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Aerosol metal relationship in air and precipitation
Environment International Vol. 2, pp. 63-65, P e r g a m o n Press Ltd. 1979. Printed in Great Britain.
Aerosol Metal Relationships in Air and Precipitation
A.W. Struempler Chadron State College, Chadron, NB 69337, U.S.A.
A one year collection study o f the concentrations o f metals in air and precipitation reveals that, on a weight-weight basis, most metals tend to be concentrated about 1000 times more in precipitation than in air. There are exceptions to this trend such as Ph, which is only concentrated by a factor o f about two. Those metals less concentrated in precipitation appear to be the more enriched atmospheric metals which are associated with smaller sized particles and higher volatilities. Also, seasonal variations in the concentrations o f some aerosol metals are evident.
Introduction
Results and discussion
At present, there is limited documented information showing the relationships o f atmospheric trace elements between precipitation (rain and snow) and air from the Northern Plains states of the United States. Such information is important before additional coal-fired power plants are constructed. Chadron, Nebraska, United States (42 ° 50'N, 103 ° 05'W, 1000 m above sea level), is located east and south and downwind from the coal reserves of Wyoming, Colorado and North Dakota where many large coalfired power plants are being constructed. Chadron's location in a non-industrial area is ideal for establishing the concentration levels o f trace metals in air and precipitation, and evaluating their relationships. During 1973 and the summer months of 1974, aerosol particulate matter was continuously collected on filters which were changed at weekly intervals. The collections were made on the campus o f Chadron State College, which is located in the extreme southeast corner of Chadron (population 6000). It is estimated that the surface winds blowing from the northwest and over the residential area o f Chadron to the sampling site occurred less than 10°70 of the time. Therefore, most collections represent surface winds from uninhabited areas. Concurrent with the above, a polyethylene funnel and bottle was set out immediately before or during the initial stages o f precipitation periods to collect the precipitation. Forty-four centimeters of precipitation fell at the collection site in 1973 (NOAA, 74). Of this amount 82°7o was collected, most o f it representing rain. More detailed sampling and analytical methods have been reported previously for the aerosols (Struempler, 75) and precipitation (Struempler, 76).
Figure ] shows the monthly trends o f aerosol AI, Mn, Zn, Cu, Pb, Cd and Ag, based on the arithmetic mean o f five weeks for January, April, July and October, and four weeks for the remaining months o f 1973, as well as the aerosol concentrations of AI, Pb and Ag for the four summer months of 1974. Of those metals tested, AI is in highest concentration in the earth's crust (Mason, 66) and is also in highest concentration in the air. The winter low for aerosol AI may be related to snow cover and a corresponding reduction in dust particles. The summer high for AI is not surprising as the Northern Plains states are characterized by dry and windy conditions. Neither A1 nor Mn shows enrichment (Table 1). On the other hand, Zn and Cu are enriched in the air as evidenced by associated enrichment factors (E) of 35 and 14, respectively, which are calculated as follows: X/AI (aerosol) Ex =
X/A1 (crust)
Table 1. S u m m a r y o f metals in precipitation and air, Chadron, Nebraska, during 1973 C o n c n i n C o n c n i n Concn earth's ppt in air crust p p m g g l-I ng kg q AI Mn Zn Cu Pb Cd Ag 63
81000 950 70 55 13 0.2 0.07
350 5.2 10 4.4 4.8 0.31 0.08
414 4.4 12 4.1 35 0.44 0.12
E air
1.0 0.9 35 14 525 430 335
W a s h o u t m m d Boiling factor W /~m point °C 845 1182 808 1073 138 704 667
5 2 1.5 1.8 0.4 2 1
2467 1962 907 2595 1740 765 2212
64
A.W. Struempler I00(
AI," IOC
Pb
I0
\ \ /
X
~ -"
~= O !
C
Cd....v " ' / " ' " " i 7 ~ -
"'\x"
// "\
0.I
"
J"/
---/',\ ./
Ag
~.
A9
0.01
"/"'\
I
d
I
F
I
M
I
A
I
M
I
J
I
J
I
A
!
S
I
0
I
N
1973 (twelve months) 1974 (three months) Fig. I. Concentration of elements in air at Chadron, Nebraska, during 1973 and 1974.
