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Observations of aerosol chemical composition and acidity in northwest and southeast regions of the United States
The Science of the Total Environment, 39 (1984) 125--133 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
125
OBSERVATIONS OF AEROSOL CHEMICAL COMPOSITION A N D ACIDITY IN NORTHWEST A N D SOUTHEAST REGIONS OF THE UNITED STATES
A.J. ALKEZWEENY and K.M. BUSNESS
Pacific Northwest Laboratory, Richland, WA 99352 (U.S.A.) (Received January 22nd, 1984; accepted March 2nd, 1984)
ABSTRACT Aircraft measurements of atmospheric aerosols and selected trace gases were performed in remote northwestern and rural southeastern regions of the continental United States. In the Northwest, the average concentrations of major constituents were sulfate, 0.15/~g/ m3; nitrate, 0.05/~g/m3; ammonium, 0.05/~g/m3; and ozone, 33 ppb. The average cloud water pH (of clouds that may form over the region) was calculated to be 4.74; 0.5 g/m 3 liquid water content was assumed. Trace metal concentrations were also measured and used in the discussion as pollutant source tracers. It was concluded that the data from the Northwest may be considered representative of background concentrations. In the Southeast, the average concentrations of major constituents were sulfate, 5.20 #g/m 3; nitrate, 0.25 pg/m 3; ammonium, 0.90 pg/m 3; ozone, 63 ppb; and pH 4.78. These large differences in each species concentration reflect the impact of anthropogenic sources in the rural southeastern region of the United States.
INTRODUCTION
During the summer of 1982, aerosol and ozone data were collected over specific northwestern and southeastern regions of the United States {Fig. 1). Detailed chemical analyses were performed and correlations between measured parameters were calculated to infer characteristics of significantly differing ambient air masses over these regions. The northwestern observations were made in remote areas that are sparsely populated and removed from significant pollution sources. These were rural farm, forested, and semi-pristine areas in Oregon, Washington, Idaho and Montana. These measurements, as will be discussed later, m a y be considered representative of background concentrations. On the other hand, observations made in the Southeast, again over rural farmland and forest areas and over water off the Atlantic Coast, can be influenced by local or long-range transport of pollutants originating from upwind sources. The reported Northwest sampling occurred during June and August 1982. The region was dominated b y high pressure off the West Coast, resulting in 0048-9697/84/$03.00
© 1984 Elsevier Science Publishers B.V.
126
NORTHWEST REGION
SOUTHEAST REGION
Fig. 1. Geographic identification of sampling regions.
clear to partly cloudy skies with light winds, moderate temperatures, low humidities, and excellent visibility. Synoptic conditions for the southeastern region during the sampling period in September 1982 were characterized by high pressure in the Northeastern region with resultant northerly flow into the sampling region. Local weather included clear to partly cloudy skies with poor visibility due to haze, high temperatures and humidities.
SAMPLING AND ANALYSIS PROCEDURES
Measurements and samples were taken within the boundary layer at elevations ranging from 100--1000 m above ground level, using the Pacific Northwest Laboratory (PNL) DC-3 aircraft (Busness et al., 1982). Generally, sampling was conducted from late morning through mid to late afternoon when the mixed layer was well established. Multiple aerosol samples were collected simultaneously on 3-pm pore-size teflon filters mounted in parallel high-volume air samplers equipped with flowmeters that continuously recorded flow rates and total volume flow. Sample volumes ranged from about 20--50 m 3. One group of filters was analyzed for trace metal content by X-ray fluorescence techniques. The companion set of filters was leached in double-distilled water to remove deposited aerosols; the pH of the resultant solutions was measured and concentrations of sulfate, nitrate and ammonium ions were determined by ion-chromatography in the PNL Precipitation Chemistry Laboratory.
127 TABLE 1 RANGES AND AVERAGE CONCENTRATIONS OF MAJOR PARAMETERS Constituent
Number of Samples
Concentration Range
Average
S.D.
