I
MANFRED J. PRAGER, EDGAR R. STEPHENS,l and WILLIAM E. SCOTT' The Franklin Institute, Philadelphia, Pa.
Aerosol Formation from Gaseous Air Pollutants In photoche mica1 nitrogen d ioxide-olefin reactions at parts-per-million concentrations, only highly substituted or cyclic olefins and diolefins form aerosols. When sulfur dioxide is added, all types of olefins produce particulates. Thus, these reactions could account for some smog in the Los Angeles area
UURING PERIODS
OF
smog in the
Los Angeles area, reduction in visibility is commonly attributed to products of reaction between hydrocarbons, especially olefins, and nitrogen oxides or ozone, I n studying these reactions ( 3 , 5 ) , a n aerosol was obtained, but concentration of reactant used were greater than those found in the atmosphere (2,8). More recently, light-scattering measurements have shown that on irradiation, automobile exhaust produces particles (7,70).Hydrocarbon-nitrogen oxide concentrations used were closer to those found in the Los Angeles atmosphere, and light scattering depended not only on olefin content of the fuel, but also on sulfur content. Therefore, work was undertaken to determine if, in the absence of other pollutants, irradiation of hydrocarbons and nitrogen oxides or dark reactions between hydrocarbons and ozone at parts-per-million concentrations produced an aerosol. Also, the effect of sulfur dioxide on aerosol production in such systems was investigated. A recently published article corroborates some of the results reported here (6). Studies of reactions discussed here were carried out in the laboratory. Further work is necessary before the relation, if any, of the results reported here to aerosol formation in the atmosphere can be determined.
viously described (9). By continuously introducing laboratory air with reactants, circulating the reaction mixture with a blower rated at 40 cubic foot per minute, and exhausting a portion of it, the chamber could be operated as a stirred flow reactor. With air intake and exhaust closed or the sidearm removed, the chamber could be used as a batch reactor. I n reactions of hydrocarbons and nitrogen dioxide, with or without sulfur dioxide, the system was irradiated with a General Electric A H 4 mercury lamp placed in a borosilicate glass tube to shield the reactants from radiation a t wave lengths less than 3000 A. (the cut-off of borosilicate glass), This simulates radiation from the sun, which is also cut off a t about 3000 A. by the atmosphere. Except for 3-methyl-l-butene, the gaseous reactants were supplied by the Matheson Co. and were C.P. grade. Obtained from the Phillips Petroleum Co. were 2-methylpentane and cyclo-
EXHAUST
hexane, research grade; 3-methyl-lbutene, 1 -pentene, 1-hexene, 2,4,4-trimethyl-1 -pentene and cyclohexene, pure grade; 2-hexene and 3-heptene, technical grade. 1,5-Hexadiene and cis-3methyl-2-pentene were obtained from the American Petroleum Institute Project 44 at Carnegie Institute of Technology. 2-Pentene and dicyclopentadiene were supplied by Matheson, Coleman & Bell; cyclopentene was supplied by the Aldrich Chemical Co. Ozo?e was prepared by electrical discharge in oxygen.
Static Experiments I n static experiments the cell was evacuated to a residual pressure of less than 1 cm. of mercury, and then filled to atmospheric pressure with extra-dry oxygen. One hundred p.p.m. of watex vapor plus the pollutants were then introduced one at a time, by filling a 1liter or 50-ml. bulb to the required pressure and then sweeping the contents into the cell with oxygen. Before and during the course of the, reaction, samples were periodically withdrawn from the cell and the extent of aerosol formation was measured with a Sinclair-Phoenix smoke photometer. Concentration of particulates was measured by forward-scattered light from the particles drawn through a dark-field illumination chamber. The photocurrent from a photomultiplier tube illuminated by the forward-scattered light is logarithmically amplified. The signal proportional to the mass concen-
BLOWER
MERCURY ARC-m
Apparafus and Materials The reactions were carried out in a long-path infrared absorption cell pre1 Present address, William E. Scott and Associates, Perkasie, Pa.
