Myron L. Corrin Department of Atmospheric Science and of Chemistry Colorado State University Fort Collins, 80523
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Atmospheric Chemistry
Atmospheric chemistry deals with the fixed and variable constituents of the atmosphere, with chemical changes occurring in the atmosphere and at the boundaries between the atmosphere. the land, and the oceans, and with the properties of atmospheric constituents natural and anthropogenic, as they affectphysical processes. Atmospheric chemistry is a domain discipline quite similar in many respects to geochemistry and chemical oceanography. Atmospheric chemistry deals with the fixed and variahle constituents of the atmosohere. with chemical chanees occurring in the atmosphere and at the boundaries between the atmosohere. the land. and the oceans. and with the~rooerties - of atm'osph&ic constituents, natural and anthropogenic, as they affect physical processes. Atmospheric chemistry is thus a subdiscipline in the general domain of the atmospheric sciences. The atmosphere is a complex system from the physical as well as chemical standpoint. Ihrine a portion of the time it receives solar radiation; at other times it emits long-wavelength radiation back into space. Photochemical processes are thus highly significant. The motion of air is complex and variahle in space and in time and is determined by a large number of factors; atmospheric mixing and transport is therefore a very complicated problem. Meteorologists speak of "scales of motion" and to some extent define sub-areas of meteorology as micrometeorology, synoptic meteorology, etc. The atmospheric chemist speaks of global, regional and local scales of chemical modeling; he also speaks of tropospheric and stratos~hericchemistrv. The variahle mixine. the variahle radiatibn and bounda4 exchange processes Giay important roles in determinine the nature and rates of chemical transformations in the atmosphere. It is thus important that the atmospheric chemist possess a solid understanding of basic atmospheric structure and behavior. This is the domain of meteorology. A suitable introductory text is that of Byers (1)
By-. H. B.,"General Mataorolw: McDraw-Hill, New York. 1959. A general 8 u m y o f m e h r o l a g y at the introductory level d~alingrvithatmmpheric atrueture, radiation. sir motion, precipitation phenomena, weather,and climate
For a somewhat more advanced treatment of the same subject see (2) Haltins, G.J.aod Martin,F. L.."DynamiealandPhysicalMctmrolagy:MEGnrsu-
Hill, NsaYork. 1957.
The atmosphere may he subdivided into a number of vertical zones. This oaoer will be concerned onlv with the chemi&y of the lower two zones, the troposph;re and the stratosphere. The chemistry of the higher regions, the meso-
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"Atmospheric Chemistry"is part of a new series of Resource Papers aimed primarily at college teachers and planned to give up-to-date summaries of the state of the science and the art in critical areas of chemistrytoday. Other papers in the series d l appear in subsequent months. The publication of Resource papers is supported in part by a grant from the Research Corporation.
210 1 Journal of Chemlcal Education
phere, thermosphere, and ionosphere is the chemistry of highly excited species--atoms, ions, highly charged particles, etc.; it is properly the domain of aeronomy. The trooos~hereextends from the surface to a heieht of about 12 km.- Temperatures range from 220 K to 310 K and exceot at the surface boundarv decrease withaltitude. Little eie~troma~netic radiation 6f wavelength less than 0.3 fim is found in the troposphere; we are thus dealing with "normal" chemistry. The stratosphere extends to about 50 km with temperatures in the 220 K range; more highly energetic electromagnetic radiation is available than in the troposphere and photochemistry probably more significant. The atmospheric chemist is faced with a number of potential problems. 1) The atmosphere is essentially never in a state of physical or chemical equilibrium. Atmospheric chemistry is thus primarily reaction rate chemistry and on occasion becomes steady state chemistry. 2) The constituents of the atmosphere with the exception of nitrogen, oxygen, argon, and frequently water and carbon dioxide exist at trace concentrations. A common cancentration unit is parts per million by volume (ppm). For ozone 1ppm is equal to 2 fig& note that the ambient air quality standard is 0.08 ppm. Many species exist at the parts per billion level while the concentration of some transient species is several orders of magnitude less. The problem of chemical analysis a t such concentrations is obvious and rather special techniques have been developed. 3) The atmosphere is hot a well-mixed system; nor is the mixing Drocess well understood. The i n ~ u tof s various materials tothe atmosphere, removals, tra&mrt, diffusion, and mixina are variahle in space and time. The atmos~heric chemist must couple his chemical rate models with Btmospheric diffusion considerations. These lead to complex calcdations usually handled through numerical modeling. 4) In addition to gases the atmosphere contains finely divided solids and liquids. Some of these are natural, some introduced through the activities of man and some formed in the atmosphere by chemical reaction. To remain suspended the particles must be smaller than a few micrometers in diameter. These particles may play an important role in atmospheric chemistry; they may act as nucleants for the phase transitions of water and as heterogeneous catalysts. Their composition is complex and analysis for chemical species difficult. Surface effects due to their small size may be significant. The Literature ot Atrnospherlc Chernldry There is a noticeable lack of inclusiue modern texts on atmmpheric chemistry; there are many specialized monographs. Some texts which may be of interest are
Atmospheric chemistry has made its greatest strides in response to crises of environmental nature. It is thus largely a missionoriented discipline with little attention or support for basic studies. One would hope that this status could change.
