Environ. Sci. Technol. 1888, 22, 41-46
sional (lateral) interaction (attractive or repulsive) of the adsorbate at the surface. As Table I illustrates, this coefficient a tends to be positive (0.6-2) for the free acids and negative (-1 to -1.5) for their conjugate bases. The repulsive interaction observed for carboxylates reflects the repulsion of the equally charged hydrophilic groups. The excess of counterions in the outer adsorption layer leads to a loss of entropy. The attraction observed for the free acids may be caused by hydrogen bonding between the carboxylic groups and by van der Waals attraction between the hydrocarbon chains. As seen in Figure 4, the adsorption of lauric acid ((212) is slow because of slow transport (diffusion) at concentrations smaller than lo4 M. In case of sodium caprylate (C,) the attainment of equilibrium is delayed most probably by structural rearrangements at the Hg surface. In case of anions, such association reactions are slower (repulsive lateral interaction) than with free acids. Apparently the longer chain fatty acid anion (C1J becomes rearranged in the surface layer somewhat faster than the shorter anion ((3,). In the case of 6-A120, the adsorption of fatty acids results from coordinative interaction (Figure lb) of the carboxylic groups with the A1 ions, the Lewis centers, in the surface layer. Mass law considerations (eq 10) on the pH dependence of the adsorbens and the adsorbate predict a maximum extent of surface binding around a pH value close to 5-6, Le., close to the pK value of the fatty acids (12). Figure 7 illustrates a pH dependence in accordance with that predicted. The values of the log 8K values obtained experimentally are comparable with those observed for other carboxylic acids; e.g., Kummert and Stumm report a value for benzoic acid of log *K = 3.7 (11). While -AGeh for the Hg surface increases systematically with chain length, this is not the case for -AGed, on the
alumina surface. Plausibly, the adsorption energy due to coordinative interaction, among short-chain fatty acids is not dependent on chain length; with molecules such as caprylic acid (C,) and larger ones a free energy contribution due to the hydrophobic effect of the longer hydrocarbon moiety becomes preponderant. As has been shown before for carboxylic acids (11),more than one layer may become adsorbed, presumably because of hemimicelle formation.
Literature Cited (1) Bockris, J. O'M.; Devanathan, M. A. V.; Muller, K. Proc. R. SOC.London, A 1963,274, 55-79. (2) Bockris, J. G. M.; Reddy, A. K. N. Modern Electrochemistry; Plenum: New York, 1970; pp 758-759. (3) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1981; pp 506-516. (4) Hansen, R. E.; Minturn, R. E.; Hickson, D. A. J. Phys. Chem. 1966, 60, 1185; 1957,61, 953. (5) Payne, R. Adv. Electrochem. Electrochem. Eng. 1970, 7, 1-76. (6) KrznariE, Do; CosoviE, B.; Kozarac, Z. Mar. Chem. 1983,14, 17-29. (7) Batina, N.; RdiE, J.; CosoviE, B. J. Electroanal. Chem. 1985, 190,21-32. (8) RdiE, J.; Ulrich, H. J.; CosoviE, B., submitted for publication in J. Colloid Interface Sci. (9) Furrer, G.; Stumm, W. Geochim. Cosmochim. Acta 1986, 50, 1847-1860. (10) Trasatti, S. J. Electroanal. Chem. 1974,53, 335-363. (11) Kummert, R.; Stumm, W. J. Colloid Interface Sci. 1980, 75(2), 373-385. (12) Stumm, W.; Furrer, G.; Kunz, B. Croat. Chem. Acta 1983, 56(4), 593-611. Received for review February 19, 1986. Revised manuscript received November 3, 1986. Accepted September 7, 1987.
