An integrated airborne particle-measuring facility and its preliminary use in atmospheric aerosol studies

J. Aerosol Sci., 1976, VoL 7, pp. 195 to 211. Pergamon Press. Printed in Great Britain.

AN INTEGRATED AIRBORNE PARTICLE-MEASURING FACILITY A N D ITS PRELIMINARY USE IN ATMOSPHERIC AEROSOL STUDIES PETER V. HOBBS a n d LAWRENCE F. RAI)KE Cloud Physics Group, Atmospheric Sciences Department, University of Washington, Seattle, WA 98195, U.S.A. and EDWARD

E. HINI)MAN,

II

Research Department, Naval Weapons Center, China Lake, CA 93555, U.S.A. (Received 28 M a y 1975)

Abstract--An integrated airborne system for studying aerosol particles and their effects on the atmosphere is described. Particles from 0.01 to 30 l~m in maximum dimensions, covering concentrations from 107 to 10-6 cm-3, can be measured and the measurements displayed in the aircraft. Particles from 5 to 100 pm are collected by impaction and their deliquescent nature and elemental compositions are determined in post-analysis. Also measured are the light scattering coefficient of the aerosol, Aitken nuclei concentrations, cloud condensation nuclei, and ice nuclei. Examples of data collected with this system over the Pacific Ocean, in the western and eastern regions of Washington State, and in the plume from a paper mill are presented. Differences in the particle size distributions and the nature of the particles from these different regions are apparent. From measurements obtained with the system in a rain-scavenged plume from a paper mill, values for the collection efficiencies of particles from I0 -2 to 51~m in size have been deduced. INTRODUCTION

The wide range of sizes and concentrations in which particles can exist in the atmosphere present f o r m i d a b l e p r o b l e m s in the m e a s u r e m e n t of their size spectra, p a r t i c u l a r l y if c o n t i n u o u s , a i r b o r n e m e a s u r e m e n t s are r e q u i r e d with i m m e d i a t e d i s p l a y in the aircraft. In this p a p e r we describe an a i r b o r n e p a r t i c l e - m e a s u r i n g facility, which we have recently developed, in which several different m e a s u r i n g techniques have been c o m b i n e d in o r d e r to o b t a i n s e m i - c o n t i n u o u s m e a s u r e m e n t s of a t m o s p h e r i c a e r o s o l particles from 0.01 to 3 0 ~ m * and over c o n c e n t r a t i o n s from 107 to 1 0 - 6 c m -3. T h e m e a s u r e m e n t s are d i s p l a y e d i m m e d i a t e l y in the aircraft. Particles from 5 to 100/~m are also collected, sized, a n d their deliquescent n a t u r e a n d elemental c o m p o s i t i o n are determined. In a d d i tion, the aircraft c o n t a i n s facilities for m e a s u r i n g the i n t e g r a t e d light scattering coefficient of the a t m o s p h e r i c aerosol, A i t k e n nuclei, the c o n c e n t r a t i o n s of c l o u d c o n d e n s a t i o n nuclei as a function of s u p e r s a t u r a t i o n , a n d the c o n c e n t r a t i o n s of ice nuclei as a function of temperature. This i n t e g r a t e d a i r b o r n e p a r t i c l e - m e a s u r i n g facility p e r m i t s detailed studies to be m a d e o f m a n y a t m o s p h e r i c processes involving a e r o s o l particles. This c a p a b i l i t y is illustrated in this p a p e r by a selection of d a t a from m e a s u r e m e n t s we have o b t a i n e d over the ocean, over the land, a n d in the effluents from a p a p e r mill. INSTRUMENTATION

Description The s p e c t r a of a t m o s p h e r i c a e r o s o l particles from 0.01 to 30 # m are m e a s u r e d semic o n t i n u o u s l y a n d are d i s p l a y e d i m m e d i a t e l y in the aircraft by integrating m e a s u r e m e n t s from the following instruments. * All sizes refer to the maximum particle dimension. 195