(31 J u w , C . E., "AtmosphetieChemistryandRadiaadinstrumentelkethods, calibration, etc.
Some of the classic wet chemical methods have recently been shown to he inaccurate. This is true for nitrogen oxides and photochemical oxidant. See, for example (14) Pit% J.N., Jr..Mdfee. J. M., Long. W. D. and Winder,A.M.,Envimn.S d Techml., 10.787 (1976). compares the ia.iom.trie detcrminationof omns with the spectm.mpicmethod.
The use of Laser techniaues in remote sensing is apromising new development and d a y be used to detectspecies at the parts per trillion level even in the transient mode. (151 Hink1ey.E. D.,Ewlmn. SEi Tschnol., 11,681 (1917). (16) Pitui, J. N.. Jr., Finlamon-Pitta.a. J., and Winn, A. M., Enuimn. Sci Tahnol., 11, 5MI (1977). Discusses the "8. of tunebl. k8.n in remote lensing app1icationa. Includes a dimcusaion of bask principles, foehniqueaand appiicetiona.
The analysis of organic vapors may be designed t o yield estimates of (a) the total organic vapor concentration, (b) the concentration of photochemically reactive hydrocarbons, or (c)the concentrations of individual organic species. For purpose (a) one normally uses flame ionization; for (b) aseparation preceding such detection and for (c) the full battery of techniaues available for senmation and identification. The technique normally involves the concentration of the species either on a solid absorbent or crvoeenicallv and then aualvsis by gas chromatography. A mass spectrometer is frequently used as the detector. For one such method see (17) auw1.J.W.,Enuimn. Sci. Tmhnol., 9,1175(1975). Usen abaomtioo to coneenvate aampie and gas chromatographyfor separationand identifieation.
In organic analysis it is essential that no change occur in composition during any of the analytical procedures. Much research remains to be done in this area; note that not infrequently more than 200 peaks can he obtained in the gas chromatographic analysis of ambient air. A valuable review of organic vapors as pollutants is given in (18) '"Vapor-PhaseOrgsniePollufanui,"NationaI Academyof Seieneea, 1376. Dimmer sources, atmaspheric reactions, biological effects, and sampling and analysis.
Analysis of Particulate Matter
The characterization of particulate matter may involve (a) the determination of particle size and particle size distribution, (b) the determination of elemental composition and (c) the determination of chemical species. The particle size is related to health effects, transport, visibility and possible catalytic activity. A great deal of attention has been given to elemental composition and little to the more significant compound composition. For s discussion of the techniques used to size aerosols see (19) M s d h d e r , S. K., "Smoke, Dust snd Haze; Wilcy-Intersience, 1917. Dlacvun miemampie method (leedin. to gwmetie sircl, h a n i o o and ektrieal mobility (aerodynamic s i i , in a thoraugh faahion. This la. comprehensive text dealing with the phyaicel and chemical properties of a e m l s , gas-to-particleconversion, light scattering,efr
The classic text on the physical and dynamical properties of aerosols is that of Fuchs
212 / Journal of Chemical Education
Collected aerosol samples may be readily analyzed for elemental composition by a large number of methods including X-ray fluorescence, neutron activation, electron probe, wet chemistrv. etc. (see references (10) and (11)).The scannine and tran&sidn electron mic&cope i$ v&able in dete; minine" oarticle size.. mornholoev . -.and elemental com~osition of single particles. Surface compositions may also be measured. &
(21) Linton,R W., W3lamqP.,Evans.C.A.,andNatuaeh.D. F.S.,AnoLChem., 49,1514 (1977). Uses variova fshniquea to demonatrste the surface predominance of eariow elem e n t e a fly ash.