Effect of Storm Type on Rainwater Composition in Southeastern North Carolina Joan D. Wllley,' Ramona I. Bennett, Jeanne M. Wllllams, Robert K. Denne, Cynthia R. Kornegay, Mark S. Perlotto, and Beth M. Moore Department of Chemistry, University of North Carolina at Wilmington, Wilmington, North Carolina 28403-3297
a function of storm origin or type. During 1983-1987, the most acidic rain and highest sulfate and nitrate concentrations occurred in rain from local summer thunderstorms, followed by rain from continental frontal storms, with the least acidic rain coming from coastal storms. Seasonal variation was observed for rainwater pH (although not for sulfate or nitrate concentrations) from continental storms, with the most acidic rain in the summer. Thunderstorm nitrate concentrations were high enough to affect seasonal averages for nitrate concentration because thunderstorms are a warm-season type of rain. Coastal storm rainwater did not show seasonal changes; this type of rainwater is similar in pH, sulfate, and nitrate concentrations to rainwater in remote areas of the world. Sulfate from sea spray was a small percentage of the total sulfate except in coastal storm rainwater. Large annual differences in rainwater composition were observed.
information was then available about rainwater composition in this area, which ieceives precipitation from several types of storm systems. The study area includes the land-sea interface, which is a source of seawater, a basic and well-buffered solution, and salt marshes, which produce gases that eventually oxidize in air to become acids. There are, therefore, natural processes that could affect rainwater pH in opposite directions. This study area also includes the city of Wilmington, an industrialized urban area with a population of approximately 109000, the only coastal city in North Carolina. Southeastern North Carolina, including Wilmington, has enjoyed extensive industrial and population growth over the last decade, as has much of the southeastern US. This region as a whole may be experiencing changes in the composition of rainwater (1-41, with resulting changes in surface water composition (4-7), so the patterns of variation in rainwater composition are of interest.
Introduction In 1983, a project was initiated to study rainwater composition and variability in coastal North Carolina. Little
Methods Sampling. Rainwater was collected on an event basis throughout this study; this allows investigation of the
rn Rainwater composition in Wilmington, NC, varies as
0013-936X/88/0922-0041~01.50/0 0 1987 American Chemical Society
Environ. Sci. Technol., Vol. 22, No. 1, 1988 41
-+ -3 4' 00' N I
77" 30'W
-Lape Fear
Flgure 1. Sampllng locatlons In the Wiimlngton area, as follows: WB = Wrlghtsville Beach, CH = Channel Haven, BC = Bradley Creek, W = WilmingtOn, and RP = Rocky Point. The National Atmospheric Deposition Program Locations used for comparison are also shown, as follows: L = Lewiston, NC, and C = Clinton, NC.
composition of rainwater from individual storms. Rain events were defined by storm systems rather than by individual showers within a storm. Precipitation amounts were measured in rain gauges at or near each collection site. Open basins were used as collectors from 1983 to 1985, so samples during this time were bulk samples that included up to 24 h of dry deposition. Aerochem Metrics automatic wet-dry samplers were used since the beginning of 1986,so samples after this time were wet-only. Sampling efficiency range from 78% during 1983to better than 95% in 1986 and 1987. A study comparing these types of collectors showed that rainwater from large rain events was not affected by dry deposition during this short time interval, so the bulk and wet-only samples were very similar. However, very small rains were often higher in ionic concentrations in the bulk samples, although the pH was not significantly dffferent in the bulk versus wet-only sample even in small rains. The process of volume-weighted averaging minimizes the effect of small rains, so the averages used in this study should not be greatly affected by sampling procedure. Rainwater has been collected continuously since 1983 (with the exception of the first half of 1984) at the Wilmington location (Figure l),with data compiled through July 25,1987. Several other locations (Figure 1) have been monitored at various times for specific purposes. All sampling sites were located in open areas away from highways and at least 12 km away from industrial sources of air pollutants. Rainwater composition data were also obtained for comparison from the two most easterly North Carolina National Atmospheric Deposition Program sites (weekly samples) in Clinton (35O01.4' N and 78'16.8' W) and Lewiston (36'07.7' N and 77'10.5' W). Analytical Methods, All instrumentation for rainwater analysis, including the ion chromatograph, and both pH meters and electrodes have been used exclusively for the analysis of rainwater throughout this study. Rainwater 42
