SYMPOSIUM OB
A I R P LLUTION Presented in part at the XIIth International Congress of Pure and Applied Chemistry, New York, September 1951
PHOTOCHEMISTRY IN THE LOWER ATMOSPHERE F. E. Blacet
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ATMOSPHERIC FLUORINE8 W. H. MacIntire, L. J. Hardin, and Winnifred Hester
CHEMISTRY AND PHYSIOLOGY OF LOS ANGELES SMOG A. J. Haagen-Smit
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IMPACTION OF DUST AND SMOKE PARTICLES ON SURFACE AND BODY COLLECTORS 1371 W. E. Ranz and J. B. Wong
COMBUSTION PRODUCTS IN LOS ANGELES Paul L. Magill and Robert W. Benoliel
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CONTINUOUS CLOUD CHAMBER Vincent J. Schaefer
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ANALYTICAL METHODS Kiagslcy Kay
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Not presented at the XIIth International Congress. Presented a t the 119th Meeting, ACS Cleveland, Ohio, April 1951. Presented at the Southwide Region21 Meeting, ACS, Wilson Dam, Ala., October 1951.
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ORGANIC PORTION OF AEROSOLS IN LOS ANGELESI P. P . Mader, R. D. MaoPhee, R. T. Lofberg, and G. P. Larson
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PINE BLIGHT IN SPOKANE COUNTY WASH.* D. F. Adams D. J. Mayhew, R. M.'Gnagy, E. P. Richey, R. K . K o p p e , k d I. W. Allen.
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Photochemistry in the Lower Atmosphere F. E. BLACET Chemistry Department, University of California, Los Angeles, Calif.
A brief
description and explanation is given of the spectrum of the sun as it is found in the lower atmosphere, which is arbitrarily thought of as the atmosphere within approximately 1 mile of sea level. High altitude photochemical reactions are considered in terms of probability of occurrence a t low altitudes. Gaseous phase reactions in unpolluted air which are permitted in the lower atmosphere are discussed on the basis of photochemical prin-
ciples. Photochemical reactions which may occur in polluted atmospheres are discussed. They include the photolysis and oxidation of sulfur dioxide and of aldehydes, ketones, hydrocarbons, and other organic compounds. In the oxidation processes i t is shown that ozone may be expected as a by-product. Emphasis is given to the role of nitrogen dioxide as a possible photosensitizer in the atmospheric oxidation of air pollutants.
I
heat the molecules, but they cannot supply enough energy t o an individual molecule t o overcome the forces which hold a t o m together. It is only in bhe visible and ultraviolet ranges of the spectrum that electrons may be excited and chemical bonds broken. I n this connection one should allay again the common misconception that chemical reactions may be catalyzed by light. A quantum of radiant energy which initiates a chemical process is consumed in the reaction. It should be regarded as a participant with enough energy t o perform its function and not as a catalyst which may remain t o serve again.
N A photochemical discussion of this nature it is well in the
beginning t o recall the important principle which is often referred to as the first law of photochemistry, namely, only .that radiant energy which is absorbed can possibly be effective in initiating a photochemical reaction. On the other hand, it should be appreciated that a high percentage of the solar radiation which is absorbed in the environment initiates no chemical changes. The bulk of the energy absorbed near the earth's surface is in the infrared spectral region. The quanta there may instigate changes in vibration and rotation and may otherwise June 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
1339,
-Air
Pollution
The lower atmosphere has no fixed upper boundary. For this treatment, however, it may well be regarded as that portion of the amosphere within 1 mile of sea level, since it is within this thin layer that most of the people of the earth carry on their daily being. I t is the visible and ultraviolet radiation which enters this layer with which this discussion is concerned. Qualitatively everyone is aware of intensive penetration of visible light to the earth’s surface. This spectrum continues with somewhat diminishing intensity until a wave length of about 2900 A . is reached, where it comes to an abrupt stop. This fact is illustrated in Table I, compiled from information given by Fabry and Buisson (IO), in which it is indicated that in going from 3150 to 2900 A., the radiation flux reaching the lower atmosphere decreases approximately a millionfold. Below 2900 A,, evidence accumulated by many investigators demonstrates that the upper atmosphere effectively filters out all solar and interstellar radiation over the complete span down to the region of cosmic rays ( 2 , 4, 6, 15, 16, 19). UPPER ATMOSPHERE
