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Wet Deposition Estimation Using Scavenging Ratios
21
Proc. First Specialty Symposium on Atmospheric Contribution to the Chemistry of Lake Waters. Internat. Assoc. Great Lakes Res. Sept. 28-0ct. 1, 1975.
WET DEPOSITION ESTIMATION USING SCAVENGING RATIOS D.F. Gatz Illinois State Water Survey Urbana, Illinois 61801 1.
INTRODUCT ION
It should be clear to everyone at this Symposium that it will first be necessary to estimate inputs of nutrients and toxic materials to the lakes, before we can infer the impacts of these materials on the lakes. This morning we are discussing atmospheric pathways of materials to lakes. There are two broad categories of processes by which materials enter the lakes through the atmosphere: wet processes and dry processes. In a sense, because deposition is on a water surface, perhaps all deposition processes are technically "we t", but we shall use the terms "we t" and "d ry " here to refer to deposition in precipitation, and that apart from precipitation, respectively. This paper discusses deposition in precipitation, and the fo110wi ng one wi 11 cover "dry" deposition. Research on wet deposition, or precipitation scavenging, received much impetus in the early 1960's when it was discovered that 80-90% of nuclear bomb fallout was reaching the ground via precipitation. There was emphasis then both on prediction of fallout deposition, as well as on understanding the fundamental physical mechanisms involved in collection of radioactive particles in rain and clouds. Even today, probably most of the information available on precipitation scavenging applies specifically to bomb debris. These results have been summarized by Engelmann (1968, 1971).
Bomb debris has a stratospheric source, whereas stable (non-radioactive) materials, which are our main concern with respect to deposition in lakes, have sources near the ground. Concentrations of materials with sources near the ground should decrease with height, in the mean, while materials with a stratospheric source should increase in concentration with height. These differing vertical distributions may result in different scavenging characteristics, so scavenging prediction methods based on bomb debris should always be tested on stable materials before they are accepted for general use. The purpose of this paper is to suggest that ratios of concentrations in precipitation and in air (also known as washout or scavenging ratios) are suitable for estimating the mean deposition of particulate matter having ground-based sources in precipitation over large areas and long (monthly or seasonal) time periods. Thus, the method should apply to those materials and sources for which the atmosphere is an important pathway to the Great Lakes. The method is not suitable for estimating the scavenging of gases or impaction scavenging of individual stack plumes. Hales (1972), Hales et a1.. (1974) and Engelmann (1968) have discussed these cases. With these purposes and limitations in mind, the suggestions given in this paper for estimating deposition in pre-
D.F.·GATZ
22 cipitation are based on two main premises: 1.
Deposition in precipitation should be proportional to the concentration of the material in air, and to the amount of precipitation, and
2.
It is appropriate, for the present state of knowledge in this field, to depend mostly on the field-observational/ empirical approach to prediction as opposed to theory and laboratory measurements.
The remainder of this paper is devoted to 1) a discussion of the characteristics of scavenging ratios of stable materials and their use in a practical problem, and 2) an outlook for the future. 2.
WET DEPOSITION PREDICTION USING SCAVENGING RATIOS
General considerations A number of natural processes exist by which air is continually cleaned of particles. These may be conveniently divided into "we t" and "d ry " processes according to whether precipitation is i nvo 1ved or not. The dry processes include gravitational sedimentation and impaction on obstacles near the earth's surface. Wet processes include capture mechanisms that occur throughout the precipitation cycle, from initial nucleation of cloud droplets on particles, through Brownian and phoretic capture of particles by cloud droplets, to impaction collection of particles by fa 11 i ng ra i ndrops. In the dry processes, particles are deposited directly, at or near the surface, but deposition by wet processes implies both collection of particles by rain or cloud, plus subsequent precipitation of the liquid water.
Several numerical approaches are generally available for calculating the removal of particles from a given parcel of air and sometimes even for calculating deposition if the populations of collecting and collected particles, and other basic physical parameters are known as a function of time. This is rarely the case in any practical problem, however. We may approach an adequate knowledge of all the necessary parameters for a well-characterized individual stack plume under certain limited meteorological conditions, but not in the case where total wet removal over a large area for periods of months to years is needed. For such situations, the scavenging (or washout) ratio (Chamberlain, 1960) W
=~ X
is recommended, where k is the concentration of any material in precipitation (~g/g), X is its concentration in unwashed air (~g/m3), and p is the dens i ty of air, taken as 1200 g/m 3 . If X can be estimated or measured, and W can somehow be specified, then k can be calculated from equation (1). Deposition follows by multiplying k by precipitation amount. This method can in principle be applied to individual places and to single precipitation events. However, the uncertainty in the estimation is large in such cases. The uncertainty should decrease as both time and deposition area increase, although the statistical relationships have not yet been speci f ied. In mos t pract i ca 1 situations, X will be known within no better than a factor of 2, so no better accuracy can be expected for k computed from equation (1). In most situations, X is measured
23
WET DEPOS ITION near the ground, and is unknown in the air entering the cloud. Clearly, it would be desirable to know X at cloud base, but it is not necessary. The empirical relationship of equation (1) can be used as long as the X used to estimate k is measured or estimated on the same basis as the X used originally to calculate W.
discussed by Engelmann (1968). Combining equations (2) and (3) we obtain an expression for the scavenging ratio in terms of both in-cloud and below-cloud collections:
Pw H L + Rt [l-exp(-At)] (4)
Theoretical aspects Although the scavenging ratio simply as a convenient empirical lationship, theory has developed lating it to both meteorological particle parameters.
