THE UN I VER S I T Y OF MI CHIGAN College of Engineering Department of Civil Engineering Technical Progress Report No. 2 RAIN SCAVENGING OF PARTICULATE MATTER FROM THE ATMOSPHERE A. Nelson Dingle UMRI Project 2921 under contract with: ATOMIC ENERGY COMMISSION Contract No. AT(11-1)-739 Lemont, Illinois administered by: THE UNIVERSITY OF MICHIGAN RESEARCH INSTITUTE Ann Arbor, Michigan January 1961

AC KNOWLEDGMENTS Without the special efforts of Messrs. Kenneth R. Hardy, Albert W. Stohrer, and Ken MacKay, and of Miss Ana Lucia Torres and Mrs. Anne C. Rivette, the compilation of this report would have been much more difficult that it turned out to be. Their work merits, and has, the special appreciation of the author.

TABLE OF CONTENTS Page LIST OF ILLUSTRATIONS v ABSTRACT vii DISTRIBUTION LIST ix INTRODUCTION 1 CHAPTER 1. Aerodynamic Raindrop Sorter Design and Construction 2 General Design Criteria 2> Design Features 4 Construction 5 CHAPTER 2. Collection and Analysis of Drop-SizeDefined Aliquots of Rain 10 Preliminary Rain Sampling Program 10 The nylon-net method 10 The rapid freezing method 10 Vertical air flow method 15 Analysis of the Rain Samples 15 The liquid nitrogen method 15 The nylon-net method 15 Raindrop-size spectra 22 CHAPTER 3. Coordination of Work with Other Units 33 Hanford Atomic Products Operation 33 Aeroallergen Project, The University of Michigan 34 CHAPTER 4. Additional Sub-Projects 36 The study of temporal variations within and between storms 36 Radioactive species analysis and decay-rate dating of radioactive contaminants 36 Deuterium and the isotopes of oxygen 37 Radar data 37 REFERENCES 39 iii

LIST OF ILLUSTRATIONS Table Page 1. Properties of some fluids having low freezing points 14 2. Nylon-net method data for radioactivity studies 17 3. Raindrop-size distributions each minute for four selected periods, 13 June 1960 26 Plate I Working drawings of the Aerodynamic Raindrop Sorter 3 Figure 1. Design criteria for depth and length of dropsorting section, and estimation of errors of resolution, for 4 m per sec air speed. 6 2. Design criteria for depth and length of dropsorting section, and estimation of errors of resolution, for 2 m per sec air speed. 7 3. Photograph of the aerodynamic raindrop sorter showing entrance fillet, straightening screens, contraction section, and rain sampling slit assembly. 8 4. Photograph of the aerodynamic raindrop sorter showing tunnel exhaust, working section (floor removed), and support structure. 9 5. Cross-section of a standard rain gauge showing nylon net in place. 11 6. Cross-section of liquid nitrogen rain-collecting apparatus. 12 v

LIST OF ILLUSTRATIONS (Concluded) Page Figure 7. Cross-section of apparatus for vertical windtunnel separation of drops. 16 8. (a) Average specific activity ([Ic per kg) of rain collected at Ann Arbor, Michigan, JuneAugust, 1960. (b) Total rainfall at Ann Arbor, June-August 1960. 19 9. (a) Weighing rain gauge chart for 12 June 1960. (b) Weighing rain gauge chart for 13 June 1960. 23 10. Rainfall amounts for periods 1, 2, 3, and 4 (Figure 9b) as computed minute-by-minute from raindrop spectra. 24 11. Raitdrop-size spectra averaged for each of the periods 1, 2, 3, and 4. (Figure 9b). 25 12. Rain intensity and cumulative intensity as functions of drop size (a) for period 1: 0935-0940 EST, 13 June 1960 (b) for period 2: 0944-0948 EST, 13 June 1960 (c) for period 3: 0953-0957 EST, 13 June 1960 (d) for period 4: 1006-1010 EST, 13 June 1960. 28 13. Weighing rain gauge chart for 16 June 1960. 29 14. Rainfall intensities as a function of time, 16041750 EST, 16 June 1960. 30 15. Typical raindrop size spectra, one-minute intervals, 16 June 1960. 31 16. Rain intensity and cumulative intensity as functions of drop size, 16 June 1960. 32 vi

ABSTRACT The design and construction of an Aerodynamic Raindrop Sorter is reported. Calibration and testing of this device remains to be done. An effort was made, by the use of expedient measures, to obtain samples of rain more or less discriminated according to drop size. Experimentation with such measures was conducted, and the nylon-net technique proved most practical although it was only partially successful. Some results of this effort are presented. Raindrop-size distributions are presented and related to raingauge records. Coordination of research plans with other research groups and efforts has progressed, and the results are discussed. vii

DISTRIBUTION LIST (One copy unless otherwise specified) Mr. J. Z. Holland, Chief (6 copies) Fallout Studies Branch Division of Biology and Medicine U. S. Atomic Energy Commission Washington 25, D. C. Technical Information Service Extension U. S. Atomic Energy Commission Oak Ridge, Tennessee Dr. Lyle T. Alexander Soil Conservation Service U. S. Department of Agriculture Beltsville, Maryland Dr. R. Appleyard, Scientific Secretary United Nations Scientific Committee on the Effect of Radiation New York 17, New York Dr. Earl W. Barrett Department of Meteorology The University of Chicago Chicago 370 Illinois Dr. L. J. Beaufait, Jr. Tracerlab, Inc. 2030 Wright Avenue Richmond 3, California Dr. Wallace S. Broecker Lamont Geological Observatory Columbia University Palisades, New York Mr. Fred Cowan Brookhaven National Laboratory Upton, Long Island, New York ix

DISTRIBUTION LIST (Continued) Dr. George A. Cowan Los Alamos Scientific Laboratory P. O. Box 1663 Los Alamos, New Mexico Dr. Charles L. Dunham, Director Division of Biology and Medicine U. S. Atomic Energy Commission Washington 25, D. C. Dr. Melvin H. Feldman Defense Atomic Support Agency Department of Defense Washington 25, D. C. Dr. Robert Goeckermann University of California Radiation Laboratory Livermore, California Dr. F. T. Hagemann Argonne National Laboratory P. O. Box 299 Lemont, Illinois Dr. John H. Harley U. S. Atomic Energy Commission Analytical Division, Health and Safety Laboratory New York Operations Office 376 Hudson Street New York 14, New York Lt. Col. James B. Hartgering Office of Chief R and D Department of Army Room 3D 442 - The Pentagon Washington 25, D. C. Mr. Jack Healy Hanford Atomic Products Operation Richland, Washington Dr. Wallace Howell Mount Washington Observatory Gorham, New Hampshire x