Environ. Sci. Technol., Vol. 22, No. 1, 1988
samples were analyzed for pH and sulfate, nitrate, and chloride concentrations during 1983, 1986, and 1987. Rainwater pH was analyzed during the summer and autumn of 1984 and throughout 1985 also. Sample pH was measured (usually within 24 h of collection) in a quiescent solution with a gel-filled combination electrode standardized with pH 4 and 7 Fisher Certified buffers (8). This procedure may yield slightly low results for rain samples that have pH above 5 (9). Beginning in the autumn of 1985,rainwater pH was also analyzed with a Ross electrode calibrated with two Orion low ionic strength buffers and with the addition of pHix (Orion) to samples to avoid liquid junction potentials, which result from the different ionic strengths of buffers and samples (10, 11). In this analysis, all solutions were stirred. The two methods usually agreed to within 0.05 pH unit. The electrodes were stored in a pH 4 solution of sulfuric acid. The coefficient of variation of pH measurements was 2.3%; analyses of three EPA artificial rainwater samples gave results within 0.05 pH units of the expected values. Since 1986, all chloride, nitrate, and sulfate analyses were performed by ion chromatography (8,12). Samples were stored refrigerated for up to 1month before analysis; because of the low pH of most of the samples, storage should have little effect on composition for the ions measured (13). Analyses of EPA synthetic rainwater solutions for these ions gave relative standard deviations of less than 6% for solutions similar to North Carolina rainwater, with recoveries of between 97 and 106% of the stated concentrations. Prior to 1986, chloride and nitrate concentrations were determined by specific ion electrodes following low concentration procedures. The relative standard deviation for the chloride analysis was 10% and for nitrate 5%, with synthetic rainwater solutions. Before 1986, sulfate was determined gravimetrically by precipitation as barium sulfate. Standard sulfate solutions were analyzed; the relative standard deviation was 7%. Eight rainwater samples were analyzed for sulfate concentration by both methods; the correlation coefficient between the results of the two methods was 0.983, the slope was 0.98, and the intercept was 0.40 pM, which shows good agreement between the data obtained by these methods. During the winter of 1986, samples were analyzed for Na+, K', and NH4+by ion chromatography and for Ca2+and Mg2+by atomic absorption spectrophotometry. These analytical results were used with others in ion balance calculations; the only samples that did not meet EPA criteria (14) were those high in sea salt. Non-sea-salt sulfate (NSS) concentration, which is an estimate of the sulfate in rainwater that does not come directly from sea salt, was calculated with the rainwater sulfate and chloride concentrations by assuming that most of the chloride in coastal rainwater comes from sea spray (15,16) and that sulfate occurs in a constant ratio to chloride in seawater (0.052:l on a molar basis). Beck et al. (17)report that the ratios of major seawater ions in coastal plain rivers in the southeastern U.S. are the same as in seawater, indicating no change in the chloride to sulfate ratio during the transition from seawater to rain to riverwater. Data Compilation. All concentration averages are reported as volume-weighted averages, and all standard deviation values related to concentrations are also volume weighted (14). Average pH values were computed from volume-weighted hydrogen ion concentrations (14). Precipitation amounts and the associated standard deviations are simple averages. For most of the seasonal data compilations, the winter and summer seasons (with seasons as defined by the calendar) were used because these represent
the extremes in this geographical area. Spring and autumn are highly variable with respect to the weather and also often have extended times of little precipitation (18). In the compilation of data according to storm type or origin, storms were grouped according to their patterns of movement and development for several days prior to the local rain event. The process of definition of storm types was verified during the GALE (Genesis of Atlantic Low Experiment) Project in the winter of 1986, when a large meteorologicalstudy was conducted in this study area, and much supporting weather data were obtained (19). Local thunderstorms are defined in this study as thunderstorms that were not associated with larger weather systems.