began rereand
Following Engelmann (1968) and Junge (1963) we reason that the portion WI of the scavenging ratio due to removal of materials from the air in cloud is given by
where kl is the concentration in cloud water (~g/m£), E is the fraction of the aerosol collected by the cloud droplets, Pw is the density of water (1 g/m£) , and l is liquid water content of the cloud (g/m3). Engelmann further suggested that E should be replaced by 'liT 1 where ''I' is the in-cloud removal rate (sec-I) and T is the lifetime of the cloud (sec). Following Esmen (1973), the portion Wz of the scavenging ratio from belowcloud (impaction) collection is given by
~t [l-exp(- At)] where H is the height of the could base (cm) , R is precipitation rate (cm sec-I), t is the duration of the rain (sec), and A is the Ilwashoutll coefficient (sec-I). Estimation of II was
Esmen (1973) has suggested how an expression similar to equation (4) can be used to estimate the relative importance of in-cloud and below-cloud scavenging. The suggestion may have some merit, although Esmen's analysis appears to have neglected the effects of raindrop evaporation during fall. This omission would have the effect of increasing the fraction of W due to incloud scavenging above its true value. Slinn (1975) has provided an extensive theoretical discussion of both particle and gas scavenging by rain and snow. He gives the following expression for the scavenging ratio k
- = X
where h is the height to the top of the cloud or the contaminant, whichever is lower, Rm is the volume-mean drop radius, a(T) is the aerosol radius, varying with non-dimensional time T, and E [a(T), Rm] is a collection efficiency for particles by cloud droplets and raindrops, varying in time as particle, cloud droplet and raindrop spectra change. To use any of the various theoretical formulations to estimate k, and subsequently deposition, requires knowledge or measurement of both particle characteristics and cloud and precipitation parameters, preferably as a function of time. For a single preci-
24
D.F. GATZ
pitation event, the use of the theory to estimate deposition would require measurement of the required input parameters -'at best a difficult and expensive task. For large areas and periods of months or longer, knowledge of the proper averages of the required parameters are required. Such data are not now avai 1able, although they conceivably could be available at some future time. For the present, the best approach appears to be empirical, through measurement of scavenging ratios and their variation with easily measurable parameters.
NEW YORK CITY
~
'">z UJ
« ~ 100 Cl « UJ --l UJ
~ V>
Observations of scavenging ratios Engelmann (1971) reviewed the available data on scavenging ratios for bomb debris and a few other materials. He concluded that mean scavenging ratios were relatively constant for a number of different fallout radionuclides and stable atmospheric constituents, and that they should therefore be useful in predicting or estimating k. For bomb debris, at least, others (Makhon1ko et al. 1970; Van der Westhuizen, 1970; Pelletier et a1. 1965; Krey and Toonke1, 1974; Hinzpeter, 1958) have generally found an inverse relationship of one sort or another between scavenging ratios and precipitation amount, for both monthly and daily samples. The same is genera11y true also for stable elements and ions. Makhon'ko et a1. (1970) presented scavenging ratio data for Ca++, Na+, HC0 3, and S04 at several different locations in the USSR. Scavenging ratios averaged within groups having similar precipitation amounts invariably decreased with increasing precipitation, although the slope of the lines varied from one species to another, and from place to place. After simi lar1y averaging their monthly data, Krey and Toonke1 (1974) found that scavenging ratios for Pb in New York City (Figure 1) decreased
lOL-_-l.._...L..-....L-.L..JL.Ll..l.:I:--_--L.._.l....--L-J......L-l...J~
1
10 MONTHLY PRECIPITATION. em
100
FIG. 1. Variation of scavenging ratio with precipitation for Pb in monthly samples at New York City (from Krey and Toonkel, 1974).
rapidly with increasing precipitation in samples collected throughout the year. The same general relationship occurs in scavenging ratios for Mg, K, Ca, Fe, and Zn measured daily in summer convective rains at St. Louis, shown in Figure 2. Thus rainfall, among meteorological parameters, appears to affect the magnitude of scavenging ratios, both from monthly and from daily samples. The particle parameter of greatest influence appears to be size, as shown next. A relationship between rainfal1weighted mean Wand particle mass median diameter (MMD) was shown by Gatz (1974) for preliminary data from METROMEX. Wwas found to increase with increasing particle size. Recently Cawse (1974) has published
25
WET DEPOSITION
VI VI
C'" 0
• Wtotal
1600
o Wsoluble
VI
<::
~
r
"t:l
'"z
1200
Co
c(
LU ~
Cl LU
l-
8 LU
As Br
I
;:::
t;
'"
0
Cr
0
oAs 0 oCs
Al
o
Ce
Fe
oCr
?
o
/ eSb
"-
"-
f!;
IV Pb~1
400
c(
U LU
Sc
Fe
~n~co Mn
800
:I:
'"az
~n
0
5
0 MMD, jJI11
RAINFALL.
II1II
FIG. 2. Variation of scavenging ratio with precipitation, for daily samples at St. Louis.
FIG. 3. Variation of scavenging ratio with particle mass median diameter, for Chilton, U.K., JUly-December, 1973. Downward-pointing arrows indicate upper limit values. (Unpublished data of P.A. Cawse; used by permission). VI VI
Ql
scavenging ratios for a large number of elements measured at a number of locations in the United Kingdom. Elemental MMDs were also measured at two sites. Figure 3 shows the relationships between rainfall-weighted Wand MMD at Chilton for July-December, 1973 (Cawse, 1975). The solid square represent scavenging ratios computed from total (soluble plus insoluble) concentrations in precipitation. Open circles represent W-values calculated using only the soluble concentration. The precipitation samples include both dry and wet deposition. The Chilton data are compared to two sampling sites near St. Louis in Figure 4. At all locations, W increases with particle size. In addition, a dependence of W on distance from urban sources, also noted by Gatz (1974), is possible. W-values (for wet deposition only) generally increase by a factor of about 2 from a site 15 km NE of St. Louis to one 50 km NW of the city. A further increase in W-values is seen
g
1600
VI
<::
.~
~
1200
~ ~
Cl
~
800
:I:
I
8LU
Mn
.1
\.OIl\S,
(;at~'l)O"'\m 1\'11
4' z 8
§
Fe 400
Gatz
st
\5
Louis, km,--:N~E_ _-
to 7.2
Mg
~
U LU
'""MMD, um
FIG. 4. Comparison of scavenging ratio variations with particle size, for Chilton (monthly samples, including dry deposition, July-December) and St. Louis (daily samples, excluding dry deposition, June-August).
in the data for Chilton, a predominantly rural site 80 km W,of London. However, this increase must be at least partially caused by the inclusion of dry deposition in the k-values at
D.F. GATZ
26
Chilton. Systematic differences in sampling duration and season could also contribute to these observed differences.
