inventoru control Sampling and Analyzing Air Pollution An Apparatus Suitable for Use in Schools Dean M. Rockwell Macomb Sr. High School, Macomb, lL61455 Tony Hansen Engineering Division, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720 Air pollution has been a major problem since the industrial revolution and the development of the internal combustion engine. One of the earliest scientific publications dealing with the analysis of soot and smoke dates back to 1896. Mankind has become so dependent on the burning of fossil fuels that the sum total of all combustion-related emissions now constitutes a serious and widespread problem not only to human health, but also to the entire global environment. This should be the concern of evervone. A logical starting place to raise such consciousness and concern would bein the schools. However, commercial air pollution analvzer ~ r i c e sare outside the limits of most school budgets. T ~ paper S describes two variations of an air sampler and analyzer that are inexpensive to construct and easy to operate. They were designed to be used in an educational program. Their extreme simplicity conveys the clear message that science does not have to be complicated. Comparison of Carbon Particles (Micrograms Per Cubic Meter of Air):
The sampler and analyzer have the additional remarkable advantage of a high degree of accuracy (Fig. 1). Most laboratory air sampling begins by drawing air through a filter using an air pump. White facial tissue (Kleenex brand works well) was found to be a suitable material as a filter because it produces a uniform spot. Two simple and inexpensive methods have been developed to draw air through the filter.
Air Pollution Sampler Construction and Calibration Vacuum Cleaner Sampler The first method uses a vacuum cleaner. Most styles of vacuums will work, especially if they have a hose. The main concern is that a reasonably tight seal must be maintained between the sampler and the vacuum cleaner once the machine is turned on. The sample holder uses three plastic disposable drinking cups. Bore a l-in. diameter hole with a cork bore through the bottom of each cup. Make sure that the holes are aligned. This will produce a colleding spot with an area of nearly 5 cm2. Next cut a ring of corrugated cardboard with a centered l-in. diameter hole. This will act as a filler. Select a suitable wire screen, and
+ J
Outer Cup
Tissue Filter Wire Screen
Figure 1. ScientificResearch Laboratory equipment and procedure vs simplaied school equipment and procedure.
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Figure 2. Air sampler.
Cardboard filler
cut it into a disk that will fit into the bottom of a cup. It should neither be too warse nor too fine (17-19 strands of wire per inch work well). This will serve as a rest for the tissue filter. Finally, cut a paper or plastic mask, also with a matching 1-in. diameter hole. This will direct the flow of air through the filter. Four-ply tissue (for exFigure 3. Flowrate: bag method ample, doubled over Kleenex) will filter the maioritv of the particles. Assemble the apparatus in the foliowing order: two inverted cups cardboard fdler screen filter
mask and cup (Fig. 2).
Position this sampler onto the hose of the vacuum cleaner. A tight seal should be produced when the vacuum is turned on. It is necessary to know the volume of air that passes through the filter during a period of time so that the wncentration of air pollution particles can be determined. Various methods may be used. Agas meter may be hooked UD to the exhaust of the vacuum cleaner while taking an air sample. However, a simpler method also provides good results. Inflate a large plastic bag (for example, a leaf sack) by holding it over the exhaust port of the vacuum cleaner. Measure the circumference along the three dimensions by placing a string or tape measure loosely around the bag (Fig. 3). Determine the volume by using the average circumference and the following formula: V = C3/6$. For convenience, measurements may be made in inches and volume computed in cubic feet that may then be converted to cubic meters (1 ft3 = 0.028 m3). Place the in~ ~ sampl,er. flated bag over the intake port ofthe F , ~ 4.L 'wer air samoler and vacuum cleaner and note thk length of time it takes to suck all the air out of the bag. This will produce the flow (vollmin).