/
L
TO SMOKE PHOTOMETER
/ INFRARED BEAM
A long-path infrared absorption cell was used as a stirred flow reactor VOL. 52,
NO. 6
JUNE 1960
521
tration of particles is given on a logarithmic scale with a range of 0 to 5. Each one-fifth deflection of full scale corresponds approximately to a tenfold change in mass concentration. The actual value of mass concentration corresponding to a reading of the instrument depends on the size, shape, and refractive index of the particles. Laboratory air generally gave readings between 0.5 and 1.5. The cell filled with dry oxygen usually gave a reading of 0.00. After adding the reactants, a value of 0.00 or 0.05 was usually obtained, but never one higher than 0.15. Straight-Chain and Branched-Chain Hydrocarbons. The reactions were carried out for a t least 1 or '/z hour after the maximum amount of light scattering was observed (Table I). The reactions had then proceeded far toward completion as determined by the disappearance rate of nitrogen dioxide in photochemical reaction with the hydrocarbon by scanning the 6.1-micron band of nitrogen dioxide ( 9 ) . In reactions between nitrogen dioxide and straight-chain mono-olefins, no particulates were formed, except small amounts by the heaviest molecules, 2hexene and 3-heptene. With 3-methyl1-butene and &-3-methyl-2-pentene, the quantities obtained were not significantly different from those for straightchain olefins having the same number of carbon atoms. The only highly branched alkene investigated, 2,4,4-trimethyl-1-pentene, formed large amounts of aerosol. None was observed for the first 20 minutes of the photolysis, after which there was a rapid accumulation and the reaction was completed
Table I. Effect of Irradiation on Aerosol Formation in Nitrogen DioxideHydrocarbon Reactions (Reactant concn.. p.p.m.: olefin, 10; NO%, 5 ; soz, 2)
Hydrocarbon
Photometer Reading Incr.a Olefin- OlefinNOz NOn-SO2
Ethylene 1-Butene 2-Butene 2-Methylpropene I-Pentene 2-Pentene 3-Methyl-I-butene 1-Hexene 2-Hexene cis-3-Methyl-2-pentene 3-Heptene 2,4,4-Trimethgl-l-pentene n-Butane 2-Methglpentane
0.1
0.05 0.05 0.0 0.1
0.0 0.1 0.05 0.25 0.2 0.45 3.3
2.2 2.85 3.0 3.05 3.0 3.35 3.05 3.35 3.3 0.1 0.0
5 Average of 3 determinations for straightchain olefins. Reading before irradiation subtracted from readings during irradiation.
522
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IRRADIATION STARTED
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Figure 1. When sulfur dioxide i s added after the nitrogen dioxide-2pentene reaction starts, fewer particles are formed
about 45 minutes later. I t is not clear whether the ability to form particulates on irradiation with nitrogen dioxide is general for extensively branched olefins. Sulfur Dioxide Enhances Aerosol Formation. Whereas no appreciable amounts of particulates were obtained in the photolysis of nitrogen dioxide with straight-chain monoalkenes, large quantities appeared when sulfur dioxide was added to the system (Table I ) . For ethylene a reading of 2.2 was obtained compared to 0.1 in the absence of sulfur dioxide. Since the photometer indicates mass concentrations on a logarithmic scale, this represents about a hundredfold increase in the amounts of particulates over that produced in the absence of sulfur dioxide. For other straight-chain alkenes photometer readings were higher by about 3 scale units in systems containing sulfur dioxide compared to the corresponding nitrogen dioxide-olefin mixtures without sulfur dioxide. This then represents a thousand fold increase in particles on addition of sulfur dioxide. Among systems containing alkenes with 4 to 7 carbon atoms there appeared to be somewhat more light scattering for particles produced by the higher molecular weight hydrocarbons. The reaction proceeded fastest with 2- and 3-olefins and, for all unsaturates except ethylene and 1-butene, it was complete in 60 to 90 minutes. With 1-butene, particulates were formed in 3 hours and with ethylene the reaction was even slower. In the 2,4,4-trimethyl1-pentene and nitrogen dioxide system, the same amounts of aerosol were formed with and without sulfur dioxide. I t has not been determined whether the same products are obtained in both reactions. Two alkanes, n-butane and 2-methylpentane did not yield an aerosol on
INDUSTRIAL AND ENGINEERING CHEMISTRY