3.
APPLICATION: ATMOSPHERIC POLLUTANT INPUTS TO LAKE MICHIGAN
The recently availability of W measurements and dry deposition measurements for individual elements suggested that a new estimate of atmospheric pollutant inputs to Lake Michigan was timely. Previous calculations (Winchester and Nifong, 1970; Robbins et al., 1972; Skibin, 1973) were based on estimates of dry deposition only. Thus, Gatz (1975) has recently calculated wet and dry inputs of pollutant aerosols to southern Lake Michigan, using available measured wet and dry deposition rates. The wet deposition portion of these earlier calculations will be reviewed here, as an illustration of the use of scavenging ratios to calculate deposition in Great Lakes. Calculations of wet deposition were performed separately for the convective half-year (March-August) and the nonconvective half-year (SeptemberFebruary) because of basic differences in the character of precipitation \systems and their expected scavenging characteristics. Experience with summer convective systems in the St. Louis area and elsewhere has shown that the heaviest deposition of urban pollutants occurs relatively close (10-15 km) to their sources. Winter precipitation, on the other hand, is relatively non-convective and generally occurs with lower average precipitation rates over larger areas, and for longer durations. This is expected to result in more uniform pollutant deposition over larger areas, although measurements to confirm these expectations are not yet avai lable. The following conceptual model of summer wet deposition of Chicago and
and northwest Indiana pollutants was used to arrive at a calculation procedure. Convective precipitation cells were envisioned to move generally eastward across urban pollution sources near the lake shore, injesting relatively undi luted urban air in their updrafts. Pollutants were scavenged primari ly in the cloud, rather than below it, and deposited in the lake approximately 20-30 min after injection when the cloud had moved off shore.
Methods Appropriate to such a scenario, spring and summer wet deposition, D, in Lake Michigan from Chicago and northwest Indiana pollutants was calculated from D = kRA where k was calculated from equation (1), using X = Xo (measured concentration in air). R is spring and summer rainfall, and A is the area of the lake that receives urban pollutants. Separate calculations were performed for a number of individual elements with contributions from two sources areas - Chicago and northwest Indiana being evaluated separately and added. Values of Wand XO chosen as input to the calculations are given in Table 1. The reasoning that led to their selection was presented by Gatz (1975). Seasonal precipitation over the southern basin of Lake Michigan was estimated from maps presented by Changon (1968) as spring and summer: 0.44 m, and fall and winter: 0.36 m. Areas assumed to be affected by spring and summer (convective) wet deposition were: Chicago - 1000 km 2 (40 km x 25 km), and NW Indiana - 250 km 2 (10 km x 25 km). Winter wet deposition was computed
27
WET DEPOSITION TABLE 1.
Element
Input parameters for wet deposition calculation
W
Xo '
Chicago Al As Cd Cr Cu Fe Mn Ni Pb Ti V
Zn
jJg/m
NW Indiana
1.5 0.02 0.01 0.02 0.14 3.5 0.10 0.04 1.2 0.2 0.08 0.3
375
llO 125 150 140 250 370 125 76 325 110 180
3
2.0 0.005 0.02 0.04 0.2 6.0 0.3 0.06 1.5 0.2 0.08 0.4
simi larly, except that X was allowed to vary with distance from sources TABLE 2.
using a simple formulation given by Turner (1969). The calculation was performed in 22~o sectors and integrated over the entire southern basin, after Qdjusting for the frequency with which winter winds carry pollutants to each sector during precipitation. Required urban source strengths were estimated from measured concentrations in urban air. Detai Is of this method were given by Gatz (1975).
Results Calculated annual and seasonal wet depositions for 12 elements are given in Table 2. Also shown are percentages of the total depositions that occur in spring and summer (convective precipitation). In general, depositions appear to be divided about equally between convective and non-convective conditions, for the conditions of the calculation. For additional results from the original calculation, see Gatz (1975).
Calculated seasonal distribution of wet deposition of Chicago and NW Indiana pollutants into S Lake Michigan Wet deposition, metric tons Spring and Fall and Total Summer Winter
Al As Cd Cr Cu Fe Mn Ni Pb Ti V
Zn
280 0.88 0.70 1.65 9.9 486 24 2.6 44 30 4.1 27
280 0.69 0.76 1.80 10.2 494 29 2.6 44 29 3.9 27
560 1. 57 1. 52 3.45 20.1 962 53 5.2 88 59 8.0 44
Spring and Summer, % of Total 50 56 46 48 49 49 45 50 50 51 51 50
D.F. GATl
28
This calculation has illustrated how scavenging ratios can be used to calculate inputs of pollutants to lakes via precipitation. Clearly, large uncertainties remain. Part of this uncertainty is associated with the particular values of the scavenging ratio used in the calculation, whi Ie another part is due to our limited knowledge of lake meteorology, especially precipitation processes. Our prospects for improving current estimates of scavenging ratios are discussed in the fol lowing section. 4.
