere (Kondratyev and Nikolsky, 1970) of 7 per cent. attributed to high-level atomic explosions, and of 3 per cent attributed to volcanic activity. are not inconsistent with the general conclusion of a gradual decline in solar intensity, based on widely distributed data from the U.S.A. and U.S.S.R. (Pivovarova. 1968 and Aslanikasvili, 1969) and the U.K.

The suggested relationship between attenuation of visible radiation and radius of particles might be usefully compared with the corresponding relationship suggested by the application of the Mie theory to the attenuation of visibility (Junge, 1963).

The M@troroloyical Office.

Brackrwll,

Berkshire, EmJland

F. N. LAWKtNCt

* Unsworth M. H. and McCartney H. A. (1973) Arnrospkr~rc~ Ewirorwwnt 7, I 173 I 185

REFERENCES

Aslanikasvili N. A. (1969) Secular variation of intensity of direct solar- radiatmn in Tbilisi. Proc. 7th Inter-Dept. Con& Actinom Optics, pp. 1699172. Leningrad, May 1968.

Junge C. E. (1963) Air Chemistry and Radioactivity: Vol. 4, pp. 141 146. Iw. Gzoph. Srrres. Academic Press. New York.

Kondratyev K. Ya. and Nikolsky G. A. (1970) Solar radiation and solar activity Q. J. R. wt. Sot. 96, 509522. Lawrence E. N. (1971a) Clean Air Act. Nature 229 (5283). 334335. Lawrence E. N. (1971b). Recent trends in solar rad$tion; maximum black-bulb and au temperatures in Britain.

Wmthrr 26 (4). 164-172. Lawrence E. N. (1972) Solar radiation trends and weather patterns. W’wrhrr 27 (X), 32tk 326. Pivovarova Z. I. (1968) The long-term variation of intensity of solar radiation according to observations of

actinometric stations. Tr. Glau. Geoph. Ohs. 233, 17737. Unsworth M. H. and McCartney H. A. (1973). Effects of atmospheric aerosols on solar radiation. Atmosphrvic

Environment 7. 117% I 185.

AUTHORS REPLY

In assessing the scale of effects of the Clean Air Act it would be useful to apply the methods of Lawrence to compare rural and urban sites in winter. However even in summer. it is unreasonable to expect that radiation measured at an urban site such as Kew should be “consistent with more general trends elsewhere”. Recent analysis (Unsworth, 1973) shows that at three rural sites in Britain over the period 19% 1972 turbidity in summer reached a minimum in the early 1960’s. Mean turbidity at Kew and Cambridge was very similar from 195991965 but since then Kew has become steadily more turbid.

If there is evidence that, on a global or regional scale, there has been a gradual decline in lrradiance at the surface, it is not clear to what extent Mr. Lawrence interprets this as a man-made effect. There is a need for