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AirWatch - Particle sampling by P.C. Manins and M.L.
Finn SUMMARY The ability to measure concentrations of fine particles in the air is the most important component of the CSIRO kit of experimental equipment for the Airwatch schools and community program of hands-on learning and investigations of air pollution issues (Manins et al., 1998). This article describes in detail how particle sampling is done with the CSIRO kit. INTRODUCTION A companion paper, AIRWATCH – The CSIRO Kit (Manins et al., 1998), describes the Airwatch program and the various experiments in it. The most important topic addressed is the measurement of fine particle concentrations using the filter soiling technique. The steps described here are
BACKGROUND INFORMATION This activity can be carried out in a school ground, in the participant's front yard, or further afield depending upon the availability of suitable sampling sites and the problem being studied. Examples of sampling sites suitable for this experiment are: a busy road (either alongside a freeway or major intersection), an industrial site or factory location, suburban street/residential block, inside a house which is heated by an open fire, in a car park (covered, rather than open air). It might be instructive to make a comparison between a school near a busy road or industrial area and one that is not. The type of sampling used in these experiments is 'active low volume sampling'. Active (as opposed to passive) refers to the presence of a pump to draw air through a filter holder. Low volume (as opposed to high volume) refers to the amount of air pumped through over a given time. A typical low volume sample period would be 24 hours; using the equipment and methods described here, it would sample 3.3 m3 of air over the 24 hour period. The measurement is of the mass of soot in the air. This is done by observing the soiling of a sample filter and relating the colour to a calibration chart that gives the mass of carbon on the filter. This is not the same as a measurement of 'total suspended particulate matter' nor PM10 nor PM2.5 (the concentration of particles with aerodynamic diameters less than 10 or 2.5 µm respectively), nor other conventional measures of particles in the air done using modern expensive monitoring equipment. The Airwatch particle measurement is similar to but not the same as a measurement of 'black smoke' by filter soiling, a measurement that has been used widely in Europe. There are current OECD, UK (BS 1747) and Australian (AS 2724.2-1987) standards and guidelines describing the methods. But the 'black smoke' measurement has been largely set aside as better understanding has led to the realisation that any relationship between filter soiling and concentration of fine particles in the air varies according to many uncontrolled factors. The Airwatch sampler measures directly the concentration of fine soot (carbon) in the air. Recent comparison with precision measurements in four cities in Australia has shown excellent agreement with Airwatch. To estimate the concentration of fine particles in general, requires additional information about the conditions in the region in which the measurements are made. Some unpublished tests suggest that if these conditions are taken into consideration, Airwatch soot measurements can be scaled by a factor to give PM10 values that can then be compared with air quality standards. Internationally, ambient air quality standards for PM10 particulate matter are in the range of 50–150 µg m-3 for samples over 24 hours. In Australia a new national ambient air quality standard has been set (NEPC 1997): a PM10 value no more than 50 µg m-3 over 24 hours. EQUIPMENT The equipment needed and possible sources are discussed in the Appendix. The most important thing to note is that the 'grey scale' calibration sheet that is used to relate filter soiling to mass of soot on the filters must be an original print for accurate use. It is available from the authors.
Figure 1 shows the essential components of the sampling setup. A cheap filter in the filter holder collects the particles. The flow meter measures how much air is sucked through the filter by the pump. The battery is necessary for operation away from mains power. If power is available and one of the recommended battery trickle chargers is used, the pump can be powered directly from the charger. In practice the setup depends on the sampling strategy. If sampling near a road or some other area, a long enough piece of tubing should be used between the filter holder and the flow meter so that the holder can be set up about 1.5 metres above the ground (at the height where you would be breathing). The filter should be pointed downwards so that dust does not fall into the inlet and so the wind cannot directly blow into it either. Similar rules should be followed if sampling from a bicycle or car, or following some other strategy.
Figure 1. Basic particle sampling equipment for Airwatch SAMPLING STRATEGIES Twenty four hours is a long time to sample, but this length of time is necessary to obtain sufficient sample in relatively clean Australian urban areas. Longer or shorter sampling times may be necessary in other regions. There might only be interest in the pollution during daytime, not night time, or only during morning peak traffic times, or while bicycling to work, or some other plan where continuous 24 hour sampling would not give the required result. This is achieved by repeatedly sampling at the times required until about 24 hours of sample has been obtained. For example for a roadside monitoring strategy this might mean sampling every morning at peak hours from 7am through to 9am (2 hours) for a total of 12 week days only. Then an adequate sample should be obtained. If there is only interest in high pollution days, then the weather needs to be considered. Particle pollution will be low on rainy days or soon after rain. Windy days are also likely to be days when (soot) particle pollution is low in urban areas. If sampling is restricted to particular kinds of days, a log of the dates, times and weather conditions should be kept. The results should be related to the weather and activity in the neighbourhood. METHOD
Table 1. PARTICLE SAMPLING TABLE OF RESULTS
CALCULATIONS The printed grey scale calibration sheet is used to compare an aerosol filter paper with a scale of greyness vs weight of carbon. The scale, shown in Manins et al., (1998; Figure 3) gives the weight of soot in micrograms (µg) for each stage of greyness.
For reference, the calibration curve for the grey scale is given in Figure
2. Using the recommended pump powered from a battery, and sampling for
24 or 12 hours, the sample volume that can be expected is 3.3 or 1.7 m3
of air respectively. The atmospheric soot concentrations follow from Step
4, and these are given in EXAMPLE : A sample was collected over 24 hours in a back yard. The printed scale number best matching the colour of the filter is 5; a sample volume of 3.3 m3 was measured. From Figure 2 there are 35 µg of aerosol on the filter. So the soot (carbon) aerosol concentration measured in the air was
Figure 2 (left) and Table 2 (right). Masses and equivalent atmospheric particle concentrations when sampled with the Airwatch pump for 24 or 12 hours, assuming a flow rate of 2.3 litre per minute.