Here, X is the concentration of the tested metal, and Al is the concentration of the reference element. Aluminum is used as the reference metal because o f its high concentration in the earth's crust and minimal pollution effects from artificial sources. An enrichment value near unity, as in the case of Al and Mn, is indicative of a crustal source. In contrast, increasing values from unity suggest increasing anthropogenic source(s). The large Cd and Ag values are in line with a compilation of over 100 aerosol studies, global in nature, which show that Cd has E values from below 100 to 10,000 (Rahn, 76). Likewise, Ag shows enrichment even though few data are present. To evaluate metal concentrations in precipitation to those in air, a washout factor (I4.') is calculated (Chamberlain, 60). W=
Concentration in rain ( # g kg -1)
concentration in air (/~g kg -1) Metal concentrations in both precipitation and air are expressed on a weight/weight basis. Most metals in
precipitation tend to be concentrated about three orders of magnitude over those in air. Similar 14s values o f about 1000 for many metals are likewise noted in an English study (Peirson, 73) where pronounced differences in climatic and population conditions prevail. Lead is one exception to this trend as it exhibits a W ratio o f 450 in the English study and an even lower 138 in the Nebraska study. It appears that the high enrichment of air Pb, over air Mn, Cu and Zn, contributes to the low washout ratio for Pb. The logical source for lead's air enrichment is leaded gasoline, which constitutes 98°70 of total lead emissions (NAS, 72); and lead's enrichment is related to gasoline consumption (Lazrus, 70). This relationship is further demonstrated by the decrease in aerosol Pb during the summer months of both years o f this study due to a decrease o f 1500 students and associated autos on the Chadron State College campus. Likewise, a supportive study in Pittsburgh, PA, indicates " t h a t local auto traffic alters concentrations o f Pb in air more than in rainwater (Chan, 76). Enriched metals having a
Aerosol metal relationships very low W value (i.e., Pb in this study) are suggestive of local sources and at low altitudes. Furthermore, the more enriched metals (i.e., Pb, Cd and Ag), regardless of sources, are associated with the smaller washout values." The source of lead's enrichment is obvious, and the Ag aerosol enrichment may be enhanced by the use of Ag salts for weather modification. But, the source of enriched Cd is less explainable. However, when enrichment factors are evaluated on the basis o f massmedian diameters (MMD) o f the aerosol particles, in conjunction with the boiling points of the elements, lheir associations on enrichments become more related. The M M D represents the particle size for which one half of the mass of the element is found on larger particles and one half is found on the smaller particles. The median MMD for most elements lies between 0.4 and 5 /am. Soil derived elements, like A1, tend to have MMD values of about 5.0. On the other hand, aerosol Pb is associated with a M M D of 0.4. The remaining tested metals have MMD values which tend to hover about 1-2 /am (Rahn, 76). Enriched elements, especially those in low crustal concentrations, are associated with small MMD values, as evidenced by a correlation coefficient o f -0.56 between E and MMD values for the seven metals tested in this study. According to Stoke's Law, the settling speed for aerosol particles o f about 5/am at rain forming altitudes is days. Particles of less than 1/am may, however, remain airborne for a month or more tTer Haar, 67), and may remain in the atmosphere almost indefinitely, especially if they are raised several km from their sources. In reality, the enriched aerosol metals are associated with the smaller particles with slow and global diffusion even in the remote area of the South Pole (Zoller, 74). Along with lower washout ratios and smaller sized particles the more enriched metals are the more volatile ones (a correlation coefficient of -0.37-exists between enrichment factors and boiling points). The volatility concept is not new (Bertine, 71; Zoller, 74; Rahn, 76) and is further borne out by the behavior o f Zn and Cu. With no known local artificial contaminating source, and having similar crustal concentrations and MMD values, Zn, with its lower boiling point, is noticeably enriched over Cu. The volatility mechanism for enrichment a n d / o r particle size o f the ambient aerosol may be the result o f elevated temperatures in the combustion zone, over the boiling point of the element, which vaporizes more o f the parent material into gases. Escaped gases, then, condense at cooler regions on solid particles at some distance from the combustion zone, and are associated with the smaller aerosol particles. On this basis, refractory dements, like AI with its associated
65 larger particle size, would not be expected to respond in the same fashion as Cd, Pb or Zn, an observation further substantiated in this study.
Summary and conclusion Seasonal differences in aerosol AI and Mn are evident and appear to be related to dust borne conditions. The source of enriched aerosol Pb appears to be largely related to the amount of gasoline consumed locally. On a weight/weight basis most metals tend to be concentrated in precipitation about a factor o f three over air. Reductions in this trend are evident particularly for those metals enriched in the atmosphere. The enriched aerosol elements are those found in least concentrations in the soil. Moreover, they are the more volatile metals and are associated with the smaller size aerosol particles. Therefore, human activity could actually increase the concentration of these elements in the atmosphere unless retained at the emission site. The author is grateful to Mike Barnes and Marie Smith for their helpful suggestions.