pop bottle housing; the "wet" and the "dry". The "wet" sampler is made by cutting the bottom off the plastic bottle. Cut a slot up the side of the removed base piece, and cut a hole on the flat surface for the electrical wire and the discharge tube to nass throueh - .(Fie. - 4). . Cut a notch at the bottom i f the bottie so that the wire and tube can pass through while the bottle stands upright. Attach the tube. Then put the aquarium pump all the way into the top of the bottle, and support it by pushing the inverted cut-off base piece up as far as it will go. Stand the bottle in a bowl with enough water to cover the notch. This forms the airtight seal. Be sure that the pump and all electrical parts are well above the level of the water. Light woking oil may be substituted for the water. The oil will eliminate evaporation and danger of electrical shock. The "dry" sampler is made by cutting off the upper portion of the bottle. In the lower section, drill two holes that are iust laree enough to nass the electrical wire and the tube througk ~ e m o i teh e plug from the end of the electrical wire. Attach the tube to the pump. Place the pump into the lower portion of the bottle, and pass the tube and the electrical wire through the holes. Reattach the electrical plug. Use a generous amount of glue to seal the wire and tube into their two holes. Use the same procedure to glue the top of the bottle back on to the base (Fig. 5). Cover the joints with several layers of tape. AU the joints must be firm. Test for air leaks in either variety sampler by plugging the pump into an electrical outlet. Be sure that any glue has dried before beginning. Check that air is coming out of the tube; then block the top of the bottle with your hand (Fi. 6). The pump should develop enough suction to begin to deform inward the thin plastic walls of the bottle.
Aquarium Pump Sampler
A simpler version of the aerosol sampler was developed using a n aquarium pump to produce the air flow. The positive pressure flow of the aquarium pump differs &om the negative pressure of the vacuum cleaner. ?b create the negative pressure necessary to draw air through a filter, the aquarium pump is housed in a sealed plastic 2-L ~ o d a - bot~o~ tle. There are two versions used to provide an airtight seal for the soda- Figure '. 'Dry" sample' Volume 71
Number 4 ADri1 1994
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finer
of water (for example 1 L). Invert the water bottle into a pan of water. Put the discharge tube into the top of this bottle (Fig. 8).Measure the length of time it takes before all the water in the second bottle has been expelled. Continually adjust the depth of the bottle so that the water levels are approximately equal between the water inside the bottle and the water in the dish outside the bottle. This will avoid any effect of pressure differences.
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Analyzer Construction and Operation Blocked opening: Water rises inside
No filter:
Water levels equal
Figure 6. Air leak check. Upper Magnet TlSSUe
Filter
Bottle Cap
Figure 7. Filter holder. this draw water into the In the ''wet" bottle. Stop before the water touches the pump. The Filter Holder
A sample of airborne particles will appear as a spot on the tissue filter. Spots are generally either brown or gray in color. Brown spots are normally caused by dust particles. The dust has a minimal affect on the light absorption readings of the analyzer. Other pollutants are oRen colorless and have no effect on the light transmission analysis. The gray is caused by soot (suspended carbon particles). Because carbon is intensely black, it may be detected in small amounts. Carbon particle emissions are common to all types of combustion. Thus, a measurement of carbon particles in the air is a good indicator of the pollution. The simplest method of determining the concentration of carbon articles is to comDare visuallv the erav s ~ otot a known sample. Several scaies are avahable-that Include samples ranging in small increments from white to black (Fig. 9). This method provides quick and satisfactory results, but it is normally not as accurate as the other analvzers because it does d e ~ e n don the ~ e r c e ~ t i o of n sthe individual. A more accurate method or determining the amount of black carbon particles on the filter involves comparing the amount of light that passes through the spot with the amount that passes through the white area of the
Filter in place Only sllght rlse
The filter holder for either variety of the simple air sampler is made in the following manner. Obtain two small flat magnets with a hole through their center. These magnets frequently are found in hobby or hardware stores. Drill a hole, that is somewhat larger than the hole through the magnet, into the cap of the soda bottle. Glue one magnet to the cap (be careful to form a tight seal). The tissue filter is then placed on top of this magnet. Twoply tissue (a regular Kleenex) works best with the aquarium pump because the pressure is not nearly as strong as generated with-the Figure 8. Flowrate: water displacement. vacuum cleaner. The second magnet is put on top to clamp the filter to the first magnet (Fig. 7). The hole in the magnets determines the aerosol spot size. Measure this hole and calculate t h e area. I t is typically around 0.2 em2. The volume of air flow for both varieties of "aquarium pump" sampler .! $2 are calibrated by water displacement. Plug the pump into an outlet and check that air is exiting the 0 2.0 4.0 6.0 8.0 tube. Screw the filter-holder cap onto the bottle top and put a piece of smcamon, n r w r a m r p r s q . c m . ( p p m ~ ) filter between the magnets. Fill a second bottle with a known volume Figure 9. Visual scale: black carbon concentrations.
