Acid Deposition Atmospheric Processes in Eastern North America (1983) / Chapter Skim
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Appendix C: Atmospheric Deposition Processes
Appendix C: Atmospheric Deposition Processes
Pages 213-373
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From page 213... ...
Wet deposition is comparatively simple to measure. As a consequence there exists a substantial and growing base of data on wet deposition from a variety of networks and field studies.
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From page 214... ...
The complexity of the individual processes involved and the variety of possible interactions among them combine to prohibit easy generalization; nevertheless, a Deposition velocity," Vd, analogous to a gravitational falling speed, is of considerable use. In practice, knowledge of vd enables fluxes, F
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From page 215... ...
In this context, the acidity of a pollutant that is being transferred to some receptor surface by dry processes is a quality of special importance that may have strong impact on the efficiency of the deposition process itself. Figure C.2-2 summarizes particle size distributions on a number, surface area, and volume basis.
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From page 216... ...
216 | AIRBORNE SOURCE | LARGE PARTICLES AERODYNAMIC | SETTLING l FACTO RS GASES I> TO R8U LENCE ~-~ TO R8U LENCE TH E RMOPH O R ESIS NEAR-SU RfACE PRORETIC ELECTROPHORESIS EFFECTS Dlf FUSI OPHO RESIS & STEFAN FLOW | I MPACTI ON QUASI-LAMINAR ~ LAYER I INTERCEPTION l f ACTO RS it= | 3ROWNIAN DlfFUSION SU R FACE PROPE RTI ES ~:r ~ STEFAN FLOW I l ~ MOLECULAR DlffUSION 1 | ORIENTATION| ~ STOMATA | | WETNESS | 1 .
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From page 217... ...
Atmospheric sulfates, nitrates, and ammonium compounds are primarily associated with the accumulation size range. Figure C.2-2 demonstrates that very little acidic or acidifying material is likely to be associated with the coarse particle fraction in background conditions.
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From page 218... ...
2.1.2 Aerodynamic Factors Except for the obvious difference that particles will settle slowly under the influence of gravity, small particles and trace gases behave similarly in the air. Trace gases are an integral part of the gas mixture that constitutes air and thus will be moved with all the turbulent motions that normally transport heat, momentum, and water vapor.
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From page 219... ...
The settling speed differential is indicated by a dynamical shape factor," a, as listed in Table C.2-2. Thus, trace gases and small particles are carried by atmospheric turbulence as if they were integral come portents of the air itself, whereas large particles are also affected by gravitational settling, which causes them to fall through the turbulent eddies.
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From page 220... ...
. However, it is not clear that the well-accepted relations governing either heat or momentum transfer are fully applicable to the case of particles or trace gases; some disagreement exists even in the case of water vapor.
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From page 221... ...
However, Figures C.2-2 and C.2-3 show that many airborne materials exist in the size range likely to be affected by inertial impaction. Since many of the chemical constituents of soil-derived particles are capable of neutralizing deposited acids, inertial impaction may have important indirect effects on acidic deposition.
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From page 222... ...
For these reasons, and because close similarlity between ideal surfaces studied in wind tunnels and natural surfaces is rather difficult to swallow, the term "quasi-laminar layer" is preferred. Wind-tunnel studies of the transfer of particles to the walls of pipes tend to support the concept of a limiting diffusive layer adjacent to smooth receptor
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From page 223... ...
These values span the value of A ~ 0.2 recommended for the case of sulfur dioxide flux to fibrous, vegetated surfaces (Shepherd 1974, Wesely and Hicks 1977)
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From page 224... ...
The process of diffusiophoresis results when particles reside in a mixture of intermixing gases. In most natural circumstances, the principal concern is with water vapor.
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From page 225... ...
For the present, it is sufficient to note that Stefan flow is capable of modifying surface deposition rates by an amount that is larger than the deposition velocity appropriate for many small particles to aero- dynamically smooth surfaces. Electrical forces have often been mentioned as possible mechanisms for promoting deposition (as well as the retention, see Section 2.1.5)
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From page 226... ...
draw attention to the effect of microscale surface roughness in promoting adhesion of particles to surfaces. Much of the experimental evidence is for particle diameters much greater than the height of surface irregularities (e.g., Bowden and Tabor 1950)
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From page 227... ...
The Plead candle. technique for detection of atmospheric sulfur dioxide is a historically interesting example of how chemical substrates can be selected to affect the deposition rates of particular pollutants.
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From page 228... ...
