The previous section described the principles of the black smoke and total acidity techniques. This section discusses the apparatus used to apply these techniques. Almost all Network sites use the 8-port sampler, a commercially available instrument used to measure daily mean concentrations of sulphur dioxide (as total acidity) and fine suspended particulate (as black smoke). Four examples of various types of 8-port samplers are shown in Fig. 3.1a, b, c, and d. It is also possible to make up an 8-port sampler from its component parts, and many such samplers are in use at Network sites. An example is shown in Fig. 3.1e.
The sampler draws ambient air, at a controlled and metered rate, first through a filter paper on which suspended particulate is collected, then through a solution in which sulphur dioxide is absorbed. Most models require mains electricity, although battery powered samplers are available for use in rural areas.
An inlet funnel, positioned outdoors, is connected via flexible tubing to the sampler. This is connected to a manifold, having 8 branches. Each branch is connected to one of eight ‘clamps’, holding a filter papers, followed by one of eight bubblers (Drechsel bottles), containing a solution of dilute, acidified hydrogen peroxide. The outlet of each bubbler is connected to one of the 8 outer ports of an 8-port valve, a device which automatically switches the air flow once every 24 hours to pass through a fresh filter and bubbler. Each filter and bubbler is ‘active’ for one day only. The central port of the valve is connected to the inlet of a dry gas meter, the outlet of which is connected to the pump. Fig. 3.2 shows a schematic diagram.
The sampler can operate unattended for upto 8 consecutive 24-hour periods; in practice it is normally set to changeover at midnight and visited weekly, when the exposed filters and solutions are replaced.
The exposed filter papers, when removed from the sampler, will exhibit a dark, circular "smoke stain" . The darkness of this stain is measured using a smoke stain reflectometer, which emits light onto the stain, and measures the reflectance. The darker the stain, the less light is reflected; this measurement is used to calculate the ambient concentration of black smoke.
The amount of acid absorbed in the exposed peroxide solutions is determined by titration with a standard alkaline solution of di-sodium tetraborate. Section 3.3 gives details of the laboratory reagents and apparatus required for this; and the titration process itself (with details on making up and testing solutions) is given in Section 6.
3.2 COMPONENTS OF THE SAMPLING INSTRUMENT
All components of the 8-port sampler (with the exception of the funnel, and the tubing connecting it to the sampler) are contained in a box or case. There is a small hole in the side of the case, through which the inlet tube enters the box.
Manifold. The inlet tube is connected to the manifold, a metal tube with eight branches (Fig. 3.3). The branches must make an angle with the inlet stem of 1350 ± 150 . This may be on the inside wall of the case, or on the floor of the box, under the bottles. Each branch is connected, via PVC tubing, to the lower section of one of eight filter clamps.
Filter clamps. These hold the filter papers on which the black smoke is collected. Various sizes are available: the size required will depend on the amount of smoke at the site, and its variability. The sizes available are 4 inch, 2 inch, 1 inch and ½ inch diameter (10, 5, 2.5 and 1.25 cm respectively). Nowadays, 1 inch clamps are used at almost all urban sites; however, larger sizes were frequently needed in the past when smoke concentrations were higher. If there is any doubt as to the appropriate clamp size for the type of district concerned, please contact AEA Technology Environment. Fig. 3.4 shows a set of filter clamps, removed from the sampler for clarity.
Clamp holder. The two sections of the clamp are most commonly held in place by a screw-down holder. When this is tightened, the paper is firmly sandwiched between them. Some commercially-available samplers have other means of securing the clamps, for example in the Airtest models the clamps are secured by operation of a lever. Most commercial sampler designs incorporate a means of ensuring that the upper and lower filter clamps are exactly aligned.
8 Bubbler (Drechsel) bottles with bubbler heads. The bottles are of 125 ml capacity and made of clear hard (borosilicate) glass (soda glass is unsuitable as its alkali content interferes with the total acidity method). The Drechsel bottle heads must be connected so that air enters the bottle through the central tube, passing down into the bottle through the top of the head and leaves through the arm inserted into the side of the head.
PVC tubing. PVC tubing (internal diameter ¼ inch (6.5 mm) nominal) connects the various parts of the apparatus. A medical grade, eg. Portex, is recommended, but not essential. The tubing must, however, be PVC. Rubber tubing must not be used as it absorbs sulphur dioxide; neither polythene or PTFE tubing must be used as they attract smoke particles electrostatically. The tubing used is normally described as "semi-hard"; it must be rigid enough to resist kinking and crushing.
8-port valve. The eight tubes leading from the bubblers connect to the back of the 8-port valve. This component incorporates a timing device, and is set to change ports each 24 hours, thus allowing the sampled air to be drawn through a fresh filter paper and bubbler each day. The single outlet from the 8-port valve leads to the inlet of the meter. Various designs are available; for example the AGL sampler incorporates a box-shaped version, with a timing disc on the front, and a light which comes on when a port-change is in operation. Fig. 3.5a and 3.5b show two widely used designs.
