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An isokinetic sampler for use on light aircraft
Atmospheric Environment Pergamon Press 1972. Vol. 6, pp. 191-196. Printed in Great Britain.
AN
ISOKINETIC ON
LIGHT
SAMPLER
FOR
USE
AIRCRAFT*
GILBERT S. RAYNOR Brookhaven National Laboratory, Upton, Long Island, New York, U.S.A.
(Received 15 September 1971) Abstract--A light aircraft-mounted isokinetic sampler was designed for obtaining accurate samples of heterodisperse particulate matter in the lower atmosphere. The sampler head is operated in undisturbed air under the wing and is drawn into the cabin along a track for changing filters. A battery-powered, high volume sampler in the cabin serves as the air moving device. The sampler can draw as much as 0.72 m3min- 1 and match aircraft speeds to 38 m s - 1 at standard temperature and pressure. The sampler is being used for measuring the vertical distribution of airborne pollens and industrial air pollutants and for determining the size distribution and composition of power plant stack effluents. INTRODUCTION
IN CONNECTIONwith two separate studies conducted by the Meteorology Group at Brookhaven National Laboratory, the need arose for a means of taking representative and quantitative samples of particulate matter at altitudes from 100 m to about 3 km. One study, in cooperation with the Department of Applied Science (TUCKER, 1969), required samples of power plant and industrial stack effluents as part of a comprehensive study of SO2 in the atmosphere and its interactions with or conversion to particulate matter. The other study, in cooperation with the New York State Botanist's Office, required samples of airborne pollens as part of an investigation into transport and diffusion of pollens over distances to 100 km. The sampler described herein was designed for use in both projects. Its usefulness in these studies suggests that it may find application in other programs requiring accurate sampling of particulates aloft. Isokinetic sampling has long been recognized as the only way to obtain an accurate and representative sample of heterodisperse particulates large enough to deviate from an air stream whose velocity is changed abruptly as by a sampler entrance (WATSON, 1954). However, the method can seldom be used when sampling from a fixed position due to the formidable problem &matching the intake flow rate to the rapid fluctuations in speed which are normal in a turbulent atmosphere. Application of isokinetic sampling to fast moving vehicles presents fewer problems and at least two such samplers designed for large, multi-engined aircraft have been described (TORGESONand STERN, 1966; TIMMONSet al., 1966). Several samplers with non-isokinetic entrances have also been used in large aircraft (KELLY et al., 1951; HOLZAPEELand GRESSITT, 1964; BURLEIGHet al., 1967). NICKOLA(1966) instrumented a twin-engine aircraft for real time sampling of fluorescent particle tracer material but did not state if sampling was isokinetic. HARRINGTON(1965) described a light aircraft-mounted pollen sampler with a non-isokinetic entrance. MCCALDIN and JOHNSON (1969) and ADAMS and KOPPE (1969) used non-isokinetic inlets to optical particle counters mounted in single and twin-engined aircraft respectively. LANGERet aL, (1968) described an instrumented * This research was carried out under the auspices of the U.S. Atomic Energy Commission and the New York State Museum and Science Service and was partially supported by Research G r a n t No. AP-81 to Dr. E. C. OGDEN from the National Air Pollution Control Administration, Consumer Protection and Environmental Health Service, U.S. Public Health Service. 191
192
GILBERT S. RAYNOR
twin-engine research aircraft whose equipment included an aerosol counter but did not specify the type of inlet. MAcCREAOY and TODD (1964) instrumented single and twin-engine aircraft with cloud particle samplers in which particles impact on Formvar coated film. WEBB et aL (1970) used a helicoper-mounted high volume sampler for air pollution studies but made no provision for isokinetic entrance. Thus, although small planes are widely available and frequently used for atmospheric studies, isokinetic sampling apparently has not been previously adapted to them. DESIGN CONSIDERATIONS Several considerations determined the basic sampler design. Since Cessna 172 and 182 aircraft, instrumented with meteorological and gas sampling equipment, were being used in related studies (TUCKER, 1969), the sampler was constructed to fit these aircraft but can easily be modified for use on other models. It was necessary that the entrance area and the flow rate through the system should produce a linear speed into the entrance within the range of aircraft cruising speeds. The largest feasible flow rate was desired to keep the sampling time reasonably short since low particle concentrations were expected at times and as many as 20-30 samples were planned for single flights. Since excess electrical power from the aircraft was limited and used for other instruments, battery power was selected for the sampler. To avoid loss of particles in tubes or ducts, the sample collection device was located as close to the air intake as possible. DESIGN The sampler which is diagrammed in FIG. 1 and illustrated in FIG. 2 consists of five sub-sections: (1) the sampling head assembly, (2) the track assembly, (3) the replacement window, (4) the air moving and monitoring unit and (5) the power source. The sampling head assembly and the air moving unit are connected by 3.8 cm i.d. flexible, plastic hose. [~