The work of Hicks and Liss is intended to apply to water bodies of sufficient size that the bulk exchange relationships of air-sea interaction research are applicable. Their considerations indicate that deposition velocities for highly soluble and chemically reactive gases such as NH3, HC1, and S02 are likely to be between 0.10 and 0.15 percent of the wind speed measured at 10-m height.
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From page 229... ...
Such wind-tunnel work indicates exceedingly low deposition velocities for particles in the size range of most acidic pollutants. As in the case of gas exchange, there are conceptual difficulties in extending these results to the open ocean.
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From page 230... ...
o ·~ 1 1 1 1 1 /1 1 o.1 PAR T ICLE ~ 10 DIAMET E R ( Am ) /° Jo / 0' 1 1 1 FIGURE C.2-4 Results of wind-tunnel studies of particle deposition to water surfaces.
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From page 231... ...
3. If the surface is wet, impinging particles will have a better chance of adhering, and soluble trace gases will be more readily "captured.
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From page 232... ...
According to Monteith (1963) , the maximum rate of dewfall is of the order of 0.07 moth, so that the maximum Stefan flow enhancement of the nocturnal deposition velocity is about 8 cm/in (see Section 2.1.4)
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From page 233... ...
Much smaller particles (in the submicrometer size range) are generated by reactions between atmospheric oxidants and organic trace gases emitted by some vegetation, especially conifers (see Arnts et al.
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From page 234... ...
This resistance is defined as that associated with the transfer of momentum; it is dependent on the roughness of the surface, the wind speed, and the prevailing atmospheric stability. The aerodynamic resistance can be written as ra = Cfn-1 ~ (c/k)
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From page 235... ...
these quantities can be clarified by relating both to the micrometeorological concept of a roughness length, zO (the height of apparent origin of the neutral logarithmic wind profile)
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From page 236... ...
The final resistances in the conceptual chain of processes represented diagrammatically by Figure C.2-5 TABLE C.2-3 Estimates of Roughness Characteristics Typical of Natural Surfacesa Approx. Canopy Roughness Neutral Fr lotion Surface Height (m)
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From page 237... ...
and Garland and Branson (1977) , who compared surface conductances of sulfur dioxide with those for water vapor over a broad range of stomata!
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From page 238... ...
/ ~t ~ r ~] 1 ~ rm rout / ~ rb PALISADE CE LLS 'I ~ GUARD CELLS FIGURE C.2-6 An illustration of the roles of different resistances associated with trace gases uptake by a leaf.
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From page 239... ...
It was recognized that collection vessels failed to reproduce the microscale roughness features of natural surfaces. However, this was not seen as a major problem, since the emphasis was on evaluating the maximum rate of deposition that was likely to occur, so that upper limits could be placed on the extent of possible hazards.
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From page 240... ...
However, accumulation-size-range particles, typically of less than 1-um diameter, do not deposit by gravitational settling at a significant rate. These small particles are transported by turbulence through the lower atmosphere and are deposited by impaction and interception on surface roughness elements, with the assistance of a wide range of surface-related effects (e.g., electrophoresis, Stefan flow)
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From page 241... ...
However, analogous studies of particle deposition using nonradioactive aerosol tracers have been carried out. In wind-tunnel experiments, Wedding et al.
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From page 242... ...
Similar work by Barrie and Walmsley tl978) yielded average sulfur dioxide deposition velocities to snow in the range 0.3 to 0.4 c~/s, with standard error equivalent to about a factor or 2.
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From page 243... ...
The ordering of deposition velocities was: hydrogen fluoride > sulfur dioxide > chlorine > nitrogen dioxide > ozone > carbon dioxide > nitric oxide > carbon monoxide. Furthermore, the chamber studies indicated a wind-speed dependence of the kind predicted by turbulent transfer theory and demonstrated a physiological effect of chlorine and ozone on stomata!
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From page 244... ...
The reciprocals of the tabulated numbers provide upper limits of the appropriate deposition velocities. Similarly informative data have been obtained about particle deposition on surfaces that can be contained in wind tunnels.
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From page 245... ...
These indicated deposition velocities some 30 percent less than the values evident in Figure C.2-7 (for particles in the 0.2- to 1.0-pm
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From page 246... ...
, as might be expected from considerations of Stefan flow and diffusiophoresis. When surface roughness was increased, deposition velocities also increased.
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From page 247... ...
indicated that deposition velocities to natural grass exceeded those to artificial grass by a factor of about 2 for particles smaller than about 5 Am. This appears to be contrary to the indication of Figure C.2-7, where vd (natural)
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From page 248... ...
Often, when sensors suitable for direct measurement of pollutant fluxes are not available, assumptions regarding the eddy diffusivity are made to
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From page 249... ...
