Seasonal trends in Denver atmospheric lead concentrations
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these metals is association with fine particles which can be expected to be deposited and resuspended many times by tidal currents. In the area of direct, alkaline, waste discharge Cd is less mobile than Ni, apparently because of the formation of an insoluble CdC03 phase.
Baseline data are provided on the distribution of Cd and Ni in the sediments of a cove, which, because of localized, in- dustrial pollution, can serve as a useful environment for the study of Cd and Ni biogeochemistry.
The toxicity and potential health hazards to humans have been the subject of much recent study (15,16) and are most dramatically expressed in the outbreak of itai-itai syndrome in Japan (17, 18). While the results of this study do not prove the existence of a public health hazard due to Cd contami- nation, they do show that the potential for such a hazard ex- ists, should be studied, and that, due to the mobility of Cd, concern for such a hazard must include the western as well as the eastern part of the cove.
Acknowledgment We thank H. Feely who served as thesis advisor for the se-
nior author, R. Bopp and C. Olsen for providing some of the analytical data, and R. Cobler for help in field sampling.
Literature Cited (1) Kneip, T . J., Cadmium in an Aquatic Ecosystem: Distribution
and Effects, 1st Annual Progress Rep. to National Science Foundation, New York Univ. Inst. for Environmental Medicine, New York, N.Y., 1974.
(2) Kneip, T. J., ibid., 2nd Annual Progress Rep., 1975. (3) Kneip, T . J., Hernandez, T., Re, G., Cadmium in an Aquatic
Ecosystem: Transport and Distribution, in Trace Contaminants in the Environment, Proc. of the 2nd Annual NSF-RANN Trace Contaminants Conf. Asilomar, Pacific Grove, Calif., 1974.
(4) Buehler, K., Hirshfield, H. I., Cadmium in an Aquatic Ecosys- tem: Effects on Planktonic Organisms, ibid.
(5) United States District Court Records, United States of America vs. Marathon Battery Company, Civil Suit 4110, Southern District of New York, 1970.
(6) Bondeitti, E. A., Sweeton, F. H., Tamura, T., Perhac, R. M., Hulet, L. D., Kneip, T. J., Characterization of Cadmium and Nickel Contaminated Sediments from Foundry Cove, New York, Proc. of the National Science Foundation Trace Contaminants Conf., 1973.
(7) Thomas. R. L.. Can. J . Earth Sci.. 10.194-204 11973). (8) Simpson, H. J., Olsen, C. R., Trier, R. hi., Williams, S. C., Science,
194,179-83 (1976). (9) Iskander, J. K., Keeney, D. R., Enuiron. Sci. Technol., 8,165-70
(1974). (10) Tourtelot, H., Huffman, C., Rader, L., Cadmium in Samples
of the Pierre Shale and Some Equivalent Stratigraphic Units, Great Plains Region, U.S. Geological Survey Professional Paper 475-D, 1964.
(11) Edgington, D. N., Robbins, J. A., The Behavior of Plutonium and Other Long-Lived Radionuclides in Lake Michigan, in Im- pacts of Nuclear Releases into the Aquatic Environment, IAEA, Vienna. 1975.
(12) Davis, J. C., Statistics and Data Analysis in Geology, Wiley, New York, N.Y., 1973.
(13) Hem, J., Water Resour. Res., 8,661-79 (1972). (14) Garrels, R. M., Christ, C. L., Solution, Minerals, and Equilib-
(15) Flick, D. F., et al., Enuiron. Res., 4, 71-85 (1971). (16) Fleischer, M., et al., Enuiron. Health Perspect., May, 253-323
(1974). (17) Kobayashi, J., Relation Between the Itai-Itai Disease and the
Pollution of River Water by Cadmium from a Mine, Proc. of the 5th Int. Conf. on Water Pollution Res., 1970.
(18) Namagata, N., Manifestation of Cadmium Poisoning in Japan and Geochemical Approach to Environmental Pollution, in Proc. of Symp. on Hydrogeochemistry and Biogeochemistry, E. Ingerson, Ed., pp 330-6, Tokyo, Japan, 1973.
ria, Freeman, San Francisco, Calif., 1965.
Received far review December 15, 1976. Accepted December 23, 1977. Financial support provided under Environmental Protection Agency contract (R803113-01, 02,03).