Acknowledgements
-
-
References Bertine, K.K. and Goldberg, E.D. (1971) Fossil fuel combustion and the major sedimentarycycle, Science 173,233-235. Chamberlain, A.C. (1960) Aspects of deposition of radioactive and other gases and particles, Int. J. AirPollut. 3, 63-88. Chan, K.C., Cohen, B.L., Frohlinger, J.O. and Shabason, L. (1976) Pittsburgh rainwater analysis by PIXE, Tellus XXVII, 24-30. Lazrus, A.L., Lorange,E. and Lodge, Jr. J.P., (1970) Lead and other metal ions in United States precipitation, Environ. Sci. Tech. 5, 55-58. Mason, B. (1966) Principles o f geochemistry, 3rd. Edn. Wiley and Sons, New York. National Academy of Sciences (1972) Lead, Airborne Lead in Perspective, Washington, DC. NOAA, Environment Data Center (1974) Annual Climatological Summary f o r Chadron, Nebraska, 1973, Federal Building, Asheville, NC. Peirson, D.H., Cawse, P.A., Salmon, L. and Cambray, R.S. (1973) Trace elements in the atmospheric environment, Nature 241,252-256. Rahn, K.A. (1976) The chemical composition of the atmospheric aerosol, Technical Report, Graduate School of Oceanography, University of Rhode Island, Kingston, RI. Struempler, A.W. (1975) Trace element composition in atmospheric particles during 1973 and the summer of 1974 at Chadron, Nebraska, Environ. Sci. Tech. 9, 1164-1168. Struempler, A.W. (1976)Trace metals in rain and snow during 1973at Chadron, Nebraska, Atmos. Environ. 10, 33-37. Ter Haar, G.L., Holtzman, R.B. and Lucas, Jr., H.F. (1967) Lead and lead-210 in rainwater, Nature 216, 353-355. Zoller, W.H. Gladney, E.S. and Duce, R.A. (1974) Atmospheric concentrations and sources of trace metals at the South Pole, Science 183, 198-200.
Aerosol Metal Relationships in Air and Precipitation
A.W. Struempler Chadron State College, Chadron, NB 69337, U.S.A.
A one year collection study o f the concentrations o f metals in air and precipitation reveals that, on a weight-weight basis, most metals tend to be concentrated about 1000 times more in precipitation than in air. There are exceptions to this trend such as Ph, which is only concentrated by a factor o f about two. Those metals less concentrated in precipitation appear to be the more enriched atmospheric metals which are associated with smaller sized particles and higher volatilities. Also, seasonal variations in the concentrations o f some aerosol metals are evident.
Introduction
Results and discussion
At present, there is limited documented information showing the relationships o f atmospheric trace elements between precipitation (rain and snow) and air from the Northern Plains states of the United States. Such information is important before additional coal-fired power plants are constructed. Chadron, Nebraska, United States (42 ° 50'N, 103 ° 05'W, 1000 m above sea level), is located east and south and downwind from the coal reserves of Wyoming, Colorado and North Dakota where many large coalfired power plants are being constructed. Chadron's location in a non-industrial area is ideal for establishing the concentration levels o f trace metals in air and precipitation, and evaluating their relationships. During 1973 and the summer months of 1974, aerosol particulate matter was continuously collected on filters which were changed at weekly intervals. The collections were made on the campus o f Chadron State College, which is located in the extreme southeast corner of Chadron (population 6000). It is estimated that the surface winds blowing from the northwest and over the residential area o f Chadron to the sampling site occurred less than 10°70 of the time. Therefore, most collections represent surface winds from uninhabited areas. Concurrent with the above, a polyethylene funnel and bottle was set out immediately before or during the initial stages o f precipitation periods to collect the precipitation. Forty-four centimeters of precipitation fell at the collection site in 1973 (NOAA, 74). Of this amount 82°7o was collected, most o f it representing rain. More detailed sampling and analytical methods have been reported previously for the aerosols (Struempler, 75) and precipitation (Struempler, 76).