The determination of chemical compounds is far more difficult than that of elements. Wet chemical methods cannot normally be used. Usually the sample is concentrated and recourse is had to X-ray or electron diffraction. (22) Olaon, K. W., and Skogerboe, R. K.,Enuiron Sei. Technal., 9.227 (1975). Lead c m m d s in contminated soils were identified by gradient density and magnetic sepsratian and identified by X-ray diffraction.
A particular class of organic particulates that has received considerable attention are the ~olvcvclics:manv of these materials have been shown to act aicarcinogens. See (23) "Polyeyelic %mic Matter," National Academy of Scienas, 1972. Sources, modes of formation, stmospherie reactivity. chemical and biological detection asmll aa biological effecte.
Gas Phase Chemical Reactions Introduction
This is a major current effort. The objective is the understanding of those processes which lead to atmospheric concentrations of various species varying in space and time. Involved are input rates, the rates of chemical transformation, removal processes, and atmospheric transport and diffusion. Understanding of this highly complex phenomenon requires cooperation between the chemist and the meteorologist. Various approaches may he taken since it is practically impossible to tackle this problem rigorously. One may use simulation in the laboratory or the field; one may parameterize a set of chemical reactions or transport relations; one may use highly simplified chemistry or highly simplified meteorology. Simulation
Many atmospheric reactions have been simulated in the laboratory. A prime example is the use of "smog chambers'' to simulate the photochemical reaction between nitrogen oxides and hydrocarbons to form photochemical oxidant. (24) Fox,D. L.,Sicklea, J.. Kohlman, M., &.st, P. C., and Wilaon, W . E.,Joum. Air Pollation Contml Aasoc.. 1049 (19751.
\.".",. Discwaea the usc of the smog chamber to study nitrogen balance. Inatrumentation ia given in soma detail.
No acrount is taken in such simulation studies of transport and diffusion: in addition wall effects are uncertain. Nevertheless valuable insights into reaction mechanisms may be obtained. Reaction Mechanisms
This is an exercise in classic physical chemistry. Usually the reactions involve a t least one ~hotochemicalstem manv involve unstable intermediates such as radicals, pe;ixide< etc. The principle is simple. On the basis of available information and good chemical intuition one develops a reaction mechanism which seems reasonable. I t is tested in the laboratory and in the free atmosphere which is a difficult problem because of transport and diffusioncom~lications.A detailed discussion of reacGon schemes is given in references (5).( 6 ) ,and (7) as well as
The validation of coupled chemical and meteorological models is one of the basic current problems in atmospheric chemistry.
(31) Seinfeld, J. H., "AiiPoUution, Physical and Chemical Fundament&," Mffimcc-Hi&
fWte. Cauneil, lW7. (27) "Ozone and a h e r Photochemical Oxidants," NationalR-d A review similar to the above. Considers dmulation erperimenta, critically with models and reaction mechanisms. Invaluable.
See also
New York, 1975. Conaidera air pollution msteomlogy, mioomefeorolw, stmaspheric diffuaion, models inclvding chemically reactive model.
For the development and validation of a single plume chemical model (35) White, W. H..Enuiron. Sci. Tochnol., 11.995 (19771.
A relative simple regional model is discussed in
For a discussion of halogen chemistry in the stratosphere see
(37) Graedei, T. E., Farrow,L. A., and Weber,T. A.,Atrn. Enuimn., 10,1095 (19761. Uses asimple sdveetive (mean e n d ) ~ansportmodel. Manyreferences.
