Composition of Organic Por= tion of Atmospheric Aerosols in the Los Angeles Area PAUL P. RSADER, ROBERT D. MAcPHEE, ROBERT T. LOFBERG, AND GORDON P . LARSON Los Angeles County Air PolIution Control District, Los Angeles, Calif.
T h e fact that only small amounts of pollutants could be collected from the atmosphere has been a handicap in the past to chemists who attempted to analyze them. A large mechanical filter, capable of collecting macro size samples of aerosols from the atmosphere, was devised. Aerosols collected by means of the large filter were analyzed and
found to contain sizable amounts of oxygenated and peroxidic organic materials. It was demonstrated that hydrocarbons constitute a large fraction of the many organic substances emitted into the atmosphere. Hydrocarbons are capable of aerosol formation in the atmosphere and are a potent factor in atmospheric pollution.
T
lected from the atmosphere were in the 0.3- to 0.8-micron range. Since the number of particles in this significant range increase markedly with resprct to other sizeq on days of intense pollution, it was apparent that the physical and chemical properties of these aerosols should be studied in order t o ascertain their oiigin.
HE identification of significant concentrations of ouygen-
ated organic materials in the gaseous and aerosol states during periods of temperature inversion, and their possible derivation from petroleum products, were reported in the 1949-50 Annual Report of the Los Angeles County Air Pollution Control District (9). The present paper discusses the nature and composition of the ether-soluble aerosol component present in the atmosphere in and around Los Angeles. Methods used in the collection of aerosols from large-volume air samples and various chemical and physical tests which led t o the identification of oxygenated hydrocarbons as one of the principal constituents are discussed. Results of efforts t o synthesize the same aerosol component of the atmosphere under controlled laboratory conditions are reported and atmospheric samples and synthetic samples are compared. PARTICLE SIZE OF ATMOSPHERIC AEROSOLS
COLLECTING AND ANALYZIh-G ATAIOSPHERIC AEROSOLS
A mechanical filtering system was developed xyhich vias capable of collecting large quantities of solid and nonvolatile liquid particles from the atmosphere (Figure 1). Filtering action r a s achieved by means of two 18 x 24 inch Whatman To. 43 filter papers in series, through which a sampling rate of about 120 cubic feet per minute was maintained. Increasing the number of papers gave a greater collection efficiency but served no important purpose for routine sampling. g n a weight basis, the tx-0 papers were found to remove about (0% of the filterable atmospheric aerosol from an air stream. The papers were pre-extracted with dry, peroxide- and acid-free ether before being used. It was recentlv reported that certain still classified filter papers have been deviloped by the Chemical Service of the U. S.Army which are capable of removing 99% of the aerosols from an air stream on one single sheet (8). These papers, however, were not available for this study.
One of the consequences of polluted air most apparent t o residents of Los Angeles is the reduction of visibility. At times of very low wind speeds and temperature inversion, and in the absence of fog, visibility has become restricted to as low as a quarter of a mile. It has long been established that maximum visible light-scattering effect is caused by particles about 0.3 to 0.8 micron in diameter, dependent on refractive index and wave A sampling station was established on the roof of a ten-story length. With the help of a Sonkin modified cascade impactor building in the downtown section of Los Angeles. During (16) it was possible t o collect rather persistent liquid drops a m p l i n g p e r i o d s , mass lets from the atmosphere. spectrometer analyses and These droplets were transchemical tests were made, parent, did not coalesce, and meteorological data and did not appear to were collected. The latter evaporate appreciably even data were particularly helpafter several days. The ful in indicating whether amounts of aerosols dethe observed reductions in poeited on caacade impactor visibility were due t o air slides were sufficient for contaminants or t o a high direct microscopic deterrelative humidity. mination of particle sizes and distribution, and as P R O C E D U FOR R E Exhas been reported ( 9 ) )these TRACTISG AEROSOLFROSI measurements showed that LARGE-YOLCSIE FIL T E R s. at times as high as 857, After the aerosol samDles of the aerosol particles colFigure 1. Large Mechanical Filter had been collected, each of
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compounds as well as organic acids and other oxygenated hydrocarbons. The sizable oxygen content, low nitrogen and sulfur, and high carbon-to-hydrogen weight ratios observed for ethersoluble aerosols collected from the atmosphere are similar to gums. This was considered particularly significant with respect to the possible origin of the natural aerosols. The synthetic aerosols gave a similar elemental analysis and carbon-to-hydrogen ratio.