Lr-L~BLEEDER ~ q I VALVE | T | HIGH ~ VOLUME SAMPLER
PRESSUREGAGE /" . PITOTTUBE FLEX HOSE
CONTROL VALVE 24 V dc POWER PACK
DIFFERENTIAL
/FILTER ~
AIR FLOW
TRACK MOUNTED FILTER HOLDER& ENTRANCE CONE
FIG. I. Block diagram of the aircraft isokinetic sampler. The sampling head assembly includes a~a aluminum base section 11.6 × 12.6 cm fitted with nylon bushings which facilitate movement along the track, and a short pipe for connecting the hose. It also includes a seat for the filter holder as well as pins and a safety catch for mounting the entrance cone. The filter holder (FIG. 3) which was designed for 102 mm dia. filters is 6.35 mm thick, with a 108-mm o.d. and a 90-mm i.d. It includes a base section, coarse and fine stainless steel screen supports, a teflon washer and a retainer ring with a groove around its outer circumference. When assembled, the filter lies above the fine screen and below the teflon washer. The assembly is secured by three set screws in the wall of the base
An Isokinetic Sampler for Use on Light Aircraft
193
section which engage the groove in the retainer ring. Cut-outs in the wall of the filter holder seat allow easy placement and removal of the filter holder. Any flat type filter which gives adequate flow rate could be used but vinyl metricel and nuclepore filters with 8/~m pores are used in the sampling programs at Brookhaven. Filter holders are pre-loaded, then stored and carried in the box shown in FIG. 2. The aluminum entrance cone is fitted with twist-lock slots which engage the pins in the base section. The cone is 29 cm long with a 2.0-cm entrance diameter. The inner diameter of the base is 9.0 cm and the outer 12.6 cm. After selection of the entrance and base diameters, the length was determined by the 7.5 degree maximum taper used in diverging nozzles to avoid generation of turbulence within the cone. In operation, the sampling head assembly is positioned at the outward end of the track near the aircraft strut (FIG. 4). Although no tests were made of flow around the aircraft, the entrance is believed to be in undisturbed air since it is well forward of the strut, about 30 cm below the wing and far beyond the region disturbed by the propellor. When not in use or when changing filters, the assembly is drawn into the aircraft cabin. The track assembly consists of two 12.7-ram dia. stainless steel rods 1.9 m long spaced 10.2 cm apart by brackets equipped with nylon rollers for easy movement of the hose. The outer end of the track terminates in a bracket which clamps around the strut of the aircraft. Near the inner end, which passes through a hole in the window, a bracket fastens the track to the window. Shockmounts are used here since the aircraft wing and strut flex somewhat in flight. Stainless steel cables 1.59 cm in diameter around pulleys at each end of the track are used to move the sampling head assembly between the cabin and the sampling position. All angles on the window and strut brackets and on the sampling head base were calculated so that the entrance cone faces directly forward in flight. The window is constructed of 6.35 mm thick Lexan (high impact, transparent plastic) and replaces the left aircraft window which is hinged up and secured under the wing. The window contains a suitable-size opening to allow passage of the sampling head assembly and a removable cover to close most of the opening in flight. Latches secure the window to the window frame. The air moving assembly consists of a 24-V d.c. high volume sampler with an adaptor piece which screws on in place of the usual filter holder and terminates in a control valve and a 9-cm length of aluminum pipe to which the hose is attached. A miniature pitot tube is mounted in the pipe and connected to a differential pressure gauge to monitor air flow through the system. A bleeder valve is used to augment the flow through the sampling system and maintain adequate cooling flow through the high volume sampler. The power source consists of two 12-V, 90-A-h lead-acid batteries connected in series, mounted in a carrying case and equipped with a plug for connection of the sampler power cord. The apparatus was designed for structural strength, ease of mounting and minimum interference with aircraft performance. Its use required FAA inspection and approval and places the aircraft in the restricted category (FAA form 337). FLOW SPECIFICATIONS The sampler was designed to match aircraft speeds of 27-38 m s-1 (60-85 m.p.h.) and can match the higher speed using several types of filters and fully charged batteries.