Much sophisticated modeling has been done in the CIAP. Volume I11 (reference (7)) is especially valuable in this connection. I t contains not onlv the modeling details hut a discussion of modeling and limitations. The use of coupled chemical and meteorological models Determination of Rate Constants involves numerical analysis and large computers. There are This again is an exercise in classical physical chemistry. mathematical as well as physical and chemical errors resulting The techniques are generally well known. See, for example in the simultaneous solition of a large number of differential equations. Sensitivity analyses are essential. The validation (30) Calvert.J.G., and Pitts, J.N,, J1.,'~Phhtachemi8tnI)) John WIey & Sonso, Inc..New of such models is difficult, requires large field programs and York, 1967. How much faith, for example, can one place in A ~ t ~ ~ d e r d ~ ~ t ~ ~ ~ h ~ t o ~ h ~ ~ i ~ t ~ d e a e r i b i " ~ ~ M y r ~ a e t i o n m e e h a n . m sisa nexpensive. deontaining a m t i o n on experimental methods. a model validation conducted a t one place and a t one time. hev validation of coupled chemical andmeteorological models For an interesting modern paper see is one of the basic current problems in atmospheric chemis(31) Caatleman, A. W.,Jr., and I. N.Tan8.J Photorhem., 6,349 (1977). try. Measures the rate mnstant for the aaroeiation reaction betweon avlfvr dioxide and the hydrorylradicai. Heterogeneous Atmospheric Chemistry A valuable collection of reference data on rate constants is This is one of the largely untouched areas in this discipline. We would include under this heading gas-particle interactions, gas-to-particle conversion processes, and heterogeneous catalysis. Atmosnheric Modeling (29) Cnrtren.P..Con.J. Chern., 52,1569(1970. A review of the photochemical proaascs accurring in the atratcaphere involving . -.~ ~hloronuorocarhona ~ ~ ~ and other chiorin8 mmh.lnllanatcrl aneeinr such~ as the pounds. Uaes a one-dimensional model to ealeulaf. steady state eoneentrationa. Many rahrences. ~
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Application of chemical reaction schemes to the real atmosphere reauires the couplinp of the chemical model with a m ~ t e o r o l o ~ c amodel l in&& transport and diffusion. These coupled models may be classified un the basis of scale as local. regional. or elohal. Thev mav he classified into one~~~~. ~.~ dimensional, two-dimensional, and three-dimensional. In one-dimensional models the variation in concentration is considered along only one axis-usually the vertical. P o dimensional models consider variations vertically and in terms of latitude. Models may also be classified in terms of the rigor of the meteorological portion. They may involve detailed consideration of the basic equations of motion or may he hizhlv parameterized. The latter t w e would include Gaussian di&& approximations as well &-models employing a single eddy diffusion coeffirient. A thorough discussion of dispersion models, either of thr chemically inert or rhemirally reactive type, is obviously heyond the scope of this paper. Normally a simple meteorological model is coupled with a chemical model. For dealing with local problems such as chemical reactions in a single plume, the meteorological input may he a simple Gaussian model. ~~~
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(33) Turner. D. E.. "Workbook of Atmaspheric DiffusionErtimates,"EPA Publication AP-26,1970. The handbook of air pauution engineem. Formulate8 the principles and presents the parameters of Gaussian diffuaion models. Does not deal with chemical tranc formation.
For a somewhat more sophisticated treatment of local models see
Gas-Paaicle Interactions
I t may be demonstrated,that under normal atmospheric conditions (possibly excluding very severely polluted areas) that physical or chemisorption of gaseous materials on aerosol particles has little effect on gaseous concentrations.
Adsorption may, however, have a profound effect, through surface coating, on the optical properties of aerosols and their behavior as water phase change nucleants. Gas-toParticle Conversion
This is important in the formation of particulate sulfates, nitrates, and organic solids. The process generally involves gas phase reactions which lead to the supersaturatiou of the vapor. This vanor mav undergo nucleation (to be discussed later) to form a iew solid particle, may condense upon the surface of an existine., particle. or mav react with such particles. Mans . qualitative ohsenrations have heen madeof this it is now beinplsubiert - . toauwtitativestudy. See reference (19) and (39) Hsisier, S. L..and Fdediander,S. K., Afmor. Enuiron.. 11.157 (19771. Laboratory simulation study of a smw forming prme.. and of a gas-to-particle canversion madei. (40) Kiang, C. S..Steuffn.D., Mohnsn. V. A.,Briaud, J.,andVi&a,D.. Afmos.Enuhn., 7.1279 119731. Oeala with water-sulhric add and water-nitric acid systems and the formation of partides through heteromolecular nudeation. The t h m is d i s c u d and some calculations made.
