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34
Methods of estimation and results — The methods of measurement used for
estimating atmospheric pollution are:—
(a) Collection and examination of total deposit in the Standard Deposit
Gauge.
(b) Estimation of the active sulphur gases in air by the Lead Peroxide
Cylinder.
(c) Estimation of the total sulphur gases in air by the Volumetric
method.
(d) Measurement of the smoke (or black suspended matter content) in
the air.
Method (a) — The deposit gauge is essentially a rain gauge. The method
claims only to measure the amount and nature of the matter deposited from the
air on a particular area during a stated period : not all the impurity present in the
air over any length of time will be deposited. The rainfall is a complicating factor
for not only the "amount" but also the "duration" of it come into account.
Nevertheless a heavier level of pollution at one place than another must be expected
to be reflected in the nature and magnitude of the deposits. This has been borne
out over a period of nearly 40 years from a study of the results of this method
obtained at a number of observation stations.
Table 39, p. 145, shows the results of some recent yearly averages and some
period averages for seven deposit gauges examined by the Chemical Branch. The
nature and sources of the components estimated need description. "Tar"
includes the more volatile matters given off in the earlier stages of heating coal or
wood. "Carbonaceous matter, other than tar" may consist of fine particles of
unburnt fuel and soot, with hairs, plant remains, etc. The "Ash of the insoluble
matter" is mainly the mineral matter from this unburnt coal together with sand,
etc., from disintegrating building materials. The "Soluble matter" is largely
derived from the leaching action of the rain on the solid matters in the gauge: the
rain brings down sulphur dioxide and carbon dioxide which assist in the solution of
the lime and other bases. The "sulphates" are almost wholly derived from the
combustion of fuel. The "Chlorides" may come originally in part from sea spray
but coal also has a chlorine content. The significance of these constituents can
easily be recognised: thus a high "tar" figure is due largely to pollution from
domestic chimneys or very inefficient furnaces with bad combustion. A high
deposit of "carbonaceous matter other than tar" indicates uneconomical adjustment
of air supply and furnace conditions by which solid combustible particles
escape in excessive amount.
Considering the figures in Table 39, it must first be pointed out that the total
rainfall for 1949 was below the yearly average and its incidence as between the
various months was very uneven: this is made apparent by the low figure shown
for the available eleven months samples at Finsbury Park as compared with the
remaining stations which included twelve months; the missing month (October)
was a very wet one.
The figures for "tar" are, of course, very small but the results for 1949 were
similar to those for former years and on the whole tended to show a slight decrease.
It might be remarked in passing that this may be due to the low rainfall since it
has been shown that a decrease in this tends to lower the proportion of atmospheric
contaminants as measured by this method.
The comparison of the "carbonaceous matter other than tar" results showed
fluctuating differences: on balance those for 1949 were rather less than those for
1948, but in some instances they were greater than the averages tor previous
periods of years. The "ash of the insoluble matter" variations were generally, as
would be expected, in line with the "carbonaceous matter other than tar" results.