Literature Cited
(IO) AACC “Approved Methods” (formerly Cereal Laboratory
(1) Page, A. L. Cincinnati, OH, 1974, EPA Report No. 670/2-74005. (2) Anderson, A.; Nilsson, K. 0. Ambio, 1972,1,176. (3) Hinesly, T. D.; Braids, 0. C.; Molina, J. E. Environmental Pro-
tection Agency Solid Waste Management Service,Report SW-30d,
1971. (4) Kirkham, M. B. In “Land as a Waste Management Alternative”; Loehr, R. C., Ed.; Ann Arbor Science: Ann Arbor, MI, 1977; pp 209-47. (5) Boesch, M. J. Compost Sci. 1974,15, 24. (6) Engdahl, R. B. “Solid Waste Processing-A State-of-the-Art Report on Unit Operations and Processes”; Report prepared for
Bureau of Solid Waste Management, Public Health Service, U.S. Department of Health, Education, and Welfare, 1969. (7) Golueke, C. G. “Comprehensive Studies of Solid Waste Management, Third Annual Report”, Interim Report (SW-lOrg); University of California, U S . Environmental Protection Agency, 1971. (8) Law, L. L.; Gordon, G. E. Enuiron. Sci. Technol. 1979,13,432. (9) Garcia, W. J.; Blessin, C. W.; Inglett, G. E.; Carlson, R. 0.J. Agric. Food Chem. 1974,22,810.
Methods), 7th ed.; American Association of Cereal Chemists: St. Paul, MN, 1962. (11) Garcia, W. J.; Blessin, C. W.; Inglett, G. E. Cereal Chem. 1974, 51,788. (12) Connor, J. J.; Shacklette, H. T. U S . Geological Survey Professional Paper 574-F, 1975, pp 40-161. (13) Watt, B. K.; Merrill, A. L. “Composition of Foods-Raw, Processed, Prepared”, US. Department of Agriculture Handbook 8 (revised 1963). (14) Dowdy, R. H.; Larson, W. E. J . Environ. Qual. 1975,4,278. (15) Fed. Regist. 1978,43,4942-55. (16) Mahaffey, K. R.; Corneliussen, P. E.; Jelinek, C. F.; Fiorino, J. A. Enuiron. Health Perspect. 1975,12,63. (17) Stenstrom, T.; Lonsjo, H. Ambio, 1974,3,87. (18) “National Research Council Recommended Dietary Allow-
ances”, 8th ed.; National Academy of Sciences: Washington, DC, 1974; pp 99. (19) Kelling, K. A.; Keeney, D. R.; Walsh, L. M.; Ryan, J. A. J. Enuiron. Qual. 1977,6, 352. Received for review May 14,1980. Accepted March 2,1982. This work was supported in part by the Environmental Protection Agency (EPA-ZAG-D-5-E763).
Oxygen Isotopy of Atmospheric Sulfates Ben D. Holt,* Paul 1.Cunningham, and Romesh Kumar Chemical Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439
The oxygen isotopy of atmospheric sulfates has been measured to determine the predominant mechanism(s) of their formation from precursor SO2.Oxygen-18 analyses were made on consecutive, continuously collected, 7-day samples of aerosol sulfate and water vapor at Argonne, IL, throughout the year of 1977. Analyses were also made on rain and snow collected for most precipitation events during the same period. As substantiated by results of analyses on samples taken intermittently during 1975 and 1976, the oxygen isotopies of precipitation sulfate and aerosol sulfate indicate that the mechanisms of sulfate formation are not identical throughout the year. Heterogeneous oxidation in the aqueous phase appears to be the prominant mechanism for the formation of sulfate associated with precipitation at all times. Aerosol sulfate also appears to be formed by the same mechanism during late fall, winter, and early spring. However, another, perhaps homogeneous, mechanism appears to predominate for the formation of aerosol sulfate during the late spring, summer, and early fall. Results of three other experiments are reported which indicate that (1) some variations in the l80 content of aerosol sulfates are continental in nature, (2) the l80content in aerosol sulfates often varies inversely with barometric pressure, and (3) the l*O content of water vapor varies inversely with altitude. The measurement of oxygen isotope ratios in atmospheric water and atmospheric sulfates is a useful analytical method for studying mechanisms of formation of sulfates from the precursor sulfur dioxide. Extensive research has been conducted by many workers in recent years on the world-wide problem of atmospheric sulfates of anthropogenic origin (1-10), particularly as it relates to acid precipitation (11-13). As part of this research, determinations of oxygen isotope content have been used to help in identifying the origins of acid sulfates that are damaging to the environment (14-161, and to investigate the predominant mechanisms of formation of sulfates in precipitation water and in aerosols (17-20). Oxygen isotopy is suited to these applications because the l80 804
Environmental Science & Technology
contents of sulfates that are formed from given S02, water, and oxidants differ, depending upon the mechanism by which they are formed (e.g., aqueous-phase air oxidation, aqueousphase H202 oxidation, surface oxidation, homogeneous gasphase oxidation, etc.). Once formed, the sulfates are extremely stable with respect to isotopic exchange with water with which they may be subsequently associated (21).We have, therefore, used these measurements to determine the predominant mechanism(s) of formation of the atmospheric sulfates. In related research to be reported elsewhere (22),we have measured the oxygen isotopy of sulfates produced in the laboratory from SOz, HzO, air oxygen, and HzOz by various heterogeneous and homogeneous reaction pathways. In this paper, we present the results of field measurements which indicate that different formation mechanisms are operative in the atmosphere for the sulfates in aerosol and in precipitation. Much is known about the oxygen isotopy of atmospheric water and factors that influence it, such as geographical location, season, plant transpiration, etc. (23-26). The SO2 in the atmosphere is probably in isotopic equilibrium with atmospheric liquid water because of the extremely high reaction rates in the hydrolysis equilibration (8,27) SO2
