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test which is normally 24 hours. This method has been used since 1931 and has
the advantage over the previous one in that it is an actual measurement of the total
amount of sulphur dioxide present in the air: but it is more time-consuming in
that a determination has to be made each day and the apparatus required is more
extensive. Table 41 shows the yearly averages of the results expressed as SO2
parts per million by volume. In the cases of Beckton and County Hall the figures
for 1949 are less than the corresponding ones for 1948 but are similar to those for
1947 and, in the former case, to those for the period averages for 1940-4 and
1945-9. At Crossness the 1949 figure, as would be expected, was of a much lower
order and generally similar to those of 1940-4 and 1945-9. In all three cases the
present figures are less than those for the seven year average 1932-38.
The volumetric sulphur determination is made on a 24 hour basis and Table 42
shows the frequency of the daily occasions upon which the degree of pollution
exceeded 0.2 parts per million by volume of SO2 or was less than 0.1; the figures
are expressed as percentages of the total number of estimations made. The results
indicate that, in this respect, atmospheric conditions were generally rather above
the average in 1949 and again demonstrate the better condition at Crossness as
compared with Beckton and County Hall.
Method (d)—For many years up to 1939 estimations of Smoke (black
suspended matter content) were made by the Owens Automatic Air filter which
gave approximately hourly readings but required a fair amount of attention and a
constant water supply. Since the war this technique has been superseded by the
installation of three smoke filters each run in conjunction with the Volumetric method
for the determination of sulphur gases: the results therefore are on a daily basis
which, once having established the normal variation over the 24 hours of the day
(to be mentioned later), has certain advantages as regards accuracy of estimation.
Table 43 shows the monthly average figures for 1949 and some previous yearly
averages of the smoke concentration in the air at the three stations operating this
test. The results are expressed as milligrammes of black suspended matter per
cubic metre. The seasonal variation through the year is clearly shown, the figure
being notably higher in the winter than in the summer. This seasonal variation in
the degree of pollution had also been demonstrated in the past by the monthly
results for all the other methods previously described. The comparison of the
results at Beckton and County Hall with those at Crossness again demonstrates
the difference between an industrial town air and one on the outer fringe. A comparison
of the few yearly average figures shows no particular trend.
Discussion—Atmospheric contaminants may be roughly divided into three
classes: (a) smoke: (b) ash and grit: (c) sulphur gases. It has been estimated
that the total amount of smoke produced in this country is about equally domestic
and industrial in origin but that ash and grit, and sulphur are mainly industrial.
(a) Smoke—This is due to incomplete combustion, and the inefficient
domestic open grate, especially the older patterns, is a major cause of its emission.
Much has been done in recent years by the installation of more efficient types to
minimise smoke from domestic fires. The great effect of these on the smoke (black
suspended matter) content of the air was proved in the early years of the
investigation by the results of the Owens Automatic Air filter which has already
been mentioned. For a winter week day the hourly records from midnight to about
6 a.m., showed low figures but then a sharp rise occurred after the fire lighting,
and a more gradual one up to 10 a.m.; a small decrease then took place followed
by a period of constant value until about 5 p.m. when a small rise again
occurred to a peak value at 6-8 p.m. which was only slightly below the 9-10
a.m. peak: after 8 p.m. there was a gradual decrease to a minimum from 2-6 a.m.
By a comparison of such results in winter and summer (domestic fires being much
fewer in the latter season), the approximately equal order of effect of them was