Dry gas meter. The meter must be readable to 0.01 m3 or 0.1 ft3 , and accurate to ± 3% at a flow rate of approximately 2 m3 per day. A wide variety of meters have been, and still are, in use. Metric meters are preferred, but many which read in cubic feet are still in use. Fig. 3.6 shows the popular Remus type.
Electric suction pump. The pump must be capable of providing a constant flowrate of 2 m3 per day (70 ft3/day or 1.5 l/min ), + 10%. Most samplers now incorporate the Capex 2LNS pump, which is made by Charles Austen Pumps Ltd, but older designs, eg. the Dymax pump, are still in use. The Capex 2LNS pump uses a flow-limiting critical orifice to provide the required flowrate. This is preceded by a "safety filter" , which traps any fine particulate remaining in the air, which would otherwise accumulate on the flow limiting orifice and gradually block it. These components are housed in a small black plastic cylinder, connected to the pump inlet by a length of reinforced tubing. The Capex pump is also supplied with a silencer on its outlet (Fig. 3.7). Older designs, eg. the Dymax pump (Fig. 3.8), are still in use at some sites.
One reflectometer and one set of chemical apparatus will serve a number of samplers.
3.3.1 For the assessment of smoke stains
Whatman No. 1 filter paper. The filter paper can be obtained conveniently in sheets 4 in x 3 in (10 x 7.5 cm) or in circles of 5.5 or 9 cm diameter, according to the size of the filter clamps being used.
Smoke Stain Reflectometer. (Suppliers: Diffusion Systems, Kemtronix).
3.3.2 For sulphur dioxide measurement
(i) Reagents
Hydrogen peroxide. This is bought in concentrated form (90-100 vol strength), and must be of analytical quality ( eg. ‘AnalaR’). It is diluted to ‘1 vol’ strength for use, in batches of 1000 ml or 2500 ml according to the number of samplers in use. 1000 ml of the dilute solution will be enough for one 8-port sampler. The concentrated peroxide is supplied in bottles of 250 and 500 ml. It is a strong oxidising agent: follow the safety instructions on the label.
BDH "pH 4.5" indicator. Supplied in 500 ml bottles. Smaller quantities can be transferred to a small dropping bottle. This indicator should be kept cool, ideally in a fridge, and because of its alcohol content is inflammable. It has a limited shelf life; refer to the packaging.
Ampoules of Concentrated Solution for preparation of 0.002 M di-Sodium Tetraborate (this may also be sold as "sodium tetraborate" or "borax"). Supplied in boxes of six ampoules, each equivalent to 500 ml 0.002 M (N/250) solution when diluted with deionised water. These have been unavailable in the UK for several years, and many site operators now obtain their 0.002 M borate solution directly from AEA Technology Environment, where it is made up from the solid (crystalline) form. Although at the time of writing the ampoules have reappeared in the supplier’s catalogue, AEA Technology Environment can still supply borate solution free of charge.
Ampoules of Concentrated Solution for preparation of 0.002M (N/250) sulphuric acid. Note: ampoules are not available for 0.002M (N/250), only for 0.01M (N/50), which is five times the required strength. However, by extra dilution, these can be used to make up the 0.002 M (N/250) sulphuric acid required. Each ampoule makes up 500 ml 0.01M solution, which is then diluted to give 2500 ml of 0.002M solution.
Deionised water. This should be obtained from an ion-exchange resin column, or from a supplier of chemicals. It need not be carbon-dioxide free. Tap water is not suitable; nor is distilled water as obtained from garages, since it is often stored in containers contaminated with acid.
(ii) Chemical apparatus
It is not necessary to use Grade A glassware. Grade B is sufficiently accurate and often much cheaper. Inert plastic such as polypropylene is suitable for beakers, volumetric flasks and some storage bottles.
For 1 vol hydrogen peroxide: dark glass, either 2500 ml or 1000 ml, depending on the quantity usually made up. Plastic screw cap, or glass or plastic stopper, not rubber or cork.
For 0.002M di-sodium tetraborate: clear glass or inert plastic 500 ml reagent bottle. Screw cap, not glass stopper, to prevent "seizing" due to drying of alkali.
For 0.002M sulphuric acid: clear glass or inert plastic, 2500 ml. Glass or plastic screw cap or stopper, not rubber or cork.
For deionised water: A large container, for example a 10 litre plastic aspirator bottle with a tap at its base is usually most convenient.
either: one spare set of Drechsel bottles per sampler, with glass or inert plastic stoppers
or: glass or inert plastic sample bottles, plastic screw caps, at least seven per sampler.
Figure 3.1a AGL 8-Port Sampler