Formation of Photochemical Aerosol from Hydrocarbons Atmospheric Analysis Robert J. O’Brien,’**James H. Crabtree, John R. Holmes, Margaret C. Hoggan, and Albert H. Bockian California Air Resources Board, 9528 Telstar Ave., El Monte, Calif. 91 731
Atmospheric aerosol samples from various locations in California (particularly the Los Angeles area) were partially analyzed for their organic content. Emphasis was placed on photochemically produced material and in particular the identification of known products of organic aerosol precursors from environmental chamber experiments. The presence of organic nitrates in atmospheric aerosols gives a good indication of their photochemical origin. Mono- and dicarboxylic acids made u p a significant portion of the organic material in aerosol samples taken in the Los Angeles area under smoggy conditions. These products have been shown previously to result from the photooxidation of olefins and oxides of nitrogen. The highly oxygenated nature of Los Angeles particulate was quite different from primarily emitted auto exhaust aerosols. Some areas of California, particularly the South Coast
(Los Angeles) Air Basin, experience severe visibility reduction resulting from light scattering by suspended particulate matter. The State air quality standard for visibility (visibility not to be reduced to less than 10 miles for relative humidity less than 70%) is exceeded on approximately two days out of every three. During periods of intense photochemical activity, prevailing visibilities are sometimes reduced to one-half mile over wide areas of the Basin. The physical mechanisms leading to the occurrence of heavy loadings of light-scattering aerosol have been elucidated to a large extent ( I , 2). The bulk of the visibility reduction seems to be due not principally to directly emitted particulate matter, but rather to condensed species formed via the photochemical reaction of gaseous primary pollutants. The aerosols produced in this fashion accumulate preferentially in the size range 0.1-1.0 pm, which is the important range of particle diameters for the scattering of visible light. The chemical composition of California atmospheric particulate matter in general and of these secondary aerosols in particular is only partially known. It has been estimated t h a t in the South Coast Air Basin approximately two thirds is anthropogenic in origin and approximately half of this or one third of the total is formed chemically in the atmosphere (3). Most analytical investigations into the composition of California particulate matter have been devoted to the inorganic constituents; very little effort has been made on the organic material. Inorganic products of photochemical or other secondary reactions are relatively well understood-sulfates and nitrates are known to be produced in the atmosphere by the oxidation of SO2 and 3 0 2 . Ammonium sulfate, for instance, is a major constituent of particulates in the eastern part of the United States. Relatively well-developed techniques exist for the analysis of these inorganic ions, and for metallic elements as well. Present address. Department of Chemistry, Portland State University, Portland, Ore. 97207.
Considerable information is available on the partial composition of atmospheric particulate matter in California ( 4 ) . Data from the National Air Surveillance Network ( 5 ) on aerosol composition in California’s South Coast Basin are summarized in Table I. Measurements for three inorganic ions associated with secondary aerosols-sulfate, nitrate and ammonium-are reported, as well as values for the total benzene-soluble organic portion. It is seen that, on a basin-wide average, the amounts of nitrate, sulfate, and benzene solubles are approximately equal and account (with ammonium ion) for about 28% of the total mass loading. However, this figure does not accurately represent the contribution of these aerosol constituents to the overall problem. The benzene-soluble fraction underestimates the total organic content of the aerosol significantly because much of the organic material present in the aerosol is highly polar and insoluble in benzene. Moreover, these aerosol constituents are to a great extent concentrated in the small particle size range and thus make a contribution to visibility reduction, or potential health hazard, which is disproportionately large relative to their mass fraction. In this paper, we describe a partial analytical scheme for the organic portion of atmospheric aerosol, estimate the relative contribution of primary and secondary sources to the organic material, and assess the overall contribution of organic material to the total atmospheric mass loading. Particular emphasis has been placed on identifying in the atmosphere known organic products of photochemical reactions (6).
