Continental-scale variations in precipitation chemistry Ion concentrations are dominated by land use and proximity to man-made emissions
J. William Munger Keck Laboratories California Institute of Technology Pasadena, Calif. 91125 Steven J. Eisenreich Department of Civil and Mineral Engineering University of Minnesota Minneapolis, Minn. 55455 Precipitation chemistry has received considerable attention over the past several years, with much of that attention focusing on the acidity of rain and snow and on the associated environmental consequences of acid deposition (1-5). Gorham (5) and Barret and Brodin (6) performed early studies of acid precipitation in nonurban areas of England and Scandinavia far from industrial centers. Likens identified the magnitude and spread of acid precipitation in the northeastern U.S. (7); reports of acid precipitation throughout the world continue to appear in the literature (4). Acid precipitation, due primarily to the presence of the strong acids H2SO4 and H N O 3 , has been implicated in watershed acidification (3, 8, 9) and in the decline of fish populations in affected lakes (3,10,11). Terrestrial effects, including soil acidification, loss of fertility, interference with the nitrogen cycle, and groundwater acidification have been suggested (1-3, 12-15). Rain-borne N H 4 + , Ca+ 2 , and Mg+ 2 are also of special interest because they act to neutralize acidity and also act as nutrients. Kilham has reported on the role acid precipitation may play in the alkalinity of some lakes (76). Lead in precipitation is derived to a large ex32A
Environ. Sci. Technol., Vol. 17, No. 1, 1983
tent from gasoline combustion, while Zn and Cd are indicative of other anthropogenic activities (17). The toxicity of these and other metals increases the importance of obtaining adequate information about their distribution in precipitation. The concentration of a particular element in precipitation is controlled by the strength of its source (either local emissions or long-range transport), the presence of sinks other than precipitation (for example, dry fallout or gaseous absorption), and precipitation-scavenging processes. Chemical concentrations in precipitation are high near large emission sources and where precipitation intensity is low. Conversely, low concentrations are indicative of a low supply of these elements (resulting from weak sources or additional sinks) or a large volume of precipitation diluting the components scavenged by the rain. Investigations of precipitation chemistry on a continental scale can reveal where the regional sources and sinks for different elements are located. Historical trends are indicated by changes in concentrations that occur over a wide area between sampling periods. Similar changes observed at an individual sampling station would not be as definitive an indication of major changes in precipitation chemistry because local conditions could be responsible. Sediment stratigraphy and the accumulation of elements in soils and vegetation (especially mosses and lichens, which are often wholly dependent on the atmosphere for their nutrient supply) provide additional information about spatial and historical variations in precipitation chemistry, most notably when those data are
calibrated chemistry
against (18-20).
precipitation
Continental-scale studies In 1955-56, Junge and co-workers investigated precipitation chemistry on a continental scale in the U.S. (21-23). One of their conclusions was that soil dust was a major source of ions in rain over continents. Marine aerosols near coastal areas are rapidly diluted in the continental air mass and become an insignificant source of ions in the precipitation farther inland. Studies covering the north-central U.S. carried out by Thornton and Eisenreich (24) and Munger (25) support this idea. In 1966-67, the National Center for Atmospheric Research analyzed major ions and trace metals in precipitation using a 32-station network (26, 27). Anthropogenic activity was implicated as the primary source of Pb, Zn, Cu, Fe, Mn, and Ni. Lead was correlated with the amount of gasoline consumed in the vicinity of the collection stations. Since the late 1970s, the Canadian Network for Sampling Precipitation ( C A N S A P ) and the U.S. National Atmospheric Deposition Program ( N A D P ) have investigated precipitation chemistry across much of North America. The distribution of sites and preliminary data for the C A N S A P , N A D P , and M A P 3 S / R A I N E precipitation networks can be found in References 28, 29, and 30, respectively. Investigative methods For this article, the concentrations of major ions and selected trace elements in precipitation were compiled from numerous regional and local precipitation studies and data collected
0013-936X/82/0916-0032A$01.50/0
© 1982 American Chemical Society
TABLE 1
Data sources of precipitation chemistry No. of sites
Location
Tim·
U.S. network NADP, U.S. network Adirondack region Florida Ontario Canadian network Minnesota Los Angeles, Calif. Northeastern U.S. Alberta Kentucky Lake Tahoe area, Nev. Central Minnesota Lake Michigan Iowa North Dakota Ontario New York Washington Manitoba-Ontario
1976-80 1/79 4/78-8/79 7/76-10/77 8/76-7/78 11/77-12/79 4/78-7/79 9/78-9/79 1976-80 5/75-4/76 1971-80 10/77-9/78 1977-78 9/75-12/76 1971-73 1981 6/73-5/74 8/70-7/71 8/75-7/76 1973-77
wet-only wet-only wet-only wet-only wet-only wet-only wet-only wet-only wet-only wet-only wet-only wet-only
monthly weekly event event variable monthly event event event event
wet-only
event monthly weekly event/monthly variable monthly
Florida
7/77-9/79
wet-only
event
1
98
Nebraska
1973
wet-only
event
1
99
Indiana
8/73-11/73
bulk
1
100
New Mexico
1976
snow
1
101
Michigan
9/72-12/74
bulk
— — —
3
102
Central Ontario
5/77-9/77
wet-only
variable
2
89
Minneapolis, Minn.
1978-79
wet-only
event
1
103
Tennessee
6/73-5/74
wet-only/bulk
monthly
1
104, 105
South central Ontario
1972-73
bulk
1
106
San Francisco, Calif.