In particular, with a typical value of u* = 40 cm/s and neutral stability, the concentration difference between adjacent levels differing in height by a factor of 2 is about 9 percent, for a 1 cm/s deposition velocity.
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From page 250... ...
concludes that "The particulate source and sink processes over natural surfaces cannot be considered as a simple unidirectional single-rate flux.. Thus, the proper interpretation of gradient data in terms of fluxes might not be possible for airborne particles, even in the best of siting circumstances, because of the role of the surface in emitting and resuspending particles.
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From page 251... ...
; it is not meant to be exhaustive but is intended to demonstrate that much of the available data on surface fluxes of trace gases refers to daytime conditions, when "canopy" resistances are usually the controlling factors. Extrapolation of these values to nighttime conditions is dangerous on two grounds; first, because of the large changes that might accompany stomata!
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From page 253... ...
253 Q' e~ .,' ~Hi. en ~ ~ t,, en 'A ._, ~ ~· ~·rl ~21 (Q· en USA toto e ~ O ~ ~ ~ ~ ~ ~ ~` ~ U 1 ~ ~ `` ~ U]
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From page 254... ...
, 1 , 1 "it, i, 17 JULY 18 JULY 20 JULY 1 1 1 1 1~ ~1 ,~ 0 12 0 12 LOCAL STANDARD TIME (h) 0 12 0 FIGURE C.2-10 Records of sulfur flux, sensible heat flux, and friction velocity through 3 days of an intensive study of dry deposition to a pine plantation (Hicks and Wesely 1980~.
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From page 255... ...
Many of the exceedingly large deposition velocities reported in the open literature appear to exceed the limits imposed by our knowledge of the aerodynamic resistance. Thus, several of the results included in the exhaustive tabulation presented by Sehmel (1980b)
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From page 256... ...
FIGURE C.2-11 Values of the maximum possible deposition velocity of trace gases, determined as the inverse of the aerodynamic resistance ra for the pine plantation experiment of Hicks and Wesely ( 1980~. C.2-12, which provides examples of the trend from nighttime, through dawn, and into the afternoon of the residual canopy resistance rc for ozone and water vapor determined using eddy correlation (Wesely et al.
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From page 257... ...
6 8 10 1 2 14 16 ~ 8 HR (C.S.T.) FIGURE C.2-12 Evaluations of the residual "canopy resistance," rc, to the transfer of ozone and water vapor, based on eddy fluxes measured above mature corn in central Illinois on 29 July 1976 (upper sequence)
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From page 258... ...
Table C.2-6 provides a list that illustrates the narrow range of available information. ~ ~ ~ The evidence points to a difference between the deposition characteristics of small particles and sulfate; the latter seems to be transferred with deposition velocities somewhat greater than the value of 0.1 cm/s that has been assumed in most assessment studies and greater than the values appropriate for small particles, on the average.
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From page 261... ...
261 1 ~ O ~ O ~ JJ ~ O C S ~ ~ ~ ~ ~1 a, ~0 ~o ~ ~ ~ ~ ·3 Ed ~ ~oJ ~ ~ ~ ~ ~ ~ e ~ ~ ~ O ~° -~ Jo .- ~ ~ ~ net s GS Q ~ 4)
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From page 262... ...
advocates the continued use of values oF O.1 cm/s or less, since experiments conducted over grass in England failed to detect a significant gradient. However, some of the experiments listed in Table C.2-6 indicate quite high deposition velocities for sulfate particles.
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From page 263... ...
measured by eddy correlation above a pine plantation in North Carolina in 1977 (Hicks and Wesely 1978 Wesely and Hicks 1979~. Note the strong diurnal cycle, with frequent extended periods of emission rather than deposition (as indicated by the negative "deposition velocities"~.
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From page 264... ...
However, small particles are transferred predominantly by turbulence, with subsequent impaction on the surface of microscale surface roughness elements; these latter factors are not easily reproduced by commonly used artificial collecting devices. The use of collection vessels to monitor the accumulation of particles in them continues to be a widespread practice; however, relating the data obtained to natural circumstances is difficult (q.v.
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From page 265... ...
Deposition to water surfaces, in general, is very slow. Similar to ~(Fowler o3 in overall deposition characteristics, but with a significant additional resistance (possibly mesophyllic, see Wesely et al.
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From page 266... ...
The evaluations given in Figure C.2-12 are especially informative, since even over a pine forest whose surface roughness operates to maximize Vd, an occasion was encountered on one evening out of eight in which atmospheric stability was sufficient to constrain the deposition velocity of all airborne material to less than 0.1 cm/s (with the exception of gravitationally settling particles)
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From page 267... ...