Volume 55, Number 4, April 1978 / 213
Further understanding of this important process demands a more complete knowledge of gas phase chemistry and photochemistry, the nucleation process and competition with other particle interaction, and growth. Surface effects will he important; the surface area of 1g of 0.1 Fm radius particles is on the order of 10 mz. Heterogeneous Catalysis
This has been a largely neglected subject in atmospheric chemistry. Some current research has not as yet appeared in the available literature. I t has been shown that nitric oxide may be catalytically oxidized to nitrogen dioxide a t room temperature and in the presence of light (41) Hsickien, J., and Cohen, N..Adu. Pholachem.. 5.157 (1968).
This is an example of photocatalysis. The adsorption of a gaseous species on a solid surface is known to change the absorption spectrum and permit activation a t wavelengths inactive in the gas phase. (42) Temnin, A,, "Electronic Swtroampy of Adsorbed Mo1ecul.s" in "Advances in Ca(alysis,"Vol. 15, Academic Press. New York, 1964. Discusses the effect of edsarption on the e1eetronie spectra of organic moieeu1es. Photodiasaciation and the formalion of ~adicelsmeyoceur.
The occurrence of photocatalysis has recently been reported for the dissociation in the troposphere of chlorofluorocarbons adsorbed on silica. Normal catalysis may also be of importance. Calculations in reference (38) indicate that heterogeneous catalysis may occur in the stratosnhere if activation enereies are a few kilocalories per mole. it has been shown that carbon may act as a catalvst a t room temnerature in the oxidation of sulfur dioxide (43) Novakov, S., Chang, G., and Harker, A. B.. Science, 186,159 (1974) Experimental observatiini.
Unnublished exneriments from the same laboratow indicate activated chkcoal also acts as a catalyst in solution I t has also been demonstrated that nitroeen comnounds may be formed on carbon surfaces from nitric oxideand ammonia (49) Chang, S.G.,andNorakm, T., Atmos. Enuiron., 9,495 (1975). This is an important area for future research. The Oxidation of Sulfur Dioxide Sulfur dioxide is converted in the atmosphere to aqueous sulfuric acid droplets and to solid particulate sulfates. These products may be formed a t considerable downwind distances from the source. are toxic in the resnirahle size ranee. and interfere with visibility. The potentiai for a major increase in the use of sulfur-containine - coal sueeests a knowledee of this oxidation is essential. Four basic processes are possible
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1) Photochemical gas phase oxidation of pure sulfur dioxide. 2) Photochemical gas phase oxidation in the presence of hydro-
carbons and nitrogen oxides. 3) Homogeneous oxidation in aqueous droplets with or without catalysts. 4) Heterogeneous oxidation at the surface of an aerosol particle. The photochemical oxidation of pure sulfur dioxide in air is slow and on the order of O.l%/hr.
This is a more rapid process than with pure sulfur dioxide but the mechanism is not well understood. Homogeneous oxidation in the aqueous phase has been investigated by a number of workers. I t is highly pH dependent and the rate increases rapidly in the presence of ammonia. See W.
V.
(48) Scott, D., and Hohb8, P. J. A l m Sci., 24-54 (1967). Goes into mnaidushle detail in analyzing the kinetics of sulfur dioxide oxidation in aqueous systems.
For catalytic effects see (49) Freiberg, J., Atmas. Enuiron.. 9.861 (1975). Diseuaaes the mmhsn.m of imn-catalyzed midation in oxygenated sw*ms.
The heterogeneous catalysis by carbon (soot) particles has been discussed. The role of these various competitive processes in the atmosphere are simply not known. The pure photochemical process may be considered insignificant and the contributions of the other three modes probably depend upon such atmospheric conditions as humidity, sunlight intensity, presence of other contaminants, etc. This is still an unsolved problem. Nucleation Chemists are aware, a t least implicitly, of the phenomenon of nucleation. The superheating of a liquid or the failure of a supersaturated solution to form crystals are indications that phase transitions do not always occur along equilibrium phase diagrams. Actually an energy barrier does exist for such transitions due to the fact that the first tiny embryo of the new ~ h a s does e not nwsess the molar free enerw -. of the bulk nhase. Considerahlp supersaturation may be required to overcome this harripr. Nucleation phenomena may he classified as 1) Homogeneous nucleation in which only a single pure species is involved. The theory is in gnod shape, calculates the free energy excess usually in terms of surface energy, and applies fluctuation theory to the calculation of nucleation rates. For a lucid treatment see (50) henkel. J.,"KinetieThwn,ofLiquida,"Douer,New Ymk, 1946.