THE FUTURE
Immediate prospects for further improving our knowledge of scavenging ratios appear bright. Major datagathering programs are underway, or recently concluded, in the U.K., the U. S. (METROMEX), and Scand i nav i a. These should yield improved estimates of scavenging ratios and their variations with meteorological and particle parameters and with distance from sources. The problems of greatest interest for the future in North America, as well as other parts of the world will likely occur on a regional scale (100lObO km). Europeans have been studying deposition in precipitation on this scale for many years, but such efforts in the U.S. and Canada have been
sporadic, at best. The greatest need for the long range (beyond 10 yr) is a continuing program of atmospheric chemistry (including precipitation deposition) measurements, on a regional scale, over at least the NE quarter of the U.S. and SE Canada. Such a program, among other benefits, would allow us to predict long-range transport and deposition of materials from the atmosphere over major areas of our countries. 5.
SUMMARY AND CONCLUSIONS
Calculating wet deposition is sti 11 an inexact science, but for monthly or seasonal estimation of wet deposition over large areas such as Great Lakes, the scavenging ratio method is probably the best available. Current evidence suggests that scavenging ratios increase with particle size and distance from urban sources, and decrease with amount of precipi tation. Additional knowledge is probably sti 11 to be gained from measurement programs currently in progress in North America and Europe. However, the problem of precipitation deposition in NE North America following longdistance transport requires operation of an appropriate sampl ing network for an extended period of time.
ACKNOWLEDGEMENTS I thank R.G. Semonin and S.A. Changnon for their review of the manuscript and for overall supervision of this work. P.A. Cawse kindly gave permission to us~ the unpublished monthly scavenging data plotted in Figures 4 and 5. This work was performed under U.S. ERDA Contract AT(II-I)-1199. REFERENCES Cawse, P.A., 1974. A survey of atmospheric trace elements in the U.K. (1972-73). AERE-R7669, United Kingdom Atomic Energy Authority, Harwell, Oxfordshire. Cawse, P.A., 1975. Personal communication. Chamberlain, A.C., 1960. Aspects of the deposition of radioactive and other gases and particles. In: Richardson, E.G., Ed., Aerodynamic Capture of Particles~ Pergamon Press, New York. 63-88. Changnon, S.A., 1968. Precipitation climatology of Lake Michigan basin. Bulletin
WET DEPOSITION
29
52, Illinois State Water Survey, Urbana. Engelmann, R.J., 1968. The calculation of precipitation scavenging. In: Slade, D.H., Ed., Meteorology and Atomic Energy-1968. U.S. Atomic Energy Commission, Division of Technical Information. TID-24100, 208-221. Engelmann, R.J., 1971. Scavenging prediction using ratios of concentrations in air and precipitation. J. Appl. Meteorol.~ 10:493-497. Esmen, N.A., 1973. Atmospheric scavenging of soluble and insoluble matter. The Science of the Total Environment~ 2: 181-189. Gatz, D.F., 1975. Estimates of wet and dry deposition of Chicago and Northwestern Indiana aerosols into southern Lake Michigan. Paper prepared for presentation at the Second Interagency Committee on Marine Sciences and Engineering Conference on the Great Lakes, Argonne, 111., 25-27 March 1975. Submitted for publication in Water 3 Air 3 and Soil Pollut' 3 July. Gatz, D.F., 1974. Scavenging ratios in METROMEX. 55-74. Hales, J.M., 1972. Fundamentals of the theory of gas scavenging by rain. Atmos. Environ. 6:635-659. Hales, J.M., Dana, M.T. and Wolf, M.A., 1974. Applications of the EPAEC scavenging model to calculations for indust~ial plumes. Paper presented at Precipitation Scavenging Symposium-1974, Champaign, 111., October. Hinzpeter, M., 1958. The influence of meteorological parameters on the propagation of radioactive fission products in the biosphere. In: Proc. Second United Nations Conf. on the Peaceful Uses of Atomic Energy, Geneva, 1958, Vol. 18, p. 284, United Nations, New York. Junge, C.E., 1963. Air Chemistry and Radioactivity, Academic Press, New York, 382 pp. Krey, P.W. and Toonkel, L.E., 1974. Scavenging ratios. Paper presented at Precipitation Scavenging Symposium-1974. Champaign, 111., October. Makhon1ko, K.P., Avramenko, A.S. and Makhon'ko, E.P., 1970. Washout of radioactive isotopes and chemical compounds from the atmosphere. In: Styra, B., Garbaliauskas, C. and Lujanas, V., Eds., Atmospheric Scavenging of RadioIsotopes. Translated from Russian. Israel Program for Scientific Translations, Jerusalem. 174-184. Pelletier, C.A., Whipple, G.H. and Wedlick, H.L., 1965. Use of surface-air concentration and rainfall measurements to predict deposition of fallout radionuclides. In: Klement, A.W., Ed., Radioactive Fallout from Nuclear Weapons Tests~ Proc. of Conf., Germantown, Maryland, November 3-6. 723-736. Robbins, J.A., Landstrom, E. and Wahlgren, M., 1972. Tributary inputs of soluble trace metals to Lake Michigan. Proc. 15th Conf. Great Lakes Res., Internat. Assoc. Great Lakes Res., 270-290. Skibin, D., 1973. Comment on water pollution in Lake Michigan from pollution aerosol fallout. Water~ Air~ and Soil Pollut.~ 2:405-407. Slinn, W.G.N., 1975. Some approximations for the wet and dry removal of particles and gases from the atmosphere. Submi tted to Water~ Air~ and Soil Pollut. ~ July. Turner, D.B., 1969. Workbook of Atmospheric Dispersion Estimates. Public Health Service Pub., No. 99-AP-26. Van der Westhuizen, M., 1970. Author's reply. (To discussion by W.E. Bradley on the author's paper "Radioactive nuclear bomb fallout.) A relationship between deposition, air concentration, and rainfal1." 1969, Atmos. Environ.~ 3:241248. Atmos. Environ.~ 4:322-323. Winchester, J.W. and Nifong, G., 1971. Water pollution in Lake Michigan by trace elements from pollution aerosol fallout. Water~ Air~ and Soil Pollut.~ 1: 50-64.