Results Storm Origin Effect. Storm origin or type is an important factor in determining rainwater composition in southeastern North Carolina. Three types of storms were defined in this study. Continental frontal systems (including warm, cold, and stationary) move in from the west or northwest and travel across the continental U.S.,and the associated air masses therefore may entrain pollutants. Coastal storm systems originate in the Gulf of Mexico or in the tropical Atlantic. Coastal storms include but are not limited to hurricanes and tropical storms. Coastal storms move toward the north or northeast along the coast. The air masses associated with these storms usually do not have as much contact with pollutants. Sampling of one continental and two marine air masses near Long Beach, CA, in August of 1977 indicated that the continental air masses had more non-sea-salt sulfate than did the marine air masses; attempts to conduct similar sampling near Morehead City, NC, were hindered by poor resolution of the air mass types during the 14-day sampling time (20). In the summertime in southeastern North Carolina, local thunderstorms (isolated convective storms) provide rain that is variable in amount and spatial distribution. Local thunderstorms do not have a defined storm track, so there is no clear way to determine a pollutant trajectory for this type of rain (21). All storms do not fit neatly into one of these three classifications, and sometimes rain events occurred in such rapid succession that the rain samples collected were from more than one event. Because of this, approximately one-fourth of the rain samples were excluded from classification. Rainwater composition data from Wilmington, NC, were grouped according to storm origin and by season, and the resulta are shown in Table I. Both the winter and summer data show that rain from continental storm events has higher hydrogen ion concentration (lower pH), higher sulfate concentration, and higher nitrate concentration than rain from coastal storms. Raynor and Hayes (22) found the highest sulfate concentrations in cold front rain on Long Island compared with rain from other types of storms, however they did not see very low total sulfate concentrations in hurricane rain; the portion of sulfate that came from sea salt was not specified in their study. In this study, the percentage of sulfate that came from sea spray was lower in rain from continental storms than coastal storms (Table I). Chloride concentrations were higher in coastal storms, although quite variable in both cases (Table I). In each of the storm types and during both seasons, the equivalent concentration of non-sea-salt sulfate is approximately twice the equivalent concentration of nitrate, which is consistent with rainwater for most of the eastern U.S. (15, 23-25). The most extreme example of a coastal storm during this study was Hurricane Diana, which brought approximately 37 cm of rain and winds in excess of 50 m/s to the Wil-
Table I. Storm Type and Seasonal Variation in Rainwater Comoosition in Wilmington, NC, for Sampling Times from 1983 to 1987 as Described in the Text"
continental thunderstorms winter summer winter summer (summer) [H+] SD n PH
30.5 4.0 31 4.52
100.4 16.3 39 4.00
13.3 1.9 16 4.88
16.0 15.1 11 4.80
103.5 15.7 18 3.99
[NSS] SD
23.2 2.6 19 5.1
20.9 3.3 20 6.9
6.8 1.9 10 30.3
12.9 4.9 8 15.8
32.4 4.0 16 2.9
17.4 3.8 15
16.9 3.3 19
4.5 1.0 8
4.4 0.9 7
32.5 3.8 15
24.0 7.2 19
29.9 6.7 20
57.1 12.2 10
46.5 17.0 8
18.5 3.3 16
10.4 10.5 31
19.1 19.2 39
34.1 27.4 16
26.5 30.4
9.8 8.9 18
n % SS
[NO,-] SD n [Cl-] SD n
P SD n
11
All concentrations and associated standard deviations (SD) are volume-weighted and in micromoles per liter. n = number of samples, NSS = non-sea-salt sulfate, and % SS = percentage of sulfate that comes from sea spray. P = simple average amount of precipitation in millimeters along with simple standard deviation of this number.
mington sampling site. Hurricane Diana made landfall 25 km south of Wilmington on September 12,1984. Rainwater during this storm had a 2.5 NM hydrogen ion concentration (pH 5.60). Salt deposition, assuming that all the chloride occurred as NaC1, was 12.5 g of NaCl/m2 at the Bradley Creek marsh site (3 km from the beach) and 4.1 g of NaCl/m2 at the Wilmington site (7 km inland). Man-made emissions of sulfur dioxide and nitrogen oxides in eastern North America exceed natural emissions by at least a factor of 10 (26,27). Nevertheless, the coastal storm rainwater received in Wilmington, NC, which is not a remote site, had pH values (Table I) within the pH range of 4.7-5.8 reported for truly remote areas of