DISTRIBUTION LIST' (Continued) Mr. Sam P. Jones. General Mills, Inc. 2003 East Hennepin Avenue Minneapolis 13, Minnesota Dr. W. W. Kellogg The RAND Corporation 1000 Connecticut Avenue, N. W. Washington 6, D. C. Dr. Paul Kruger Nuclear Science and Engineering Corporation P. O. Box 10901 Pittsburgh 36, Pennsylvania Dr. J. Laurence Kulp Columbia University Lamont Geological Observatory Palisades, New York Dr. Paul K. Kuroda University of Arkansas Fayetteville, Arkansas Dr. Wright Langham Los Alamos Scientific Laboratory P. O. Box 1663 Los Alamos0 New Mexico Mr. Kermit Larson USAEC Project, UCLA P. O. Box 4164 West Los Angeles, California Dr o Luther B. Lockart, Jr. Chemistry Division Naval Research Laboratory Washington 25, D. C. Dr. S. Allan Lough New York Operations Office U. S. Atomic Energy Commission 376 Hudson Street New York 14, New York xi

DISTRIBUTION LIST (Continued) Dr. Lester Machta U. S. Weather Bureau Washington 25, D. C. Dr. Edward A. Martell Air FOrce Cambridge Research Center Bedford, Massachusetts Mr. Harry Moses Argonne National Laboratory P. O. Box 299 Lemont, Illinois Mr. Doyle L. Northrup AFTAC, DSC/O, USAF Temp. T. Building Washington 25, D. C. Mr, Thomas F. O'Leary United States Atomic Energy Commission New York Operations Office 376 Hudson Street New York. 14, New York. Mr. Jack Reed Sandia Corporation Albuquerque, New Mexico Mr. John Rosinski Fine Particles Research Armour Research Foundation Chicago 16, Illinois Dr. L. Silverman Harvard University Boston 15, Massachusetts Dr. Hans A. Suess Scripps Institution of Oceanography La Jolla, California Professor Victor P. Starr Department of Meteorology Massachusetts Institute of Technology Cambridge, Massachusetts xii

DISTRIBUTION LIST (Continued) Dr. Herbert Volchok Isotopes Incorporated 123 Woodland Avenue Westwood, New Jersey Dr. Alan Walton Isotopes, Incorporated 123 Woodland Avenue Westwood, New Jersey Dr. Francis Weber U. S. Public Health Service Washingtcn 25, D. C. Dr. A. Nelson Dingle (5 copies) Meteorological Laboratories 5062 East Engineering Building The University of Michigan Ann Arbor0 Michigan The University of Michigan Engineering Library Ann Arbor, Michigan The University of Michigan Research Institute File Cooley Building Ann Arbor, Michigan Dr. Bert Bolin Institute of Meteorology University of Stockholm Lindhagensagatan 124 Stockholm 8, SWeden Dr. K. Edvarson Research Institute of National Defense 2nd Division Stockholm 80, Sweden Dr. W. G. Marley Atomic Energy Research Establishment Harwell, Didcot, Berks England

DISTRIBUTION LIST (Concluded) Dr. T. A. Rafter Department of Scientific and Industrial Research Wellington, New Zealand xiv

INTRODUCTION The objectives of the present research have been set forth as follows: 1. To construct a full-scale aerodynamic raindrop sorter along the lines of the present pilot model, and incorporating additional features of design suggested by experimental tests. 2. To collect, for radiochemical analysis, drop-sizedefined aliquots of naturally falling rain, by means of the aerodynamic raindrop sorter, and to coordinate with these collections detailed raindrop-size spectra obtained using the photoelectric raindrop-size spectrometer, together with other pertinent information, with a view toward detailed evaluation of the natural scavenging function of rain. 3. To coordinate the activities under this project with those of other concerned units, and in particular, to arrange for joint field experiments with the unit under Mr. Fuquay's direction. Progress toward the realization of each of these objectives is reported in the first three chapters of the report. Chapter 4 contains a discussion of additional sub-projects which have been undertaken.but which were not anticipated when the proposal of research was preparedo 1

CHAPTER 1 Aerodynamic Raindrop Sorter Design and Construction The problem of sampling rain according to raindrop-size defined categories was studied in some detail during the first year of the present project. The results of these studies are contained in Technical Progress Report No. 1.1 The engineering design of a full scale aerodynamic raindrop sorter was projected upon the basis of those findings, compromised as favorably as possible to meet practical considerations. Working drawings are included herewith as Plate I. General Design Criteria The analysis showed that reasonably useful impingement patterns should be obtained by the use -of an upward-tilted windtunnel, and that flaring of the tunnel should also be beneficial. However, the degree of flare required to produce a substantial effect is too great to preserve laminar flow, hence the decision was made to use a constant cross-section design for the raindrop sorter. The upward tilt of 45~ was retained in the basic design for the reasons set forth earlier.' The unevenness of flow observed in the pilot model had been attributed largely to inadequacies of the intake design and of the fan arrangement in that model. Pr~actice in wind tunnel design 23 has led to criteria for a contraction section and straightening screens at the intake as a means of attaining laminar flow in the test section. The intake of the aerodynamic raindrop sorter was designed according to these criteria. Study of the air flow in the light of wind tunnel experience also led to the decision to place the fan downstream from the laminar-flow section. This largely prevents the vorticity generated by the fan from entering the test (or raindrop-sorting) section o 2

It was necessary to consider the relative merits of closedcircuit versus open-end wind tunnel design for the present purpose. Whereas the closed-circuit design would serve to isolate the wind tunnel as completely as possible from dynamic influences of the free wind field, and would make possible the maintenance of a clean and saturated condition of the wind tunnel air, it would require a much more bulky structure, and rather elaborate temperature control apparatus would be needed to prevent evaporation or condensation from occurring in the tunnel. Aside from wind-tunnel design, for the moment, the requirement that raindrops fall vertically through the entrance slit into the sorting section, requires some means of eliminating horizontal momentum from the falling rain above the entrance slit. In the field situation, this may be done by providing a settling column, open at the top to receive a representative rain sample, and sheltered from horizontal motion of air. Considering that the raindrop sorter must be located in such a settling column for these reasons, the additional isolation of a closed-circuit wind tunnel appeared to be superfluouso It was therefore concluded that an open-end wind tunnel should more simply serve the desired purpose. Because it is anticipated that the unit will be used at various sites separated by appreciable distance, and because the unit can conveniently be divided into structurally independent sections, a final general design criterion was that of convenient disassembly for packing and shipping. Desiqn Features A 4 to 1 ratio was finally used in the specification of the contraction section. Four screen sections of.015 in. wire spaced at.062 ino centers in both directions are used in the contraction sectiono These are designed to be assembled separately and to bolt to the entrance fillet. The sections of the unit are so designed that each part weighs less than one hundred pounds. This allows for ease of dismantling, moving and reassembly by a crew of two men. Consideration of the speed of flow desired (4 m per sec maximum) and the pressure losses in the system led to the specification of a six-blade vane axial fan driven by a 1/3 hp, 1750 rpm, single phase, 110 v motoro Turning vanes were designed to prevent flow separation and turbulence in the test section. 4