Since it has a direct bearing on that which occurs near the ground, it is well to review briefly some of the photochemical processes of the upper atmosphere. The available evidence indicates that the elements to be found a t high altitudes are essentially the same as those present in pure air, as it is known a t the earth’s surface. The molecular, atomic, and ionic constituents, however, are greatly different. This is because high energy photons in vast numbers from the sun and elsewhere are absorbed in this outer atmosphere. Individually, they have enough energy t o shatter even the most stable molecules, so that matter in that zone consists predominantly of atoms and ions. For example, because of slow recombination a t low pressures, at heights above 50 miles oxygen exists almost exclusively in the monatomic form (4,80). Dropping t o lower levels, where a t the steady state considerable diatomic oxygen exists, the conditions become favorable for still another combination reaction to occur. This is the formation of the triatomic molecular compound, ozone. The region of greatest ozone concentration is reported to be betweeen 10 and 20 miles high (20). The principal molecular air constituents-diatomic oxygen, nitrogen, water, carbon dioxide, etc.-are all transparent to the visible and ultraviolet down t o a t least 2000 A. units. It is absorption below 2000 A. which causes the dissociation processes of the topmost atmosphere referred t o previously. Ozone in quantity, however, represents a new filter and is responsible for the sharp termination of the solar spectrum a t 2900 A. By absorption in this region it dissociates into the substances from which it was formed in the first place, namely, diatomic molecules and oxygen atoms (6). I t actually absorbs slightly in the visible spectrum with dissociation resulting; however, the absorption is too slight to be considered a significant factor in low altitude photochemistry. It absorbs strongly down t o about 2200 -4., at which point diatomic oxygen takes over. In high mountain regions there is some evidence that a very small amount of sunlight leaks through between the ozone filter and the oxygen filter ( 1 6 ) , but in the lower atmosphere this radiation has not been found. The oxygen role in the upper atmosphere is summarized in the following equations: 0 2 hr(x2000) +0 0 (1)
+
+
0 +O+M+02+M 0 0 2 +0 3 0 3 hr(X2900 - 2000) ---+
(2)
+
+
0 2
+0
(3)
(4)
At extremely high altitudes Equation 1 readily occurs, but Equations 2 and 3 do not because of the improbability of collisions and the fact that third bodies are needed, a t least in Equation 2. A recent study has indicated that these reactions are so slow that only 16% of the atoms present in the upper atmosphere at the end of a day disappear during the night ( 1 7 ) . At moder-
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ately high altitudes where Equation 1 still may take place, Equntion 3 predominates over 2 and the ozone layer is formed. However, as a consequence of Equation 3, Equation 4 becomes possible. Thus, fluctuating with radiant energy, a continuous tendency exists t o establish a steady state involving the three forms of oxygen, 0, 0 2 , and 03. LOWER ATMOSPHERE
From the above resume it is apparent that when considering photochemistry which may take place in the lower atmosphere, only visible and long wave-length ultraviolet light is important. This may be thought of as approximately in the range 8000 to 2900 A. Reactions requiring radiant energy below 2900 A. cannot be significant. This, of course, rules out photochemical ozone formation from diatomic oxygen absorption and the photochemical synthesis of oxides of nitrogen from their elements. In fact, it eliminates all of the high altitude reactions. Consequently, a t low altitudes photochemical processes of significance can only occur when they involve one or more substances not normally regarded as being present in pure air.
TABLE
AT GROUND LEVELO F I. ABRUPT TERMINATIOX
SPECTRUM IN
THE
sUK’5
ULTRAVIOLET
Intensity (Arbitrary Units) 22400 10200 1300 76 11 2.2 0.30 0.02
Wave Length, A. 3143 3052 2997 2956 2936 2922 2912 2898
This implies that in uninhabited areas, in the absence of volcanoes, forest fires, and similar natural sources of air contamination, no strictly gas phase photochemical reactions of importance may occur in the lower atmosphere. Attention may be directed, therefore, to gas phase photochemical processes involving man-made air pollutants. One of these contaminants is sulfur dioxide, a compound produced to a greater or lesser extent in the combustion of most fuels. I t exhibits moderate banded absorption in the ultraviolet end of the spectral range. This radiation does not represent sufficient energy to disrupt a bond in the molecule and it must be assumed, therefore, that initially only activated molecules are created. These energized molecules may either revert to their original state, dissipating the absorbed energy, or the.; may react with surrounding molecules. I n laboratory experiments involving pure sulfur dioxide vapor, elementary sulfur and sulfur trioxide are formed, but in the atmosphere where the oxygen concentration is high in comparison to sulfur dioxide, ozone and sulfur trioxide appear to be the over-all reaction products. The mechanism may be represented as follows: SO2 hr --+ SOz* (5) so** 0 2 +so4 (6) so4 0 2 +so3 Oa (7) Ha0 SO3 +HBOa (8)