Northwest
SO~- (pg/m 3) NO3+(,ug/m3) NH4 (pg/m °) 03 (ppb) Equiv. pH
9 9 9 9 6
0.04--0.24 0.01--0.22 0.01--0.14 28--40 4.59--5.03
0.15 0.05 0.05 33 4.74
0.06 0.07 0.04 4 0.18
0.45--19.82 0.03--0.84 0.07--3.53 45--90 3.75--6.52
5.20 0.25 0.90 63 4.78
4.48 0.25 0.64 -0.84
S o u theast
SO~- (pg/m 3) SO~ (pg/m 3) NH4 (pg/m 3) 03 (ppb) Equiv. pH
22 16 17 22 16
RESULTS AND DISCUSSION
Ranges and average concentrations of SO~', NO~, and NI-Fa ions and ozone for the two regions are shown in Table 1. The average ozone concentrations are time averages over corresponding filter-sampling times, followed by the usual averaging over the number of observations. The table includes equivalent pH, which is defined as the pH of water in clouds t h a t may be formed over a region characterized by aerosols of similar chemical composition; the calculation is based on an assumed liquid water c o n t e n t of 0.5 g/m 3. Equivalent pH values were calculated from the H + concentrations in the leached filter aerosol solutions as determined from laboratory pH measurements. It should be noted that if nitric acid were present during sampling, the equivalent pH will be lower than what we calculated. The reasons are that nitric acid is highly soluble in water and will be incorporated in clouds during their formation. Secondly, nitric acid did n o t contribute to our H ÷ concentration measurement because teflon filters do n o t capture HNO3. The concentrations of sulfate from the Northwest observations, in the range of 0.04--0.24/ag/m 3, are among the lowest values reported in the literature. For example, Alkezweeny and Laulainen (1981) measured values of 0.17--0.87 pg/m 3 in air masses over Lake Michigan. The air masses were very clean as evidenced by very low aerosol light scattering (bseat : 0.05-0.09 × 10-4m - 1). Hoffer et al. (1979) found the sulfate concentration as low as 0.09 pg/m 3 in the southwestern desert region of the United States. In central Antarctica, Cunningham and Zoller (1981) reported average values of 0.09 and 0.23 pg/m 3 for winter and summer, respectively. Nitrate values of
128
0.01--0.22 pg/m 3 for the Northwest are also among the lowest reported for background or remote area aerosols. McMuUen et al. (1970) analyzed National Air Surveillance Network samples taken at remote areas of the United States and obtained a value of 0 . 4 6 p g / m 3 for nitrate ion. Other values are 0.08--0.51 pg/m 3 found over Lake Michigan in clean air masses (Alkezweeny and Laulainen, 1981; Alkezweeny et al., 1982). Similar values were measured by Huebert and Lazrus (1980) in boundary-layer air over remote continental areas of North America. Possible reasons for the low nitzate values are losses of NH4NO3 by volatilization or the reaction with sulfuric acid, leading to the liberation of nitric acid gas (Appel and Tokiwa, 1981; Shaw et al., 1982) that will pass through the teflon filter. Northwest ammonium ion concentrations are in the range of 0.01-0.14~g/m 3, lower than the values reported b y McMuUen et al. (1970) for remote areas of the United States. Ozone concentrations of 28--40 p p b in the Northwest, are typical of values for clean air masses. For example, Kelly et al. (1982) found an average value of 41 p p b in remote areas in South Dakota. Alkezweeny and Laulainen (1981) measured ozone levels of 26 and 30 ppb over Lake Michigan. An interesting feature of the Northwest data is the aerosol acidity as represented by the equivalent pH described earlier. The calculated pH's are in the range of 4.59 to 5.03 (Table 1). These values suggest that clouds which form in clean air masses may have cloud water acidity comparable to that found in precipitation in industrial regions. It should be noted that 8
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5
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I 6
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10
5
r 1
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I 2
SO,(/~rnole/m3xlO -s)
Fig. 2. (a)Ammonium ion versus sulfate and nitrate plot for the northwestern sampling
region. (b) Hydrogen ion versus sulfate plot for the northwestern sampling region.
129
Galloway et al. (1982) and Sequeira (1982) report pH values in the range of 3.7 to 7.2 for precipitation in remote and unpolluted areas of the world. If background acidity values like these are frequently found, this fact should be considered in determining the impact of industrial emissions on acid deposition. To speculate on the origin and causes of aerosol acidity, we have performed a linear regression analysis of the relationship between [NH~] and 2[SO~-] + [NO~] (molar units) and found a modest correlation (correlation coefficient = 0.60). The slope of the fit {Fig. 2a) is very close to 1.0, indicating that sufficient ammonia existed to neutralize sulfuric and nitric acids. Thus, in this situation, sulfate ions do not appear to contribute to aerosol acidity. This conclusion is supported by the lack of correlation between H + and SO~- concentrations (Fig. 2b). This suggests that other acidic substances such as organics may be responsible for the resultant low pH. In contrast, the slope for the southeastern region data (Fig. 3a) is 0.56, an indication of excess nitrate and/or sulfate ions over ammonium ions. Furthermore, very strong correlation, R = 0.92, is found between H ÷ and SO~- (Fig. 3b). This high correlation suggests that sulfate is the major contributor to aerosol acidity in the Southeast. The range of sulfate concentrations in the southeastern region is 0.45-19.82/~g/m a, which is more than an order of magnitude greater than the range in the Northwest. High sulfate values of similar magnitude, about 20#g/m a, have been measured by us in rural areas of the Northeast when meteorological conditions were favorable for long-range transport. It is interesting to note that Altshuller (1973) examined the SO: and SO~-results ).05
I
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).04
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/..-
o 50
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100
150
2[S04]+[NO,](/lmole/m 'xlO-')
200
0.00
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0.05
I
0.10
0.15
SO,(~mole/m')
Fig. 3. (a) A m m o n i u m ion versus sulfate and nitrate plot for the southeastern sampling region. (b) Hydrogen ion versus sulfate plot for the southeastern sampling region.