This approach has not been extended to other gaseous pollutants. 2.4.2 Particles Modeling of particle deposition is complicated by three major factors: (1)
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From page 268... ...
Sehmel's work has been updated recently to provide an estimate of deposition velocities to canopies of a range of geometries in different meteorological conditions (see Sehmel, 1980a)
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From page 269... ...
Slinn and Slinn (1980) invoke the rapid growth of hydroscopic aerosol particles in very humid air to propose rather rapid deposition to open water; deposition velocities of the order of 0.5 cm/s appear possible in this case.
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From page 270... ...
Their growth as they enter a region of high humidity and their liquid nature when they strike the surface are both potentially important factors that might work to increase otherwise small deposition velocities. Moreover, there is evidence that acidic particles, especially sulfates, might be carried by larger particles; the rates of deposition of such complicated particle structures are essentially unknown.
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From page 271... ...
European work has been fairly uniform in producing deposition velocities close to 0.1 cm/s, while North American experience has generated larger values. It is informative to consider the flux of any airborne quantity to the surface underneath in terms of an electrical analog, the so-called resistance model developed initially in studies of agrometeorology.
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From page 272... ...
While sulfate particles have received most of the recent emphasis, the general question of acid deposition requires that equal attention be paid to nitrate and ammonium particles. There is little information regarding the deposition velocities of these particles.
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From page 273... ...
Once again field observations are lacking. While larger deposition velocities of so uble trace gases to open water surfaces appear quite likely, water bodies are frequently sufficiently small that air-surface thermal equilibrium cannot be achieved.
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From page 274... ...
For example, the use of average deposition velocities is inappropriate when it is desired to look at time- or space-resolved details of deposition fluxes. Sufficient is known about the processes that control the deposition of trace gases that in many instances deposition velocities can be considered to be known functions of properties such as wind speed, atmospheric stability, surface roughness, and biological factors such as stomata!
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From page 275... ...
For both case-study measurement purposes and for long-term monitoring, accurate measurements of pollutant air concentrations are necessary. For monitoring purposes, measurement of airborne pollutant concentrations in a manner carefully designed to permit evaluation of dry-deposition rates by applying time-varying deposition velocities specific to the pollutant and site in question appears to be the most attractive option.
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From page 276... ...
1978. A study of sulphur d ioxide deposition velocities to snow in northern Canada.
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From page 277... ...
1982. A comparison of surrogate surfaces for dry deposition collection, Proceedings, Fourth International Conference on Precipitation Scavenging, Dry Deposition, and Resuspension, Santa Monica, California, 29 November-3 December.
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From page 278... ...
1974. Aerosol particles on tobacco trichomes.
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From page 279... ...
1977. The dry deposition of sulphur dioxide to land and water surfaces.
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From page 280... ...
1982. An experimental study of sulfur deposition to grassland Proceedings, Fourth International Conference on Precipitation Scavenging, Dry Deposition, and Resuspension, Santa Monica, California, 29 November-3 December.
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From page 281... ...
1976. Laboratory measurement of SO2 deposition velocities on selected building materials and soils.
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From page 282... ...
1974. Concentrations, size distribution and chemical nature of atmospheric aerosol particles in remote oceanic areas.
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From page 283... ...
1979. Estimated dry deposition velocities of sulfur over the eastern United States and surrounding regions.
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From page 284... ...
1974. Analytical Investigations of Inertial Deposition of Small Aerosol Particles from Laminar Flows into Large Obstacles -- Parts A and B
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From page 285... ...
In Proceedings, Fourth International Conference on Precipitation Scavenging, Dry Deposition, and Resuspension, Santa Monica, California, 29 November-3 December. Wesely, M.L., J.A.
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From page 286... ...
"Initial portions of this section will treat precipitation scavenging in a general sense, with limited reference to specific types of atmospheric material. The reader should continue to note, however, that the "natural or pollutant molecules" of primary concern in the present context are species associated with acid-base formation, such as SO2, HNO3, NH3, sulfate, chloride, and metallic cations.
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From page 287... ...
287 ~ _ 4< G ~ i You at: an JO _:-~ 1~1 i_; ~I 1 _ _ ~ o Ct CO Q)
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From page 288... ...
This effectively eliminates gaseous Resorption from the condensed water and thus has a strong tendency to enhance the overall scavenging rate as a result. From Figure C.3-2 one can note also that precipitation scavenging of pollutant materials from the atmosphere is intimately linked with the precipitation scavenging of water.
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From page 289... ...