+ + + +
+
The evidence available indicates that the quantum efficiency is low (14). Based upon sulfate production the quantum yield is between 10-2 and 10-8. If this information is coupled with the fact.that the absorption coefficient also is low in the available spectral range, one can understand why molecular sulfur dioxide may remain detectable in the atmosphere for many hours. The above process illustrates a way in which ozone may be created in the lower atmosphere as a by-product of a photochemical reaction. With the high concentrations of sulfur dioxide normally employed in laboratory experiments, the ozone may react rapidly with another molecule of sulfur dioxide to give sul-
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
Vol. 44, No. 6
Air Pollution fur trioxide and oxygen. However, in the atmosphere where the sulfur dioxide is present in concentrations of the order of 1 part per million ( I ) , ozone may coexist with it for extended periods. Possibly the cleaner the air the longer ozone will remain. It is a good oxidizing substance and is generally regarded as a germicidal agent. Accordingly, in areas of relatively high organic-type air pollution, it may be anticipated that ozone concentration would be kept low and, indeed, this has been reported t o be a fact in air on the lee side of at least one large city. It should not be inferred from this discussion that sulfur dioxide may not be oxidized by other means, In moist climates oxidation may occur in aqueous aerosols with or without t h e aid of absorbed radiant energy. Combined nitrogen is another constituent of most fuels and, hence, the oxides of nitrogen are t o be expected as combustion products in industrial areas. Also, there is considerable evidence that atmospheric nitrogen is fixed in the form of oxides in automobile and truck motors (8, 9,15, 18). The research of Daniels and his coworkers (11)has shown clearly t h a t under conditions of high temperature followed by rapid cooling some air is converted t o nitric oxide. Similar conditions are attained in internalcombustion motors and it appears probable t h a t automobiles, in addition t o contributing nitrogen oxides from the fuel which is burned, contribute them by fixation. I n any event, irrespective of their source, the oxides of nitrogen have been found in the Los Angeles atmosphere in concentrations comparable t o those found for sulfur dioxide ( 1 ) . The most prevalent of these oxides may be regarded as nitrogen dioxide, NOZ. This compound absorbs radiation from 6000 A. down t o well below 2900 A. From 6000 to 3700 A. the spectrum indicates only the formation of excited molecules, and the dissociation quantum yield is low. Below 3700 A., however, the absorption bands are diffuse and the primary quantum efficiency of dissociation is near unity (7, 13). In the atmosphere where the oxygen-nitrogen dioxide ratio is very large, molecular oxygen may take an important role in the following over-all process: NOz+hy+NO+O 0 -k 0 2 +03 NO f oz --+ NO3 NO3
+
0 2
--+ NOz
(9) (3)
+
(10) (11)
0 3
I n summary, 302 +203 (12) This mechanism is speculative and needs considerable experimental testing before it may be regarded as much more than a hypothesis. However, the postulated steps are reasonable and indicate a way in which nitrogen dioxide may serve as an intermediate in making solar radiation available for use in the synthesis of ozone from atmospheric oxygen. I n the final step (Equation 11) nitrogen dioxide is regenerated and, therefore, is available t o repeat the process. I n this respect nitrogen dioxide behaves very differently than does sulfur dioxide. Nitrogen dioxide is reproduced and thus may be regarded as a catalyst, while sulfur dioxide is converted t o sulfur trioxide, which does not give up its third oxygen atom t o form a second molecule of ozone. It appears, therefore, that nitrogen dioxide may be much more effective than sulfur dioxide in forming ozone in the lower atmosphere, both because of a much higher primary quantum efficiency and because of its catalytic effect, Another, perhaps minor, source of ozone may be the photochemical oxidation of aldehydes and other oxygenated organic compounds which are produced in vast quantities as industrial a n d domestic wastes and from incomplete combustion in engines, incinerators, etc. The following type mechanism may pertain:
+ hr RCHO* + --+ RC03H RCOiH + +RCOOH + ECHO ECHO*
(13) (14)
0 2
0 2
0 3
(15)
Activated aldehydes are known t o oxidize readily t o the corre-
lune 1952