TABLE111. AVERAQEELEMENTALANALYSESFOR NATURAL AND SYNTHETIC ETHER-SOLUBLE AEROSOLS A N D GASOLIXE Gum (Per cent by weight) Ether-Soluble Aerosols Synthetio, Natural from gasoline Carbon 67 9 69.1 Hydrogen 9.2 9.0 Nitrogen 1.2 Nonea . Oxygen 20 7b 14.2b Sulfur 0 62} C / H ratio 7.4 7.7 5 Limit of detection 0.5y weight nitrogen. b Values obtained b y di8erence.
:
Gasoline Gums 69.8(a) 75.2 (17) 8.7 7.6
...
21.1 0.4 8 0
0.15
15.5 1.5 9.9
Specific chemical tests were made on a large number of samples of ether-soluble aerosols collected on days representative of various intensities of air pollution. These analyses indicated the presence of aldehydes and ketones (dinitrophenylhydrazine test), and organic acids (hydroxamic acid method). A positive test for methyl ketones ('Ea>C=0)
was obtained using the sodium
nitroprusside procedure described by Feigl (S), while liberation of iodine from a buffered potassium iodide solution was characTABLE I. WEIGHTOF ETHER-SOLUBLE MATERIALCOLLECTED teristic of the presence of peroxidic materials (6). Every one of USINQTwo FILTER PAPERS the many samples tested showed the same positive results for the (Milligrams of aerosol per thousand cubic meters of air) various oxygenated organic types. This, together with the uniVisibility Visibility Visibility form elemental analyses, indicated a certain similarity of the below 1.5 Miles 1.5 to 3 Miles More than 3 Miles ether-soluble aerosols regardless of the visibility conditions. 76.1 60.1 26.3 84.5 41.3 24.2 A similarity was found between ether-soluble aerosols collected 89.3 59.3 24.1 on different days by infrared spectrophotometric analyses. AS TABLE 11. ELEMENTAL ANALYSESOF ETHER-SOLUBLE AEROSOLS certain shifts in position of absorption bands are commonly observed in a great number of compounds, depending upon whether COLLECTED FROM ATMOSPHEREO N LARGEFILTERS the spectrogram was obtained in solution or in the absence of (Per cent by weight) solvents, it was advisable to analyze the ether-soluble aerosols Visibility Visibility below Visibility More both in solution inside the customary rock-salt cell and as an 1.5 Miles 1.5 to 3 Miles Than 3 Miles Average oily residue after evaporation of the ether solvent on the outside Carbon 68.50 66.7 66.3 69.8 68.2 67.9 surface of the cell. All samples thus analyzed exhibited considerHydrogen 9.2 8.8 8.8 9.7 9.4 9.2 Nitrogen 1.8 ' 0.94 1.0 1.1 1.4 1.2 able absorption in the 5.Ei-micron region. This absorption band, Oxygen 19.50 22.70 22.9 17.7 .. 20.7 Sulfur 0.58 0.55 0.41 0.95 .. 0.62 in conjunction with the chemical evidence previously mentioned, Halogen 0.44 0.35 0.52 indicated the presence of significant concentrations of oxygenC/H ratio 7.50 ated organic materials. I n addition to the specific absorption band at 5.8 microns for carbon-to-oxygen double bonds, which was by far the strongest and the most stable in position, a rather rial decreases. However, even on relatively clear days apprestriking similarity was observed in the general absorption pattern ciable quantities of aerosols are suspended in the atmosphere. of every sample analyzed. Each pattern contained prominent This latter fact was borne out by the observation that oily dropbands in the regions of 2.9, 3.4, 6.15, 6.9, 7.3, and 7.9 and a broad lets were impacted on cascade impactor slides not only on clear band a t 11.4 microns. days but sometimes even after a rain. Although a difference is apparent in the quantity of etherSYNTHETIC SMOGS soluble aerosols collected from the atmosphere, dependent on visibility conditions during the collection periods, no trends with It has been established by careful statistical survey that hydrovisibility can be noted for the elemental analyses (Table 11). carbons represent one of the largest single groups of pollutants Regardless of the atmospheric condition under which the samples discharged into the Los Angeles atmosphere. They are larger were collected, the elemental composition of the ether-soluble by far in quantity than organic compounds of all other types aerosols stayed constant within rather narrow limits. ( I O ) . Many saturated and unsaturated hydrocarbons in the C P It is of interest to compare the average elemental analysis of to range have been individually determined in gaseous atmosether-soluble aerosols from the atmosphere, and synthetic aeropheric samples by mass spectrometer analyses (19-14). These sols, with gasoline gums reported in the literature. This is done hydrocarbons could arise from a variety of sources, such as the in Table 111. It is known that cracked gasolines form viscous large petroleum production, refining, and marketing operations gums upon exposure to air-oxygen and sunlight. These gums are in this area as well as the considerable amount of automobile excommonly recognized as being composed of polymerized peroxidic haust gases. It was long ago established by Fudakowski (4) ~
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that petroleum fractions form oxidation products and evcn acquire oxidizing properties when exposed t o air and sunlight. When certain indicator plants such as spinach, endive, chard, sugar beets, etc., were exposed a t the Earhart Laboratories of the California Inst,itute of Technology to the oxidation products of hydrocarbons, the same characteristic symptoms of crop damage viere produced as are found in the Los Angeles area after days of air pollution (6). The question arose as t o whether the aerosols generated, under controlled laboratory conditions, from hydrocarbons knoll-n to be escaping into the atmosphere, were similar t o those collected from the air on the large-volume filter system. Saturated aud unsaturated hydrocarbons as well as hydrocarbon niixtures were exposed to sunlight alone and in combination with other air con-
.. .