196

PHliR V. HOBBS, LAWRIN('Ii F. RAI)KI and EI)\VARI) E. HI'
(a) A~ Electrical Aerosol Anal rser* (EAA). This instrument, which has been described by Liu et al. (1973), was used to measure semi-continuously the size spectra of particles from 0.01 to 0.36"t. It is based on the relationship between electrical charging, particle size and particle mobility described by Whitby and Clark (1966). The number concentrations of particles are measured in eight size ranges: 0.01 0.(t178, 0.0178-0.026, 0.026~0.036, 0.036-0.070, 0.070 0.120, 0.12(>0.185, 0.185-0.260 and 0.260 0.360 ltm. A complete set of measurements over all of these size ranges is obtained in two minutes: if only alternate size ranges are measured the time required is one minute. Laboratory' tests on homogeneous aerosol samples showed that particle concentrations measured with this instrument are stable and reproducible to within about a factor of 1.8. The calibration of the EAA given by. Liu and Piu (1974) was tlsed in this study. (by A Model 220 Ro)'co Optical Particle Coutm, r{ {OPC). This instrument, which has been described by' Zinky (19621, is used to measure contintously the size spectra of particles from 0.3 to 15 Ima. Particles are sized by measuring the amount of energy' they scatter at 90 from an incandescent light beam. We have moditied the original instrument by using a low-noise photo-multiplier system for detecting the scattered light and replacing the maker's pulse-height analyzer with a high-speed 16 channel pulse-height analyzer. We calibrated the modified instrument using polstyrene latex spheres of known sizes following the technique described by' Whitby and Liu (19681. This size range 0.3 15 itm was sub-divided into 15 discrete channels, particles greater than 15 #m were recorded in the 16th channel. The OPC is cquipped with a system to dilute the sampled air with particle-free air in order to prevent coincidence counting. This is necessary when the concentration of particles between 0.3 and 15/tm in the sampled air exceeds 100 cm 3 (c) A Knollenber,q Axialh' Scattering Spectrometer Probe§ {ASSP). The instrument was designed by R. Knollenberg but has not been described in the open literature. It measures i1~ silu the size distribution of particles {or liquid drops) from 2 to 30 lira by measuring the amount of light energy they scatter out of a lascr beam. The instrument was calibrated in our laboratorx using crown glass spheres of known sizes. The size range over which the instrument was sensitive was sub-divided into 15 discrete channels and a 16th channel was used either for oversized or undersized particles, In addition to the above automatic aerosol size-measuring instruments, the following techniques are employed on the aircraft: (d) Particle impaetioll ol7 .slides. Particles in excess of 5 tan are captured by: direct impaction on microscope slides (7 or 14 mm wide and 76 mm long) coated with a thin (101tin) layer of silicone grease. Circular carbon planchets, 14mm dia., are glned to the 14 mm wide slides and their upper surfaces are coated with vaseline. Particles collected on the planchets arc subsequently' studied with a scanning electron microscope and their elemental composition is determined b~ energy' dispersive analysis of X-rays (EDAXt. The particles collected on the slides are stored at a relative humidity of 50°,, and then photographed, sized, and counted with an optical microscope. The slides are then placed in a closed petri dish where they are surrounded by a moat of a saturated-solution of potassium nitrate which maintains the r.h. at 95°;. After 10 min in this environment, the slides are again photographed. Comparison of the photographs taken at 50 and 95!!,, r.h. reveal which particles deliquesce at the higher humidity. To check if the silicone grease inhibits deliquescence, one half of a slide was coated with silicone grease and the other left clean. Pure sodium chloride was then crushed down to particles l 100 Iml in size and these were blasted onto the slide with a high pressure jet of air. The slide was then photographed at relative humidities of 50 and 95°,. Since no differences were * Manufactured by Thermo-Systems Inc., North St. Paul MN 55113, U.S.A. "I"The instrument was designed to measure particle spectra from 0.0032 to [.0/ti'il. but we have not extended our measurements to the upper and lower limits. +Manufacturcd by Ro~co Instruments Inc., Menlo Park. CA, U.S.A Manufactured by Particle Measuring Systems, Boulder, (70 80301, U.S.A.

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detected between the number of particles which deliquesced at 95~o humidity on the silicone-coated and on the clean halves of the slide, it was concluded that the silicone grease did not inhibit deliquescence (probably because the particles were not completely imbedded in it). (e) A Gardner small particle counter*. This is a rapid expansion counter (REC) in which the number of droplets which form by condensation are detected photoelectrically. It measures the total concentration of particles in the air with diameters greater than 0.002/~m. The instrument is operated manually and the data are recorded by hand. These data were adjusted according to the calibration reported by Ruskin and Kochmond (1971). (f) An integrating nephelometer?. (Charlson et al., 1969). This automatic instrument measures the integrated light scattering coefficient of the atmospheric aerosol from which the total mass of aerosol may be deduced. (g) An automatic Cloud Condensation Nucleus (CCN) counter. This instrument, which was designed and built in our laboratory (Radke and Hobbs, 1969; Radke and Turner, 1972), provides automatic measurements every 0.25 minutes of the concentrations of CCN in the air which are active at a pre-determined supersaturation (0.2-1.57/o) with respect to water. (h) Ice nucleus measurements. Three different techniques are used for measuring the concentrations of ice nuclei in the air active at various temperatures. Two of them, the NCAR acoustical counter (Langer et al., 1967) and an optical counters are automatic and provide immediate display of the measurements. In the third method, samples of air are drawn through Millipore fillers which are subsequently processed in the laboratory to determine the number of active ice nuclei they contain using the technique described by Stevenson (1968).