Proc. First Specialty Symposium on Atmospheric Contribution to the Chemistry of Lake Waters. Internat. Assoc. Great Lakes Res. Sept. 28-0ct. 1, 1975.
WET DEPOSITION ESTIMATION USING SCAVENGING RATIOS D.F. Gatz Illinois State Water Survey Urbana, Illinois 61801 1.
INTRODUCT ION
It should be clear to everyone at this Symposium that it will first be necessary to estimate inputs of nutrients and toxic materials to the lakes, before we can infer the impacts of these materials on the lakes. This morning we are discussing atmospheric pathways of materials to lakes. There are two broad categories of processes by which materials enter the lakes through the atmosphere: wet processes and dry processes. In a sense, because deposition is on a water surface, perhaps all deposition processes are technically "we t", but we shall use the terms "we t" and "d ry " here to refer to deposition in precipitation, and that apart from precipitation, respectively. This paper discusses deposition in precipitation, and the fo110wi ng one wi 11 cover "dry" deposition. Research on wet deposition, or precipitation scavenging, received much impetus in the early 1960's when it was discovered that 80-90% of nuclear bomb fallout was reaching the ground via precipitation. There was emphasis then both on prediction of fallout deposition, as well as on understanding the fundamental physical mechanisms involved in collection of radioactive particles in rain and clouds. Even today, probably most of the information available on precipitation scavenging applies specifically to bomb debris. These results have been summarized by Engelmann (1968, 1971).
Bomb debris has a stratospheric source, whereas stable (non-radioactive) materials, which are our main concern with respect to deposition in lakes, have sources near the ground. Concentrations of materials with sources near the ground should decrease with height, in the mean, while materials with a stratospheric source should increase in concentration with height. These differing vertical distributions may result in different scavenging characteristics, so scavenging prediction methods based on bomb debris should always be tested on stable materials before they are accepted for general use. The purpose of this paper is to suggest that ratios of concentrations in precipitation and in air (also known as washout or scavenging ratios) are suitable for estimating the mean deposition of particulate matter having ground-based sources in precipitation over large areas and long (monthly or seasonal) time periods. Thus, the method should apply to those materials and sources for which the atmosphere is an important pathway to the Great Lakes. The method is not suitable for estimating the scavenging of gases or impaction scavenging of individual stack plumes. Hales (1972), Hales et a1.. (1974) and Engelmann (1968) have discussed these cases. With these purposes and limitations in mind, the suggestions given in this paper for estimating deposition in pre-
D.F.·GATZ
22 cipitation are based on two main premises: 1.
Deposition in precipitation should be proportional to the concentration of the material in air, and to the amount of precipitation, and
2.
It is appropriate, for the present state of knowledge in this field, to depend mostly on the field-observational/ empirical approach to prediction as opposed to theory and laboratory measurements.
The remainder of this paper is devoted to 1) a discussion of the characteristics of scavenging ratios of stable materials and their use in a practical problem, and 2) an outlook for the future. 2.
WET DEPOSITION PREDICTION USING SCAVENGING RATIOS
General considerations A number of natural processes exist by which air is continually cleaned of particles. These may be conveniently divided into "we t" and "d ry " processes according to whether precipitation is i nvo 1ved or not. The dry processes include gravitational sedimentation and impaction on obstacles near the earth's surface. Wet processes include capture mechanisms that occur throughout the precipitation cycle, from initial nucleation of cloud droplets on particles, through Brownian and phoretic capture of particles by cloud droplets, to impaction collection of particles by fa 11 i ng ra i ndrops. In the dry processes, particles are deposited directly, at or near the surface, but deposition by wet processes implies both collection of particles by rain or cloud, plus subsequent precipitation of the liquid water.
Several numerical approaches are generally available for calculating the removal of particles from a given parcel of air and sometimes even for calculating deposition if the populations of collecting and collected particles, and other basic physical parameters are known as a function of time. This is rarely the case in any practical problem, however. We may approach an adequate knowledge of all the necessary parameters for a well-characterized individual stack plume under certain limited meteorological conditions, but not in the case where total wet removal over a large area for periods of months to years is needed. For such situations, the scavenging (or washout) ratio (Chamberlain, 1960) W
=~ X
is recommended, where k is the concentration of any material in precipitation (~g/g), X is its concentration in unwashed air (~g/m3), and p is the dens i ty of air, taken as 1200 g/m 3 . If X can be estimated or measured, and W can somehow be specified, then k can be calculated from equation (1). Deposition follows by multiplying k by precipitation amount. This method can in principle be applied to individual places and to single precipitation events. However, the uncertainty in the estimation is large in such cases. The uncertainty should decrease as both time and deposition area increase, although the statistical relationships have not yet been speci f ied. In mos t pract i ca 1 situations, X will be known within no better than a factor of 2, so no better accuracy can be expected for k computed from equation (1). In most situations, X is measured
23
WET DEPOS ITION near the ground, and is unknown in the air entering the cloud. Clearly, it would be desirable to know X at cloud base, but it is not necessary. The empirical relationship of equation (1) can be used as long as the X used to estimate k is measured or estimated on the same basis as the X used originally to calculate W.
discussed by Engelmann (1968). Combining equations (2) and (3) we obtain an expression for the scavenging ratio in terms of both in-cloud and below-cloud collections:
Pw H L + Rt [l-exp(-At)] (4)
Theoretical aspects Although the scavenging ratio simply as a convenient empirical lationship, theory has developed lating it to both meteorological particle parameters.