Figure ] shows the monthly trends o f aerosol AI, Mn, Zn, Cu, Pb, Cd and Ag, based on the arithmetic mean o f five weeks for January, April, July and October, and four weeks for the remaining months o f 1973, as well as the aerosol concentrations of AI, Pb and Ag for the four summer months of 1974. Of those metals tested, AI is in highest concentration in the earth's crust (Mason, 66) and is also in highest concentration in the air. The winter low for aerosol AI may be related to snow cover and a corresponding reduction in dust particles. The summer high for AI is not surprising as the Northern Plains states are characterized by dry and windy conditions. Neither A1 nor Mn shows enrichment (Table 1). On the other hand, Zn and Cu are enriched in the air as evidenced by associated enrichment factors (E) of 35 and 14, respectively, which are calculated as follows: X/AI (aerosol) Ex =
X/A1 (crust)
Table 1. S u m m a r y o f metals in precipitation and air, Chadron, Nebraska, during 1973 C o n c n i n C o n c n i n Concn earth's ppt in air crust p p m g g l-I ng kg q AI Mn Zn Cu Pb Cd Ag 63
81000 950 70 55 13 0.2 0.07
350 5.2 10 4.4 4.8 0.31 0.08
414 4.4 12 4.1 35 0.44 0.12
E air
1.0 0.9 35 14 525 430 335
W a s h o u t m m d Boiling factor W /~m point °C 845 1182 808 1073 138 704 667
5 2 1.5 1.8 0.4 2 1
2467 1962 907 2595 1740 765 2212
64
A.W. Struempler I00(
AI," IOC
Pb
I0
\ \ /
X
~ -"
~= O !
C
Cd....v " ' / " ' " " i 7 ~ -
"'\x"
// "\
0.I
"
J"/
---/',\ ./
Ag
~.
A9
0.01
"/"'\
I
d
I
F
I
M
I
A
I
M
I
J
I
J
I
A
!
S
I
0
I
N
1973 (twelve months) 1974 (three months) Fig. I. Concentration of elements in air at Chadron, Nebraska, during 1973 and 1974.
Here, X is the concentration of the tested metal, and Al is the concentration of the reference element. Aluminum is used as the reference metal because o f its high concentration in the earth's crust and minimal pollution effects from artificial sources. An enrichment value near unity, as in the case of Al and Mn, is indicative of a crustal source. In contrast, increasing values from unity suggest increasing anthropogenic source(s). The large Cd and Ag values are in line with a compilation of over 100 aerosol studies, global in nature, which show that Cd has E values from below 100 to 10,000 (Rahn, 76). Likewise, Ag shows enrichment even though few data are present. To evaluate metal concentrations in precipitation to those in air, a washout factor (I4.') is calculated (Chamberlain, 60). W=
Concentration in rain ( # g kg -1)
concentration in air (/~g kg -1) Metal concentrations in both precipitation and air are expressed on a weight/weight basis. Most metals in
precipitation tend to be concentrated about three orders of magnitude over those in air. Similar 14s values o f about 1000 for many metals are likewise noted in an English study (Peirson, 73) where pronounced differences in climatic and population conditions prevail. Lead is one exception to this trend as it exhibits a W ratio o f 450 in the English study and an even lower 138 in the Nebraska study. It appears that the high enrichment of air Pb, over air Mn, Cu and Zn, contributes to the low washout ratio for Pb. The logical source for lead's air enrichment is leaded gasoline, which constitutes 98°70 of total lead emissions (NAS, 72); and lead's enrichment is related to gasoline consumption (Lazrus, 70). This relationship is further demonstrated by the decrease in aerosol Pb during the summer months of both years o f this study due to a decrease o f 1500 students and associated autos on the Chadron State College campus. Likewise, a supportive study in Pittsburgh, PA, indicates " t h a t local auto traffic alters concentrations o f Pb in air more than in rainwater (Chan, 76). Enriched metals having a
Aerosol metal relationships very low W value (i.e., Pb in this study) are suggestive of local sources and at low altitudes. Furthermore, the more enriched metals (i.e., Pb, Cd and Ag), regardless of sources, are associated with the smaller washout values." The source of lead's enrichment is obvious, and the Ag aerosol enrichment may be enhanced by the use of Ag salts for weather modification. But, the source of enriched Cd is less explainable. However, when enrichment factors are evaluated on the basis o f massmedian diameters (MMD) o f the aerosol particles, in conjunction with the boiling points of the elements, lheir associations on enrichments become more related. The M M D represents the particle size for which one half of the mass of the element is found on larger particles and one half is found on the smaller particles. The median MMD for most elements