289 m I ;~ ~1 POLLUTANT IN CLEAR AIR cot _ ~{.r~ em <=l ~ cat x 0 POLLUTANT AND CONDENSED WATER INTERMIXED IN COMMON AIRSPACE POLLUTANT ATTACHED TO CONDENSED WATER ELEMENTS l . o o _ 4 ATTACHED POLLUTANT MOD I F I ED BY AQUEOU S - PHA SE PHY S l OCOCHEM I CAL REACT I ON S 1 POLLUTANT DEPOSITED ON EARTH'S SURFACE FIGURE C.3-2 Scavenging sequence: interaction diagram.
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From page 290... ...
Successful scavenging models must emulate these steps effectively and tend to reflect the structure of Figure C.3-2 as a result. This point will re-emerge later when scavenging models are examined specifically.
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From page 291... ...
Based on in-cloud versus below-cloud scavenging estimates (Slinn 1983) it is not unreasonable to estimate that, as a global average, roughly 90 percent of all precipitation scavenging occurs as the ~^n~'l~n-" of a Type-2 Process.
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From page 292... ...
Different types of aerosol particles possess different capabilities to nucleate cloud elements and grow by the condensation process. As a consequence there is typically a competition for water molecules among the aerosol and associated cloud particles; some will capture water with high efficiency and grow substantially in size.
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From page 293... ...
Finally, the competitive nature of the cloud-nucleation process results in significant impacts by the pollutant on the basic character of the cloud itself. If the local aerosol were populated solely by a relatively small number of large, hydroscopic particles, for example, would expect any corresponding cloud to be composed chiefly of low populations of large droplets.
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From page 294... ...
294 U1 Hi: ~ 41: 6 J ~ a: a: Z Z At o . o o £ Urn/ 513~dO~G 'NOIlV~lN3ON03 13~do~a Gnome U
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From page 295... ...
Diffusional attachment is of utmost importance for scavenging of gases and very small aerosol particles. For all practical purposes it can be ignored for aerosol particle sizes above a few tenths of a micrometer.
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From page 296... ...
Some of the aerosol particles (because of their mass) cannot move sufficiently rapidly with the flow field to avoid the hydrometeor and thus are impacted.
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From page 297... ...
Diffusiophoretic attachment to a capturing element can occur whenever the element grows via the condensation of water vapor. In effect the flux of condensing water vapor "sweeps" the surrounding aerosol particles to the element's surface.
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From page 298... ...
Electrical attachment of aerosol particles to cloud and precipitation elements has been the subject of continuing study over the past three decades. Understanding of this process is currently at a state where
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From page 299... ...
This aspect will continue to emerge throughout this section, especially during the discussion of scavenging models. 3.1.4 Aqueous-Phase Reactions (Step 3-4)
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From page 300... ...
Net condensation will occur to the surface or a cloud element whenever water vapor molecules can find a more favorable thermodynamic state in association with it; and because clouds contain varieties of makeup elements having different thermodynamic characteristics, a competition for water vapor usually exists. Such interactions are discussed at length in standard textbooks (Mason 1971, Pruppacher and Klett 1978)
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From page 301... ...
. In mixed-phase clouds the Bergeron-Findeisen process can be expected to enhance the coagulation process by widening the droplet size distribution, as well as contributing to precipitation growth in a direct sense.
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From page 302... ...
Classification of fog-bound pollutant deposition is problematic for two major reasons. The first of these is that no sharp demarcation exists between n fog droplets" and "watercontaining aerosols~; thus the choice of considering fog deposition as simply the dry deposition of wet particles, or the wet deposition of contaminated water, depends primarily on personal preference.
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From page 303... ...
Regardless of this it is appropriate to conclude that fog-deposition processes probably play an important, if secondary, role in pollutant delivery on a regional basis. In the future more effort should be addressed to this important research area.
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From page 304... ...
3.2 STORM SYSTEMS AND STORM CLIMATOLOGY* 3.2.1 Introduction From the preceding discussion it is not difficult to imagine that scavenging rates and pathways will be dictated to a large extent by the basic nature of the particular storm causing the wet removal to occur.
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From page 305... ...
Characterized schematically in Figure C.3-5, the Bjerknes model can be understood most easily by considering a cool northern airmass, separated from a warm southern air mass by an east-west front, as indicated in Figure C.3-5A. The progression of figures represents a typical result of the atmosphere's natural \ \
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From page 307... ...
307 tendency to exchange heat from southern to northern latitudes. This is often referred to as a "tongue" of warm air intruding into the cold air mass.
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From page 309... ...