Seasonal Trends in Denver Atmospheric Lead Concentrations
Harry W. Edwards and Harovel G. Wheat Department of Mechanical Engineering, Colorado State University, Fort Collins, Colo. 80523
During the 42-month period from January 1972 to June 1975, monthly average atmospheric lead concentrations at five metropolitan Denver sampling sites displayed maxima during winter months that coincided with minima in mixing heights. Monthly atmospheric lead inputs estimated on the basis of city-wide consumption of leaded gasoline showed a general downward trend with maxima during summer months. At- mospheric lead concentrations correlated well with the dis- persion factor, the product of the mixing height and the mean wind. Although the data suggested a long-term declining trend a t three sites, monthly atmospheric lead concentrations did not correlate with estimated monthly atmospheric lead in- puts.
The major source of lead in urban atmospheres is com- bustion of leaded gasoline ( I ) . Although there is a divergence of scientific opinion regarding some of the possible environ- mental and health effects of automotive lead (2, 3), there is widespread agreement that a general trend toward rising urban atmospheric lead exposures would be undesirable. The National Academy of Sciences Panel on Airborne Lead (1)
reported in 1972 that urban atmospheric lead concentrations in the United States had increased very little over several decades in spite of substantial increases in the consumption of lead antiknock additives. Shorter-term increasing trends have been reported for certain sites, however ( 4 , 5 ) . The general implication is that meteorological factors, urban ge- ometry, and traffic routing may enter strongly in determining urban atmospheric lead concentrations. However, very little quantitative information has been compiled concerning the relative sensitivity of atmospheric lead concentrations in a specific urban center to changes in local lead additive con- sumption. The need for quantitative information has assumed major importance since regulations have been adopted to re- duce ambient atmospheric lead exposures in the United States by phasing down the lead content of the total gasoline pool (6). These regulations are based upon a linear rollback formula, Le., changes in atmospheric lead concentrations are directly proportional to changes in lead additive consumption.
While no totally satisfactory mathematical model has been developed for predicting pollutant dispersion within an urban complex, the atmospheric concentration of a pollutant emitted from an area source is often proportional to the emission rate and inversely proportional to the product of the mean wind
0013-936X/78/0912-0687$01.00/0 @ 1978 American Chemical Society Volume 12, Number 6, June 1978 687
and the vertical mixing height ( 7 ) . This relationship serves as the basis of the linear rollback model and is often useful for correlating trends in pollutant concentrations a t specific sites with trends in emission rates and meteorological parameters. Even though fine resolution in time and space is not always present, the model has been justified on the basis of simplicity and predictive success.
Application of the linear rollback model to atmospheric lead presents a t least three difficulties. The first is that of accu- rately determining the lead source strength. Both the rate and size of lead particles emitted are markedly dependent upon the mode of operation and history of the vehicle (8-10). These studies indicate that the amount of lead emitted can range from 10 to 2000% of the lead burned. The latter value corre- sponds to full-throttle acceleration, which apparently can dislodge lead particles previously deposited in the exhaust system. Changing traffic patterns may also affect the spatial and temporal characteristics of the lead source strength. The second difficulty is that because lead is emitted primarily in particulate form, some gravitational settling of the larger particles takes place. This is evident from field studies which show that when samplers are placed a t least 100 m from a traffic source, further dispersion of the lead plume can be described by gaseous pollutant transport relationships ( 1 1 ) . Closer to the roadway, an empirical approach is needed to deal with the apparent weakening of the source strength with distance due to particle depletion (12). The third difficulty is that urban geometry can profoundly influence pollutant dispersion. Both field and wind tunnel studies demonstrate the inhomogeneous mixing of automotive emissions within a city street canyon ( 1 3 ) .
In this paper we present data that show definite seasonal patterns in monthly average atmospheric lead concentrations in metropolitan Denver, Colo., during the 42-month period from January 1972 through June 1975. These data are ex- amined in terms of the existence of statistically significant correlations between atmospheric lead concentrations, com- puted atmospheric inputs of automotive lead, and meteoro- logical parameters. The metropolitan Denver area was se- lected for this study largely on the basis of its virtual isolation in terms of air pollution sources (14). In 1974 there were ap- proximately 1.5 X 106 inhabitants and 1.1 X lo6 registered automobiles in the Denver metropolitan area. In addition to the automobile, restrictive meteorology enters strongly in the Denver air pollution situation, particularly during winter months when thermal inversions are common (15). There is no primary lead smelting in the Denver metropolitan area, and private incineration of solid waste has been prohibited. Nat- ural gas is the dominant fuel for space heating. Electrical power is generated locally, largely by coal combustion.