lies between 0.4 and 5 /am. Soil derived elements, like A1, tend to have MMD values of about 5.0. On the other hand, aerosol Pb is associated with a M M D of 0.4. The remaining tested metals have MMD values which tend to hover about 1-2 /am (Rahn, 76). Enriched elements, especially those in low crustal concentrations, are associated with small MMD values, as evidenced by a correlation coefficient o f -0.56 between E and MMD values for the seven metals tested in this study. According to Stoke's Law, the settling speed for aerosol particles o f about 5/am at rain forming altitudes is days. Particles of less than 1/am may, however, remain airborne for a month or more tTer Haar, 67), and may remain in the atmosphere almost indefinitely, especially if they are raised several km from their sources. In reality, the enriched aerosol metals are associated with the smaller particles with slow and global diffusion even in the remote area of the South Pole (Zoller, 74). Along with lower washout ratios and smaller sized particles the more enriched metals are the more volatile ones (a correlation coefficient of -0.37-exists between enrichment factors and boiling points). The volatility concept is not new (Bertine, 71; Zoller, 74; Rahn, 76) and is further borne out by the behavior o f Zn and Cu. With no known local artificial contaminating source, and having similar crustal concentrations and MMD values, Zn, with its lower boiling point, is noticeably enriched over Cu. The volatility mechanism for enrichment a n d / o r particle size o f the ambient aerosol may be the result o f elevated temperatures in the combustion zone, over the boiling point of the element, which vaporizes more o f the parent material into gases. Escaped gases, then, condense at cooler regions on solid particles at some distance from the combustion zone, and are associated with the smaller aerosol particles. On this basis, refractory dements, like AI with its associated
65 larger particle size, would not be expected to respond in the same fashion as Cd, Pb or Zn, an observation further substantiated in this study.
Summary and conclusion Seasonal differences in aerosol AI and Mn are evident and appear to be related to dust borne conditions. The source of enriched aerosol Pb appears to be largely related to the amount of gasoline consumed locally. On a weight/weight basis most metals tend to be concentrated in precipitation about a factor o f three over air. Reductions in this trend are evident particularly for those metals enriched in the atmosphere. The enriched aerosol elements are those found in least concentrations in the soil. Moreover, they are the more volatile metals and are associated with the smaller size aerosol particles. Therefore, human activity could actually increase the concentration of these elements in the atmosphere unless retained at the emission site. The author is grateful to Mike Barnes and Marie Smith for their helpful suggestions.
Acknowledgements
-
-
References Bertine, K.K. and Goldberg, E.D. (1971) Fossil fuel combustion and the major sedimentarycycle, Science 173,233-235. Chamberlain, A.C. (1960) Aspects of deposition of radioactive and other gases and particles, Int. J. AirPollut. 3, 63-88. Chan, K.C., Cohen, B.L., Frohlinger, J.O. and Shabason, L. (1976) Pittsburgh rainwater analysis by PIXE, Tellus XXVII, 24-30. Lazrus, A.L., Lorange,E. and Lodge, Jr. J.P., (1970) Lead and other metal ions in United States precipitation, Environ. Sci. Tech. 5, 55-58. Mason, B. (1966) Principles o f geochemistry, 3rd. Edn. Wiley and Sons, New York. National Academy of Sciences (1972) Lead, Airborne Lead in Perspective, Washington, DC. NOAA, Environment Data Center (1974) Annual Climatological Summary f o r Chadron, Nebraska, 1973, Federal Building, Asheville, NC. Peirson, D.H., Cawse, P.A., Salmon, L. and Cambray, R.S. (1973) Trace elements in the atmospheric environment, Nature 241,252-256. Rahn, K.A. (1976) The chemical composition of the atmospheric aerosol, Technical Report, Graduate School of Oceanography, University of Rhode Island, Kingston, RI. Struempler, A.W. (1975) Trace element composition in atmospheric particles during 1973 and the summer of 1974 at Chadron, Nebraska, Environ. Sci. Tech. 9, 1164-1168. Struempler, A.W. (1976)Trace metals in rain and snow during 1973at Chadron, Nebraska, Atmos. Environ. 10, 33-37. Ter Haar, G.L., Holtzman, R.B. and Lucas, Jr., H.F. (1967) Lead and lead-210 in rainwater, Nature 216, 353-355. Zoller, W.H. Gladney, E.S. and Duce, R.A. (1974) Atmospheric concentrations and sources of trace metals at the South Pole, Science 183, 198-200.