reaction with nitrogen dioxide and sulfur dioxide. Unsaturated hydrocarbons are apparently required to produce particulates in these systems. Effect of Sulfur Dioxide Addition after the Start of Photolysis. The greatest amounts of particulates formed when sulfur dioxide was present at the start of the photolysis (Figure 1). The longer the reaction had proceeded before the sulfur dioxide was introduced, the less aerosol was formed. Addition of sulfur dioxide 10 minutes after irradiation of nitrogen dioxide and 2-pentene had begun reduced the amount of light scattering to 10%. When the sulfur dioxide was added 20 minutes after the start of the photolysis, the extent of aerosol formation was reduced to about 1%. I n the photolysis of nitrogen dioxide and 2-pentene, after 10 minutes the nitrogen dioxide concentration is reduced to about 40Yo of the original and to about 10% after 20 minutes ( 9 ) . The olefin disappears more slowly. Aerosol is therefore produced by reaction of sulfur dioxide with an intermediate in the nitrogen dioxide-olefin photolysis. This species is neither nitric oxide nor ozone, because on irradiation of these substances and sulfur dioxide no aerosol was obtained. 2Pentene and sulfur dioxide produced similar quantities of particulates when irradiated with nitrogen dioxide and in the absence of any oxide of nitrogen. However while in the presence of nitrogen dioxide the reaction was complete in about an hour, in its absence particulates formed for 5 to 6 hours. The aerosol in the latter case may have been produced by the photolysis of sulfur dioxide because particulates are made slowly by irradiating sulfur dioxide in air. At high concentrations in the liquid phase sulfur dioxide and olefins react in the dark ( 4 ) . In the part-permillion range the reaction proceeds photochemically. When 10 p.p.m. of 2-pentene and 2 p.p.m. sulfur dioxide were in contact for about 16 hours in the dark no aerosol at all was made. I n experiments performed with 2pentene, nitrogen dioxide, and sulfur dioxide, nature of the diluent gas and the amount of water vapor in the reaction vessel did not materially affect the extent of light scattering. The same amounts have been obtained in an atmosphere oflaboratory air at 707, relative humidity, dry oxygen, and oxygen containing 100 p.p.m. water. Delay in Aerosol Formation with Nitric Oxide. Stephens and Schuck (7, 70) observed that on irradiation of automobile exaust there was a delay of 30 to 60 minutes before aerosol appeared. The nitrogen oxide in unirradiated exahust is primarily nitric oxide which on irradiation is converted to nitrogen dioxide. Amount of nitrogen dioxide is
AEROSOL F R O M A I R P O L L U T A N T S
1
maximum when the nitric oxide has completely disappeared and the rate of its formation depends on the olefin present in the system. To determine whether the delay in aerosol formation on irradiation of automobile exhaust is related to the rate of photoconversion of nitric oxide to nitrogen dioxide, several experiments were performed with olefins, nitric oxide, and sulfur dioxide. On irradiation an aerosol appeared very rapidly in the system containing nitrogen dioxide (Figure 2). I n 15 minutes about 45Yo of the particulates had formed. I n the reaction with nitric oxide, on the other hand, an aerosol only started to appear after about 15 minutes and after that time continued to form at about the same rate and to the same extent as in the system with nitrogen dioxide. By repeatedly scanning the ozone band at 9.6 microns, ozone was first noticed at about the same time that aerosol appeared. Because ozone does not form until all the nitric oxide is converted to nitrogen dioxide (7, 70), it appears that Iittle or no aerosol was formed until the nitric oxide had been changed to nitrogen dioxide. Once ozone was produced, particulates accumulated rapidly. Reactions of Ozone and Monoalkenes. Ozone and hydrocarbons react at high concentrations to produce a dense smoke (3). In these experiments, however, straight-chain mono-olefins formed few or no particulates, as was the case in reactions between straight-chain monoalkenes and nitrogen dioxide. With 2,4,4-trimethyl-1-pentene somewhat greater amounts of particulates were obtained, but not nearly as much as in reactions with nitrogen dioxide (Table 11). It appears that the concentrations must be appreciably greater to produce an aerosol in reactions of ozone, at least with straightchain mono-olefins. Diolefins. Reactions have been carried out between two diolefins, 1,3-butadiene or 1,5-hexadiene, with nitrogen di-
Table ll. Aerosol Formation for Ozone-Mono-olefin Reactions (Nonphotochemical reactions; olefin concn., 10 p.p.m.; ozone concn., 5 p.p.m.)