Atmospheric Lead Inputs Monthly atmospheric inputs of automotive lead in the
Denver metropolitan area were estimated on the basis of monthly consumption of leaded gasoline. Through its Oil Inspection Section, the Colorado Department of Labor and Employment maintains monthly records for tax purposes of the total consumption of all grades of gasoline sold a t retail in the Denver metropolitan area (16). Gasoline transported out of the Denver metropolitan area for resale elsewhere is not included. Gasoline sold in Boulder, Colorado Springs, Long- mont, Greeley, Fort Collins, and other nearby cities outside of the Denver metropolitan area is also not included. Infor- mation concerning the average lead content of the total gas- oline pool sold a t retail in the Denver metropolitan area was obtained from the Ethyl Corp., a major local supplier of lead additives (17).
The average lead content of the total gasoline pool, in- cluding low-lead, unleaded, and all grades of leaded gasoline,
is given in Table I. Lower lead contents during winter months reflect in part the effects of reblending gasoline feedstocks for improved winter starting characteristics. The product of the monthly gasoline consumption and the lead content of the gasoline pool thus yields the total monthly lead additive consumption. Data reported by Hirschler et al. (10) indicate that averaged over 50 000 miles of consumer driving condi- tions, an automobile typically exhausts 78% of the lead burned and retains 22% of the lead burned. Some of the exhausted lead is associated with large particles that would settle rapidly in still air but may be temporarily suspended by traffic-in- duced turbulence. Multiplication of the above product, e.g., monthly consumption of lead additives, by 0.78 yields the estimated monthly atmospheric input of automotive lead. These data are shown in Figure 1 and reflect a downward trend in lead additive consumption during the 42-month pe- riod of the study. The downward trend is consistent with in- creasing consumption of unleaded gasoline since introduction of catalyst-equipped automobiles in the fall of 1974. Com- puted atmospheric inputs of automotive lead in the Denver metropolitan area show peaks during summer months and minima during winter months.
For purposes of comparison, monthly atmospheric inputs of lead from combustion of coal for electrical power generation were also estimated. Electricity consumed in the Denver metropolitan area is generated by the Public Service Co. of Colorado a t three plants, two of which, Arapahoe and Cher- okee, are fueled by coal. The Zuni plant is fueled by natural gas on an interruptive basis with oil serving as the back-up fuel. The Zuni plant therefore does not enter into the com- putations of atmospheric lead inputs. For the coal-fired plants, monthly coal consumption and lead-content data were ob-
Table 1. Lead Content of Gasoline Sold in Denver, g/gal
1972 1973 1974 1975
January-March 1.75 1.75 1.45 1.40 April-September 2.05 2.05 1.75 1.70 October-December 1.75 1.75 1.45 . . .
I . , . . . . . . . . . I . . . . . . . I . . . I . . . . . J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J
1972 1973 1974 I975 Month
Figure 1. Computed atmospheric lead inputs in Denver from January 1972 to June 1975
688 Environmental Science & Technology
tained on a unit-by-unit basis from the Public Service Co. of Colorado (18) . The ash content of the coal was 6-8%, and the average lead content of the ash was 0.0031%. These data then permitted cornputation of the monthly lead input to each of the coal-fired units.
The particle collection efficiencies (mass basis) were ob- tained for each of the gas cleaning devices on a unit-by-unit basis. The collection efficiencies ranged from 90 to 99% de- pending on the type of particulate removal devices employed. Studies by Davison e t al. (19), Lee et al. (20), and Klein e t al. (21) provide ample evidence that the smaller fly ash particles which escape collection in a coal-fired power plant are mark- edly enriched in lead and other toxic elements. Using these studies as a guide, we have employed a factor of 10 for the lead enrichment ratio. The estimated monthly lead release by each coal-fired unit is then obtained as the product of the monthly ash production, the lead content of the ash, one minus the collection efficiency, and the lead enrichment factor of 10. Results of these computations are also shown in Figure 1 and indicate that the computed monthly atmospheric lead input from coal combustion is typically several hundred times smaller than the computed atmospheric lead input from gasoline combustion in the Denver metropolitan area. Even though great accuracy is not claimed for these estimates, the computations indicate that coal combustion contributed negligibly on a relative basis to Denver atmospheric lead burdens during the 42-month period of this study.
Atmospheric Lead Concentrations Monthly average atmospheric lead concentrations in met-
ropolitan Denver were determined by the Colorado Depart- ment of Health (22) as a part of a broad air quality monitoring program. The six sites employed in this study provide a variety of traffic conditions, urban geometries, and locations. Ap- proximate daily traffic volumes 011 str...