For a much more sophisticated approach (51) Abraham, F. F.. "Homogeneow Nudeation Theory," Aeademic P-. New York, 1974, Considers thermodynsmies in the simple fornulation hut emphasimthe atstiaticalappmach.
The homogeneous condensation process from vapor t o llquid or the homoeeneous freezine nrocess from liouid or vanor to solid does not-occur for water-& the atmospheie. ~ufficiknt heterogeneous nucleants are present to allow the transition to occur at lower supersaturations than those required by the homogeneous process. Homogeneous nucleation may he important in some gas-to-particle conversions. 2) Heteromolecular nucleation in which two or more species simultaneously undergo a phase transition. This occurs, for example, in the formation of a sulfur acid solution droplet from water vapor and sulfuric acid vapor. The theory has been developed by
(45) Gerhard, E. R.. and Johnstona,H. F., bd.En& C h m . . 47,972 (19551.
Possible mechanisms have been examined.
Application to ternary systems and t o gas-to-particle conversion is discussed in
(46) Bufalini, M., Enuimn. Sci. Technol., 5,686 (1971).
A review of the photachemistry of aulfur dioxide.
Photochemical oxidation in the presence of nitrogen oxides and hydrocarbons has been studied extensively. See (47) W h " , W . E. Jr.,Lm, &and Wimman. B., J. AirPollutian Confml Asam., 22. 27 (1972). Primarily anexperimental study.
214 1 Journal of Chemical Education
(53) Kiang.C.S.,Cadle.RD.,HmiU,P.,M~hnhnhn,V,A.,~dYue,G.K.,J.A~r~ol.Sci., 6,464 (1975). Cmsidengas-to-psrtieieeonv.rsion inv01vingUlreegasnwsp.cie. DeveIopment of the theomand some applications.
3) Heterogeneous nucleation in which some foreien material acts as acatalyst. This is the type of nucleation &curring in the atmosphere for the phase transitions of water. It is
probably also a significant nucleation mode in some gas-toparticle conversion processes. The nucleant particle acting to promote the formation of the liquid from the vapor is termed a "condensation nucleant;" that promoting the formation of ice from liquid is termed a "freezing nucleant;" and that involved in the transition from vapor to ice is termed a "deposition nucleant!' Condensation nucleants for water may act through two mechanisms. (a) a water wet solid may adsorb a duplex film of liquid which may, if the critical size is exceeded, grow into and a hulk water drodet. (h) the presence of a hvarosco~ic potentially wate;soluhle suhs&nce in the nucleant can cause a decrease in water vawr aressure over the resultinn saturated solution embryo and pro&ce an effectively increased supersaturation of the vapor. Condensation nucleation as well as heterogeneous ice nucleation are discussed by (54) Mason, 8.J.. "ThePhysiraofClouda." Oxford, 1957. Agmd aemunt of doud physies,a dwiptivetcsatment of nucleationphenomena from the 8te"dpointof f mfteoteoteoI~giit.
Ice nucleation can occur through three modes. (a) the freezing of a supercooled liquid droplet by a nucleant immersed in the drop (h) The freezing of a supercooled liquid d r o ~ l ehv t a nucleant on its surface (contact nucleation). (c) Formation of ice from the vapor on a nurleant particle.'rhe relative contribution of these modes to the ice forming o w s s in the natural atmosphere or in the "seeded" atmoiphere is unknown. The interest in ice nucleationarises from the fact that it is pussihle to enhance precipitation from supercooled clouds hy the introduction of "cloud seeding agents." These same agents may, according to some sources, be used to decrease the size of hailstones and hence reduce crop damage or to dissipate cold fogs at airports. This is the domain of weather modification. (55) Hem W. N., editor, "Weather and Climate Mdifieation," Wiley-lntcrwiene, ,a,. .