Aircraft installation The instruments described above are mounted on-board a Douglas B-23 aircraft. This twin-engine aircraft is ideal for aerosol measurements in that it is large enough to carry a substantial load and a crew of six but it also cruises at about 6 0 m s-1 and has a stall speed of 30 m s-1. These relatively low speeds reduce the problems associated with sampling small particles from an aircraft. The locations of the instruments on the aircraft are shown in Fig. 1. In addition to the aerosol-measuring equipment the aircraft is equipped with a comprehensive range of instruments for meteorological and cloud physics research which have been described by Hobbs et al. (1971). Here we are concerned only with the aerosol instrumentation. The air sampled by the EAA, the OPC, the nephelometer and the REC, enters the aircraft through a tube 2.2 cm dia. which has an inlet 0.95 cm dia. The entrance to this tube is situated 19 cm from the skin of the aircraft. This probe feeds a 301. capacity plenum chamber. The chamber is also connected to a vacuum pump the pumping rate of which is controlled by a pressure transducer which measures the differential pressure between the plenum chamber and the static free-air. In this way the vacuum pump maintains the plenum chamber at close to the static free-air pressure which, in turn, results in isokinetic sampling at the inlet. At an aircraft speed of 60 m s - l , the air in the plenum chamber is replaced in 8 s. During this time the air is heated from 3 to 5°C which causes its relative humidity to be reduced by about 30~. Therefore, any deliquesced particles in the air sample should effloresce before leaving the plenum chamber. The largest particles in the air flowing through this system which can be measured (by the OPC) are 15/~m in maximum dimensions. Particles of this size settle about 16 cm in 8 s. Since the plenum chamber is 46 cm deep, some of the 15/~m sized particles will be lost in the chamber. The tubing * Manufactured by Gardner Associates Inc., NY 12303, U.S.A. t Our version of this instrument was built in-house. The instrument is manufactured by Meteorology Research Inc., Altadena, CA 91001, U.S.A. :t: Manufactured by Mee Industries Inc., Rosemead, Los Angeles, CA 91770, U.S.A.

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which connects the isokinetic probe to the plenum chamber is 3.9 cm dia., but the residence time in this tube is only 0.25 s, The aerosol sample travels from the plenum chamber to the OPC through a tube 84cm long and 4 c m wide. A vacuum pump is attached to the tube in order to decrease the residence time of the particles to 6 s. In this tube further losses of 1(~15 /~m particles occur. The plenum chamber is connected to the EAA by 9l cm of tygon tubing 0.95 cm dia. The residence time of the sampled air in this tube is 0.8 s. The loss of aerosols to the walls of this tube by diffusion was estimated to be such that there should be a reduction of 30'}i~ in the concentration of particles 0.0032 Izm in naaximum dimensions. When operating this instrument above ground level, allowance is made for the decrease in air density with altitude in order to maintain the required mass flow rates. The aerosol sample travels from the plenum chamber to the rapid expansion chamber through tygon tubing 40cm long and 0.95cm dia. The air sample is drawn into the instrument by a hand-operated pump; its residence time in the tube is too short for any appreciable losses of particles too occur. The CCN and ice nucleus counters are fed through a separate sampling probe (the entrance of which is located outside of the boundary layer of the aircraft) and a tube 2 m long by 4 c m dia. The flow rate through this tube is 4 0 m s t which produces a 0.05 s residence time. This short time produces insignificant aerosol losses. Furthermore, the rapid flow results in virtually isokinetic sampling. The slides, on which airborne particles are caught by impaction, are attached to a wand 1.2 m in length which is manually deployed from the aircraft (Fig. 1). Compari-

Airborne particle-measuring facility

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sons between the concentrations of particles between 5 and 30tim dia. collected on the slides and those measured with the ASSP, showed that the sticking coefficient for these particles on the silicone grease-coated slides was 100%. Furthermore, we believe that the sticking coefficient for particles greater than 30pro is also high because the shape of the number distribution function deduced from particles collected on these slides is the same as that obtained by Noll (1970).

DATA C O L L E C T I O N The electrical signals produced by the particles passing through the light beam of the OPC are passed through a pulse height detector and then stored in the memory of a pulse-height analyzer. Histograms of total voltage (which corresponds to the number of particles counted) versus channel number (which corresponds to the size range of particles) are plotted out periodically on two channels of a 6-channel pen recorder. The electrical pulses produced by particles passing through the laser beam of the ASSP are also passed through the pulse-height detector and stored in the memory of the pulse-height analyzer. The signals from this instrument occupy the first 16 channels of the memory while those from the OPC occupy the next 16 channels. Histograms of particle size distributions from both the OPC and the ASSP are plotted simultaneously on a pen recorder after one of the thirty-two channels in the memory of the pulse-height analyzer reaches 100 counts. The memory of the pulse-height analyzer is then purged and operations repeated. Since the OPC counts the smaller more numerous particles, one of its 16 channels would reach 100 counts before the ASSP had sampled a sufficient number of particles for a good statistical sample. This problem can be overcome by diluting the air sample entering the OPC by a factor of ten with aerosol-flee air. During typical flight three sets of histograms from the OPC are collected without dilution; the air to the OPC is then diluted by a factor of 10 until three histograms from the ASSP are obtained. This cycle of operations is then repeated as often as necessary. The output from the EAA consists of two voltages. One is the voltage on the center rod (the analyzer voltage) and the other the voltage output from particles collected on the absolute filter (the analyzer current). Data from the EAA are collected only when the aircraft is flying at constant altitude. When the altitude is changed, the flow rate to the EAA is adjusted to restore standard values prior to gathering data. The CCN counter is operated automatically at 0.5~o supersaturation except during periods when the supersaturation is changed manually to 0.2, 1.0 and 1.5~ in order to obtain a CCN supersaturation spectrum. During the latter measurements, which take about 10 rain, the aircraft is flown at constant altitude along a prescribed track which is designed to sample the same air mass. When aerosol measurements are required on a relatively small sample of air, a 1901. capacity polyethylene bag is filled with air from outside the aircraft in about 2 s. Air from this bag is subsequently fed to the OPC, EAA, CCN counter and REC (the latter is operated manually and the data logged by hand). The direct impactor surfaces (slides) are exposed to the air stream for a sufficient period of time to obtain a reasonably dense distribution of particles. At a airspeed of 60 m s 1 this period is generally between 3 and 5 minutes, during which time about 1 m 3 of air is sampled. After exposure the slides are placed in a closed microscope slide box. One slide with a coating of silicone grease is not exposed to the airstream but in all other respects is treated the same as the other slides. This slide is used as a "blank" for estimating the degree of erroneous contamination of the slides due to handling. Prior to analysis, the slides in the box are kept at a relative humidity which does not exceed 50~,, in order to avoid deliquescence. Figure 2 summarizes schematically the sampling periods of the aerosol-measuring instruments.