began rereand
Following Engelmann (1968) and Junge (1963) we reason that the portion WI of the scavenging ratio due to removal of materials from the air in cloud is given by
where kl is the concentration in cloud water (~g/m£), E is the fraction of the aerosol collected by the cloud droplets, Pw is the density of water (1 g/m£) , and l is liquid water content of the cloud (g/m3). Engelmann further suggested that E should be replaced by 'liT 1 where ''I' is the in-cloud removal rate (sec-I) and T is the lifetime of the cloud (sec). Following Esmen (1973), the portion Wz of the scavenging ratio from belowcloud (impaction) collection is given by
~t [l-exp(- At)] where H is the height of the could base (cm) , R is precipitation rate (cm sec-I), t is the duration of the rain (sec), and A is the Ilwashoutll coefficient (sec-I). Estimation of II was
Esmen (1973) has suggested how an expression similar to equation (4) can be used to estimate the relative importance of in-cloud and below-cloud scavenging. The suggestion may have some merit, although Esmen's analysis appears to have neglected the effects of raindrop evaporation during fall. This omission would have the effect of increasing the fraction of W due to incloud scavenging above its true value. Slinn (1975) has provided an extensive theoretical discussion of both particle and gas scavenging by rain and snow. He gives the following expression for the scavenging ratio k
- = X
where h is the height to the top of the cloud or the contaminant, whichever is lower, Rm is the volume-mean drop radius, a(T) is the aerosol radius, varying with non-dimensional time T, and E [a(T), Rm] is a collection efficiency for particles by cloud droplets and raindrops, varying in time as particle, cloud droplet and raindrop spectra change. To use any of the various theoretical formulations to estimate k, and subsequently deposition, requires knowledge or measurement of both particle characteristics and cloud and precipitation parameters, preferably as a function of time. For a single preci-
24
D.F. GATZ
pitation event, the use of the theory to estimate deposition would require measurement of the required input parameters -'at best a difficult and expensive task. For large areas and periods of months or longer, knowledge of the proper averages of the required parameters are required. Such data are not now avai 1able, although they conceivably could be available at some future time. For the present, the best approach appears to be empirical, through measurement of scavenging ratios and their variation with easily measurable parameters.
NEW YORK CITY
~
'">z UJ
« ~ 100 Cl « UJ --l UJ
~ V>
Observations of scavenging ratios Engelmann (1971) reviewed the available data on scavenging ratios for bomb debris and a few other materials. He concluded that mean scavenging ratios were relatively constant for a number of different fallout radionuclides and stable atmospheric constituents, and that they should therefore be useful in predicting or estimating k. For bomb debris, at least, others (Makhon1ko et al. 1970; Van der Westhuizen, 1970; Pelletier et a1. 1965; Krey and Toonke1, 1974; Hinzpeter, 1958) have generally found an inverse relationship of one sort or another between scavenging ratios and precipitation amount, for both monthly and daily samples. The same is genera11y true also for stable elements and ions. Makhon'ko et a1. (1970) presented scavenging ratio data for Ca++, Na+, HC0 3, and S04 at several different locations in the USSR. Scavenging ratios averaged within groups having similar precipitation amounts invariably decreased with increasing precipitation, although the slope of the lines varied from one species to another, and from place to place. After simi lar1y averaging their monthly data, Krey and Toonke1 (1974) found that scavenging ratios for Pb in New York City (Figure 1) decreased
lOL-_-l.._...L..-....L-.L..JL.Ll..l.:I:--_--L.._.l....--L-J......L-l...J~
1
10 MONTHLY PRECIPITATION. em
100
FIG. 1. Variation of scavenging ratio with precipitation for Pb in monthly samples at New York City (from Krey and Toonkel, 1974).
rapidly with increasing precipitation in samples collected throughout the year. The same general relationship occurs in scavenging ratios for Mg, K, Ca, Fe, and Zn measured daily in summer convective rains at St. Louis, shown in Figure 2. Thus rainfall, among meteorological parameters, appears to affect the magnitude of scavenging ratios, both from monthly and from daily samples. The particle parameter of greatest influence appears to be size, as shown next. A relationship between rainfal1weighted mean Wand particle mass median diameter (MMD) was shown by Gatz (1974) for preliminary data from METROMEX. Wwas found to increase with increasing particle size. Recently Cawse (1974) has published
25
WET DEPOSITION
VI VI
C'" 0
• Wtotal
1600
o Wsoluble
VI
<::
~
r
"t:l
'"z
1200
Co
c(
LU ~
Cl LU
l-
8 LU
As Br
I
;:::
t;
'"
0
Cr
0
oAs 0 oCs
Al
o
Ce
Fe
oCr
?
o
/ eSb
"-
"-
f!;
IV Pb~1
400
c(
U LU
Sc
Fe
~n~co Mn
800
:I:
'"az
~n
0
5
0 MMD, jJI11
RAINFALL.
II1II
FIG. 2. Variation of scavenging ratio with precipitation, for daily samples at St. Louis.
FIG. 3. Variation of scavenging ratio with particle mass median diameter, for Chilton, U.K., JUly-December, 1973. Downward-pointing arrows indicate upper limit values. (Unpublished data of P.A. Cawse; used by permission). VI VI
Ql
scavenging ratios for a large number of elements measured at a number of locations in the United Kingdom. Elemental MMDs were also measured at two sites. Figure 3 shows the relationships between rainfall-weighted Wand MMD at Chilton for July-December, 1973 (Cawse, 1975). The solid square represent scavenging ratios computed from total (soluble plus insoluble) concentrations in precipitation. Open circles represent W-values calculated using only the soluble concentration. The precipitation samples include both dry and wet deposition. The Chilton data are compared to two sampling sites near St. Louis in Figure 4. At all locations, W increases with particle size. In addition, a dependence of W on distance from urban sources, also noted by Gatz (1974), is possible. W-values (for wet deposition only) generally increase by a factor of about 2 from a site 15 km NE of St. Louis to one 50 km NW of the city. A further increase in W-values is seen
g
1600
VI
<::
.~
~
1200
~ ~
Cl
~
800
:I:
I
8LU
Mn
.1
\.OIl\S,
(;at~'l)O"'\m 1\'11
4' z 8
§
Fe 400
Gatz
st
\5
Louis, km,--:N~E_ _-
to 7.2
Mg
~
U LU
'""MMD, um
FIG. 4. Comparison of scavenging ratio variations with particle size, for Chilton (monthly samples, including dry deposition, July-December) and St. Louis (daily samples, excluding dry deposition, June-August).
in the data for Chilton, a predominantly rural site 80 km W,of London. However, this increase must be at least partially caused by the inclusion of dry deposition in the k-values at
D.F. GATZ
26
Chilton. Systematic differences in sampling duration and season could also contribute to these observed differences.