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Journal of Chemical Education
11.0
['%y
Tissue Filter
/
Black Electricians Tape With Hole Silicon Solar Cell Mounted to Lld 01 Clear Plastic Box
Figure 11. Simplified black carbon analyzer. Simpler Analyzer
Wwden Base Sillcon Solar Cell W~redto Nail Terminals
Figure 10. Black carbon analyzer: light box method filter. This can be accomplished with the use of a silicon solar cell. Two variations of housing the silicon solar cell may be used. Light Box Method
A wooden base (12 cm x 12 em) houses a silicon solar cell in a shallow de~ression.The actual dimensions of the base pieces, filter holders, and light box are not critical. Six screws are placed on the upper surface to standardize the position of the filter holder. The filter holder is made of two sheets of metal (9 cm x 9 cm) with a hole drilled through both. An indented thumb space also may be cut into the top surface to make positioning of the holder easier. The wires of the cell pass through a hole in the base. The upper base is mounted on a second (18 cm x 18 cm) base of wood that has a shallow groove cut into its upper surface for passage of the wires. The wires are soldered to two small nails that will act as terminals (Fig. 10). The top light box (12.5 cm x 12.5 cm x 16.5 cm) houses a ceramic ceiling fixture and a 10-Watt light bulb. The box may be made of metal or wood. If wood is used, be careful not to have it overheat by leaving it plugged in for long periods of time. Slight fluctuations in readings may be caused by voltage irregularities from the electrical outlets. These variations may be acceptable and may even prove a useful tool for teaching the value of averaging. One may avoid the problem by either using a constant voltage transformer, or by substituting a battery as a power supply. The purpose of the base and light box method is to supply a constant intensity of light at a constant distance from the sample. Analysis of a sample begins by positioning the gray spot between the two holders and placing the holder into position above the solar cell. The light is turned on and placed into position on the base. Attach a digital multimeter to the terminals and set the readout to measure direct current in milliamperes [DCmAI. Change the position of the filter so that a second reading is taken from the white area.
A second and simpler method also will provide reasonable results. Connect the wires to the silicon solar cell and mount the cell to the lid of the plastic box in which it comes. Melt a small groove into the plastic to allow the wires to pass out of the box. Punch a small hole into black electricians tape. The opening must be slightly smaller than the size of the pollution spot collected by the sampler. Place the tape over the lid of the box with the bole directly above the solar cell. Alight (desk lamp or even bright ceiling lights) should be positioned above the analyzer. Attach the multimeter to the wires protruding from the plastic case (Fig. 11).Set the digital readout multimeter to measure direct current in milliamperes [DCmAl. Readings are taken by positioning areas of the pollution sample and areas of the white tissue above the hole and solar cell while keeping the position of the cell constant relative to the light source. An average of five sets of readings produced statistically valid results. Mathematical Formulas
The s t e ~of s the mathematical m a n i d a t i o n of the sample follows below Do NOTcalculatc any quantities to more precision than two significant figures. T h s is meaningless. Flowrate: Vacuum Cleaner Method This is determined by using either the plastic bag technique or the gas meter. One can either directly use metric measurements to determine volume or convert from cubic feet. Flow rate = volume (ft3)x 60 dmin x Vtime x 0.028 conversion = cubic meters per min
where time = s needed to deflate the bag or run the meter. Flow Rate: Water Displacement Method Flow rate = volume (L)x 60 dmin. x 1 /time x 1m3/1000~ = cubic meters per minute where time = s needed to displace the volume of water. Attenuation Attenuation = 100 in (blanWspot) where blank =milliampere reading of the white area ofthe filter, and spot = milliampere reading of the pollution spot collected. Note: this must be the natural logarithm which is 2.3 times the common logarithm of base 10. There are calculators that do this with just one touch of the button. Black Carbon Density Black Carbon Density = Attenuation112 The units of this are micrograms per square centimeter. I t is amazing that you can see a'microgram. Volume 71 Number 4 April 1994