194
GILBERT S. RAYNOR
The system is normally operated at 35.7 m s-1 (80 m.p.h.). At standard temperature and pressure (STP), this linear speed at the entrance is given by a flow rate of about 0.67 m a min-~ (23.7 ft 3 min-1). This flow rate gives a speed of 14.3 m s -~ past the pitot tube which corresponds to a pressure differential of 0.45 in. of water at a pressure drop of 1.5 in. Hg. across the filter. When sampling is conducted at higher altitudes or at non-standard temperatures, varying air density must be accounted for and two modes of operation are possible. In mode one, the pilot flies at true air speed and the flow rate through the sampler is adjusted to match. This necessitates knowledge of temperature and pressure but results in the same volume of air space sampled per unit time under all conditions. In mode two, which is normally used, the indicated air speed and indicated flow rate are kept constant. True air speed, true flow rate, and true entrance speed then remain proportional to each other since they vary by the same ratio with density changes. Thus, sampling is always isokinetic although the air speed being matched by the entrance speed and the volume of air space sampled do change with altitude. However, the volume is calculated after the flight by use of temperature and pressure-altitude measurements. OPERATION
The sampler can be mounted in the Cessna aircraft without removing seats although the left front seat is normally removed for mounting associated meteorological instruments and recorders on the seat track. The batteries are secured in the luggage compartment behind the rear seats. Installation takes about 15 min. Filters are loaded in their holders in a clean area before each flight and carried in the box shown in FIG. 2. Upon arrival at a sampling location, afilter holder is inserted, the entrance cone attached, and the sampling head assembly pulled into sampling position. The sampler is operated for a pre-selected length of time while the pilot circles at a constant altitude over the desired location or maintains a track in a plume. The assembly is then pulled in and the filter holder replaced while the pilot proceeds to the next altitude or location. Meteorological measurements may be taken simultaneously. Flights are limited to about 5 h without refueling. Life of the batteries is about the same length at continuous operation, but shorter when the sampler is turned off and on frequently. Several flights have been made to sample ragweed pollen, industrial air pollutants and power plant stack effluents. Five-minute samples were found adequate for pollen up to 6000 ft and 1-5 rain samples for stack effluents depending on dispersion conditions and distance from the stack. A N A L Y S I S "OF S A M P L E S
Treatment of the exposed filter depends on the material collected and the type of information desired from the sample. For pollen analysis, the 102-ram membrane filter is dissolved in a suitable liquid in a filter funnel and the liquid filtered through a 25-mm filter of a type not affected by the liquid used to dissolve the large filter. The small filter is then mounted on a microscope slide in pre-stained glycerin jelly for microscopic examination. If the sample is taken on a filter with adequate tensile strength such as the nuclepore, the filter can be reverse flushed without being removed from the holder and the material concentrated on a smaller filter.
FIG. 2. Photograph of the aircraft isokinetic sampler and the filter holder carrying box. Components are described in the text.
Fie. 3. Assembled and disassembled filter holder. Components include in order: base section, coarse and fine screen supports, falter, teflon washer and retainer ring.
(facing page 194)
FI~. 4. Aircraft isokinetic sampler mounted on Cessna 182 aircraft.
An Isokinetic Sampler for Use on Light Aircraft
195
Stack e&tents and air pollutants collected on nuclepore filters can be examined under either a scanning or transmission electron microscope to determine particle shape and size distribution. They may also be analyzed chemically or evaluated gravimetrically if collected in sufficient quantity. Other aids to particle identification which might be used are electron microprobe microscopy, neutron activation analysis, X-ray fluorescence and emission spectroscopy. PRELIMINARY
RESULTS
Two flights have been made to measure the variation in ragweed pollen concentration with height over fixed locations. Results are shown in FIG. 5 where the solid lines represent the time-weighted means of 5 and 10 min samples and the dashed lines the actual values of the two samples at each altitude. Differences between 5 and 10 min samples were not systematic and almost identical concentrations were found over both time periods at the highest altitude reached in each fight. During flight 2, simultaneous measurements were taken at 1.5 and 108 m on the Brookhaven meteorology tower and are also shown.