The physical aspect of the unit together with the basic requirement of adequate sample size led to the preliminary specification of a 2 cm x 50 cm entrance slit for the rain sample. Studies of the drop trajectories -in the test section then indicated that, with a 4 m per sec air speed, sufficient separation of the largest drops would be obtained within a tunnel depth of 43 cm, and 0.6 mm diameter drops would strike the floor of such a wind tunnel 1.41 m downstream from the impaction point of an undeflected drop.- These considerations are shown graphically in Figure 1. In the case of a 2 m per sec air speed, the separation of the largest drops is poorer, but the smaller drops do not move so far downstream before impaction (Figure 2) In either case, the smallest drops are carried out of the tunnel unless special provision is made to collect them. For this purpose turning vanes and collecting screens are to be used. The turning vanes will collect the larger sized drops in this size category by inertial impaction, and the collecting screens (e.g., see Twomey4) will intercept a proportion of the smallest drops. There appears no simple means to avoid a considerable evaporative loss from this smallest drop category. Independent data on the drop-size distribution will be used to estimate the error and to make allowance for the evaporation. Construction Upon completion of the design (see Plate I), estimates were obtained of the cost of constructing the unit. The Plant Department, The University of Michigan, was selected as the best bidder, and the job was placed thereo Construction was completed on January 20, 1961, and photographs of the assembly are shown in Figures 3 and 4. Experimental tests of the performance of this unit will be performed in the laboratory. Refinements of design will be derived from these tests and incorporated into the instrument in the next phase of this work. Every effort will be made to prepare the unit for field operations by mid-March-. 5

U =400cm/sec; 8 = 450 SLOT EDGE - UP STREAM x SLOT EDGE - DOWN STREAM 2 cmUNDEFLECTED TRAJEC TORY -4'.8mm DROP 0.7mm DROP — O.5 mm DROP 0.9mm DROP I.Omm DROP 1.4 mm DROP 2.0mm DROP 3.Omm DROP I i I I, I 10 20 30 40 50 60 70 80 90 00 110 120 SCALE IN CM Figure 1. Design criteria for depth and length of dropsorting section, and estimation of errors of resolution, for 4 m per sec air speed.

U0 = 200 cm/sec; 8 = 450 a SLOT EDGE - UP STREAM 2 cm t W. =,45~ x SLOT EDGE - DOWN STREAM UNDEFLE'CTED, TRAJECTORY 43cm 0.8mm DROP t I~Omm DROP 0.4mm DROP 0.mm 2-0mm DROP DROP 5.0 mm DROP J I I! I I'!,I I I I I I' I I' I 10 20 30 40 50 60 70 80 90 100 110 120 SCALE IN CM Figure 2. Design criteria for depth and length of dropsorting section, and estimation of errors of resolution, for 2 m per sec air speed.

Figure 3. Photograph of the-aerodynamic raindrop sorter showing, entrance fillet, straightening screens, contraction section, and rain sampling slit assembly.

Figure 4. Photograph of the aerodynamic raindrop sorter showing tunnel exhaust, working section ( floor removed), and support structur e. 9

CHAPTER 2 Collection and Analysis of Drop-Size-Defined Aliuots of Rain Preliminarr Rain Samplinq Program To gain experience with the problems of sampling rain, of handling the samples and analyzing them, and of managing all the details that go with such experimental procedures, and also to gain some initial information on scavenging effectiveness of large versus small drops, a preliminary program of rain sampling was established early in' the summer of 1960. In this program experiments have been conducted with several different, relatively crude, methods of separating drops according to size in the sampling procedure. The nylon net method. One such method is to stretch a net of nylon over the mouth of a standard rain gauge and to hang a weight from the center of it thus forming a conical shaped net surface (Figure 5)o Drainage from the lower point of the net cone is then caught in the inner container of the rain gauge and the part of the rain water which does not flow along the net cone to its center is caught in the outer portion of the rain gauge. It is supposed that the portion of the rain caught in the inner part of the rain gauge is biased toward the small-drop fraction of the rain, and that that collected in the outer part of the rain gauge mainly represents the large-drop portion of the raino Beyond this statement it is very difficult to tell just exactly how the rain is divided by such a neto Our experience assures us that some diviision is accomplished The rapid freezing method. Recognizing the extreme crudeness of the nylon net method, we considered whether some other techniques might not worko One idea which came forward was that of freezing the raindrops very quickly upon impact and separating the resulting ice pellets by means of a.set of standard aggregate sieves The samples resulting from this method should then be quite precisely separated according to drop sizeo An added attraction of this method was that using liquid nitrogen, as we dido in a rather wide-mouthed container (Figure 6), we could expand the area of sampling some ten-fold over that of an ordinary rain gauge, and we could do this very conveniently. Although we 10

NYLON MESH / /'''I X WEIGHT OUTER CYLINDER INNER CYLINDER Figure 5. Cross-section of a standard rain gauge showing nylon net in place. 11

LIQUID NITROGEN LEVEL OUTER SHELL INSULATION'-:~ INNER BUCKET 4 76 2.38 wait A 42 limit A-..-" Figure 6. Cross-section of liquid nitrogen rain-collecting apparatus. 12

had misgivings about this method because of the likelihood of drop shatter upon impact, and because of the extreme thermal stresses which must be present in a rapidly freezing ice pellet, we were not entirely prepared for what we observed. Among other things, we found that the ice pellets tended to form clusters, that is, a number of small very nearly spherical ice pellets would be found stuck together. Because the specific gravities of water and of ice are nearly 25 per cent larger than that of liquid nitrogen, we had assumed that gravity would prevail, and that the water substance would fall below the surface of the nitrogen and remain there throughout the freezing phase. What we found out upon dripping water drops into a container of liquid nitrogen was that the drop, if dropped from a relatively low elevation, would fall down into the liquid nitrogen pool as a single particle, but would immediately -emerge again on the surface of the liquid nitrogen. There it would scoot about the surface causing the nitrogen below it to boil until thermal equilibrium was reached between the ice and the nitrogen, at which time it would sink down into the liquid, and onto the sieveso This activity on the surface of the liquid nitrogen was most fascinating, but of course it defeated our purpose because of the clustering it produced. The freezing technique still had a number of attractive features about it. We therefore considered that if we could find a fluid of low surface tension which would not be soluble in water and which would remain fluid to quite low temperatures, that we might arrange a cooling system to maintain such a liquid at temperatures well below freezing, and thus salvage some of the benefits of this technique. Table L gives pertinent data on fluids that appear to meet these physical requirements. Of the various materials tried, the most nearly successful was Dow Corning 200 Fluid. This was placed in a container and chilled to about -780C, using dry iceo Drops were dripped into the container from an eyedroppero In this case the behavior of the drops was still somewhat similar to that in the liquid nitrogen. First they sank, then they came back to the surface and moved about making a sound somewhat similar to that made by-a flat knife blade held on a piece of dry ice. Finally the respective ice pellets formed an equatorial-plane split as the freezing went to completion and the noise stopped. Obviously, the hemispheres thus produced are nicely related to the size and number of the drops from which they are formed and from this point of view, they are quite satisfactory. However, it was still necessary to examine the impact shattering problem. 13