. Typically the warm air supplying moisture tor this purpose has been advec ted from deep within the southern air mass, carrying water vapor and pollutant over extensive distances.
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From page 311... ...
of a cold-front storm is shown in Figure C.3-8. This differs substantially from the warm-front situation in the sense that, instead of flowing over the frontal surface, the warm air is forced ahead by the moving cold air mass.
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From page 313... ...
of an occluded system demonstrates this point. Typically the Awry resow or warm air alort maintains a relatively stable environment to the east of the occlusion, and clouds and precipitation occur in this region largely as a consequence of ascending flow from the south.
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From page 315... ...
An isolated air mass, for example, is totally capable of acquiring sufficient energy and water vapor to induce a convective disturbance on its own accord. Perturbations arising from fronts, however, often contribute to the creation of convective activity -- if for no other reason than supplying a n trigger" to initiate convection in a conditionally unstable atmosphere.
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From page 316... ...
Typically such storms occur during fall and early winter periods when land surfaces tend to be cooler than their adjoining water bodies. Considering an air parcel moving on an easterly course across Lake Michigan, for example, the warm lake surface tends to supply both heat and water vapor as it proceeds.
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From page 317... ...
3.2.5 Storm and Precipitation Climatology The subject of storm climatology is exceedingly complex and will be discussed here only to the point necessary to describe some key attributes and indicate references for more detailed pursuit. Factors especially important in the context of precipitation scavenging are temporal and spatial precipitation patterns, storm~trajectory behavior, and storm-duration statistics.
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From page 318... ...
StormrDuration Statistics In the preparation of regional scavenging models it often is desirable to create some sort of statistical average of storm characteristics so that ~average" wet-removal behavior can be defined. Although little activity has been devoted to this area until very recently, the
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From page 319... ...
over Lake Michigan and environs. Adapted from Changnon (1968~.
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From page 320... ...
320 NORMAL MONTHLY TOTAL j Espy city 1 a!
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From page 321... ...
321 PRECIPITATION (Inches) FIGURE C.3-12 Climatological summary of U.S.
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From page 322... ...
322 ad\ ? \\N ~< ~ \ \~4 \ \ \ ~ ~,:~ 5t,;~f 1~`,~ ~ ~ - _ ;_g _ FIGURE C.3- 13 Major climatological storm tracks for the North Amencan Continent.
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From page 323... ...
323 uo!
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From page 324... ...
n Field studies of precipitation scavenging began in earnest during the early 1950's to gain an understanding of radioactive fallout. Pioneering studies in this area were performed in England by Chamberlain (1953)
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From page 325... ...
presents a comprehensive account of this early stage of scavenging field studies. During the late 1960s field-experiment emphasis shifted to more conventional pollutants, with the general recognition of precipitation scavenging's importance in preserving atmospheric quality and its potential adverse impacts of deposition on the Earth's surface ecosystem.
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From page 327... ...
Reactive scavenging of plumeborne sulfur dioxide to form rainborne sulfate is difficult to differentiate from primary sulfate removal. The previously noted findings of low excess sulfate in below-plume rain samples, however, suggests that neither process is particularly effective in near-source plume depletion.
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From page 328... ...
an enhancement of sulfate and nitrate precipitation scavenging seems co occur, presumably because the precursors have had more opportunity to dilute and to react under these circuit stances. Hogstrom (1974)
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From page 329... ...
behavior of sulfate-ion concentrations in contrast to the largely disorganized behavior of nitrate-ion concentrations has been suggested to occur as a consequence of an aqueous-phase oxidation of sulfur dioxide, which proceeds more rapidly during summer months. Whatever the cause, it is readily apparent from this figure that scavenging mechanisms for these two species differ appreciably.
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From page 330... ...
330 uo!
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From page 331... ...
Source attribution is particularly uncertain on a regional scale, and the basic data obtainable from standard precipitation-chemistry networks are of limited help in this regard. Combined with the lack of data from well-designed regional field studied, this aspect poses one of the most important and uncertain questions facing the acid deposition issue at present.
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From page 332... ...
Satellite observations Precipitation-Chemistry Data OSCAR network: Sequential measurements of rainfall, field pH, laboratory pH, conductivity, NOT, NO2, SO4, SOT, C1-, NHt, Ca++, Mg++, 1~, Na+, Al ++, PO{, total P b Additional networks: Time-averaged data as available from sources such as NADP, CANSAP, CCIW, and APN Special rainborne H2O2 measurements Aircraf t Data Trace gases: O3, NO/NOx, SO2, HNO3, NH3 Aerosol parameters: scattering coefficient (bScat) , Aitken nuclei, aerosol sulfur, sulfate size distribution, aerosol size distribution, aerosol acidity Cloud water chemistry: NO-3, NO-2, SO4, Sit, pH, NET, conductivity, C1-, Ca++, Mg++, K+, Na+, total Pb Meteorological parameters: Temperature, humidity, liquid-water content, wind speed and direction, cloud droplet size di str ibution Position parameters: Latitude, longitude, altitude, time Surface Air Chemistry Data Emissions .