0
IO
20
30
CONCENTRATION
40 (GRAINS
50
60
70
n?,
FIG. 5. Graph showing ragweed pollen concentration variation with height on two sampling flights. The solid lines represent the time-weighted means of 5 and 10 min samples and the dashed lines the actual values.
Two flights have been made to collect power-plant stack effluents at varying distances from the source. A variety of particulates, some as small as 1 x 10m2pm, werephotographed in a preliminary examination of the filters with an electron microscope. These collections are now being analyzed to determine particulate size distribution and composition. Acknowledgenlents-LEsrER A. COHEN of the Meteorology Group and ROBERT W. DILLINGHAM Instrumentation and Health Physics Department assisted in the design and construction sampler. Funds for construction were provided by the Department of Applied Science.
of the of the
REFERENCES ADAMS D. F. and KOPPE R. K. (1969) Instrumenting light aircraft for air pollution research. J. Air Poll&. Control Ass. 19, 410415. BURLEIGH J. R., KRAMER C. L. and COLLINS T. J. (1967) A spore sampler for use in aircraft. Phytopathol. 57,434-436.
196
GILBERT
S. RAYNOR
HARR~NGTONJ. B. (1965) Atmospheric Pollution by Aeroallergens: MeteorologicalPhase (1 March 1962 to 28 February 1965), Atmospheric Diffusion of Ragweed Pollen in Urban Areas. Vol. II: text. The University of Michigan, Ann Arbor. HOLZAPFEL, E. P. and G~~ssrr-r, J. L. (1964) Airplane Trapping of Organisms and Particles. Proc. AtmosphericBiology Co& Univ. of Minnesota, 13-15 April, 1964, pp. 151- 162. NASA, Washington, D.C. KELLY C. D., PADY S. M. and POLUNIN N. (1951) Aerobiological sampling methods from aircraft. Can. J. Bot. 29, 206-214. LANCER G., BITER C. and DAXHER A. (1968) An automated aircraft instrumentation system for cloud nucleation studies. Bull. Am. met. Sot. 49,914-917. MA&READY, P. B., JR. and TODD C. J. (1964) Continuous particle sampler. J. appl. Met. 3,450-460. MCCALDM, R. 0. and JOHNSON, L. W. (1969) The use of aircraft in air pollution research. J. Air Pollut. Control Ass. 19, 405-409. NICKOLA P. W. (1966) Instrumenting of the Queen Air Aircraft for Sampling of a Zinc Sulfide Atmospheric Tracer. In: Pacific Northwest Lab. Annual Report for 1965 in the Physical Sciences. Vol. 1, Atmospheric Sciences, pp. 11-13. Report No. BNWL-235 I. Pacific Northwest Lab., Richland, Wash. TWMONS D. E., FIJLTON J. D. and MITCHELL R. B. (1966) Microorganisms in the upper atmosphere. I. Instrumentation for isokinetic air sampling at altitude. Appl. Microbial. 14, 229-231. TORGESON W. L. and STERN S. C. (1966) An aircraft impactor for determining the size distributions of tropospheric aerosols. J. appl. Meteorol. 5, 205-210. TUCKER W. D. ed. (1969) The Atmospheric Diagnostics Program at Brookhaven NationaI Luboratory: Second Status Report. BNL 50206 (T-553) Brookhaven National Laboratory, Upton, New York. WATSON H. H. (1954) Errors due to anisokinetic sampling, Am. Znd. Hyg. Ass. Quart. 15, 21-25. WEBB J. C., KINCHEN J. C. and SCARBERRYJ. E. (1970) Aerial sampling by helicopter using a high volume sampler. J. Air Pollut. Control Ass. 20,453-455.
AN
ISOKINETIC ON
LIGHT
SAMPLER
FOR
USE
AIRCRAFT*
GILBERT S. RAYNOR Brookhaven National Laboratory, Upton, Long Island, New York, U.S.A.