sponding acid; peroxyacids are probable intermediates ( 3 ) . Just as in the cases of sulfur dioxide and nitrogen dioxide, with very low concentration of the light-absorbing species and relatively high concentration of oxygen, ozone is a logical by-product of the oxidation process. However, in some instances, the peroxyacids may be sufficiently stable t o resist air reduction and would possibly remain to react with other air contaminants. Some organic air contaminants dissociate into odd molecules or free radicals upon the absorption of radiation. These fragments doubtless react with oxygen, giving alcohols, aldehydes, and acids. From the limited available data, however, it does not seem probable that in the spectral range available this class of reaction is as important as the process represented by Equations 13, 14, and 15, because of low dissociation quantum yields generally observed in the laboratory. OTHER CONTAMINANTS
The remaining molecularly dispersed compounds not discussed previously and which commonly are reported as air contaminants are ammonia, hydrogen sulfide, carbon monoxide, and volatile hydrocarbons. Ammonia does not absorb in the range of 8000 to 2900 A. and probably can be chemically removed from the lower atmosphere only by reaction with an acid or acid-forming oxide Hydrogen sulfide is likewise transparent t o this spectral range and can be removed only by well-recognized nonphotochemical reactions. Ammonia, hydrogen sulfide, and other water-soluble gases, of course, may be removed from the atmosphere in rain and by absorption at moist surfaces. Carbon monoxide is a transparent and comparatively inert substance. I t s fate in t h e atmosphere‘ has not been followed but since there is no evidence t h a t it accumulates over the years, presumably it is slowly oxidized to carbon dioxide. Probably this occurs for the most part a t high altitudes where the ozone and atomic oxygen mole fractions are greater than at ground level and where carbon monoxide itself may absorb short wave-length radiant energy. Hydrocarbons are the remaining important gaseous air pollutants to be considered. The low molecular weight, volatile hydrocarbons, both saturated and unsaturated, are transparent in the range 8000 to 2900 A. They cannot serve as primary absorbers of radiant energy of photochemical importance. Nevertheless, evidence being accumulated by Haagen-Smit and Mader and their coworkers shows that sunlight promotes the air oxidation rate of unsaturated hydrocarbons (1,l a ) . They have shown also that traces of nitrogen dioxide greatly accelerate this oxidation. The explanation which appears probable involves the cyclic nitrogen dioxide-catalyzed ozone formation described previously. The ozone, in a straight thermal reaction, attacks the double bonds forming ozonides, then possibly peroxyacids and aldehydes. According t o this scheme a trace of nitrogen dioxide may in time make possible t h e generation of largeamountsof organic lachrymators. The only limiting factors may be sunshine and suitable organic compounds. I n summary, none of the high altitude photochemical reactions can occur in the lower atmosphere. The photochemical reactions which do occur in the immediate environment are initiated as a result of sunlight absorption by pollutants produced primarily in the activities of man. Many of these may be of minor importance. Perhaps the most significant will prove to be the conversion of diatomic oxygen to ozone through the medium of nitrogen dioxide. The over-all mechanism contains the elements of a cyclic process and a small amount of nitric oxide may generate much ozone. The ozone thus produced may react with many contaminants, including unsaturated hydrocarbons and some of the oxygenated organic compounds resulting from incomplete combustion of fuels and of industrial and domestic wastes. LITERATURE CITED
(1) “Annual Report, 1949-50,” Air Pollution Control Districi, County of Los Angeles, Calif.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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r Pollution
(2) Bamford, C. H., Repte. Progress Phus., 9, 5-89 (1942). (3) Blacet, F. E., and Volman, D. H., J . Am. Chem. SOC.,61, 582-4 (1939). (4) Chapman, S., Proc. R o g . SOC.(London), A132, 353-74 (1931). (5) Chapman, S., Repts. Progress Phys., 9, 92-101 (1942). (6) Colange, G., J . phys. radium, 8 , 254-6 (1927). (7) Dickinson, R. G., and Baxter, W. P., J . Am. Chem. Soc., 50, 774 (1928). (8) Elliott, M. A., and Berger, L. B., IXD.ENG.CHEX.,34, 1065-71 (1942). (9) Elliott, M. A., and Davis, R. F., S A E Quart. Trans., 4 , 330-46 (1950). (10) Fabry, C., and Buisson, H., J . p h g s . radium, 2, 6th ser., 197-226 (1921) . (11) Gilbert, h-.,and Daniels, F., IND.ENG.CHEX., 40, 1719-23 (1948).
(12) Haagen-Smit, A. J., Eng. Sei. (Calif. Inst. Tech.), 14, 1 (nccember 1950). (13) Holmes, H. H., and Daniels, F., J . Am. Chem. Soc., 56, 630-7 (1934). (14) Kornfeld, G., and Weegmann, E., Z . Elektiochem., 36, 7 8 - 9 4 (1930). (15) Mader, P., private communication. (16) Meyer, E., Schein, M., and Stoll, B., Hela. Phgs. Acta, 7, 670-1 (1934); Nature, 134, 535 (19341. (17) Penndorf, R., J . Geophys. Rea., 54, 7-38 (1950). (18) "Third Interim Report," Stanford Research Institute, Los Angeles, Calif., 1950. (19) Wulf, 0. R., J . Optical SOC.Am., 25, 231-6 (1935). (20) Wulf, 0. R., and Deming, L. S., Terrestrial Magnetism a d Atmospheric Electricity, 41, 299-312 (1936). RECEIVED f o r review November 2 i , 1051.