TABLE 1 Properties of materials that miqht be used in the cold sampling method Surface Tension Density Freezing Boiling Solubility Compound dyne/cm gm/ml Pt ~C Pt ~C gm/100 gm H20 Remarks Anyl benzene 0.860(4-22~C) -78.25 202.1 i Butyl benzene 0.860(20~C) -81.2 183.2 i Propyl benzene 30.6(4.5~C) 0.862(20~C) -101.6 159.2.006(15~C) 2, 2 Dimethyl butane 18.2(0~C) 0.649(4-20~C) -98.2 49.7 i Flammable, boiling pt too low 2, 3 Dimethyl butane 19.4(0~C) 0.668(17~C) -135.1 58.1 i Flammable 2 Metiyl butane 19.4(-20~C) 0.621(19~C) -160.5 28 i Very flammable Cumene 0.862(20~C) -96.9 152.4 i Tried Cyclolexene 0.810(20~C) -103.7 83 i Too expensive Dow Corning "200 Fluid" 16.8(25~C) 0.818(25~C) -123(~F) 100(~F) i Tried Buthyl ethyl ether 0.752 -124 91.4 i 2-5 Dimethyl; 2-4 Hexadiene 0.716(4-21~C) -91.3 102.5 i 1 Chlorohexane 0.872(4-20~C) -83 132.4 i 2-5 Dimethyl hexane 0.699(4-20~C) -91 108.2 i 2 Methyl hexane 0.679(4-20~C) -119.1 90.0 i 3 Methyl hexane 0.687(20~C) -119.4 91.8 i 1 Hexene 0.673(4-20~C) -98.5 63.5 i 1 Hexyne 0.736(0-4~C) -150 71.5 i Octylene 0.722(17~C) -104 123 i 2, 2 Dimethyl pentane 0.674(20~C) -125 79.2 i 2, 4 Dimethyl pentane 0.673(20~C) -123.4 80.5 i Ethyl methyl sulfide 0.837 -104.8 66 i Valeronitrile 0.801 -96.0 141 i

Serious shattering was observed when the dropls were released from 3 ft and higher elevations. At this point, the freezing method was set aside. The suggestion has been made that a braking layer of a viscous gas, or a freezing process that precedes impaction or shatter-producing braking might overcome the difficulty. These possibilities are under study. Vertical air flow method. Still a third method of separating out a certain drop-size fraction of the rain was considered. This was to be accomplished by means of a small vertical wind tunnel in which the air speed would determine the drop sizes rejected from the entrance (Figure 7). After some consideration of this method and of the problems involved in getting a uniform air stream at the mouth of the sampler, of wall wetting, and so forth, this scheme was abandoned because of (a) the excessive costs of activating it and (b) its relatively low yield of information. Analysis of the Rain Samples The preliminary rain sampling program was begun in June 1960. The liquid nitroqen method. Four storms were sampled using the liquid nitrogen method before that particular technique was given up. In the first of these, on June 16, a total rainfall of 1.24 in. was sampled, and 118 gm of ice pellets were collected distributed in six different size samples which ranged in weight from 8.8 gm to 31.2 gm. The radioactivity counts for these samples varied from 0.6 to 1.7 Arc with counting error estimates ranging from 19 to 58 per cent and the average count was 46 Arc per kg ~45 per cento The second of these storms, on June 22, gave a total rainfall of 0.85 in. and the samples totaled 47.8 gm. The size fractions for this rain were mostly too small to give measurable beta counts. The third and fourth storms sampled by the liquid nitrogen method gave too little rain for analysis by the available instrument. Because of the errors of sizing and counting in this method (see above), no interpretation of the results is attempted. The nylon net method. This technique has been used in some thirty storms. Of these, many produced rain too light to provide a sample. Experience with the method indicates that a storm must yield at least 0.4 to 0.5 in. of rain to give useful data. Information on samples collected by this method is presented in Table 2. 15

AIR FLOW CONTRACTION SECTION COLLECTOR _/| |, oX STRAIGHTENING FA N | VANES IIOv. VARIAC Figure 7. Cross-section of apparatus for vertical windtunnel separation of drops. 16

TABLE 2 Radioactivity of rain samples collected by nylon net method 1 2 3 4 5 6 7 8 9 Mass Radioactivity Date Sample Size Mass Fraction Total Fraction Error Average 1960 No. Category gm % c % % cAc/kg 1 large 627.6 32 7.7 32 13 12.1 6/12-14 3 small 365.8 18 4.7 19 18 12.8 38 large 988.3 50 12.0 49 10 12.1 1981.7 24.4 12.3 6/14-15 33 large 143.8 90) Not measurable 35 small 16.1 10) 159.9 7/2-3 2 large * 9.5 93 12 - 32 small * 0.7 7 54 - 757.7 * 10.2 13.5 40 small 35.6 7 0.5 12 60 14.0 41 large 464.9 93 3.6 88 20 7.7 500.5 4.1 8.2 42 small 24.7 10 0.1 4 80 4.0 43 large 216.0 90 2.4 96 28 11.1 240.7 2.5 10.4 45 small 140.3 11 1.0 6 52 7.1 8/10 46 large 767.1 60 10.0 58 10 13.0 47 large 379.1 29 6.1 36 15 16.1 1286.5 17.1 13.3 8/15 48 small.53.3 11 1.7 16 36 31.9 49 large 434.2 89 9.0 84 11 20.7 487.5 10.7 21.9 51 small 72.3 12 52 large 544.6 ~ 88 616.9 (53 small 288.1 26 (54 large 824.7 74 10/14-15 1112.8 D(55 small 177.5 16 (56 large 922.4' 84 1099.9 61 large 127.0 62 11/15-17 63 small 78.1 38 205.1 * By an oversight, these samples were not weighed. Total sample mass is estimated from independent rain gauge information. 17

The radiological analysis, which is performed by The National Sanitation Foundationa School of Public Health, The University of Michigan0 provides a thallium-204 equivalent gross beta count (column 6) together with the 95 per cent confidence error estimate (column 8) for each sample submitted. The fraction of the radioactivity (column 7) and the specific radioactivity (column 9) for each sample are then computed. The changes of specific radioactivity between storms are of interest0 particularly as they pertain to the transfer of radioactive debris from the stratosphere to the troposphere and to its dispersion within the troposphere. Figure 8 shows the average specific radioactivity levels for rains sampled by the nylon net method, and also the rainfall amounts recorded for all rains at Ann Arbor during the period June 12 to August 15, 1960. There appears a general level of radioactivity near 12 to 13 plc per kg of rain. The samples of August 3 gave counts below, and those of August 15 gave counts considerably above this value. The rain of August 3 essentially terminated a hot dry period of eight days' duration, at the end of which unirrigated lawns were completely brown. That of August 15, however had been preceded more closely by substantial rain on the 10th (of 13 Arc per kg specific activity) and a light rain on the 14th. The implications of the synoptic situations are not clear, and considerable compilation and study are needed to clarify them. The large counting error associated with these samples strongly suggests that such a study on these data is not justifiable; nonetheless, it is appropriate to consider whether the excess of radioactive materials present on August 15, and the relative deficit on August 3 might be explained in terms of stratosphere-troposphere exchange. Bleichrodt, et a15 have reported a dramatic change of radioactivity associated with a cold frontal passage on October 31, 1958. This change is related through the -air trajectories traced in the respective air masses to the high-latitude tests conducted in the earlier part of October. Dyer and Yeo6 infer from their observations of radioactivity levels in weekly samples of rain that the original cloud of contamination injected at low latitude remains discrete in the lower stratosphere for six to nine months. They cite evidence that suggests a poleward drift of the debris cloud at a mean rate of about lo.1 m per sec from near the equator to 380S accompanied by a lateral spread of about 1800 of longitude seven months after its introduction into the stratosphereo These authors do not attempt to postulate a mechanism whereby the contamination enters the troposphere from the stratosphere. 18