(Received 15 September 1971) Abstract--A light aircraft-mounted isokinetic sampler was designed for obtaining accurate samples of heterodisperse particulate matter in the lower atmosphere. The sampler head is operated in undisturbed air under the wing and is drawn into the cabin along a track for changing filters. A battery-powered, high volume sampler in the cabin serves as the air moving device. The sampler can draw as much as 0.72 m3min- 1 and match aircraft speeds to 38 m s - 1 at standard temperature and pressure. The sampler is being used for measuring the vertical distribution of airborne pollens and industrial air pollutants and for determining the size distribution and composition of power plant stack effluents. INTRODUCTION
IN CONNECTIONwith two separate studies conducted by the Meteorology Group at Brookhaven National Laboratory, the need arose for a means of taking representative and quantitative samples of particulate matter at altitudes from 100 m to about 3 km. One study, in cooperation with the Department of Applied Science (TUCKER, 1969), required samples of power plant and industrial stack effluents as part of a comprehensive study of SO2 in the atmosphere and its interactions with or conversion to particulate matter. The other study, in cooperation with the New York State Botanist's Office, required samples of airborne pollens as part of an investigation into transport and diffusion of pollens over distances to 100 km. The sampler described herein was designed for use in both projects. Its usefulness in these studies suggests that it may find application in other programs requiring accurate sampling of particulates aloft. Isokinetic sampling has long been recognized as the only way to obtain an accurate and representative sample of heterodisperse particulates large enough to deviate from an air stream whose velocity is changed abruptly as by a sampler entrance (WATSON, 1954). However, the method can seldom be used when sampling from a fixed position due to the formidable problem &matching the intake flow rate to the rapid fluctuations in speed which are normal in a turbulent atmosphere. Application of isokinetic sampling to fast moving vehicles presents fewer problems and at least two such samplers designed for large, multi-engined aircraft have been described (TORGESONand STERN, 1966; TIMMONSet al., 1966). Several samplers with non-isokinetic entrances have also been used in large aircraft (KELLY et al., 1951; HOLZAPEELand GRESSITT, 1964; BURLEIGHet al., 1967). NICKOLA(1966) instrumented a twin-engine aircraft for real time sampling of fluorescent particle tracer material but did not state if sampling was isokinetic. HARRINGTON(1965) described a light aircraft-mounted pollen sampler with a non-isokinetic entrance. MCCALDIN and JOHNSON (1969) and ADAMS and KOPPE (1969) used non-isokinetic inlets to optical particle counters mounted in single and twin-engined aircraft respectively. LANGERet aL, (1968) described an instrumented * This research was carried out under the auspices of the U.S. Atomic Energy Commission and the New York State Museum and Science Service and was partially supported by Research G r a n t No. AP-81 to Dr. E. C. OGDEN from the National Air Pollution Control Administration, Consumer Protection and Environmental Health Service, U.S. Public Health Service. 191
192
GILBERT S. RAYNOR
twin-engine research aircraft whose equipment included an aerosol counter but did not specify the type of inlet. MAcCREAOY and TODD (1964) instrumented single and twin-engine aircraft with cloud particle samplers in which particles impact on Formvar coated film. WEBB et aL (1970) used a helicoper-mounted high volume sampler for air pollution studies but made no provision for isokinetic entrance. Thus, although small planes are widely available and frequently used for atmospheric studies, isokinetic sampling apparently has not been previously adapted to them. DESIGN CONSIDERATIONS Several considerations determined the basic sampler design. Since Cessna 172 and 182 aircraft, instrumented with meteorological and gas sampling equipment, were being used in related studies (TUCKER, 1969), the sampler was constructed to fit these aircraft but can easily be modified for use on other models. It was necessary that the entrance area and the flow rate through the system should produce a linear speed into the entrance within the range of aircraft cruising speeds. The largest feasible flow rate was desired to keep the sampling time reasonably short since low particle concentrations were expected at times and as many as 20-30 samples were planned for single flights. Since excess electrical power from the aircraft was limited and used for other instruments, battery power was selected for the sampler. To avoid loss of particles in tubes or ducts, the sample collection device was located as close to the air intake as possible. DESIGN The sampler which is diagrammed in FIG. 1 and illustrated in FIG. 2 consists of five sub-sections: (1) the sampling head assembly, (2) the track assembly, (3) the replacement window, (4) the air moving and monitoring unit and (5) the power source. The sampling head assembly and the air moving unit are connected by 3.8 cm i.d. flexible, plastic hose. [~