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Comparison of airborne aerosol measurements orer the PaL'(tic' O('eaJl and the Puget Sound Airborne aerosol measurements were made from 15:30 to 16:30h local time (PDT) on 5 August, 1974, over the Pacific Ocean at a point located approx. 113 km west-northwest of Hoquiam, Washington. Three sets of measurements were made at this location at altitudes of 9 m (1020 mb), 190 m (980 rob) and 1524 m (825 rob). The aircraft then flew east to a point over the Puget Sound located 28 km northwest of Seattle where aerosol measurements were made at altitudes of 310 m (970 rob) and 1524 m (825 rob) between 18:00 and 18:15 h PDT. Air parcel trajectory analysis on the 700 and 850mb constant pressure surfaces showed that the air in both of the regions where measurements were made had originated almost due west over the Pacific Ocean. The aerosol size-distribution measurements* made at 190 m above mean sea level (MSL) over the Pacific Ocean and 310 m MSL over the Puget Sound arc shown in * Note: Figs. 3, 7, 11 and 14 are log log plots of (dN/dllog Dej)(= vl against D~,,where N is the concentration ol' particles with equivalent diameter greater than or equal to /)p. On lifts plol the number of particles A\~between the limits of the interval ,&flogDp) is A.'\' -- yA(log1)~,t.

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Fig. 3. We note first that these results (and also those to be described later) show that there are generally correlations and consistencies between the measurements obtained with the different instruments. However, some differences do exist. For example, in the overlap region between the OPC and the ASSP (2-15/~m) the OPC gives lower number concentrations. This difference may be due to the fact the particles measured by the OPC are at a lower relative humidity than those measured in the ambient air by the ASSP. Comparing the particle size distribution over the ocean with those over the Puget Sound, we see that particles between about 1 and 10/~m dia. are present in concentrations significantly higher over the ocean than over the Sound. The extra particles over the ocean may be attributed to an oceanic source. This conclusion is supported by the results shown in Figs. 4 and 5. It can be seen from Fig. 4 that many of the particles collected above the ocean deliquesced at 95~o r.h., and elemental analyses of these particles (Fig. 5) revealed that they consisted primarily of sodium and chlorine. We may conclude, therefore, that they are sodium chloride. Particles between 0.02 and 0.5/~m appear in higher concentrations over the Sound than over the Pacific Ocean. We attribute this to the role of combustion in producing

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particles over the Puget Sound. It should be noted that for particles larger than about 0.01 pm, dN/d{log Dp) is approximately proportional to Di,-~. The average values of the integrated light scattering coefficients for the two sets of data shown in Fig. 3 were 2.6 × 10 S m 1 for the ocean air and 3.9 x 1 0 - S m - ~ for the air over the Puget Sound. CCN spectra, which were measured simultaneously with the aerosol size measurements over the Ocean and over tile Sound, are shown in Fig. 6. The straight lines which are fitted to the experimental points are least-square best fits. The larger scatter in the experimental points for the Puget Sound data reflects the more heterogeneous spatial composition of the air in this region compared to that over the Pacific.

Comparison of airborne aerosol measurements in western and eo.s'tem~ Washington West of the Cascade Mountains the State of Washington is covered by large expanses of coniferous forests: the main industrial and urban areas are located on the eastern shores of the Puget Sound. East of the Cascades is a much drier area which supports less lush vegetation; cities are relatively small and isolated. Airborne aerosol measurements were made from 14:21 to 14:29 h P D T on 28 May, 1974, at 1100 m (860 rob) above mean sea level 16 km east of Seattle in western Washington. The aircraft was then flown across the Cascade Mountains and measurements were taken at 1840m (790 mb} and 2896 m (680 mb) above mean sea level 28 km northcast of Wenatchee in eastern Washington from 16:05 to 16:32 h PDT. Air parcel trajectory analysis on the 700 mb pressure surface showed that the air over both western and eastern Washington on this day was continental in origin, having come from the northwest along a track just inland of the Pacific Coast of Canada. The aerosol size distribution measurements at 1100 m MSL in western Washington and 1840m MSL in eastern Washington are shown in Fig. 7. It can be seen that for particles greater than about 1.5 t~m the two size distributions are similar but there were higher concentrations of smaller particles in western Washington than in eastern Washington. The higher concentrations of smaller particles in western Washington probably reflects greater numbers of combustion particles from the industrial areas of western Washington. Figure 8 shows the results of exposing the particles collected on the slides

Fig. 4. Optical microscope photographs of aerosol particles collected at 9 m MSL over the Pacific Ocean on 5 August 1974. (a) at 50% r.h. and (b) at 95% r.h.