Olefin Ethylene 2-Butene 2-Hexene 2,4,4-Trimethyl- 1-pentene
Photometer Reading Trial 1 Trial 2 0.0 0.0
0.25 0.5
0.2
Table 111. Aerosol Formation by Systems Containing Diolefins (Reactant concn., p.p.m.: hydrocarbon, 10; NO?, 5 ; 801,20a, 5 ) System
Photometer Readings Trial 1 Trial 2 Trial 3
1,3-Butadiene, NOzl" 1,3-Butadiene, NOz, S O P 1,3-Butadiene, Osb 1,5-€€exadiene, NOza 1,5-Hexadiene, NOz, SO?" 1,5-Hexadiene, Osb Irradiated.
1.0
1.3
3.2 0.25
0.0
3.7
3.6
3.9 3.2
3.95 3.2
0.0
Dark.
oxide, with nitrogen dioxide and sulfur dioxide, and with ozone (Table 111). Whereas no particulates were produced during irradiation of straight-chain mono-olefins and nitrogen dioxide, with diolefins and nitrogen dioxide an aerosol was obtained, however slowly. With 1,3-butadiene particles appeared only after 15 minutes of irradiation and accumulated slowly for 2 to 3 hot rs. With 1,5-hexadiene the particulates appeared somewhat earlier and much larger quantities were obtained. The reaction was complete in about 2 hours. I n the presence of sulfur dioxide a much larger increase in the amounts of aerosol was obtained for 1,3-butadiene than for 1,5-hexadiene. The reactions were somewhat faster but still slower than with straight-chain mono-olefins. While no particulates were produced in dark reactions between ozone and 1,3-butadiene a large number formed with 1,5hexadiene. The difference in behavior between 1,3-butadiene and I,5-hexadiene in these reactions is not clear. Cyclic Hydrocarbons. As a result of experiments performed at high concentrations Haagen-Smit (3) concluded that cyclic olefins are especially effective in yielding particulate products. In these experiments, the photolysis of nitrogen dioxide with cyclic saturates produced no aerosol. However irradiation of nitrogen dioxide with cyclic olefins gave particulates whether sulfur dioxide was present or not (Table IV). These reactions were much more rapid than those with straight-chain olefins and were complete in '/z hour or less. Sulfur dioxide did not significantly affect the rate of aerosol formation or the amounts obtained. In the ozoneolefin reactions the aerosol appeared rapidly and within 5 minutes after the reactants had been mixed the maximum amount was present in the reaction chamber. With cyclohexene, nitric
oxide, and sulfur dioxide, about as many particulates were produced as with nitrogen dioxide. No aerosol at all was obtained in the first half hour and then in the next half hour the maximum amount was attained. With nitrogen dioxide the maximum amount of light scattering was observed in the first half hour. It has been stated previously that in the reaction between 2-pentene, nitric oxide, and sulfur dioxide, the first signs of an aerosol came about 15 minutes after irradiation had begun. With cyclohexene about 30 minutes was required. The rate of photoconversion of nitric oxide to nitrogen dioxide depends on olefins present in the system and is known to be faster for 2pentene than for cyclohexene. This is another indication then that an aerosol is not formed until the nitric oxide has been changed to nitrogen dioxide.
Dynamic Experiments Experimental Procedure. In static runs concentrations of the reacting species continually decreased. To simulate more closely atmospheric conditions reactions were also carried out dynamically and at lower concentrations. The reactants were measured into 7-liter borosilicate glass bulbs in such quantities that when swept into the cell they would be at the desired concentrations. The bulbs were pressurized usually with tank oxygen and the reactants were then passed separately with oxygen at controlled flow rates into the cell. There they were further diluted by laboratory air which was continuously flowed
3.2
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OXIDE
IX