Lr-L~BLEEDER ~ q I VALVE | T | HIGH ~ VOLUME SAMPLER
PRESSUREGAGE /" . PITOTTUBE FLEX HOSE
CONTROL VALVE 24 V dc POWER PACK
DIFFERENTIAL
/FILTER ~
AIR FLOW
TRACK MOUNTED FILTER HOLDER& ENTRANCE CONE
FIG. I. Block diagram of the aircraft isokinetic sampler. The sampling head assembly includes a~a aluminum base section 11.6 × 12.6 cm fitted with nylon bushings which facilitate movement along the track, and a short pipe for connecting the hose. It also includes a seat for the filter holder as well as pins and a safety catch for mounting the entrance cone. The filter holder (FIG. 3) which was designed for 102 mm dia. filters is 6.35 mm thick, with a 108-mm o.d. and a 90-mm i.d. It includes a base section, coarse and fine stainless steel screen supports, a teflon washer and a retainer ring with a groove around its outer circumference. When assembled, the filter lies above the fine screen and below the teflon washer. The assembly is secured by three set screws in the wall of the base
An Isokinetic Sampler for Use on Light Aircraft
193
section which engage the groove in the retainer ring. Cut-outs in the wall of the filter holder seat allow easy placement and removal of the filter holder. Any flat type filter which gives adequate flow rate could be used but vinyl metricel and nuclepore filters with 8/~m pores are used in the sampling programs at Brookhaven. Filter holders are pre-loaded, then stored and carried in the box shown in FIG. 2. The aluminum entrance cone is fitted with twist-lock slots which engage the pins in the base section. The cone is 29 cm long with a 2.0-cm entrance diameter. The inner diameter of the base is 9.0 cm and the outer 12.6 cm. After selection of the entrance and base diameters, the length was determined by the 7.5 degree maximum taper used in diverging nozzles to avoid generation of turbulence within the cone. In operation, the sampling head assembly is positioned at the outward end of the track near the aircraft strut (FIG. 4). Although no tests were made of flow around the aircraft, the entrance is believed to be in undisturbed air since it is well forward of the strut, about 30 cm below the wing and far beyond the region disturbed by the propellor. When not in use or when changing filters, the assembly is drawn into the aircraft cabin. The track assembly consists of two 12.7-ram dia. stainless steel rods 1.9 m long spaced 10.2 cm apart by brackets equipped with nylon rollers for easy movement of the hose. The outer end of the track terminates in a bracket which clamps around the strut of the aircraft. Near the inner end, which passes through a hole in the window, a bracket fastens the track to the window. Shockmounts are used here since the aircraft wing and strut flex somewhat in flight. Stainless steel cables 1.59 cm in diameter around pulleys at each end of the track are used to move the sampling head assembly between the cabin and the sampling position. All angles on the window and strut brackets and on the sampling head base were calculated so that the entrance cone faces directly forward in flight. The window is constructed of 6.35 mm thick Lexan (high impact, transparent plastic) and replaces the left aircraft window which is hinged up and secured under the wing. The window contains a suitable-size opening to allow passage of the sampling head assembly and a removable cover to close most of the opening in flight. Latches secure the window to the window frame. The air moving assembly consists of a 24-V d.c. high volume sampler with an adaptor piece which screws on in place of the usual filter holder and terminates in a control valve and a 9-cm length of aluminum pipe to which the hose is attached. A miniature pitot tube is mounted in the pipe and connected to a differential pressure gauge to monitor air flow through the system. A bleeder valve is used to augment the flow through the sampling system and maintain adequate cooling flow through the high volume sampler. The power source consists of two 12-V, 90-A-h lead-acid batteries connected in series, mounted in a carrying case and equipped with a plug for connection of the sampler power cord. The apparatus was designed for structural strength, ease of mounting and minimum interference with aircraft performance. Its use required FAA inspection and approval and places the aircraft in the restricted category (FAA form 337). FLOW SPECIFICATIONS The sampler was designed to match aircraft speeds of 27-38 m s-1 (60-85 m.p.h.) and can match the higher speed using several types of filters and fully charged batteries.