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Fig. 8. Optical microscope photographs of aerosol particles collected at 1840 m MSL 28 km northeast of Wenatchee in eastern Washington on 28 May 1974. (a) at 50% r.h. and (b) at 95% r.h.

Fig. 9. (a) Scanning electron microscope photograph of aerosol particles collected at 2896 m MSL, 28 km northeast of Wenatchee in eastern Washington on 28 May, 1974. (b) Elemental analysis of particle X in (a) using energy dispersive analysis of X-rays with the scanning electron microscope. The particle consists primarily of silicon, potassium and iron.

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in eastern Washington to relative humidities of 50-95~o. It can be Seen that compared to the particles collected over the Pacific (Fig. 4), most of the particles collected in eastern Washington did not deliquesce. Elemental analysis of the larger particles from eastern Washington showed that most of them were probably silicates (Fig. 9) which must have originated from the soil. The CCN spectra, which were obtained simultaneously with the data shown in Fig. 7, show that there were fewer CCN at all supersaturations in eastern Washington than in western Washington (Fig. 10). This is consistent with the low concentrations of deliquescent particles in the air from this region (Fig. 8). The average values of the integrated light scattering coefficients for the samples collected in western and eastern Washington were 4.5 x 10- s m- 1 and 3.5 x 10- 5 m - 1, respectively. Airborne aerosol measurements in the effluents from a Kraft-process paper mill

We describe now aerosol measurements obtained in a large Kraft paper mill located at Pt. Townsend on the Olympic Peninsula in Washington State. The effluents from this mill and their atmospheric effects have been studied extensively by our research group over the past six years (Hobbs et al., 1970; Eagan et al., 1974). Here we summarize 106

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Up (vm) Fig. 7. Aerosol particle size distribution measurements obtained on 28 May, 1974. The open symbols (connected by a dashed line) are averages of measurements made between 14:21 and 14:29 h PDT at 11:00 m MSL (860 mb) 16 km east of Seattle in western Washington. The closed symbols (connected by a solid line) are averages of measurements made between 16:05 and 16:14 h PDT at 18:40 m MSL (790 mb) 28 km northeast of Wenatchee in eastern Washington. Electrical aerosol analyzer, EAA (ll, E3); optical particle counter, O P C (e, O); axially scattering spectrometer probe, ASSP (V, V); direct impactor surfaces, DIS (&, A).

204

PI!TtR V. HoBI~S, LAWRI!N('I F. RADKE and [~I)WARD E. [-tlNI)MA\, I1

104

l

,

\o%@N = 4240S1"2

103

E z

N = 1656S 1 ~ 102

101 2.0

I 1.0

015

S (%)

\• \

0.1

Fig. 10. Numbers of cloud condensation nuclei N at supersaturation 5" on 28 May, 1974 measured simultaneously with aerosol size distributious shown m Fig. 8. Tile open symbols (connected by the dashed line) are lhe data for western Washington and the solid symbols [connected by the solid line) are the data for eastern Washington.

just one of several sets of aerosol measurements of the effluents from this mill which we have obtained recently using our airborne facility. On 20 August, 1974, aerosol measurements were taken from 12:22 to 12:33 h PDT in the visible plume from the Pt Townsend paper mill while the aircraft flew at an altitude of 396 m MSL at 2-5 km downwind from the mill. The results are shown as the solid symbols in Figs. 11 and 12. Also shown in these figures are measurements obtained in air upwind of the mill. It can be seen that for particles greater than about 0.15ym the concentrations of particles in the plume from the mill were greater than in the air unaffected by the mill; this is attributed to the emission of large particles by the mill. However, surprisingly, there were fewer particles with diameters less than 0.15 #m in the plume than in the ambient air. We suggest that this may be due to the rapid removal of small particles in the plume as they coagulate with the numerous larger particles. The average values of integrated light scattering coefficients measured in the plume and upwind of the plume were 21 x 10 Sm 1 and 2.6× 10 Sm ~.respectively. The CCN spectra, measured simultaneously with the aerosol size distributions, are shown in Fig. 13. It can be seen that the slope of the line for the data upwind of the paper mill is steeper than that for the data in the plume of the m}ll. This indicates that there were more larger particles but fewer smaller particles acting as CCN in the plume than in the ambient air. This agrees with thc measurements of the particle size distributions. It can be seen from the photographs shown in Fig. 14 that many more particles from the samples collected in the plume of the mill deliquesced than did particles from the samples collected upwind of the mill. Table 1 shows the results of elemental analyses of some particles 1 80 t~m dia. collected in the plume and upwind of the plume. Ten particles from each source were analyzed. The numbers of particles containing the indicated element are given in parenthesis. The most frequent occurring element in the particles from the plume is sodium while the most frequent element in the particles collected upwind of the plume is silicon. These results add further weight to the evidence previously presented by Hobbs et al. (1970), Eagan et al. (1974) and Hindman and Hobbs (1974) that the effluents from