20 0 t 16 410 C) 06> C... 1 1 1 6/12-14 7/2-3 8/3 8/10 8/15 DATE (a) Average specific activity ([pic per kg) of rain collected at Ann Arbor, Michigan, June-August, 1960. Horizontal bar indicates time of accumulation of rain sample. 3.0 z I Ut 2.0 z -J 01.0 O L0 I0 I I I. I I I I I I I I I I I 6/13 19 25 7/1 7 13 19 25 31 8/6 12 18 DAT E'(b) Total rainfall at Ann Arbor, June-August, 1960. Figure 8 19

Most speculation on the question of the synoptic distribution of radioactive contaminants in the atmosphere tends to the notion that in the absence of renewed nuclear tests, the distribution of radioactive contaminants should in time become quite uniform as the concentration diminishes. Although it is likely in the present case that sampling and counting errors contribute effectively to the observed variations of concentration between storms, the possibility remains that real changes may occur and may be related to irregular stratosphere-troposphere exchanges. The hypothesis in this case would be that irregular injections of radioactive material from the stratosphere into the troposphere may produce a non-uniform distribution of the materials in the troposphereand result in changes of the type shown by our data. A rather low general background level of radioactivity should then prevail, and relatively infrequent and brief increases above this level should be associated with such localized injections of stratospheric air as suggested by ReedJ7 The occurrence of such an intrusion of stratospheric air into the troposphere in the formation of a katafront may or may not place this air directly into a rain-generating situation in the troposphereo A substantial synoptic investigation, such as Reed's, would be necessary to establish this point, and of course this will be justified for those cases in which the radiochemical data are sufficiently accurate. The differences of radioactivity levels within storms, as shown by the values for the respective drop-size fractions (columns 6 and 9, Table 2) also deserve some discussion. Initially it is necessary to consider the role of the nylon net in separating the drop size fractions. Since the inner cylinder of the rain gauge has a cross-sectional area one tenth as large as the outer container, ten per cent of the rain should enter the inner container without benefit of any biasing effect of the conical net. In addition, small drops that impinge upon the net without penetrating it contribute water which tends to flow along the net surface to the inner cylindero Laboratory tests indicate that about 50 per cent of the 008 mm diameter drops which strike the net at their terminal speed penetrate it. Data obtained using the raindropsize spectrometer show that about 10 per cent of the total mass of a typical summer rain is contained in drops less than 0.8 mm diameter. If the biasing effect of the net were limited to this drop size class0 depending upon the applicability of the 10 per cent mass estimate for the specific rainfall in question, 19.0 per cent of the total catch of rain should be found in the inner containero In the storms of June 12-14, October 14-15, and November 15-17 (Table 2) the small drop fractions comprise about this proportion of the total sample. The problem of explaining an 20

inner cylinder catch of less than 10 per cent is raised in the case of the morning rain of August 3. The most ready explanation is the possibility of a handling mistake or a weighing error. The alternative possibility of strong wind and of evaporation from the net producing this effect should not be overlooked, however. The other storms gave samples somewhat less biased than expected by the above reasoning. More complete explanation of these mass proportions is contemplated following reduction of the raindrop-size data. The partitioning of the radioactivity among the rain fractions is of special interest. Column 9, Table 2, gives the respective specific activities for the samples that have been analyzed. Interpretation is tenuous because of the large counting errors (column 8) associated with the small samples. In the first storm (June 12-14), however, these errors are only moderate, and computation of the respective levels of radioactivity of the drop fraction smaller than 0.8 mm diameter and of the large drop fraction may be made under the assumptions discussed above. The result is a specific activity of 12.1 Arc per kg for each part of the large drop sample and 13.5 44c per kg for the small drops. The corresponding figures for the August 15 storm are much more extreme: 20.7 and 139 Arc per kg for the large drop and small drop fractions, respectively. There appears little point in attempting such estimates for the other higher error samples, but-evidently the strongest evidence of the present data is that the small-drop fraction of the rain is characterized by higher specific radioactivity than the large-drop fraction. The most obvious mechanism available to produce this effect is evaporation from the small drops during their fall and from the surface of the collecting net. An analysis of the evaporative losses will be essayed for a future report. Bleichrodt, et a18 found a negative correlation between rainfall rate and specific radioactivity of their samples of rain water. Similar observations have been made by Cowan and Steimers9 and by Isotopes, Inc.,l and the latter group has pursued the question further to show an inverse climatic correlation between rainfall amount and cumulative Sr90 in soils. in the latitude zone of 300 to 600N. The possibility that the radioactive content of rain was concentrated by an evaporative process operating more effectively upon the light than the heavy rains was pointed out by Bleichrodt, et al.8 Because they also observe that "... gross activity is usually proportional to the volume of water in the sample..." the assumption that in these latitudes the cloud water antecedent of all rains tends to contain a relatively uniform 21

concentration of radioactive material appears reasonable. This assumption further implies that the radioactive materials are uniformly mixed throughout the rain-forming air of middle latitudes. The climatological evidencel~ is somewhat less amenable to direct interpretation. The variations of annual precipitation totals within the 30~ to 600N zone are definitely not primarily attributable to evaporation from falling rain. This means that another mechanism must be operative. It is possible that the leaching effects of the heavier rains may serve to reduce the soil Sr900 but this effect must also depend upon soil typeo This question deserves further careful study. Raindrop-size spectra. Difficulties were encountered in getting complete coordination of raindrop size data with the nylon net sampling datao Further, the raindrop-size data have not yet been completely reduced to numerical form. Nonetheless, a few of these data are available8 and brief inspection of them in relation to the rain sampling data is appropriate. Raindrop spectra were recorded on June 12 and 13 and portions of these records were selected for analysis. The rain of June 12 was attributed to overrunning of warm air with a stationary front south of the stationo This rain was light and quite constant in character, being composed entirely of drops less than. 2 mm in diameter. The standard weighing rain gauge, equipped with a tenfold-enlarged collector funnel0 produced the record shown in Figure 9(a) under this conditiono The rain intensity averaged about 1.0 mm per hr and accumulated to 4.07 mm (00160 in.) during this period. In contrast8 the rain on the 13th was considerably heavier as shown by the ten-fold-rain gauge chart in Figure 9(b). In this case the intensities become moderate, and are quite variable. To characterize the rain under different intensity regimes, four periods were chosen from the records of the 13th for preliminary analysis: (1) 0935-0940 (2) 0944-0948, (3) 0953-0957, and (4) 10061010, all ESTo The rain amounts as read from the record and as computed from the raindrop spectra for these four periods are shown in Figure 10. Raindrop-size spectra for these four periods are shown in Figure 11. Characteristically0 in this range of rainfall intensities the number of drops in each size range increases, and the maximum drop size increases, as the intensity riseso. There are, however, clear variations of this general tendency. Table 3 gives the minute-byminute drop-size spectrum data in which these features are somewhat more prominento The rain intensity contribution of each drop 22