the world (1, 28-32) and within the theoretical pH range 4.5-5.6 calculated by Charlson and Rodhe (33)for uncontaminated rain. Wilmington coastal rain was similar to remote rain in nitrate concentrations, but slightly higher in non-sea-salt sulfate concentrations (1,29,32). These data indicate that the coastal storm air masses that bring rain to Wilmington do not entrain much continental air pollution as they move along the southeast US. coast. Rain from summer thunderstorms in Wilmington was acidic, with relatively high sulfate and nitrate concentrations (Table I). High nitrate and sulfate concentrations have been reported for thunderstorm rain in eastern North Carolina and Virginia (34,35);the rainwater data apparently must be compiled on an event basis in order for this to be apparent (36). Lightning was observed to be a principal source of nitrogen oxides during a study of one thunderstorm on June 15,1985, near the Oklahoma-Arkansas border (37). Thunderstorms can also transport ground-level pollutants high up into the clouds by convection, where they encounter higher concentrations of ozone (37), along with moisture, intense sunlight, and turbulent mixing, all of which may contribute to acid formation. Significant production of both sulfuric and nitric acids has been reported in warm frontal clouds, along with subsequent removal in rain (38). Clearly more studies of individual storms are needed in order to interpret this Environ. Sci. Tschnol., Vol. 22, No. 1, 1988
43
Table 11. Volume-Weighted Non-Sea-Salt Sulfate Concentration (NSS) and Standard Deviations (SD) in Micromoles per Liter for Rainwater from the Two Sites Indicated
Table 111. Volume-Weighted Average Concentrations and Standard Deviations (SD) in Micromoles per Liter for Rainwater from Wilmington, NC, and the Surrounding Area for 1983a
winter
summer 1983 1986 1983 1986 [NSS] SD [NSS] SD [NSS] SD [NSS] SD Wilmington Bradley Creek
24.5 24.1
2.7 6.6
9.3 10.0
2.6 3.2
21.7 41.3
3.9 6.5
18.8 3.4 21.4 3.7
'Number of samples in each data set was between 6 and 29. There were 18 samples from the Bradley Creek site during the summer of 1983. During the winter of 1986,43% of the rain came from a single rain event that was very dilute.
""1
I
1984
\ '\
75/
e-- S E A S ON->
Flgure 2. Volume-weighted seasonal average hydrogen Ion concentratlons converted to pH values for rainwater from the Wilmington, NC, sampling site for the years indicated.
type of rainwater composition data with confidence. Seasonal Variation. Thunderstorms in this geographical location occur primarily during the warm seasons, and thunderstorm rainwater is chemically different than rainwater from other storms. This storm variation contributes to the seasonal variation in rainwater composition by adding a more acidic and higher nitrate concentration rain in the warm seasons only. During 1986 at the Wilmington site, winter rain had 8.7 pM nitrate (SD = 3.3 pM),and in the summer this was 21.1 pM (SD = 3.3 pM) for 18 and 26 samples, respectively, grouping all storm types together. No simple seasonal variation was observed for non-sea-salt sulfate concentrations when grouped this way (Table 11). Rainwater was more acidic in the summer (Figure 2), which results in part from the thunderstorm rain effect; the hydrogen ion concentration in continental storm rainwater was also greater in the summer. The concentrations of sulfate and nitrate did not change seasonally for continental storm rain in this same data set (Table I). Summer rainwater from continental storms had excess hydrogen ion concentration on the average compared with the sum of sulfate plus nitrate equivalent concentrations, which suggests the presence of an acid other than sulfuric or nitric in this type of rainwater. Approximately 20% of summer rainwater samples collected on Long Island, NY, during 1983-1985 also indicated the presence of an acid other than sulfuric and nitric; this was observed only during the summer (39). Coastal storm rainwater in Wilmington during this study was slightly 44
Environ. Sci. Technol., Vol. 22, No. 1, 1988
[H+] SD Wilmington Rocky Point Bradley Creek Channel Haven Wrightsville Beach Clinton Lewiston
70.6 79.6 92.2 94.7 42.5 25.0 36.4
14.7 12.9 15.9 16.0 12.0 4.3 5.5
[NSS] SD [NO,] 23.9 30.3 26.7 28.8 27.1 16.3 21.5
2.4 3.6 3.3 3.8 3.8 2.2
2.7
12.8 12.6 4.7 9.2 16.1 12.6 18.5
SD
7.8 5.8 0.9 2.0 6.3 1.8
2.6
"NSS = non-sea-salt sulfate; sulfate data from Clinton and Lewiston are total sulfate because these locations are not coastal. All data are annual averages except for the nitrate concentration data from the Wilmington locations, which are summer and autumn data onlv.