Airborne particle-measuring facility

205

paper mills contain copious numbers of CCN which may modify the microstructure of clouds and thereby affect precipitation processes. The measured size distributions depicted in Figs. 3, 7 and 11 all tend to follow a Dp 3 relationship d o w n to the smallest diameter particles measured (0.01/lm). Junge (1963) first observed this relationship for particles with diameters greater than about 0.1 /~m. There are, however, significant and recurring deviations between our results and the Dp 3 relationship. These can be seen clearly in the volume distribution plot shown in Fig. 12, where major modes are apparent between 0.1 and 1/~m and I and 100 #m. These m o d e s have been observed previously by W h i t b y and Liu (1973) who attribute them to the growth by coagulation or condensation of smaller particles into the 0.1 l # m size range and the production of particles 1 - 1 0 0 # m dia. by mechanical processes.

Plume scavengin 9 by a rain shower Finally, to demonstrate the power of our integrated airborne particle-measuring facility, we describe the results of measurements of particulate scavenging of an industrial plume by a rain shower which has led to estimates of collection efficiencies for particles from 5 to 10 2 ILm in size (two orders of magnitude lower in particle size than any previous scavenging measurements).

106

I

I

1

L

105

,04 l°3 -

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102

_

~ ' ~

101 _

loo _

\\

_

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_

,o, o,

_

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10 2 .

O0~'~r'.Y~

10-3 --

10-4 -lO-6 -

\~_~1 r

-"t .zc ~

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0PC

/10-6

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--

I

[

10-2

10-1

~

A,_Ip'l ~\D!~~

[_ L

100

~-

--

.[ I=

I

101

102

Op (~m)

Fig. l 1. Aerosol particle size distribution measurements obtained on 20 August, 1974. The solid symbols (connected by the solid line) are averages of measurements made from 12:22 to 12:33 h PDT at an altitude of 396m MSL and 2-5 km downwind of the Pt. Townsend paper mill in the visible plume from the mill. The open symbols (connected by the dashed line) are averages of measurements made from 13:15 to 13:27h PDT at an altitude of 470 m MSL 35 km upwind of the Pt. Townsend paper mill. Rapid expansion chamber, REC (I,<1); electrical aerosol analyzer, EAA (i, El); optical particle counter, OPC (O, O); axially scattering spectrometer probe, ASSP (Y, •); direct impactor surfaces, DIS (A, A).

206

PETER V. HOBBS, LAWREN('E F. RAI)KI and ~I)%VARI) E. HINI)MAN. II

6°t

'

[

'

I

,

[

7

I

'

t

50

I I

4O E E =30 g

20

VV

10

0

10-3

10-2

10-1

100

Dij(~ill)

101

102

Fig. 12. Aerosol particle volume distributions for some data as in Fig. i I. The error in the mean is indicated where it is greater than the size of the symbols used. See caption to Fig. l I for key to symbols.

104

103

I

"°~"~c ~ ~ = 790S0"95

A

i

E

=.

\ 102

10'2.

N = 900S1"6""'~ X~

I

1.0

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0.1

Fig. 13. N u m b e r of cloud condensation nuclei N at supersaturation S on 20 August, 1974, measured simultaneously with aerosol distributions shown in Fig. 13. The closed symbols (connected by a solid line) are the data collected in the plume of the Pt, Townsend paper mill and the open symbols (connected by a dashed line) are data collected upwind of the mill.

Airborne particle-measuring facility

207

Table h Results of elemental analyses of 10 particles collected in the plume of the paper mill and 10 collected upwind of the mill. The number after each element indicates the number of particles in which that element was detected Mill aerosol

Upwind aerosol

Na CI S K Si

(9) (8) (8) (6) (3)

Si S Al Fe Ca

(9) (6) (5) (4)

Ca

(2)

Mg

(4) (3)

A]

(l)

C]

(2)

Mg Ti

(1) (1)

K Mn

(2) (1)

The m e a s u r e m e n t s to be described were made on 13 May, 1974, in the plume of the Pt. T o w n s e n d p a p e r mill. A series of particle size distributions were obtained in the visible plume from the mill before and during the period that a convective rain shower scavenged the plume. The time taken for aerosols to travel from the source

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10-1

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_l I

101

102

Dp (.m) Fig. 15. Airborne measurements of the aerosol particle size distributions of the effluent from the paper mill at Pt. Townsend, Wa. Both datum sets were obtained at a distance of approx. 5 km downwind in the visible plume from the mill. The first distribution (open symbols connected by a dashed line) was measured from 14:43 to 14:50h PDT prior to the rain shower. The second distribution (solid symbols connected by a solid line) was measured from 15:36 to 15:39 h PDT after the plume had been scavenged by a rain shower. Electrical aerosol analyzer, EAA (1, D); optical particle counter, O P C (O, O); axially scattering spectrometer probe, ASSP (~7); direct impactor surfaces, DIS (A).