194
GILBERT S. RAYNOR
The system is normally operated at 35.7 m s-1 (80 m.p.h.). At standard temperature and pressure (STP), this linear speed at the entrance is given by a flow rate of about 0.67 m a min-~ (23.7 ft 3 min-1). This flow rate gives a speed of 14.3 m s -~ past the pitot tube which corresponds to a pressure differential of 0.45 in. of water at a pressure drop of 1.5 in. Hg. across the filter. When sampling is conducted at higher altitudes or at non-standard temperatures, varying air density must be accounted for and two modes of operation are possible. In mode one, the pilot flies at true air speed and the flow rate through the sampler is adjusted to match. This necessitates knowledge of temperature and pressure but results in the same volume of air space sampled per unit time under all conditions. In mode two, which is normally used, the indicated air speed and indicated flow rate are kept constant. True air speed, true flow rate, and true entrance speed then remain proportional to each other since they vary by the same ratio with density changes. Thus, sampling is always isokinetic although the air speed being matched by the entrance speed and the volume of air space sampled do change with altitude. However, the volume is calculated after the flight by use of temperature and pressure-altitude measurements. OPERATION
The sampler can be mounted in the Cessna aircraft without removing seats although the left front seat is normally removed for mounting associated meteorological instruments and recorders on the seat track. The batteries are secured in the luggage compartment behind the rear seats. Installation takes about 15 min. Filters are loaded in their holders in a clean area before each flight and carried in the box shown in FIG. 2. Upon arrival at a sampling location, afilter holder is inserted, the entrance cone attached, and the sampling head assembly pulled into sampling position. The sampler is operated for a pre-selected length of time while the pilot circles at a constant altitude over the desired location or maintains a track in a plume. The assembly is then pulled in and the filter holder replaced while the pilot proceeds to the next altitude or location. Meteorological measurements may be taken simultaneously. Flights are limited to about 5 h without refueling. Life of the batteries is about the same length at continuous operation, but shorter when the sampler is turned off and on frequently. Several flights have been made to sample ragweed pollen, industrial air pollutants and power plant stack effluents. Five-minute samples were found adequate for pollen up to 6000 ft and 1-5 rain samples for stack effluents depending on dispersion conditions and distance from the stack. A N A L Y S I S "OF S A M P L E S
Treatment of the exposed filter depends on the material collected and the type of information desired from the sample. For pollen analysis, the 102-ram membrane filter is dissolved in a suitable liquid in a filter funnel and the liquid filtered through a 25-mm filter of a type not affected by the liquid used to dissolve the large filter. The small filter is then mounted on a microscope slide in pre-stained glycerin jelly for microscopic examination. If the sample is taken on a filter with adequate tensile strength such as the nuclepore, the filter can be reverse flushed without being removed from the holder and the material concentrated on a smaller filter.
FIG. 2. Photograph of the aircraft isokinetic sampler and the filter holder carrying box. Components are described in the text.
Fie. 3. Assembled and disassembled filter holder. Components include in order: base section, coarse and fine screen supports, falter, teflon washer and retainer ring.
(facing page 194)
FI~. 4. Aircraft isokinetic sampler mounted on Cessna 182 aircraft.
An Isokinetic Sampler for Use on Light Aircraft
195
Stack e&tents and air pollutants collected on nuclepore filters can be examined under either a scanning or transmission electron microscope to determine particle shape and size distribution. They may also be analyzed chemically or evaluated gravimetrically if collected in sufficient quantity. Other aids to particle identification which might be used are electron microprobe microscopy, neutron activation analysis, X-ray fluorescence and emission spectroscopy. PRELIMINARY
RESULTS
Two flights have been made to measure the variation in ragweed pollen concentration with height over fixed locations. Results are shown in FIG. 5 where the solid lines represent the time-weighted means of 5 and 10 min samples and the dashed lines the actual values of the two samples at each altitude. Differences between 5 and 10 min samples were not systematic and almost identical concentrations were found over both time periods at the highest altitude reached in each fight. During flight 2, simultaneous measurements were taken at 1.5 and 108 m on the Brookhaven meteorology tower and are also shown.
0
IO
20
30
CONCENTRATION
40 (GRAINS
50
60
70
n?,
FIG. 5. Graph showing ragweed pollen concentration variation with height on two sampling flights. The solid lines represent the time-weighted means of 5 and 10 min samples and the dashed lines the actual values.
Two flights have been made to collect power-plant stack effluents at varying distances from the source. A variety of particulates, some as small as 1 x 10m2pm, werephotographed in a preliminary examination of the filters with an electron microscope. These collections are now being analyzed to determine particulate size distribution and composition. Acknowledgenlents-LEsrER A. COHEN of the Meteorology Group and ROBERT W. DILLINGHAM Instrumentation and Health Physics Department assisted in the design and construction sampler. Funds for construction were provided by the Department of Applied Science.
of the of the
REFERENCES ADAMS D. F. and KOPPE R. K. (1969) Instrumenting light aircraft for air pollution research. J. Air Poll&. Control Ass. 19, 410415. BURLEIGH J. R., KRAMER C. L. and COLLINS T. J. (1967) A spore sampler for use in aircraft. Phytopathol. 57,434-436.
196
GILBERT
S. RAYNOR
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