.30.25 n.20 (.15.10 I0:00.05 0500 0530 0600 0630 0700 0730 0800 0830 0900 0930 TIME, EST. 12 JUNE 1960 (a) Weighing rain gauge chart for 12 June 1960. Amount is amplified ten-fold by means of an enlarged receiving funnel. Maximum intensity is 2.3 mm per hr..60.60 C50 -.70.40.80 WI 4.30.90.20 1.00.10 1.10.00 1.20 0800 0830 0900 0930 1000 1030 1100 1130 1200 TIME, EST. 13 JUNE 1960 (b) Weighing rain gauge chart for 13 June 1960. Note periods 1, 2, 3, and 4 discussed in text. Figure 9 23

15 14 12 Z I O10 8-J -J z6 LL Z 4 cc 2 -— 4 —- I IiI 0932 35 40 45 5'0 55 100 05 1 15 20 TIME, EST. 13 JUNE 1960 Figure 10. Rainfall intensities each minute for periods 1, 2, 3, and 4 (Figure 9b) as computed from raindrop spectra.

4000 3000 E0 PERIOD I, AV INTENSITY 4.76 MM HR' 2000 PERIOD 2, AV INTENSITY 1.96 MM HR-' A PERIOD 3, AV INTENSITY 10.38 MMHR-' 0 PERIOD 4, AV INTENSITY 1.18 MM HR-l 1000 500Z El a I00Z )0 I0- El o100E 050 A 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 DROP DIAMETER (MM), 13 JUNE 1960 Figure 11. Raindrop-size spectra averaged for each of the

TABLE 3 Raindrop-size distributions each minute for four selected periods 13 June 1960 Time Drop Diameter Intensity EST mm mm hr~ 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 0935 1384 607 375 196 107 62 71 36 0 9 9 5.39 0936 1197 330 312 161 188 71 9 18 9 9 0 9 4.67 0937 1179 491 339 205 71 36 27 18 27 9 18 5.06 0938 1232 366 420 241 134 89 18 27 18 4.60 0939 1009 411 384 188 107 98 18 18 0 0 9 4.08 Average 1200 441 366 198 121 71.2 28.6 23.4 10.8 5.4 7.2 1.8 4.76 0944 777 447 116 80 18 36 9 1.38 0945 705 259 152 89 54 18 18 9 18 0 9 2.60 ~0 0946 857 268 107 89 28 9 9 9 1.29 0947 723 339 152 89 45 18 27 18 9 9 2.55 Average 766 328 132 86.8 36.3 20.2 15.8 9.0 6.8 2.2 2.2 1.96 0953 1116 518 339 259 250 134 89 18 27 9 6.98 0954 1375 643 634 411 268 152 80 89 45 9 18 18 12.10 0955 1697 732 920 697 357 107 116 98 18 18 9 0 9 12.98 0956 1358 652 822 286 170 134 36 0 18 9.46 Average 1386 636 679 413 261 132 80.2 51.2 27.0 9.0 6.8 4.5 2.2 10.38 1006 893 401 366 80 18 0 9 1.45 1007 929 295 312 125 54 1.51 1008 750 313 277 89 9 1.09 1009 679 304 170 18 9 0.69 Average 813 328 281 78 22.5 0.0 2.2 1.18

size category and the curve of accumulated rainfall against drop diameter for each of the four periods are shown in Figure 12. In these periods the percentages of water received in drops up to 0.8 mm diameter range from 6.6 to 31.3 per cento Although the radiochemical data for the storm of 16 June are not subject to firm interpretation because of the sampling method (liquid nitrogen) used, complete raindrop-size spectra were obtained for this storm. The type of rain in this storm was vastly different from that of the June 12-13 storm, hence the data are pertinent here to document more extreme rains. The ten-foldrain gauge record is shown in Figure 13. Because of the extreme sharpness of the showers, it is not feasible to estimate intensities of rainfall from the record chart. The minute-by-minute intensities as computed from the drop-size data show the violently variable character of the storm (Figure 14). Some of the more extreme raindrop size spectra from this storm are shown in Figure 15. The intensity and the total rainfall are plotted against drop size in Figure 16. These figures show quite clearly the degree to which, with increasing intensity of rainfall, the raindrop-size spectra depart from currently accepted empirical relations (Best,ll Marshall and Palmerl'). Further, it is quite evident that, as rainfall intensity increases, the variability of the drop-size spectra for any given intensity of rainfall increases. The significance of these findings as, regards the various components and the total of scavenging effectiveness is under study. 27

14 ---- 14 PER MEAN ITENSITY PERIOD 2 MEAN INTENSITY ~~~~~~~~~~~~~~~~~~>- PEID2>-NINEST >- 4.76 mm HR-I I - 1.96 mm HR-I U) Z Z e W — Z F zO I0 z IV0 IOOZ 10 100 w - 0' o z O0 O~~~~~~~~~~~ - 0 w - CD~~~~~~~~~~~~~~~~~~~~~~~~~C ~ ~~~~5 75 ~u:D~-~~~~~~~~~~I 45' I~~~~ ~~~~~~~~~~~~~~~~~- 0 I-.Z z Z w o w 0 0 0 ~~~~~~~~~~~~~~~5Ow 5- 50 > w Z~~~~~~~~~~~~~~~~~~~~~~~~~~ w 4 ~~~~~~~~~~~~~~~~~~~Z 4J o _ o nw5 -Z w ~~~~~~~~~~~~~~~25 2 Cr 25 -50 5-w/o 0 0.5 1.0 1.5 2.0 2-5 3.0 0.5 1.0 1.5 2.0 2.5 3.0 DROP DIAMETER (mm) DROP DIAMETER (mm) (a) for period 1: 0935-0940 EST, 13 June 1960 (b) for period 2: 0944-0948 EST, 13 June 1960 t\.) PERIOD 3 MEAN INTENSITY co 15 -I la 38 mm HR-' PERIOD 4 MEAN INTENSITY I I I,, I 1.18 mm HR-' Z 50- z z > w o. I-z - z 10 IOOZ 40 100 w z o z0 0~~~~~~~~~~ o 0 0c~~~~~~ 0.~~75w 30 =715. Q z~~~~~~~~~~~~~~~~~~~~~~~Q z z w w o (3 C0o (a 5 5o 200 50 w 4 a~w w a wP 4 w )-w > w w4 25 _ 10 - 25 -J 0 > z- I0- _1 i I I I 1Ou ~~~~~ 0 0.5 1.0 1.5 2.0 25 3.0 0 0.5 1.0 1.5 2.0 DROP DIAMETER (mm) DROP DIAMETER (mm) (c) for period 3: 0953-0957 EST, 13 June 1960 (d) for period 4: 1006-1010 EST, 13 June 1960 Figure 12. Rain intensity and cumulative intensity as functions of drop size.