208

PIi'I'IR V. H()Bt~S, L A W R I N ( ' I : F'. RAI)KIi a n d

I~]])WARI) E. HINDMAN,

[I

to the aircraft was about 16 minutes. The size spectra of particles in the unscavenged and rain scavenged plume are shown in Fig. 15. The two sets of measurements are very similar for particles 0.1 1 #m in size but they differ significantly for particles outside of this range. This is demonstrated more clearly in Fig. 16 which shows the percentages 10 6

7

T~ ........

T~

.....

~ . . . . . . .

~a lO 5

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~> 103

lO2

:z: 101

ua 'E 100

1o- 1 .,=,1o-

J

10~3

10-2

t

/

10-1

I

100

101

10 2

Dp (~m)

(b)

100 --

z

1

I

I

I

u.l

c~ ~

-

~. s o -

...i

ua u.I

o 10-3

10-2

10-1

I

10 0

I lO 1

lO 2

Dp (urn)

Fig. 16. Repeat of data in F'ig. 14 to show (a) the number concentrations and (bl the percentages of particles removed from the plume of the paper mill b~ the rain shower,

Airborne particle-measuring facility

209

and numbers of particles in each size class removed by the rain shower. It can be seen from these plots that virtually no particles about 0.1 and 1.0#m in size were removed from the plume by the rain shower. The rain scavenging rate A(Dp) is defined by

z(Dp) = Zo(Dp)exp[-A(Op)t], where z(Dp) is the concentration of particles with diameters from Dp to Dp + dD r at time t and Zo(Dr) is the concentration at time t = 0. Using the data shown in Fig. 15 and the estimate of a 16 min rain scavenging period, values of A(Dr) can be deduced. These are shown in Fig. 17. The rain scavenging rate is related to the collection efficiency E(Dr, D) of raindrops of dia. D for particles of dia. Dr by

A(Dr) =

fo

A(D)E(Dr,D)V(D)N(D) dD,

where A(D) is the effective cross-sectional area of a drop of dia. D, N(D) dD the concentration of raindrops with diameters between D and D + dD and V(D) the terminal velocity of a raindrop of dia. D. If it is assumed that E(Dr, D) is only a function of the aerosol particle size D r, the collection efficiency is given approximately by

E(Dr, D ) = A(Dp)I~i Ai(D)V~(D)Ni(D)ADII-', where i indicates a size interval for the raindrops. The size spectrum of the raindrops in the shower was measured from the aircraft (they ranged from 0.1 to 3 mm dia.). Using these values, and those calculated previously for A(Dr), values of D(Dr, D) were I

/

15-

lO< c~

z ~u

5m

m

I

o lO-3

10-2

10-1 Dp (urn)

100

Fig. 17. Rain scavenging rates deduced from experimental data.

101

PETER V. H{}BBS, LAWRtN('I! F. RAI}KE a n d El)WARD E. HI',,I)MA'-,. II

210

01

I

lO l-

"'"

I

•

~

.".... .

,o-,=~ = =

~

'

~// (Fuchs, 1964)

\ \ w--.,, " ~ ~ °

~ 10-,-

h_4/,,

__

.,0,.u,,0n ~

~

~ =0o,)~/! ° °~ °m'N~-""......-~

~ " - - " g ¢ ~

= 0~,.; °=.1

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° -,.,...,

11

~-"

-

---L

~

//I

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-

/ .k_.,...,.,~ ~, -Is,,.. t;% I

i

pacti

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•

I

'

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I

I

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10-1

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~

t

]

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(,urn)

Fig. 18. Collection efficiencies deduced from our experimental data ( . I " ~; previous measurements (X--Starr and Mason (1966)), D = 0.05 cm; C> Walton and Woodcock (1970), D = 0.05 cm; + -Adam and Semonin (1970); • - Sood and Jackson (1970). D - 0.05 cm; • ---Engelmann (1965), D = 0.02 cm) and theoretical studies (Fonda and Herne (1957); Slinn (1971. 19741: Fuchs (1974)). calculated. These are shown in Fig. 18 where they are c o m p a r e d with previous measurements a n d some theoretical calculations. W e note first that whereas the previous m e a s u r e m e n t s of collection efficiency only covered particles from a b o u t l . ( ~ 2 0 y m in size, our m e a s u r e m e n t s e x t e n d e d from 0.01 to 5 t~m. In the region of overlap, o u r m e a s u r e m e n t s agree fairly well with previous m e a s u r e m e n t s a l t h o u g h they are s o m e w h a t higher. The theoretical curves shown in Fig. 18 are those c o m p o s i t e d by D a n a and H a l e s (1974). F o r particles in excess in I y m these theoretical results are in r e a s o n a b l e a g r e e m e n t with o u r e x p e r i m e n t a l measurements. F o r smaller particles the theoretical estimates are a b o u t two o r d e r of m a g n i t u d e low and do not predict the low scavenging singularity at a b o u t 0.2 a n d 1.0 I t m shown by o u r measurements. However, calculations by S p a r k s a n d Pilat (1970) of the effects of diffusiophoresis indicate that under e v a p o r a t ing c o n d i t i o n s the collection efficiency falls sharply from u n i t \ for 4 f u n particles to zero for 1/,zm particles, which is similar to our e x p e r i m e n t a l results. It should be noted that practically all of the particles in the p l u m e of the Pt. T o w n s e n d p a p e r mill deliquesce at 95~i; r.h. (Fig. 14), but since the particles were w a r m e d in the aircraft p r i o r to measurements they p r o b a b l y effloresced. Therefore, deliquescent particles m a y have been scavenged from the p l u m e as larger solution d r o p l e t s than the sizes at which they were measured. ('ONCLUSIONS W e have described in this p a p e r an integrated a i r b o r n e p a r t i c l e - m e a s u r i n g facility. T h e facility is c a p a b l e of m e a s u r i n g particles over the entire size range of interest in h u m a n health p r o b l e m s , air quality studies a n d m e t e o r o l o g y . Several e x a m p l e s of d a t a collected with this facility have been described in o r d e r to d e m o n s t r a t e its utility. Acknowledgements We thank our flight crew (Messrs. D. Atkinson, L. Engels. J. Russcl, K. Biswas and