ACCEPTED
.4Pril 2. 1052.
A. J. HAAGEN-SMIT
.,
Calgornia Institute of Technology, Pasadena, Calif and Las Angeles County Air Pollution Control District, Los Angeles, Calif.
Air pollution in the Los Angeles area is Characterized by a decrease i n visibility, crop damage, eye irritation, objectionable odor, and rubber deterioration. These effects are attributed to the release of large quantities of hydrocarbons and nitrogen oxides to the atmosphere. The photochemical action of nitrogen oxides oxidizes the hydrocarbons and thereby forms ozone, responsible for rubber cracking. Under experimental conditions, organic peroxides formed in the vapor phase oxidation of hydrocarbons have been shown to give eye irritation and crop damage resembling closely that observed on smog days.
1 he aeiosols formed in these oxidations are contributors to the decrease in visibility. The odors observed in oxidation of gasoline fractions are similar to those associated with smog. Hydrocarbons present in cracked petroleum products, harmless in themselves, are transformed in the atmosphere into compounds highly irritating to both plants and animals, and should therefore be considered 2 1 s potentially toxic materials. A proper evaluation of the contributionof air pollutants to the smog nuisance must include not only the time and place of their emission, ~ L I L also their fate in the air.
A
damage in this area. It has long been known that ozone liits LP. characteristic cracking action on r a v or vulcani~edrubber when rubber is under strain in a bent or stretched condition ( 3 ) . Low concentrations of ozone are used commercially for comparative aging tests on rubber compounds, but when a standardized rubber is used, this method can serve as a sensitive measuie of ozone concentration ( 2 ) . By comparing the time necessary to obtain just visible cracking, it was noticed that under smog conditions the minimum cracking time mas reduced sharply. Whereas normally 45 minutes are necessary to observe this effect, hevcie smog conditions reduced the cracking time to only 6 minutes. These times correspond to the action of 0.03 and 0.22 p.p.ni. of ozone, respectively. In this case, the cracking time was determined every hour, and the results were plotted in ozone conceiitrations which would give the same response (Figure 1). The sharp rise corresponds closely to the subjective judgment of the severity of the smog. The presence in smog air of strong oxidizing agents clowly resembling ozone in its action on organic compounds could he expected to lead to the formation of peroxides and their degradation products. I n the determination of the peroxidic reaction products in the air, chemical methods are not suitable because of the presence of other oxidants and reducing substances, and advantage was therefore taken of the specificity of enzymatic reactions (9). The enzyme peroxidase, in the presence of hydroperoxide groups, catalyzes the oxidation of phenols and amines to colored derivatives. The oxidation of guaiac tincture produces a blue color which can be measured colorimetrically. NO color appears when the enzyme is poisoned with small quantities of hydrogen cyanide ( 1 6 ) . This inhibition effect tiistinguishcs
IR pollution in the Los Angeles area is characterized by a
decrease in visibility, crop damage, eye irritation, objectionable odor, and rubber deterioration. Although dust and fumes were materially reduced through the combined action of the Los Angeles County Air Pollution Control District and the industries located in the area, the smog conditions were not alleviated proportionally. A program studying the chemical aspects of air pollutants was started at the Air Pollution Control District, and its efforts were combined with those of the California Institute of Technology. It was found that the most characteristic property of the Los Angeles air during smog periods was the oxidizing capacity demonstrated in the release of iodine from neutral buffered potassium iodide solutions. Condensates of smog air collected by passing air through traps cooled t o -80" and -180" C. showed the same oxidizing action. This iodine release could not be explained by the very small amounts of metal oxides present, and only partially by the presence of nitrogen oxides. The conclusion was drawn t h a t peroxides of organic nature were present, which could originate in oxidation of organic material in the air by other air constituents such as ozone and nitrogen oxides ( 6 ) , and it was demonstrated t h a t the vapor phase reactions of ozone and nitrogen oxides with unsaturated hydrocarbons produced aerosols of eye-irritatjng properties. When a cracked gasoline was used as a source of olefins, aerosols were produced which also simulated the odor usually associated with smog ( 5 ) . DETERMINATION OF OZONE AND PEROXIDES IN AIR '
Evidence t h a t these reactions play a role in the Los Angeles smog problem came from the study of rubber cracking and crop
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 6
