This section describes the methods of measurement used in the UK Smoke and Sulphur Dioxide Network. Since the original Instruction Manual was written in 1966, these techniques have been standardised in several British Standard Specifications, and in some cases International Standards. These are:
These Standards provide the definitive description of the techniques, and Network site operators may wish to obtain copies for reference. However, this manual is more concerned with the application of these methods to the Network. The current section gives an overview of the underlying principles; the apparatus and procedures are described in greater detail in subsequent sections.
The term "black smoke" refers to any fine dark suspended particulate which can be measured by this smoke stain technique, not necessarily particulate resulting from combustion sources. Black smoke is defined as "strongly light absorbing particulate material suspended in the ambient atmosphere...… The major contributor to black smoke is soot particles; ie particles containing carbon in its elemental form" 2.4. Such particles remain suspended in the air as they are too small to fall under their own weight. In this important respect, the black smoke method differs from deposit gauge techniques, which measure deposited particulate.
The British Standard technique for monitoring fine suspended particulate matter as "black smoke" is well established, and is also defined as a reference method for monitoring compliance with the EC Directive 80/779/EEC2.1. The principle of the method involves drawing air at a constant, measured flowrate through a paper filter. Suspended particulate matter is collected on the filter, forming a dark stain. An instrument known as a reflectometer is used to measure the darkness of the stain, and this reflectometer measurement is then used to calculate the concentration of particulate matter in the sampled air from a standard calibration (in the UK, the British Standard calibration).
The concentration of sulphur dioxide in the air is estimated by passing the same measured sample of filtered air through a dilute, acidified solution of hydrogen peroxide in a Drechsel bottle (bubbler). Sulphur dioxide in the sampled air reacts with the hydrogen peroxide to form sulphuric acid in solution. The solution is acidified to pH 4.5, so that strongly acidic compounds will be absorbed in preference to weakly acidic compounds (such as carbon dioxide). The amount of acid in the exposed sample is determined by titration with a standard alkaline solution. Other strong acids or alkalis in the air will affect the result, so technically this method measures total acidity rather that SO2. However, in normal circumstances the concentration of such substances is very much less than that of sulphur dioxide. Therefore the result obtained is usually a good approximation to the concentration of sulphur dioxide.
References for Section 2