?
J
Figure 2.
Plastic Chamber
taminants. Chemic-a1and physical characteristics of the resulting organic aerosol end products were conipared with those of atmospheric aerosols. M'hile the spectra of natural aerosol eainpleP remain rather constant betmen themselves, this is not al-ivays the case with synthetic samples. The oxidation products formed and the resulting infrared spectra depend on the hydrocarbon mixture used and to a large extent on the t'iine of exposure t o ultraviolet, radiat,ion, humidity inside the Plexiglas chamber, etc. Studies to determine 100 the effects of these factors on the formation of w 0 various oxidation products are being made a t z 4 the present t,ime. Gaseous products formed cesimultancously with the generation of the sj-nI 50 thetic aerosols also appear to have properties z e Il-hich are highly significant to the problem of tatmospheric pollution. The nature of t,he gase8 ous air p o l l u t s n t ~has been reported (5,6,12-14) 0 and is being studied further. PREPARATIO OFX SYNTHETIC AEROSOLS.A Figure 3 , Plexiglas chamber was built with a capacity of 8 cubic feet, (Figure 2). Plexiglas is \vdl known for its ability t o transmit a very large percentage of ground-level solar radiations. The Plexiglas chamber m-as flushed of all foreign gases and aerosols by drawing through air a t a high rate. The incoming air was purified by the use of sodium hydroxide t,raps to prevent moisture, carbon dioxide, aldehydes, organic acids, and certain other materials from entering the chamber. A cleaning period of 1.5 hours v a s maintained, as it was shoxn that after such a procedure no aerosols could be detected on cascade impactor slides. Hydrocarbons were ineawred into the chamber by means of a syringe and a hypodermic needle adapted to fit. glass-stoppered openings. ,411 the work was carried out, a t atmospheric pressure and room temperature. I n order to receive a sizable sample of the synthetic aerosol using an 8-cubic-foot chamber, t8he concentration of hydrocarbons int'roduced had to be somewhat, higher than those that might be found under practical circumst,:mces in t,hc atmosphere. 1354
A rapid aerosol formation was observed when 0.1 ml. of a rcgular brand of gasoline (containing 25.4% of unsaturated hydrocarbons) and 1ml. of gaseous nit'rogen dioxide (tetroxide) were introduced into the chamber. h slower formation of aerosols Tvas observed when straight-run gasoline (containing 2% unsaturated hydrocarbons) \vas tested, and the quantity of aerosols formed was much less. Oxides of nitrogen xvere added because they have been found in the Los Bngeles atmosphere and because t,hr: oxidation of hydrocarbons is catalyzed by their presence ( 1 ) . TVithout addit,ion of oxides of nitrogen, aerosol format,ion is not so rapid and not quite so dense. After the regular gasoline-nit,rogen oxide mixture was exposed t,obright sunlight for 30 minutes, using the plastic chamber, the aerosols formed were removed onto SEVera1 layers of the same type of pretreated filter papers, used for the collection of atmospheric samples. The deposits were etherextracted as described previously. I f t e r evaporation of the solvent the oily residue gave the same chemical reactions as rcported for the aerosols collected from thc atmosphere. Positive chemical tests for aldehydes, ketones, and organic acids indicated that oxygenated organic materials had been formed. INFRARED STUDIES.It is nominail;- possible, by mearis of infrared absorption spectra, to identify various types of organic functional groups. Although the mixtures of compounds which made up the synt'het'ic and natural ether-soluble aerosols a-ere 110 doubt cxtremely complex, it was deemcd more than a, coincidence that certain infrared bands could be observed a t positions that would have been expertcd h s r d on chemical teats (Figure 3). -4bsorption a t 2.9 (hydroxyl group) and 5.8 microns (carbonyl group) was compatible with a positive hydroxaniic acid test for organic acids in the aerosols (Table IVj. The presence of unsat,urated hydrocarbons in both act,ual and svnthetic samnles IT-as indicated by absorption a t 6.15 microns and by positive tc with phosphomolybdic acid reagent. This could not be considered a complete identification for unsaturates, however. Ketones, aldehydes, and many other substances show a collective C=O vibration in the 5.8-micron region, and consequently these compountis had to be identified further by chemical means, as indicated in Table IY. Esters exhibit a strong carbonyl absorption near 8 microns and a C-0-C group absorption a t about 9.0 microns.