Experimental
Sampling of Atmospheric Aerosol. Aerosol samples were collected with standard high-volume air samplers and Gelman Type A glass fiber filters (without binder). Sampling times were usually 24 hr, but in some cases were shorter or longer. Preliminary work indicated t h a t commercial glass fiber filters, although labeled as flash fired, contained considerable extractable organic material which interfered with the chemical analysis (see below). Consequently, all filters were prewashed in a large Soxhlet extractor for two days with absolute alcohol and subsequently dried a t room temperature. The filters were pre- and postconditioned a t 21” and 55% relative humidity before the initial and final weighings. Recovery of Organic Aerosol. Soxhlet extraction of the glass fiber filters was employed for recovery of the organic material. Generally, one quarter of the 8 x 10 in. filter was extracted for two or more hours in a 250-ml Soxhlet. The solution containing the sample was reduced in volume to about 5 ml by boiling, transferred to a n aluminum weighing pan and reduced to dryness in a stream of warm N2. The sample was weighed to the nearest 0.1 mg, redissolved in the pan in 1 or 2 ml of solvent and transferred to a vial. The sample was again dried with warm Nz and then made up as a 10% (wt/vol) solution in the extracting solvent. Finally, the aluminum pan was reweighed to determine the amount of residue. Volume 9,Number 6,June 1975
577
Table I. National Air Surveillance Networks Data for 1968, California South Coast Basin (Maximum and arithmetic mean of particulate constituents, fig in+) Suspended particulate
Burbank Glendale Long Beach Los Angeles Ontario Pasadena Riverside San Bernardino Av % of total av
NOjMax
Mean
Max
Mean
Max
Mean
Benzene soluble, mean
340 216 290 272 389 195 255 213
122 98 132 135 135 113 130 108 122 -
26 21 18 21 72 17 38 22
8
39 29 52 41 18 41 37 36
11
10 4 31 9 6 8 14 10
1 1 3 2 1 1 2 1 2 2
14 12 13 15 8 12 lo 7 11 9
7 7 4 12 7 13 9 9 7
% r e m o v e d = 100 x
total c a r b o n residue total extract wt - "IN03 wt. + total c a r b o n residue
i
Most samples in this study were extracted from individual quarters of the filter in benzene and in absolute alcohol (95% ethanol, 5% propanol). As mentioned above, the glass fiber filters were found to contain very appreciable amounts of oxygenated organic material. A blank 8 X 10 in. filter (Gelman Type A) extracted in a Soxhlet with benzene yielded 2.4 mg of material. Extraction with ethanol removed 14.8 mg. A prewash with alcohol lowered these background organics t o acceptable levels, less than 2.0 mg per filter with alcohol. Solvents were predistilled to remove any nonvolatile material. Infrared Analysis of Extracts. Infrared spectra of aerosol extracts were obtained with a Perkin-Elmer 521 equipped with a beam condenser. Samples were prepared for analysis by placing a n aliquot of the aerosol solution (generally 1-10 11) on a small agate mortar. When the solvent had evaporated, about 10-20 mg of KBr were added and the mixture was ground to intimacy. A portion of the mixture was pressed into a 1.5-mm diameter micropellet. In this way, high-resolution, full-scale spectra could be obtained from less than 100 pg of aerosol. It was not generally possible to keep traces of water out 578
NHa-
Mean
Most studies of atmospheric aerosol in the past have employed either benzene, methylene chloride, or cyclohexane extractions of organic material from the sample. Due to the presence of very highly polar organic compounds in these aerosol samples, however, it was necessary to use alcohol (95% ethanol, 5% propanol) as a solvent. For example, a single filter sample extracted in a Soxhlet successively with cyclohexane, benzene, methylene chloride, and alcohol yields appreciable organic material in all stages of the extraction. The alcohol extract also contains considerable inorganic material, particularly nitrates, thus complicating the analysis in some cases. The efficiency of the alcohol extraction in removing organic material was tested on four samples collected in different areas of the South Coast Air Basin. The filters were extracted in the usual fashion in alcohol and then total noncarbonate carbon remaining on the filters was determined by a n independent laboratory. The results showed 68, 80, 84, and 87% of the organic material had been removed in the extraction. These percentages were calculated as:
t-
s04s-
Max
Environmental Science & Technology
11
15 13 10 12 10 10 12 10
of the KBr pellets in the process of grinding and pressing. Thus, the characteristic water bands at 3400-3500 cm-I and at 1630 cm-I were always present. In addition, a band at 1385 cm-I was sometimes present in the blank. Since these bands were variable in magnitude, interpretation of these spectral regions was made with caution. Chromatographic Separation of Organic Constituents. Previous work (6) has indicated that carboxylic acids are a major aerosol product of the photochemical reaction of olefinic hydrocarbons. Further, preliminary analysis of atmospheric aerosol samples by infrared spectroscopy showed the presence of strong carbonyl bands a t tributable to carboxylic acids. Consequently, a paper chromatographic technique for separating these acids was developed (7). Whatman No. 41 paper was cut into 9-in. square sheets and washed in a large Soxhlet with absolute alcohol to remove organic impurities. The paper was dried and then washed in the Soxhlet with azeotropic 2% aqueous acetic acid to prevent tailing of the acid spots. Ten percent solutions of the extracts were spotted in one corner and the paper was coiled and placed in a glass jar for elution. The solvent front was allowed to move 15 cm past the spot. Twoway chromatography was employed. The solvent in the first direction was alcohol-water-ammonia in the proportions 190-10-2. The second direction solvent was ethoxyethanolbutylether-acetic acid-water, 84-84-29-4. The first direction solvent moved the acids as their ammonium salts and in the second direction they were converted to free acids by the acetic acid. Although optimized for acids, this system also adequately separated less polar materials and inorganic salts as well. The results of separations of several reference compounds, as well as some synthetic photochemical aerosols, have been presented previously ( 6 ) . The chromatogram was allowed to dry overnight to remove the acetic acid. Nonvolatile acidic compounds were then detected by spraying with bromcresol purple (40 mg in 100 ml of formaldehyde-ethanol (1:5) adjusted to p H 10 with NaOH). Exposing the chromatogram to ammonia vapors helps to visualize the spots. This spray reagent detected mono- and dicarboxylic acids, acidic or basic salts, and amines. Long-chain fatty acids were not detected. The detection limit for mono- or diacids in the c 5 - C ~ range was about 0.1 pmol. In the solvent system used, many inorganic salts apparently underwent a metathetical reaction with aqueous ammonia. Thus, a salt such as N a N 0 3 gave two spots: a blue (basic) spot due to NaOH and a yellow (acidic) spot due to N H 4 N 0 3 . Ammonium nitrate gave a single acidic spot. In addition to the chromatographic analysis, inorganic nitrates were measured by specific ion electrode and ali-
phatic aldehydes by MBTH test (8).Total carbon and molecular weight determinations were performed by the Schwarzkopf Laboratory, Woodside, N.Y.