+ H2O e HSOs- + H+
Because of the high rate of this reaction, the bisulfite ion, which is a transient form of S(1V)in precipitation water of pH 3-6 (8),is isotopically dominated by the large excess of water in which it is dissolved. (In our laboratory studies of the transformation of SO2 to sulfate by aqueous-phase air oxidation (22), we have confirmed that three of the four oxygen atoms in the sulfate ion are isotopically controlled by the oxygens of the solvent water.) Virtually all of the isotopic contribution of the SO2 is lost in the equilibration (28). The l80content of air oxygen is essentially constant at -23.5% (29).T o our knowledge, the l80contents of other atmospheric oxidants (such as 03, HzO2, and NO,) and of free radicals (such as OH and HO2) have not been determined. Measurement of the oxygen isotopy of samples collected during 1975 and 1976 at the Argonne meteorological tower
0013-936X/81/0915-0804$01.25/0 @ 1981 American Chemical
Society
indicated that the ls0/l6O ratios in water vapor, in precipitation water, and in sulfate present in the precipitation water vary seasonally (higher in summer, lower in winter). The seasonal variations in the oxygen isotopy of precipitation sulfates paralleled the isotope variations in atmospheric water (vapor and precipitation), indicating that the sulfate in precipitation is formed predominantly by the oxidation of SO2 in the aqueous medium. In contrast, the 180/160 ratio in the atmospheric aerosol sulfate appeared to vary randomly with season (17,18). The 1975-1976 data on water vapor and particulate sulfate were obtained on 24-h field samples, collected -6 m above ground level on three consecutive days in each month. Since the samples were collected intermittently, they did not necessarily represent continuous, temporal variations of the oxygen isotopes, except during the 3-day sampling period within each month. Since the 2-yr averages of lSO content in aerosol sulfate and precipitation sulfate were about equal, it was postulated that both sulfate varieties were probably formed by the same aqueous-phase oxidation mechanism in clouds. It was further proposed that relatively long residence times and transport distances of aerosol sulfates may provide sufficient mixing among sulfates of various origins to obscure the seasonal influence that was observed in the relatively short-residenced precipitation sulfates (I7 ) . During 1977, sequential 7-day samples of water vapor, aerosol sulfate, and sulfur dioxide were collected continuously throughout the four seasons. Samples of rain and snow were also collected for most of the precipitation events during this period. The results of isotopic studies made during this more intensive sampling program are now available (30)for an alternative and possibly more accurate interpretation of the observed differences in seasonal isotopic variability between precipitation and aerosol sulfates. The purpose of this paper is to report and discuss these new results and results of other experiments related to the oxygen isotopy of atmospheric sulfates. The other experiments were (1)a comparison of seasonal changes in the lSO content of aerosol sulfates sampled in different geographical regions during 1975, (2) a comparison of daily changes in isotopic content of aerosol sulfate with changes in barometric pressure, and (3) an assessment of the dependence of oxygen isotope ratio in aerosol sulfates and in water vapor on altitude.
the filter cassette and the cold trap. During the 3-week experiment, 12-h diurnal samples of water vapor were collected continuously a t ground level. A few samples of precipitation water a t ground level and lake water from nearby Lake Michigan were also collected.
Results and Discussion Sequential 1-Week Samples. The isotopic results obtained from the 1977 continuous sampling program are shown in Figure 1. The data are given in units of P O ,which is the deviation in parts per thousand (%o) of the 1sO/160ratio of the sample from that of standard mean ocean water (SMOW). Curve A shows the temporal variation of 6lSO in atmospheric water vapor collected -6 m above ground level. Solid lines connect the data points representing consecutive 1-week samples; broken lines connect points interrupted by missing or improperly processed samples. Besides the strong seasonal variation that is displayed by the curve, it may be noted that the P O was at times nearly constant for several weeks, e.g., in August and September; other occasional perturbations of a smooth seasonal variation occurred, as, for example, near the end of October; and the transitions between winter and the other three seasons of the year were comparatively abrupt. Curve B gives the corresponding results for atmospheric water sampled as rain or snow. The open-circle data points represent rain and the crossed circles represent snow. The shape of the plot approximates that of the curve for water vapor, the average level being -9%0 higher because of the isotopic separation that is characteristic of the two-phase system. The abrupt change between winter and the warmer
30
1
AEROSOL SULFATE
Experimental Section The methods used for sample collection and analysis have been described elsewhere ( I 7-20). The sampling procedures were slightly modified to accommodate the 7-day samples acquired in 1977. The hi-vol air sampler, loaded with a cassette of two filters (the upstream glass-fiber filter (Gelman A) to collect particulate sulfate, and the downstream cellulose filter, pretreated with &C03 and glycerol, to collect SOz), was maintained at a relatively low flow rate of -14 m3/h. We estimate that the amount of artifact sulfate formed from oxidation of SO2 on the Gelman A glass-fiber filter was