2
4
6 WAVELENGTH,
8 MICRONS
10
I2
Representative Spectrograms for. Synthetic and Natural
Smogs
They can be detected chemically by the hydroxamic acid procedure, omitting the esterificat,ion step which precludes an!. intcrference by organic acids in the sample. AR only a weak band in the region of 8 microns ? o d d IIC observed in the case of the natural aerosol. and esters could not be detected by chemical test in &her the synthetic or' natural aerosols, they were reported in Table IT- as absent. Organically bound nitro groups usually exhibit absorption in the regions of 6.2 t,o 6.6 and 7 . 2 to 7.6 microns. S o band in the 6.2- to 6.6micron range was present for the natural aerosol, but chemical analyses demonstrated the presence of a very small amount of organic nitrogen (Table 111). The synthetic aerosol exhihitcd absorption a t both 6.2 to 6.6 and 7.2 to 7.6 microns. Organically bound nitrogen could not be detected, hoxever, in the synthctic aerosol by microanalytical techniques (Table 111). S i t r o groups,
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-Air TABLE IV. CHEMICAL TESTSSUPPORTED B Y INFRARED SPECTROPHOTOMETRY Group or Type of Compound Synthetic Smog Natural Smog Hydroxyl group Present Present Organic acidsa Present Present Unsaturated (C=C) linkagea Possible Possible Carbonyl group Present Present Aldehydes and/or ketones 6 Present Present Absent Absent Esters5 Organic nitro group Possible small amount Absentd Peroxidic organic oompoundse Present Present a I n conjunction with hydroxamic acid test (9). b I n conjunction with phosphomolybdic acid test ( f 1 ) . C I n conjunction with dinitroohenvlhvdrazine and sodium nitrourusvide . - tests (3). . d Small amount of organic nitrogen detected by micro-Dumas (Table 111). e I n conjunction with liberation of iodine from buffered potassium iodide solution ( 5 ) .
therefore, if present in the synthetic aerosol were exceedingly small in quantity. It has recently been shown that hydroperoxides exhibit a broad band in the region of 11.4 microns, or in the case of concentrated samples, two narrow bands at approximately 11.3 and 11.8 microns (16). When more concentrated samples of synthetic aerosols were prepared, both the 11.3- and 11.8-micron bands were distinguished. However, in the case of natural aerosols and synthetic aerosols, prepared by use of practical concentrations of gasoline and oxides of nitrogen, only the broad band in the 11.4-micron region was clearly discernible. If the ether-soluble aerosol is tested soon after collection, iodine liberation can be obtained from a buffered potassium iodide solution. By passing atmospheric air during smog days through a peroxidase and gum guaiac solution (18), a positive test for hydroperoxides was obtained. All evidence appears to confirm the presence of peroxidic organic compounds in both natural and synthetic ether-soluble aerosols. Infrared spectrophotometer studies with the artificially produced aerosols from gasoline uncovered various points of general similarity between the spectrograms for synthetic and natural smogs, although there were certain differences in detail. Unexplained differences in wave length of several bands too large to ignore, may reflect some significant differences in the structure of compounds t o which they are related and may be indicative of differences in the relative proportions of certain compounds responsible for the peaks.