INTERPRETATION To interpret the reading of soot concentration in terms of air quality standards, the reading must be scaled to be an estimate of general fine particles, not just carbon. There are many other kinds of particles in the air, and these depend on the season, condition of motor vehicles and fuel types, and other things such as use of wood for heating, bushfires, etc. The multiplier factors presented in Table 3 have been derived from data in Australian cities during the winter of 1997. Table 3. Scaling factor for particle sample to estimate PM10.
* 'fires' includes wood heaters, scrub or rubbish burning. A description that most closely matches the general region and conditions where the measurements were made should be picked from Table 3 — the surrounding kilometre or so if outdoors, the whole house if indoors. Then the concentration measured with the Airwatch sampler should be multiplied by the factor from Table 3; the result is an estimate of the PM10 fine particle concentration. EXAMPLE : The measured soot concentration was 11 µg m-3 over 24 hours in a busy neighbourhood with some litter burning in the evening. The multiplier is 6 from Table 3. The estimate for PM10 is PM10 = 66 µg m-3. Now the findings can be compared with those from the local EPA and with national and international guideline values and standards. If the local standard is that PM10 should be no more than 50 µg m-3 over 24 hours, then the example indicates unacceptable conditions. REFERENCES Manins, P.C., Finn, M.L. and Anderton, J. (1998), ‘AIRWATCH – The CSIRO kit’, this publication. NEPC (1997), ‘Draft National Environment Protection Measure and Impact Statement for Ambient Air Quality’, National Environment Protection Council Service Corporation, Adelaide, November. APPENDIX Equipment for Particle Sampling Prices mentioned are in Australian dollars; they and the suggested suppliers are given as a guide only. Filter Holder The inlet diameter and geometry is important since it acts as a size-selective inlet. A screw-down type filter holder is adequate for the particle sampling filter. A clip-down type would be better since it is less likely to cause damage to the filter. There are several suppliers. One is:
A brass nipple is required for each filter holder so that they can be connected to other parts of the setup via plastic hose. Part required : 1 x ¼ inch female / ¼ bsp brass fitting. $1.95 each
Filters These are low-cost glass fibre filters with a nominal pore size of 1 µm and with no binder. Whatman type 934AH, Millipore type APFA and Gelman type A/E are all suitable. One supplier is Gelman Sciences. Product Number 61631. Type A/E. Glass Fibre Filter. 47 mm. 100/pkg
Fish Tank Pump A 12 volt DC type with an inlet port is the easiest to use, but any voltage may be used if power can be maintained for long enough. Pumps with no inlet port may be useable if they can be disassembled and the valves changed over so the pump sucks rather than blows. A flow rate of 3–4 litres per minute is probably adequate. Other flow rates may require a change in the sampling time to give filter soilings within the range of calibration of the Sampler. Aerator pump with inlet port. Rate approximately 3.3 litres/minute. 12 volt DC 3 Watt. $38 domestic postage included.
Gas Meter This is a small version of a domestic gas meter, and is used here to measure the volume of air passed through the filter. They seem to be specified by flow rate. Approximately 2 m3 hr-1 is the smallest available in Australia. One suitable meter is:
The price includes the brass fittings needed to connect the meter to the filter holder and to the pump. Modification of the fittings is required. Screw the fittings into place on the inlet and outlet ports of the meter. Cut off the thread on both sides. Braze a 6 mm copper tube to each connection, bending it outwards. This enables a 6 mm plastic hose to be connected to and from the flow meter. Another type is made in Australia by Email Gas. It is rated at 6 m3 hr-1 and is physically larger too. Part Number Domestic 610 $121 plus postage
Battery While an ordinary car battery can be used and will work well for a few experiments, it will fail prematurely. It is not designed to supply current for long periods without recharging. Better is a deep-discharge sealed lead-acid battery. The Portalac PE12V17 17 A hr sealed lead-acid battery works well. Approx. cost $59 - $85 each
Battery Recharger, Wires and Alligator Clips 500 mA charge rate at 12volts (a 17 Ahr battery will take at least a day and a half to recharge from a fully discharged state with this charger). Approx. cost $30 each. A local dealer will be in the phone book SUPPLIERS: eg. Jaycar Electronics or Dick Smith Electronics Battery Maintenance A fully charged lead-acid battery should read approximately 14.7 volts open circuit on a multi-meter. The life of the battery will be greatly diminished if the battery is allowed to fall below 12 volts. Testing of the recommended battery with the 12 V pump has shown that the battery provides enough power to sample continuously for 24 hours. It is important to note that a trickle charger should be used when recharging the battery (charge current less than 1 Amp for the specified deep-discharge battery). It is too easy to damage the battery with a high-current charger. Ensure that the battery is fully charged before the sampling period commences. At the end of a sampling period the battery will need recharging; and this will take at least 24–48 hours using the trickle charger. Plastic Tubing Specifications : 6 mm clear plastic hose - available from hardware suppliers, plastic specialists. Plastic Bags Snap lock resealable bags. 16 cm x 17 cm. packs of 30 - available from some supermarkets. Grey-Scale Calibration Sheets These are accurately calibrated colour prints matched to calibration filters. Original prints must be used. Any other rendering of the calibration sheets such as by photocopying will make the accurate determination of concentrations impossible. Contact the Airwatch organisers to obtain the calibration sheets.
Contact: peter.manins@csiro.au Return to The Airwatch Index |
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Modified: 16 May 2002 |