3.
APPLICATION: ATMOSPHERIC POLLUTANT INPUTS TO LAKE MICHIGAN
The recently availability of W measurements and dry deposition measurements for individual elements suggested that a new estimate of atmospheric pollutant inputs to Lake Michigan was timely. Previous calculations (Winchester and Nifong, 1970; Robbins et al., 1972; Skibin, 1973) were based on estimates of dry deposition only. Thus, Gatz (1975) has recently calculated wet and dry inputs of pollutant aerosols to southern Lake Michigan, using available measured wet and dry deposition rates. The wet deposition portion of these earlier calculations will be reviewed here, as an illustration of the use of scavenging ratios to calculate deposition in Great Lakes. Calculations of wet deposition were performed separately for the convective half-year (March-August) and the nonconvective half-year (SeptemberFebruary) because of basic differences in the character of precipitation \systems and their expected scavenging characteristics. Experience with summer convective systems in the St. Louis area and elsewhere has shown that the heaviest deposition of urban pollutants occurs relatively close (10-15 km) to their sources. Winter precipitation, on the other hand, is relatively non-convective and generally occurs with lower average precipitation rates over larger areas, and for longer durations. This is expected to result in more uniform pollutant deposition over larger areas, although measurements to confirm these expectations are not yet avai lable. The following conceptual model of summer wet deposition of Chicago and
and northwest Indiana pollutants was used to arrive at a calculation procedure. Convective precipitation cells were envisioned to move generally eastward across urban pollution sources near the lake shore, injesting relatively undi luted urban air in their updrafts. Pollutants were scavenged primari ly in the cloud, rather than below it, and deposited in the lake approximately 20-30 min after injection when the cloud had moved off shore.
Methods Appropriate to such a scenario, spring and summer wet deposition, D, in Lake Michigan from Chicago and northwest Indiana pollutants was calculated from D = kRA where k was calculated from equation (1), using X = Xo (measured concentration in air). R is spring and summer rainfall, and A is the area of the lake that receives urban pollutants. Separate calculations were performed for a number of individual elements with contributions from two sources areas - Chicago and northwest Indiana being evaluated separately and added. Values of Wand XO chosen as input to the calculations are given in Table 1. The reasoning that led to their selection was presented by Gatz (1975). Seasonal precipitation over the southern basin of Lake Michigan was estimated from maps presented by Changon (1968) as spring and summer: 0.44 m, and fall and winter: 0.36 m. Areas assumed to be affected by spring and summer (convective) wet deposition were: Chicago - 1000 km 2 (40 km x 25 km), and NW Indiana - 250 km 2 (10 km x 25 km). Winter wet deposition was computed
27
WET DEPOSITION TABLE 1.
Element
Input parameters for wet deposition calculation
W
Xo '
Chicago Al As Cd Cr Cu Fe Mn Ni Pb Ti V
Zn
jJg/m
NW Indiana
1.5 0.02 0.01 0.02 0.14 3.5 0.10 0.04 1.2 0.2 0.08 0.3
375
llO 125 150 140 250 370 125 76 325 110 180
3
2.0 0.005 0.02 0.04 0.2 6.0 0.3 0.06 1.5 0.2 0.08 0.4
simi larly, except that X was allowed to vary with distance from sources TABLE 2.
using a simple formulation given by Turner (1969). The calculation was performed in 22~o sectors and integrated over the entire southern basin, after Qdjusting for the frequency with which winter winds carry pollutants to each sector during precipitation. Required urban source strengths were estimated from measured concentrations in urban air. Detai Is of this method were given by Gatz (1975).
Results Calculated annual and seasonal wet depositions for 12 elements are given in Table 2. Also shown are percentages of the total depositions that occur in spring and summer (convective precipitation). In general, depositions appear to be divided about equally between convective and non-convective conditions, for the conditions of the calculation. For additional results from the original calculation, see Gatz (1975).
Calculated seasonal distribution of wet deposition of Chicago and NW Indiana pollutants into S Lake Michigan Wet deposition, metric tons Spring and Fall and Total Summer Winter
Al As Cd Cr Cu Fe Mn Ni Pb Ti V
Zn
280 0.88 0.70 1.65 9.9 486 24 2.6 44 30 4.1 27
280 0.69 0.76 1.80 10.2 494 29 2.6 44 29 3.9 27
560 1. 57 1. 52 3.45 20.1 962 53 5.2 88 59 8.0 44
Spring and Summer, % of Total 50 56 46 48 49 49 45 50 50 51 51 50
D.F. GATl
28
This calculation has illustrated how scavenging ratios can be used to calculate inputs of pollutants to lakes via precipitation. Clearly, large uncertainties remain. Part of this uncertainty is associated with the particular values of the scavenging ratio used in the calculation, whi Ie another part is due to our limited knowledge of lake meteorology, especially precipitation processes. Our prospects for improving current estimates of scavenging ratios are discussed in the fol lowing section. 4.