A single-beam, automatic recording Beckman IR2 spectrophotometer was used in the determinations. All the analyses reported in this section were carried out on oily residues made by placing comparable volumes of ether solutions of the aerosols on a rock-salt plate and allowing the solvent to evaporate. I n the 1- to %micron region, the scanning time used was 45 minutes and the response time 8 seconds. The 9- to 15-micron region was run with a scanning time of 22 minutes and an &second response time. I n plotting the spectrograms in Figure 3 corrections were made for the rock-salt plate blanks. The natural aerosol shown in Figure 3 was collected when visibility averaged 3.5 miles. This might be considered a moderately smoggy day. DUST CAM ERA
Aerosols collected from the atmosphere and those synthesized in the Plexiglas chamber from gasoline were compared, using the Chaney and Hall dark-field aerosol camera (7), with which it is possible to determine the sizes and distribution of aerosols. A suspension of aerosols in a small volume is photographed and the size range of the photographed particle is compared after magni-
fication with a series of aerosols of known and homogeneous size produced by means of a La-Mar Sinclair aerosol generator. Results demonstrated that the major proportion of particles from both naturally occurring and synthetically produced aerosols fell in the size range of 0.3 t o 0.8 micron. CONCLUSIONS
Ether-soluble aerosols in significant amounts were collected from the atmosphere with the help of a mechanical filtering system. They were shown t o contain sizable amounts of oxygenated and peroxidized organic substances. The quantities of aerosol per unit volume of air differed from day to day, depending on the degree of pollution, but the chemical composition remained rather uniform. Synthetic aerosols of the same kind were produced in the laboratory from hydrocarbon mixtures in conjunction with other atmospheric pollutants and sunlight. Although many types of organic materials emitted into the atmosphere in the Los Angeles area are capable of being oxidized, hydrocarbons are by far the largest in quantity. The oxidation of certain hydrocarbons which are emitted into the atmosphere from a variety of sourees, contributes in substantial part to the organic aerosols collected on mechanical filters in and around Los Angeles. When considered along with dust, fumes, and other pollutants already identified (9),the ether-soluble aerosols in the atmosphere provide a convenient criterion for indicating the extent of air pollution and the progress of control activities. LITERATURE CITED
(1) Blncet, F. A., “Photochemistry in the Lower Atmosphere,”
presented in the Symposium on Air and Stream Pollution, XIIth International Congress of Pure and Applied Chemistry. New York, Sept. 10 to 13, 1951. ( 2 ) Ellis, Carleton, “Chemistry of Petroleum Derivatives,” Vol. 11, New York, Reinhold Publishing Corp., 1937. (3) Feirrl, F., “Qualitative Analysis by Spot Tests,” 3rd ed., New York, Elsevier Publishing Co., 1949. * (4) Fudakowski, H., Be?., 6, 106 (1873). (5) Haagen-Smit, A. J., Research contract reports t o Los dngeles County Air Pollution Control District, 1951. (6) Haagen-Smit, A. J., Darley, E. F., Zaitlin, >I., Hull, H., and Noble, W. M., Plant Physiol.. 27, 18-34 (1952). (7) Hall, S. R., “Instrumentation for the Recording of Air Pollution Levels,” presented before Air Pollution and Smoke Prevention Assoc. of America, Roanoke, Va., May 7 to 10, 1951. (8) Johnstone, H. F., J . I n d . Hy?. Tosicol., 30, 358-69 ,(1948); also private communications. (9) Larson, G. P.. Los Angeles County iiir Pollution Control District, Technical and Administrative Report on Air Pollution Control in Los Angeles County, Annual Report (1949-50). (10) Ibid. (1950-51) (to be published). (11) Polis, R. D., Berger, L. B., and Schrenk, H. H., U. S. Bur. Mines, Rept. Invest. 3785 (November 1944). (12) Shepherd, M., Research contract report to Los Angeles County Air Pollution Control District, Jan. 12, 1951. (13) Shepherd, M., and Rock, S. M., “Identification of Gas Phase Atmospheric Pollutants with Mass Spectrometry.” presented before Consolidated Engineering Corp. Mass Spectrometer Symposium, Pasadena, Calif., May 18, 1951. (14) Shepherd, M., and Rock, S. M., Howard, R., and Stormes, J., Anal. Chem., 23,1431 (1951). (15) Shreve, 0. D., Heether, M . R., Knight, H. B., and Swern, Daniel, Ibid.,23, 282-5 (1951). (16) Sonkin, L. S.,J . Ind. H y g . Tozicol., 28, 269 (1946). ENG. (17) Walters, E. L., Minor, H. B., and Yabroff, D. L., IND. CHEM.,41, 1723-9 (1949). (18) Willsttitter, R., Ber., 59, 1871 (1926). Willstatter’s test method was adapted for atmospheric sampling by A. J. Haagen-Sniit, California Institute of Technology, Pasadena, Calif. RECEIVEDfor review August 28, 1951.
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ACCEIPTED April 7, 1952.
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