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Micrograms of Black Carbon in the Spot Sample Micrograms =Densityx Area of the original collection spot (em2) Example: total micrograms collected on the 5-cm2 spot using the vacuum cleaner method. Concentration of Blazk Carbon in the Air Micrograms Concentration = [Flow rate x Duration of air sample (min)l =Micrograms per cubic meter of air v p i c a l levels are: less than 1, clean rural areas 1-10, cities or towns with fireplaces over 10, heavily polluted areas Classroom Applications Samples of air pollution obtained by this method may be used from the early grades of school to advanced levels of research. Some suggestions for the use of the sample will be given. You are encouraged to develop your own ideas. Young grade school children may be introduced to pollution of the air by looking a t the spot that was collected. They may then be able to visualize how "dirty" the air is. The students may design their own shape for the mask placed over the filter. For example, they might use their initials. Thus, after collecting a sample, they can see their name written in pollution. Older grade school children may conduct some simple scientific observations. Along range study can be done by comparing samples collected for the same length of time on different days. For example, the sample may be taken every day for a month; or once a week for a year. Arrange the samples side by side on a bulletin board or chart so that they may be compared. Does the blackness change? What a r e some reasons that cause the change? The students could monitor the direction of the wind and relate pollution to weatherconditions. Does the mot chanee whenthe wind is blowing from a factory or a large city? Does the spot change before and after a rain? Is there a change in the spot from fall to winter (when we heat our homes) to s~rinz? -students in the middle grades may determine carbon particle concentrations by comparing the intensity of the
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Journal of Chemical Education
spot with a chart that has a range from white to black. They may want to perform similar observations to those already mentioned, or conduct some experiments. Is there a difference between the exhaust of a car and that of a truck? Does a car with a catalytic converter burn cleaner than one that does not? Does a wood fire uroduce more soot than a coal or a gas fire? How would ihe air in a home where people smoke cigarettes compare with one where the family does not smoke? Finally, advanced students may analyze the spots using the silicon solar cells and perform the necessary mathematical calculations to determine carbon concentrations. These students may perform and analyze the observations or experiments previously mentioned; or better still, they may develop their own. Remember that a scientific experiment is simply a n attempt to find the answer to a question. What questions would they like to ask? Other topics may be related to the study of pollution a t this level. The students could discuss the classic example of soot pollution and the "evolution" of the peppered moth (Biston betularia). Students also could research articles on pollution and health problems. Acknowledgment This apparatus was developed during DR's summer residency at the Lawrence Berkeley Laboratory, Berkeley, California. The work was supported by the US. Department of Energy Summer Teacher Research Associate Program, 1989. Our thanks to the members of the Atmospheric Aerosol Research Group for their support. This article is based in part on a paper presented at the Environmental Conference for Chemistry Teachers of Slovenia, May 1992. General References 1. Gerber,H.E.: Hindman, E. E. Ljghf A b q t i o n by AemsdPodlcks. Sp-Preaa: Hampbton,VA. 1982: p420. 2. Goldbere. E. D. Block C o r b n in fhr E n u i m m n t . Wilev: New Ymh.1985: D 198. 3. Gnndel, L. A ; h d , R. L.; h a e n , H.: Nwakov, T. Seieieop of thr Totd Envimnmrnt 1984,36,197-202. Dod,R.L.; Navakav.T.Atrnoapk%Auml~rehAnnual 4. Hanaen, A,; Roam, H.; Rappon. 1919. LawreneeBerkeleyLaborataryRepartIgL11650UC Berkeley: CA 1979; pp 816821. 5. h e n , H.:Hanaen, A U . A: Dod,R.L.: Nwakov, T. Seiena 1880,208,741-744. 6. &sen, H.: NovaLov, TApplrod Optics 1989,22,126&1267. 7. Wolff, G T;Klimisch, R. L. Portieulate Carbon: Amspheric Lib Cyclp. Plenum Press: NY,1982: p 412.
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