12/74-4/75
bulk
— —
1
75, 76
North Carolina
7/75-4/77
bulk
variable
1
107
at individual sites operated over the past 10 years. From these data, and the NADP, CANSAP, and MAP3S information, isopleths of the concentrations of major elements were constructed. In addition to providing insight into continental-scale patterns of precipitation chemistry, the geographical variations in atmospheric fluxes may be used to investigate metal and nutrient accumulation in ombrotrophic bogs throughout eastern North America {31). From the data base was created a file of concentration values in precipitation at specific locations identified by longitude and latitude. Matrices of concentrations at regularly spaced coordinates corresponding to a 90° by 50° longitude-latitude grid were generated by interpolating from the original data file. The WORLD Projection and Mapping Program was
bulk bulk wet-only bulk bulk bulk bulk
7 33 7 1 1 48 3 9 8 1 1 1 1 21 6
bimonthly
used to construct concentration isopleths from the array of interpolated data (32). The contour lines and map outline were stored in a longitudelatitude coordinate system and projected onto an x-y plotter. Ideally, the only data that should be compared are those for samples collected and analyzed by identical techniques. A variety of data sources, however, was used to generate a data base sufficiently large to make trends apparent and statistically significant. Most of the data were derived from wet-only precipitation samples collected on an event, weekly, or biweekly schedule, but some data were derived from bulk precipitation samples. The inclusion of dryfall in bulk samples yields higher concentrations of soilderived elements and lower H + concentrations than those measured in wet-only precipitation (33, 34).
83, 84 29 85 86 87, 88 89 24, 25 69 30 90 TVA, unpublished 91 92 55, 56 93 51 94 95 96 97
Analysis of variance indicates that the mean concentrations of K + , NH4+, Mg+2, Ca+2, SO4- 2 , and N 0 3 " in the bulk precipitation samples were significantly higher than in the wet-only samples (p < 0.01) for regions having exposed, calcareous soils. But the mean concentrations of Na + , Cl~, and H + were not significantly different for the two types of samples in coastal and forested areas and in regions where dustfall was not dominant. This distinction is not totally definitive because the sites for which bulk precipitation data were used were clustered; geographical differences in precipitation chemistry could be responsible for part of the observed differences between the two groups. For the ions that showed significant differences between bulk and wet-only data, isopleths were generated with and without the bulk data. For S O J - 2 , Environ. Sci. Technol., Vol. 17, No. 1, 1983
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NC>3~, and N H 4 + , the differences between the two sets of isopleths were minor; without including bulk precip itation data, concentrations differed only around Iowa by ~ 1 0 μ ε ς L _ 1 . However, maxima in C a + 2 and Mg+ 2 were evident only when the bulk pre cipitation data were included in the isopleths, and therefore wet-only data were used for these two ions. The inclusion of some data for col lection periods other than whole (or multiple) year(s) may be a source of bias because precipitation chemistry varies significantly from season to season. Different collection frequen cies (event, weekly, monthly), sampler
34A
designs, and analytical procedures may also introduce biases in the data set. Inspecting the isopleths shows, how ever, that the computing routine is not strongly influenced by individual data that are in disagreement with regional data trends. Furthermore, the magni tude of the concentration trends ob served is greater than could be credibly accounted for by sampling and ana lytical errors (35). The existence of large regions with generally uniform concentrations further supports the validity of these data. One additional difficulty is that few data on precipitation chemistry were available for some regions of North
America (Figure 1, Table 1). More data should be obtained for the Gulf Coast-Lower Mississippi valley and for the plains and Rocky Mountain states in the U.S. Lewis and Grant (36) and Lewis (37) recently pub lished information on precipitation chemistry at Como Creek, Colo., and several new N A D P sites have been established in some of these areas. Results The isopleth maps for C a 2 + , Mg 2 + , and K + all show midcontinental con centration maxima centered over the northern prairies with decreasing concentrations towards the coasts
FIGURE 1
FIGURE 2B
Distribution of precipitation sampling sites in North America used in constructing isopleths A Bulk samplers; " · wet-only samplers
Average concentrations of Mg''• in North American precipitation (μβα, L 1 )
Average concentrations of Ca 2 in North American precipitation (/u«q L"1)
Average concentrations of Κ in North American precipitation (/tieq L~')
Environ. Sci. Technol., Vol. 17, No. 1, 1983
FIGURE 2C
FIGURE 3A
FIGURE 3B
Average concentrations of Na in North American precipitation (μ-eq L~1>
Average concentrations of CI in North American precipitation (μβς L_1)
(Figures 2a-c). Calcium concentra tions range from ~ 1 0 0 μ ε ς L _ 1 in the prairies to
The isopleths that have been pre sented for Pb, Zn, and Fe are prelimi nary because insufficient data exist for most areas. Many of the studies in cluded in the data base were conducted at sites influenced by sources of an thropogenic metals. A network of background sites is required to con struct a complete picture of the re gional patterns of trace-metal con centrations and deposition. Further more, the sites where trace metals in precipitation have been studied are not well dispersed over the continent. Only around the Great Lakes have trace metals in precipitation been studied in
detail, and much of that data was not presented in a form that could be used for constructing concentration iso pleths (24, 50, 55-57). Discussion Major cations and anions in pre cipitation may be separated into three important groups based on projected sources derived from isopleth maps shown here and in the literature: 1) Ca 2 +, M g 2 + , K+; 2) Na+, Cl~; and 3) S 0 4 2 , N O r , N H 4 + , and H+. The sources of these ions are wind-blown soil, dust, sea spray, and natural and anthropogenic emissions of precursor
gases and elements. In their geographical distribution in the precipitation over North America C a 2 + and M g 2 + show a striking simi larity (r = 0.83;/?