R. Spurling) for their help in collecting the data described in this paper and Messrs. ,1. Friederichsen and M. Eltgroth for help in data reduction. This work was supported by the RANN division of the National Science Foundation under Grant GI-31759 and the Electric Power Research Institute under contracl RP-330-1. This is Contribution No. 338 from the Departmem of Atmospheric Sciences. Uni',ersity of Washington. REFERENCES Adam, J. R. and Semonin, R. G. 11970) Proceedi~1(lsolthe Precipitariopl Scort'lltlill¢l Sl's111)OsItlt11. p. 151. Richland, WA. Charlson, R. J., Alquist, N. C., Selvidge, H. and Mac('ready. P. B.. Jr. (19691 J. liv Pollar. Colm.ol A.s~. 19. 937.

Airborne particle-measuring facility

211

Dana, M. T. and Hales, J. (1974) Proceedings of the Precipitation Scavenging Symposium, Champaign, IL. (in press). Eagan, R. C., Hobbs, P. V, and Radke, L F. (1974) J. appl. Meteor. 13, 535. Engelmann, R. J. (1965) J. Atmos. Sci. 22, 719. Fonda, A. and Herne, H. (1957) Nat. Coal Board Mineral Research Establishment Rpt. No. 2068. Fuchs, W. A. (1964) The Mechanics of Aerosols, 408 pp. Pergamon Press, Oxford. Hindman, E. E. II and Hobbs, P. V. (1974) Preprints of 4th Conference on Weather Modification, p. 401. Am. Meteor. Soc., Boston, MA. Hobbs, P. V., Radke, L. F. and Shumway, S. E. (1970) J. Atmos. Sci. 27, 81. Hobbs, P. V., Radke, L. F., Frazer, A. B., Locatelli, J. D., Robertson, C. E., Atkinson, D. G., Farber, R. J., Weiss, R. R. and Easter, R. C. (1971) Contributions from the Cloud Physics Group, Research Report VI, 306 pp. Dept. Atmos. Sci., Univ. Washington, Seattle, WA. Junge, C. E. (1963) Air Chemistry and Radioactivity, 382 pp. Academic Press, New York. Langer, G., Rosinski, J. and Edwards, C. P. (1967) J. appl. Meteor. 6, 114. Liu, B. Y. H. and Pui, D. Y. H. (1974) J. Aerosol Sci. 6. 249. Liu, B. Y. H., Whitby, K. T. and Pui, D. Y. H. (1973) Particle Technology Laboratory Pub. No. 201, Dept. Mechanical Eng., University of Minn., Minneapolis, Minn., 20 pp. Noll, K. E. (1970) Atmospheric Environment 4, 9. Radke, L. F. and Hobbs, P. V. (1969) J. appl. Meteor. 8, 105. Radke, L. F. and Turner, F. M. (1972) J. appl. Meteor. 11,407. Ruskin, R. E. and Kockmond, W. C. (1971) Proceedings of 2nd International Workshop on Cloud Condensation Nuclei and Ice Nuclei, p, 92. Dept. of Atmospheric Sciences, Colorado State Univ., Ft. Collins, CO. Slinn, W. G. N. (1971) Battelle Pacific Northwest Laboratories Annual Report to the Atomic Energy Commission BNWL-1551. Slinn, W. (3. N. (1974) Proceedings of the Precipitation Scavenoing Symposium, Champaign, IL. (in press). Sood, S. K. and Jackson, M. R. (1970) Proceedings of the Precipitation Scavengin 9 Symposium, p. 121. Richland, WA. Sparks, L. E. and Pilat, M. J. (1970) Atmospheric Environment 4, 651. Starr, J. R. and Mason, B. J. (1966) Q. J. R. met. Soc. 92, 490. Stevenson, C. M. (1968) Q. J. R. met. Soc. 94, 35. Walton, W. and Woodcock, A. (1970) In Aerodynamic Capture of Particles, p. 121. Pergamon Press, Oxford. Whitby, K. T. and Clark, W. E. (1966) Tellus 18, 573. Whitby, K. T. and Liu, B. Y. H. (1968) Atmospheric Environment 2, 103. Whitby, K. T. and Liu, B. Y. H. (1973) Particle Technology Laboratory Publication No. 216, Dept. Mechanical Eng., University of Minnesota, Minneapolis, Minn. 34 pp. Zinky, W. T. (1962) J. Air. Pollut. Control Ass. 12, 578.