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From page 334... ...
One early analysis, conducted for the April 22-24, 1981, storm, is presented in Figure C.3-19. Backtrajectories of this type are currently being combined in diagnostic scavenging models with aircraft and surface data to evaluate source-receptor relationships in greater detail (Easter and Hales 1983a,b)
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From page 335... ...
.: 1 (it' o At o a: At c~ I: o .
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From page 338... ...
. and Prediction of the impact on precipitation chemistry of proposed new sources, source modifications, and alternate emission-control strategies; · Prediction of long-range trends in precipitation chemistry; Estimation of the relative contributions of specific sources to precipitation chemistry at a chosen receptor point; Estimation of transport of acidic-precipitation precursors across political borders; Estimation and prediction of air-quality modifications occurring as a consequence of the scavenging process; Site selection for precipitation-chemistry network sampling stations; Design of field studies of precipitation scavenging;
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From page 339... ...
Currently there is a tendency for newer models to accommodate larger numbers of these species; but complete modeling coverage will not be achieved in the foreseeable future. Mechanistic detail is another important feature determining the basic composition of a scavenging model.
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From page 340... ...
The approach, rather, will be to develop a basic understanding of the fundamental elements of a scavenging model and then provide a systematic procedure for choosing and locating appropriate models from the literature. The following subsection discusses the basic conservation equations, which constitute the conceptual bases for scavenging models in general.
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From page 341... ...
Material balances thus form the underlying structure for all scavenging models. To formulate a material balance one simply visualizes some chosen volume of atmosphere and sums over all inputs and outputs of the substance in question.
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From page 342... ...
In selecting a scavenging model one often is confronted with the problem of deciding whether to estimate precipitation attributes and these related terms independently on the basis of assumptions or previous information or to attempt to compute the desired entities directly by solving appropriate forms of Equations (C.3-2)
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From page 343... ...
for scavenging modeling purposes is given by Hales (1983)
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From page 344... ...
3.4.3 Definitions of Scavenging Parameters Four key parameters often arise in the context of scavenging models, and it is appropriate at this point to define these terms and indicate their general application. Reference to these entities as "parameters" is consistent with the usage applied in the previous section, in that they serve to ~lump" the effects of a number of mechanistic processes in a simple formulation.
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From page 345... ...
Efficiencies can be negative if the element is releasing pollutant to the surrounding atmosphere, such as in the case of pollutant-gas Resorption. Typical efficiencies for aerosol particles collected by raindrops are shown in Figure C.3.4.
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From page 346... ...
From this one can note that ~ essentially extends the paramaterization over the total spectrum of hydrometeor sizes. Atmospheric aerosol particles are typically distributed over extensive size ranges, and because of this it is often desirable to possess some sort of an effective scavenging coefficient, which represents a weighted average over the aerosol size spectrum.
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From page 348... ...
A simple example of scavenging-ratio application is given in the following section. It is useful for the sake of visualization to discuss briefly the qualitative features of the scavenging parameters noted above.
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From page 349... ...
The expected magnitudes and uncertainty levels associated with the scavenging parameters listed in this section depend strongly on the substance being scavenged and the environment in which the scavenging takes place. Large aerosol particles in below-cloud environments, for example, are characterized by scavenging efficiencies in the range of 1.0 (cf.
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From page 350... ...
Uncertainties associated with macroscopic modeling applications will be discussed at some length in a later section. 3.4.4 Formulation of Scavenging Models: Simple Examples of Microscopic and Macroscopic Approaches As noted previously, the description given in this document will refrain in general from deriving and applying -~ an: ~ ~1 ~ ~v~1; ha;-1 w scavenging lllo"=l~ =~l`~l~^y.
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From page 351... ...
Here, in contrast to the differential-element approach, the material balances are formulated around a large volume element, in this case a total storm. If one denotes concentrations and flow rates of water and pollutant as follows: cAy = airborne concentration of pollutant, H = airborne concentration of water vapor into cloud'
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From page 352... ...
352 ~ ~ 1l / in ~,~5, <'l If W\ ~ ~C Z _ _.
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From page 353... ...