0.6 0.6 0.5 - - - - - -0.7 1830, 16 JUNE 0.4 - 0.8 1719, 16 JUNE EMPTIED BUCKET 16050X 1 JUNE 0.1 - - - 1. 1721, 16 JUNE 0.0 - 1.2 1500 1600 1700 1800 1900 TIME EST, 16 JUNE 1960 Figure 13. Weighing rain gauge chart for 16 June 1960. Same collector as described above (Figure 9).

80 3 70 7 LULL WHEN RAIN GAUGE BUCKET WAS EMPTIED X (SEE FIG. 13) 60 6 E z >- 50 5 z w z 400 z 0 30aw a- 20 2 PAPE R 1604 1615 1620 1625 1630 1635 1705 1710 1715 1720 1725 1730 1735 1740 175 70 TIME EST,16 JUNE 1960 Figure 14. Rainfall intensities as a function of time, 16041750 EST, 16 June 1960.

4000 3000 E 2000 \v 1708 AV INTENSITY 22.8 mm HREl 1710 AV INTENSITY 38.6 mm HR V"-'"~~\~ \~0 1716 AV INTENSITY 70.2 mm HR-' 1000 0O 1730 AV INTENSITY 6.66 mm HR'500 E 0 E 0.4 1.0 2 3.0 4.0 4.4 00 16 June1960. o_ 100 ~ ~ ~3 0_ El I0 31

12 (~~~~~~~~~~~~~~~~~~~~~~~~~~Iu/) z WI0 -- ZH 0~~~~~~~~~~~~~~~~~~~ Z7H z ~~~~~~~~~~~~~~~~~~~~ H 0 D~~~~~~~~~~~~~~~~~~~~~ H Z 5 H 0 0 W- 0 z(-) a::D 0.0 0.0 0.5 DROP DIAMETER (m m), 16 JUNE 1960 Figure 16. Rain intensity and cumulative intensity as functions of drop size, 16 June 1960.

CHAPTER 3 Coordination of Work with Other Units Hanford Atomic Products Operation Discussion of collaborative efforts to be undertaken with the Hanford Atomic Products Operation meteorological group has led to the planning o.f a series of field experiments at the Hanford site. As a preliminary step, a comprehensive climatological study of rain and fog occurrences at the Hanford site was made. The results of this study indicate that the best time to study natural rains at the Hanford site is during October and November. Fog occurrences are at a maximum in November and December. Thus it is concluded that the period October-December should prove most fruitful.for field experiments on natural cloud and rain scavenging. Several preliminary experiments designed to test the rain wash-out and the Brownian and turbulent motion scavenging mechanisms are planned for the summer period, 1961. In connection with these experiments, the feasibility of various techniques of air sampling and of rain collecting will be explored in anticipation of the fall season experiments. An effort will be made to model the wash-out situation by using controlled sprays upon artificially generated clouds of particulate. The sampling grid at the Hanford site will provide a well-documented basis for monitoring the particulate cloud; the cloud generating equipment there will serve as source; and the towers will provide elevated mountings for spray generators to simulate rain. Samples collected at ground level will be evaluated in terms of the drop sizes involved and the number and size distribution of the particles collected. The fogs that occur at the Hanford site offer an opportunity to study the scavenging effectiveness of the Brownian motion process combined with small scale atmospheric turbulence. In general outline, it is clear that the fog presents conditions such as prevail in stable clouds prior to the production of rain. One experimental approach requires that such a fog be contaminated with an identifiable particulate of suitable size, and that it be sampled sequentially to determine the rate at which the particulates are "captured" 33

by fog dropletso Studies will be made to determine the practical feasibility of available techniques for performing this type of experiment o Field experiments under natural conditions are then contemplated for the October-December period at the Hanford siteo In these experiments, raindrop-size spectra and size-defined samples of rain will be collectedo Additional data on the contamination of the atmosphere before0 during and after rain and fog will be collected utilizing the Hanford sampling grid6 The feasibility of obtaining useful data from aircraft on contamination at higher levels, and on cloud water inside and outside the rain area0 etc, will be exploredo These experiments will provide necessary background for the design of more comprehensive field studies of the scavenging effectiveness of rain under natural conditionso Under the existing circumstance that the level of radioactive contamination of the atmosphere is very low and decreasing0 the possibility of advancing knowledge in the field of rain cleansing of the atmosphere by working with other contaminants deserves study. Inasmuch as most atmospheric contaminants originate at the earth0s surfaces the problem of finding an identifiable substance, the behavior of which might simulate that of radioactive debris moving downward from the stratosphere, is complexo However, the physical behavior of tiny particulates present in the troposphere, independent of their origin, is pertinent to the scavenging function, and the study of this behavior should lead to knowledge that can be applied to the problem of radioactive contamination. Accordingly0 some study of natural contaminants of the atmosphere and of rain will be undertaken in connection with the field experiments at the Hanford site~ Aeroalleren Project The University of Michiqan The attention of the Aeroallergen Project has been focused upon plant pollens as air contaminants0 and has especially dealt with ragweed (Ambrosia artemisiifolia) polleno Because substantial monitoring of the atmospheric distribution of ragweed pollen is accomplished under the Aeroallergen Project0 it is both logical and economical for the Rain Scavenging Project to take advantage of the opportunity thus presented to study the rain scavenging of this specific particulate. The pollen of A. artemisiifolia is a sphere with small bumps on its surface having a diameter of 18 pi +2 p and a density of 34

about 1.3 gm per cm3. Its source is within about 18 in. of ground level, and by and large, it is not carried to great heights, although in strong convective currents, some pollen are carried to tropopause-level. Thus it is a large particle as air contaminants go, and it is removed from the atmosphere primarily by gravitation, turbulent impaction, and rain wash-out. The opportunity to make evaluations of existing theories of the rain wash-out process is thus presented by the juxtaposition of the pollen-monitoring data, counts of the pollen found in the various rain samples, and drop-size distributions of the rain. Because the circulation of the pollen grains in a storm must be governed by continuity considerations, these evaluations should provide a basis for estimating the extent to which low level air is entrained in the rainstorm circulations and participates in the rain processes. The mechanics of the wash-out of particles of this size have been quite well delineated by Langmuir,l3 Chamberlainl4 and Greenfieldl5 and the analysis we are doing represents a straightforward application of their equations. Preliminary work with this approach has been undertaken under the present contract. The data from the nylon net separator are, however, considered inadequate to deal realistically with the problems we wish to attack, because of mechanical interference of the net with the falling rain. The more adequate data that will be obtained with the aerodynamic raindrop sorter will provide a basis for numerical comparisons and a complete report of this phase of the work. 35