salty water that did not vary on a seasonal basis (Table 1). Seasonal variation in rainwater composition apparently has a geographical dependence in eastern North America (40). The higher nitrate in the summer rain is most pronounced in the southeast compared with the northeast or midwest US.(41) and has also been reported for Florida rain (42). These southern regions have much thunderstorm acitivity and little snow. Snow is a factor in seasonal variability in some areas (43)because snow tends to have a higher nitrate concentration than rainwater (44, 45), which would make the winter nitrate average higher. Snow was not a factor in this study because only trace amounts were received, which is the normal pattern for southeastern North Carolina. Several areas report higher sulfate concentrations and lower pH in summer rain (1,46). Reasons suggested for this seasonal variation include more stagnant air in the summer; smaller, more concentrated rains in the summer; and more extensive formation of acids during the warm seasons both from natural and man-made sources. Annual Variations. Figure 2 shows the lower pH values in rainwater received in Wilmington during the summer and autumn of 1983 compared with the other seasons in 1983 and in subsequent years. Table I11 shows rainwater compositionaldata for several sampling locations in the Wilmington area during 1983. The most acidic rain fell on the salt marsh locations (Bradley Creek and Channel Haven), however the whole Wilmington region had rain more acidic and higher in non-sea-salt sulfate than predicted on the basis of continental scale studies (23-25) or on the basis of comparison with other nearby North Carolina National Atmospheric Deposition Program locations at Clinton or Lewiston (Figure 1 and Table 111). Several factors may contribute to this annual variation in Wilmington rainwater; these include differences in the number of tropical storms or hurricanes each year and variation in salt marsh activity. On the basis of fuel consumption data supplied by the major sulfur dioxide producing industries within 150 km of Wilmington, industrial output of sulfur dioxide did not change enough between 1983 and 1984 to account for the observed rainwater differences. The year 1983was anomalous for Atlantic named storms (tropical storms plus hurricanes) because there were very few [4 versus an average of 10 (47)], and none of them brought rainwater to Wilmington. The Wilmington area received rain from three named storms in 1984, six in 1985, and two in 1986. The small number of named storms in 1983 and the lack of rainwater from any of these storms may have contributed to the acidity of the rainwater received during the summer of that year. Large coastal
storms affect rainwater composition by providing good dispersion of pollutants and by giving large quantities of uncontaminated rain. The two Wilmington area salt marsh sites (Bradley Creek and Channel Haven) had the most acidic summer averages of the sampling sites (pH 3.82 and 3.78, respectively) and the highest non-sea-salt sulfate concentrations (41.3 and 36.9 wM, respectively) during the summer of 1983. This suggests some salt marsh effect on local rainwater composition during the summer of 1983. This difference between salt marsh and nonmarsh locations was not observed in 1986 (Table 11).
Cone lusions (1) Storm origin is an important factor in determining rainwater composition in southeastern North Carolina. (2) The most acidic rain comes in the summer from local thunderstorms and continental frontal storms. (3) Coastal storms bring rainwater similar in composition to rainwater in remote areas of the world. (4) With the exception of coastal storm rainwater, only a small percentage of the sulfate in rainwater in southeastern North Carolina comes from sea spray. ( 5 ) Rainwater in this area is highly variable. Proximity to the beach and to salt marsh areas affects rainwater composition. Large annual variations occur and may result from differences in storm patterns. Acknowledgments
The assistance during the winter of 1986 of M. E. Bowers, D. C. Coleman, B. J. Devereux, and T. V. Sams is appreciated. Registry No. H+, 12408-02-5.
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Glass, G. E.; Loucks, 0. L. Environ. Sci. Technol. 1985, 20(1), 35-43. Bowersox, V. C.; De Pena, R. G. J.Geophys. Res. C: Oceans Atmos. 1980,85(ClO), 5614-5620. Summers, P. W.; Bowersox, V. C.; Stensland,G. J. Water, Air, Soil Pollut. 1986, 31, 523-535. Lindberg, S. E. Atmos. Environ. 1982, 16(7), 1701-1709.
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Received for review March 30,1987. Revised manuscript received August 4, 1987. Accepted September 9, 1987. This work was supported i n part by NSF Grant ATM-8512537.