THE FUTURE
Immediate prospects for further improving our knowledge of scavenging ratios appear bright. Major datagathering programs are underway, or recently concluded, in the U.K., the U. S. (METROMEX), and Scand i nav i a. These should yield improved estimates of scavenging ratios and their variations with meteorological and particle parameters and with distance from sources. The problems of greatest interest for the future in North America, as well as other parts of the world will likely occur on a regional scale (100lObO km). Europeans have been studying deposition in precipitation on this scale for many years, but such efforts in the U.S. and Canada have been
sporadic, at best. The greatest need for the long range (beyond 10 yr) is a continuing program of atmospheric chemistry (including precipitation deposition) measurements, on a regional scale, over at least the NE quarter of the U.S. and SE Canada. Such a program, among other benefits, would allow us to predict long-range transport and deposition of materials from the atmosphere over major areas of our countries. 5.
SUMMARY AND CONCLUSIONS
Calculating wet deposition is sti 11 an inexact science, but for monthly or seasonal estimation of wet deposition over large areas such as Great Lakes, the scavenging ratio method is probably the best available. Current evidence suggests that scavenging ratios increase with particle size and distance from urban sources, and decrease with amount of precipi tation. Additional knowledge is probably sti 11 to be gained from measurement programs currently in progress in North America and Europe. However, the problem of precipitation deposition in NE North America following longdistance transport requires operation of an appropriate sampl ing network for an extended period of time.
ACKNOWLEDGEMENTS I thank R.G. Semonin and S.A. Changnon for their review of the manuscript and for overall supervision of this work. P.A. Cawse kindly gave permission to us~ the unpublished monthly scavenging data plotted in Figures 4 and 5. This work was performed under U.S. ERDA Contract AT(II-I)-1199. REFERENCES Cawse, P.A., 1974. A survey of atmospheric trace elements in the U.K. (1972-73). AERE-R7669, United Kingdom Atomic Energy Authority, Harwell, Oxfordshire. Cawse, P.A., 1975. Personal communication. Chamberlain, A.C., 1960. Aspects of the deposition of radioactive and other gases and particles. In: Richardson, E.G., Ed., Aerodynamic Capture of Particles~ Pergamon Press, New York. 63-88. Changnon, S.A., 1968. Precipitation climatology of Lake Michigan basin. Bulletin
WET DEPOSITION
29
52, Illinois State Water Survey, Urbana. Engelmann, R.J., 1968. The calculation of precipitation scavenging. In: Slade, D.H., Ed., Meteorology and Atomic Energy-1968. U.S. Atomic Energy Commission, Division of Technical Information. TID-24100, 208-221. Engelmann, R.J., 1971. Scavenging prediction using ratios of concentrations in air and precipitation. J. Appl. Meteorol.~ 10:493-497. Esmen, N.A., 1973. Atmospheric scavenging of soluble and insoluble matter. The Science of the Total Environment~ 2: 181-189. Gatz, D.F., 1975. Estimates of wet and dry deposition of Chicago and Northwestern Indiana aerosols into southern Lake Michigan. Paper prepared for presentation at the Second Interagency Committee on Marine Sciences and Engineering Conference on the Great Lakes, Argonne, 111., 25-27 March 1975. Submitted for publication in Water 3 Air 3 and Soil Pollut' 3 July. Gatz, D.F., 1974. Scavenging ratios in METROMEX. 55-74. Hales, J.M., 1972. Fundamentals of the theory of gas scavenging by rain. Atmos. Environ. 6:635-659. Hales, J.M., Dana, M.T. and Wolf, M.A., 1974. Applications of the EPAEC scavenging model to calculations for indust~ial plumes. Paper presented at Precipitation Scavenging Symposium-1974, Champaign, 111., October. Hinzpeter, M., 1958. The influence of meteorological parameters on the propagation of radioactive fission products in the biosphere. In: Proc. Second United Nations Conf. on the Peaceful Uses of Atomic Energy, Geneva, 1958, Vol. 18, p. 284, United Nations, New York. Junge, C.E., 1963. Air Chemistry and Radioactivity, Academic Press, New York, 382 pp. Krey, P.W. and Toonkel, L.E., 1974. Scavenging ratios. Paper presented at Precipitation Scavenging Symposium-1974. Champaign, 111., October. Makhon1ko, K.P., Avramenko, A.S. and Makhon'ko, E.P., 1970. Washout of radioactive isotopes and chemical compounds from the atmosphere. In: Styra, B., Garbaliauskas, C. and Lujanas, V., Eds., Atmospheric Scavenging of RadioIsotopes. Translated from Russian. Israel Program for Scientific Translations, Jerusalem. 174-184. Pelletier, C.A., Whipple, G.H. and Wedlick, H.L., 1965. Use of surface-air concentration and rainfall measurements to predict deposition of fallout radionuclides. In: Klement, A.W., Ed., Radioactive Fallout from Nuclear Weapons Tests~ Proc. of Conf., Germantown, Maryland, November 3-6. 723-736. Robbins, J.A., Landstrom, E. and Wahlgren, M., 1972. Tributary inputs of soluble trace metals to Lake Michigan. Proc. 15th Conf. Great Lakes Res., Internat. Assoc. Great Lakes Res., 270-290. Skibin, D., 1973. Comment on water pollution in Lake Michigan from pollution aerosol fallout. Water~ Air~ and Soil Pollut.~ 2:405-407. Slinn, W.G.N., 1975. Some approximations for the wet and dry removal of particles and gases from the atmosphere. Submi tted to Water~ Air~ and Soil Pollut. ~ July. Turner, D.B., 1969. Workbook of Atmospheric Dispersion Estimates. Public Health Service Pub., No. 99-AP-26. Van der Westhuizen, M., 1970. Author's reply. (To discussion by W.E. Bradley on the author's paper "Radioactive nuclear bomb fallout.) A relationship between deposition, air concentration, and rainfal1." 1969, Atmos. Environ.~ 3:241248. Atmos. Environ.~ 4:322-323. Winchester, J.W. and Nifong, G., 1971. Water pollution in Lake Michigan by trace elements from pollution aerosol fallout. Water~ Air~ and Soil Pollut.~ 1: 50-64.