CHAPTER 4 Additional Sub-Projects The study of temporal variations within and between storms. Bleichrodt's work5'8 shows that temporal variations of specific radioactivity of rain can be correlated with changes of the rate of rainfall on the one hand, and with changes of air mass on the other. In the first instance, the difference is attributed largely to the increasing effect of evaporation as rainfall rate decreases, whereas in the second the origin and age of the radioactive contaminants play a part. The data of Dyer and YeO6 are interpreted in terms of a postulated "cloud" of radioactive contamination which appears to remain distinct for an extended time. The pertinence of this kind of information to the present studies is apparent, and we have taken steps preparatory to assembling similar data. A pair of large rain-collecting..pans have been made for the purpose of procuring sequential samples in.heavy rains, and measurable samples in light rains. The design was in a measure controlled by the standard size of galvanized metal sheets, i.e., 4 ft x 8 ft. Each of two such sheets is formed into a 4-in. deep pan turning up the edges and sealing the joints. A 2-in. diameter drain is made in each pan in such a way that the two can be conveniently manifolded into a single drain. Each pan thus presents a collecting area of 28.1 ft2 (206 m2) The method of Bleichrodt, et a18 will be adapted for obtaining sequential samples cf rain within stormso The pans will be set upon a wooden platform designed to slope slightly toward the drain-corner of each pan. The bottoms will thus be maintained quite flat so that a squeegee procedure may be used in sampling light rains. This procedure is obviously superfluous under heavy rainfall. A complete record of the collecting time and time interval for all samples will be maintained, and samples of the order of 1 galo of rain water (about 0.035 in. of rain over the total area of the collecting pans) will be taken. Radioactive species analysis and decay-rate datingq of radioactive contaminants. The instrumentation and technique required to identify radioactive species present in sufficiently large samples of rain are available to our analyst. The large collecting 36

pans will make possible the regular collection of sufficient sample for species determinations, and it is planned that these shall be made on samples collected in 1961. In addition, in those cases in which the drop-size-discriminated samples are large enough, species determinations will be made. The method used by Dyer and Yeo6 to estimate the date of origin of radioactive samples is also of interest. Since we need to develop as complete information as possible about the variations in radioactivity, it is appropriate to attempt this type of identification of our samples. Accordingly, we plan to make sequential counts of representative samples over a period of months for this purpose. Deuterium and the isotopes of oxygen. It has been suggested that analyses of some of our rain samples to determine the 018/016 ratios and the relative amounts of deuterium present may be accomplished simply, and at the same time, may contribute highly interesting additional information, particularly relating to the phase-change history of the water substance. This suggestion is under study, and will be activated if it proves feasible within the context of the project. Radar data. The raindrop-size data provide all necessary details of the physical nature of each rainfall as it reaches the ground. A good deal of additional information about the processes of growth of the raindrops, and the nature of the storms in which they are generated is required to construct reasonably complete descriptions of the drop growth and scavenging processes. The means immediately available for obtaining observations within the rainforming clouds is present in weather radar. To promote this aspect of the research, an effort was made toprocure a suitable surplus radar unit. Such a unit was located in the possession of the Huron Portland Cement Company of Detroit. This unit, a Westinghouse MU-1 type marine radar, had been used on one of the company's Great Lakes ships. The characteristics of this type of radar unit are well adapted to our needs and situation. It operates on 110 v, 60 cycle electric power. It is rated at 40 kw power output in the 3 cm (X-band) wave-length region. The pulse-length is 1/4 fii sec, and the pulse repetition rate is 1100 per sec. Whereas these characteristics are not ideal for general weather radar work, they are well-adapted for short-range study of 37

the structure of rainstorms, and for relatively detailed resolution of rain masses aloft. Douglas, Gunn and Marshalll6 showed that a zenith-pointing radar, capable of good resolution at close range, could add valuable information about the origins of snow to that available by means of PPI and RHI presentations and soundings. It is our opinion that, used in conjunction with other radar data and synoptic information, the raindrop-size spectra and the records obtained from a zenith-pointing radar of high resolution will provide a powerful means of penetrating the precipitation-forming processes. These, of course, are intimately involved with the rain scavenging function. In lthe present instance, the MU-1 unit has been transferred gratis to our possession through the good offices of Mr. Charles M. Adams, Vice President, The Huron Portland Cement Co. We propose to modify and adapt this unit to serve as a zenith-pointing narrowbeam radar and to install it near the site of the raindrop sorter, the raindrop-size spectrometer, and the rain-collecting pans. The work of servicing and adapting the MU-1 radar is under way. 38

REFERENCES 1. Dingle, A. N., and F. V. Brock, 1960. An aerodynamic raindrop sorter. Technical Progress Report No. 1, Contract No. AT(11-1)-739, Atomic Energy Commission, The University of Michigan Research Institute, Ann Arbor. 2. Pankhurst, R. C., and D. W. Holder, 1952. Wind tunnel techniques. London, Sir Isaac Pitman and Sons Ltd., pp. 647-649. 3. Pope, Alan, 1954. Wind tunnel testing, 2nd ed. New York, John Wiley and Sons, Inc., pp. 31-77. 4. Twomey, S., 1956. The collection of cloud water by a vertical wire mesh. Bull. de l'Obs. du Puy de Dome, 3, 65-70. 5. Bleichrodt, J. F., W. Beeker, and F. A. Schmidt, 1960. Changes in the radioactivity regime during the passage of a cold front over the Netherlands. Tellus, 12, 2, 188-194. 6. Dyer, A. J., and S. A. Yeo, 1960. A radioactive fall-out study at Melbourne,Australia. Tellus, 12, 2, 195-199. 7. Reed, R. J., 1955. A study of a characteristic type of upper-level frontogenesis. J. Meteor., 12, 3, 226-237. 8. Bleichrodt, J. F., J. Blok, R. H. Dekker, and G. J. H. Lock, 1959. The dependence of artificial radioactivity in rain on rainfall rate. Tellus, 11, 4, 404-407. 9. Cowan, F. P., and J. Steimers, 1958. The distribution of fallout activity in rainfall at Brookhaven National Laboratory, June to September, 1957. BNL 496(T-117), Upton, New York. 10. Isotopes, Inc., 1960. Studies of nuclear debris in precipitation. 4th Quarterly-Progress Report, Contract No. AT(30-1)2145, Westwood, New Jersey. 11. Best, A. C., 1950. The size distribution of rain drops. Quart. J. r. Meteor. Soc., 76, 327, 16-36. 39

12. Marshall, J. S., and W. M. Palmer, 1948. The distribution of. raindrops with size. J. Meteor., 5, 4, 165-166. 13. Langmuir, I., 1948. The production of rain by a chain reaction in cumulus clouds at temperatures above freezing. J. Meteor., 5, 5, 175-192. 14. Chamberlain, A. C., 1953. Aspects of travel and deposition of aerosol and vapour clouds. Atomic Energy Research Estab., HP/R 1261, Harwell, Berks, England. 15. Greenfield, S. M., 1957. Rain scavenging of radioactive particulate matter from the atmosphere. J. Meteor., 14, 2, 115-125. 16. Douglas, R. H., K. L. S. Gunn, and J. S. Marshall, 1957. Pattern in the vertical of snow generation. J. Meteor., 14, 2, 95-114. 40

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