A Composite Receptor Method Applied to Philadelphia Aerosol Thomas G. Dzubay" and Robert K. Stevens Atmospheric Sclences Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 2771 1
Glen E. Gordon, Ilhan Olmez,+and Ann E. Sheffleld Department of Chemistry and Biochemistry, Universlty of Maryland, College Park, Maryland 20742
Willlam J. Courtney$ Northrop Services Inc., Research Triangle Park, North Carolina 27709
A composite of chemical mass balances, multiple linear regression, and wind trajectory receptor models was developed to apportion particulate mass into source categories. It was applied to 156 aerosol samples collected in dichotomous samplers at three sites in the Philadelphia area and analyzed by X-ray fluorescence, instrumental neutron activation, ion chromatography, and pyrolysis. The largest component accounted for 49-55% of the mass of 110 hm diameter particles and consisted of sulfate plus related ions and water. Other components were crustal matter (17-24% of the mass) and vehicle exhaust (4-6% of the mass). Less than 5% of the mass was attributed to primary emissions from five types of stationary sources. Wind-stratified data indicated that 80 f 20% of the sulfate was from a regional background. Multiple linear regression attributed 72 f 8 and 16 f 5 % of S to coal- and oil-fired power plants, respectively. Introduction
Receptor models are used to resolve the composition of atmospheric aerosol into components related to emission sources (1-3). When the chemical mass balance (CMB) receptor model is applied to element concentrations measured by X-ray fluorescence (XRF), four to eight components can typically be resolved on the basis of known chemical signatures (4). A difficulty is that the required signatures are not accurately known for two major components: sulfate and vehicle exhaust. For the former, there is an uncertain amount of water associated with sulfate in particles. For vehicle exhaust, the Pb abundance is difficult to specify in the U.S. because measurements are sparse, and the P b content in gasoline has been declining rapidly during recent years. Multiple linear regression (MLR) has been used to determine the P b abundance when Pb is a unique tracer for vehicle exhaust, but as this report demonstrates, motor vehicles are not the only significant source of Pb. We explore the feasibility of overcoming such problems by using a composite receptor method (combined use of ~~
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t Present address: Nuclear Reactor Laboratory, Massachusetts
Institute of Technology, Cambridge, MA 02139. t Present address: International Business Machines Corp., Dallas, TX 75234. 46
Environ. Sci. Technol., Vol. 22, No. 1, 1988
several receptor models). I t was tested with data from a field study designed according to recommendations from the Quail Roost I1 Conference ( 5 ) and conducted as part of a larger study to evaluate dispersion models (6, 7). Ambient aerosol was collected in the PM-10 size range (particle diameter 110 pm) at three sites in the Philadelphia area during summer 1982. Source emissions were collected by a dilution-cooling technique at two oil-fired power plants, a coal-fired power plant, a municipal incinerator, an Sb ore roaster, a fluidized catalytic cracker at a refinery, and a secondary Al smelter (8). Surface soil and dust were collected at 30 sites and suspended by an aerosol generator (9). Ambient, source, and soil samples were collected by dichotomous samplers and analyzed by XRF, instrumental neutron activation analysis (INAA), ion chromatography (IC), and pyrolysis with the exception that INAA was not applied to the soil samples. Here we report the ambient aerosol measurements and illustrate the precision and accuracy of concentrations by comparing results of different analytical methods. We present a mass apportionment based on the following steps: (a) Wind-trajectory analysis (IO) was used to identify sources to include in CMB calculations. (b) A new composite method described below used a preliminary CMB to derive a set of adjusted Pb and S concentrations that were then used in MLR to determine abundances of Pb in vehicle exhaust and S in the sulfate component. (c) Final CMBs, based on results of the composite method, were used to apportion mass into nine components. (d) Wind direction stratification was used to detect the relative influences of local and regional sources. (e) MLR of S vs Se and V was used to estimate the particulate S contributions from coal and oil burning. Measurements
Aerosol samples were collected nearly continuously at the three sites shown in Figure 1 for 12-h periods during the day (0600-1800 EDT) and the night (1800-0600 EDT) between July 14 and August 13, 1982. Site 28, at the Institute for Medical Research in Camden, NJ, was in an industrial, commercial area within 10 km of several large emission sources (Figure 2) and was within 400-500 m of several major roads. Site 12, at Northeast Airport, was in a residential area of Philadelphia with isolated light in-
0013-936X/88/0922-0046$01.50/0
0 1987 American Chemical Society