. CA Pw win wOut fin fout W F concentration of scavenged pollutant in rainwater, density of condensed water, flow rate of water vapor into the storm, flow rate of water vapor out of the storm, flow rate of pollutant into the storm, flow rate of pollutant out of the storm, low rate of precipitation out of the storm, flow rate of scavenged pollutant out of the storm, then extraction efficiencies for water vapor and pollutant can be defined, respectively, as W P Win and (C.
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From page 354... ...
For practical conditions involving aci8-forming aerosols, however, the scavenging of water vapor and pollutant appear to be sufficiently related to allow sp ~ ~ to be employed as an approximate rule of thumb.
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From page 355... ...
More elaborate systems, involving reactive pollutants, gases, and nonhomogeneous systems are discussed in references given in the following section. 3.4.5 Systematic Selection of Scavenging Models: A Flow-Chart Approach Hales (1983)
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From page 356... ...
356 r -- ~ l 1 0 flu W O ~ 4 < \/ L_ r -- ~~1 ~ Jon Hi :: ~ · I T ~_: ~ T: _# ~ I,,| ,,9]
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From page 357... ...
. · The predictive capability of a scavenaina morph ~: By contingent on its desired application AS notes in section 3-4~1, there exists a variety of different applications of scavenging models, and some are much more difficult to fulfill than others.
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From page 358... ...
Mechanisms Irreversible attachment Irreversible attachment Irreversible attachment Reversible attachment Reversible attachment with aqueous-phase reaction Irreversible attachment Irreversible or reversible attachment Transport, reaction, and deposition Irreversible or reversible attachment with chemical reaction Transport, reaction, and deposition Transport, reaction, and deposition Transport, reaction, and deposition Transport, reaction, and deposition Irreversible attachment, nonreactive All modes of scavenging including chemical reaction
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From page 359... ...
359 Typical Application Be logic loud scaveng i ng of aerosols and reactive gases Below-cloud scavenging of size-d istr ibuted aerosols Condensation-enhanced be low c loud scaveng i ng of aerosols Be low-c loud scaveng i ng of nonreactive gases Below-cloud scavenging of reactive gases Scavenging in storm systems (nonreactive ) Scaveng ing in storm sys tems Scoping studies Interpretation of study data Reg ional-s cafe deposition Reg tonal-scale depos ition Regiona 1-scale depos ition Scoping studies and life-t ime assessment In-cloud scaveng ing analysis I n-c loud scaveng i ng analys is Pertinent References Chamberlain (1953)
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From page 360... ...
As noted previously, this is likely to increase uncertainty levels appreciably. · The predictive capability of a scavenging model is contingent on its temporal averaging time.
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From page 361... ...
This observation should be tempered by the fact that, in addition to random errors, scavenging models can be expected to possess substantial systematic biases. In general these biases do not decrease with averaging time and in fact many lead to cumulative discrepancies on occasion.
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From page 362... ...
Moreover, we possess little competence in identifying specific sources responsible for wet deposition at a given receptor site. Finally, the order-of-magnitude predictive capability noted above can hardly be judged satisfactory for most assessment purposes.
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From page 363... ...
Besides providing required knowledge of basic physical phenomena, such research is important in providing valid parameterizations of wet removal for subsequent use in composite regional models. As a final note, it is useful to reflect once again on the fact that scavenging modeling research -- as treated in this section -- has been in a rather continuous state of development over the past 30 years.
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From page 364... ...
1979. A model for the reversible washout of sulfur dioxide, ammonia, and carbon dioxide from a polluted atmosphere and the production of sulfate in raindrops.
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From page 365... ...
1975. Precipitation scavenging of power plant effluents: rainwater concentrations of sulfur and nitrogen compounds and evaluation of rain samples Resorption of SO2.
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From page 366... ...
Proceedings Fourth International Conference on Precipitation Scavenging, Dry Deposition and Resuspension, Santa Monica, Calif. Easter, R.C., and J.M.
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From page 367... ...
In Precipitation Scavenging 1974. ERDA Symposium Series 41.
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From page 368... ...
1979b. Regional scale deposition of sulfur dioxide by precipitation scavenging.
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From page 369... ...
1977. Precipitation scavenging in rainout assessment: the ACRA system and summaries of simulation results.
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From page 370... ...
1974. A one-d~mensional numerical model of precipitation scavenging with application to rainout of radioactive debris.
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From page 371... ...
1978. Precipitation scavenging of aerosol particles.
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From page 372... ...
1983. Precipitation scavenging.
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From page 373... ...
1977. An experimental determination of the efficiency which aerosol particles are collected by water drops in sub-saturated air.
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Key Terms
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