Results and Discussion T h e ir spectra of the extracts of a typical aerosol c01lected on a smoggy day in downtown Los Angeles are shown in Figure 1. In this case, the filter sample was extracted successively in benzene followed by absolute alcohol. (The usual procedure was to extract separate portions of the filter in these two solvents.) The necessity of a n alcohol extraction to more completely remove the organic material is illustrated in Figure 1: The benzene extraction removed 12% of the total amount of material on the filter and the subsequent alcohol extraction removed an additional 20% of the total. It can be seen from Figure l a that the benzene extract is characterized by strong carbonyl and organic nitrate bands. The alcohol extraction removed further organic material, as seen by the CH and carbonyl bands, and dissolved a large amount of ammonium nitrate as well. Apparently all of the organic nitrate, but only part of the organic acid, was removed in the benzene extraction. A single extraction in ethanol would remove close to the total 3270, however. The benzene extract, Figure l a , gave a relatively simple spectrum similar to those obtained from the aerosols generated from mono- or diolefins under simulated atmospheric conditions (6). All five bands characteristic of organic nitrates are present (700, 7 5 5 , 860, 1280, 1630 c m - l ) . Sharp bands indicating the aliphatic organic nature of the aerosol exist in the CH, regions a t 1385, 1460. and about 2900 c m - l . Carboxylic acid is indicated by the strong carbonyl bands a t 1720 cm-1 and the broad OH band about 3450 c m - I . There is no indication of any aromatic content in this sample, but the ir does not conclusively rule this out. The alcohol extract, Figure I b consists primarily of ammonium nitrate. Note the characteristic doublet a t 830 c m - I and the bands a t 1385 and 3160 c m - l . Overtones are present a t 1760 and 2420 c m - l . However, appreciable organic material is present as indicated by the carbonyl and C-H bands. Paper chromatography indicated the presence of appreciable amounts of dicarboxylic but no detectable monocarboxylic acid in the alcohol extract. In redissolving the extracts in 1 or 2 ml of alcohol, much of the inorganic salt was lost, as these are of limited solubility. Most organic material did redissolve. In contrast to this ambient smog aerosol, the ir of directly emitted auto exhaust particulate is shown in Figure 2 . Motor vehicles contribute 42% of all directly emitted particulates in the Los Angeles Basin, according to data of the Los Angeles County Air Pollution Control District (LAAPCD, 1971). Figure 2a shows the infrared spectrum of an alcohol extract of an auto exhaust sample. This sample was taken during a seven-mode (California) cycle on a dynamometer from a 1964 vehicle burning low-lead fuel. The aerosol is primarily aliphatic in nature and exhibits only a small carbonyl band (relative to the C-H intensity). The aliphatic nature is also reflected in the fact either hexane or alcohol would dissolve most of the material present. Figure 2b shows a 2-hr hi-vol sample taken during morning rush hour in a tunnel in downtown Los Angeles. During this period ( 6 to 8 a.m.) secondary aerosols should not yet be present to a large extent. The enrichment of this sample by directly emitted auto exhaust is evidenced by the fact that the aerosol mass loading in the tunnel was three times the mass loading obtained concurrently outside the tunnel. Here again the aliphatic organic na-
WAVENUMBER I C M -
1
Figure 1 . Infrared spectra of two successive solvent extractions
of the same filter sample Etght-hour sample taken 8 30-16 30 9 / 1 4 , 7 2 at downtown Los Angeles
I 4m
3100
xa
2100 .."T*Y"liR
>om
5m
wo
I 500
IC".,
Infrared spectra of organic fraction of (a) auto exhaust particulate and (b) particulate from vehicular tunnel in downtown Los Angeles Figure 2.
ture is apparent, with a small carbonyl to C-H band intensity ratio, about 0.3-1. In addition to the organic material, ammonium nitrate is present which is probably from the ambient atmosphere. As part of this preliminary characterization study, atmospheric aerosol samples were obtained from the South Coast Air Basin and elsewhere in California. These samples were extracted in alcohol and in benzene and were analyzed by infrared spectroscopy, paper chromatography, and for inorganic nitrate by specific ion electrode. The results of these analyses are presented below. In addition, aliphatic aldehydes were determined by MBTH test but were found to be negligible (