130 025
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0.15
::L
r~-0.42
o" U"}
0.10
0.05
/
/ II 0 0,00 25
=
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35
40
03(ppb)
0 45
40
I
I
I
i
I
50
60
70
80
90
100
O,(ppb)
Fig. 4. (a) Sulfate versus ozone plot for the northwestern sampling region. (b) Sulfate versus ozone plot for the southeastern sampling region.
from the U.S. National Air Surveillance Network and concluded that the regional background due to transport of urban SO: and SO~- is 5.0 pg/m a. It is possible that the southeastern region was under the influence of long-range transport, as reflected by the wide range in concentrations of ionic species, ozone, and equivalent pH. Since ozone is formed b y a photochemical reaction, a correlation of ozone with sulfate may lead to interesting speculation a b o u t the origin of the measured sulfate. For the Northwest, such a correlation is insignificant (Fig. 4a). If the sulfate formation by photochemical reaction is ruled out, then it may have been either directly emitted or the result of aqueous sulfur oxidation in nonprecipitating clouds. This kind of cloud is very c o m m o n in this region during the summertime. Furthermore, high-sulfate formation in such clouds has been indicated by the measurements of Alkezweeny and Hales (1981) made in the vicinity of fair weather cumulus clouds. On the other hand, the calculated correlation between ozone and sulfate from the southeastern region data is 0.55 (Fig. 4b). Alkezweeny and Berkowitz (1981) found a similar value for data collected over Lake Michigan in air masses of different origins. However, when these data were classified as being of urban and rural origin, they found the correlation coefficient to be 0.90 for urban air and close to zero for rural air. The rural air, moving over Lake Michigan, more likely traversed clean, remote regions which were not significantly influenced by pollutant sources. Therefore, this modest correlation in the southeastern data may be the result of anthropogenic and background sources.
131
TABLE 2 AVERAGE
R E G I O N A L E L E M E N T A L C O M P O S I T I O N (ng/m 3)
Element
Northwest
Southeast
A1 Si S C1 K Ca Ti V Cr Mn Fe Ni Cu Zn Br Pb Sa Na a C1 a
0 -+ 1 3 2 1 2 1 + 57 1 1 7 + 15 0+8 14+3 27 + 3 5 -+2 0-+0.5 0 1 48 + 2 0 0-+2 2-+1 0 0-+1 5 2 -+ 9 0 0
16 298 2081 260 55 64 17 2 8 4 109 7
Ka
NH4 a
0
42
+ 16 + 274 + 1548 + 283 + 28 -+ 3 3 + 14 + 1 + 21 + 5 + 99 + 12
8+3 2+1 1 5 + 11 2069 + 1712 1 7 5 -+ 2 1 4 115 + 171 15 + 3 0 1227 + 940
aConcentrationsacquiredvia ion-chromatography.
Mass ratios of sulfate to nitrate are quite different in the two regions, the Northwest ratio of a b o u t 3 is substantially smaller than the ratio of 21 in the Southeast. It is possible that our values for nitrate, at least in the Southeast, are underestimated because of ammonium nitrate losses from the filter as discussed earlier. Our nitrate of 0 . 0 3 - - 0 . 8 4 # g / m 3 for the Southeast are much lower than the values reported by Shaw et al. (1982) (0.6--1.2 #g/m 3 ) for surface samples obtained in rural areas in North Carolina. The results of X-ray fluorescence analyses of filter samples acquired in the two regions are shown in Table 2. The uncertainty shown for the Northwest data (a small data set) is the c o m p o u n d error of both analytical results and sampling. Standard deviations are indicated for the larger Southeast data set. The Northwest filters were exposed in eastern Oregon. The lower section of Table 2 shows the results of analyses of companion filters by ion-chromatography, indicating the water-soluble portion of the aerosol collections. If we consider the trace metals as tracers for pollutant sources, it appears that the Northwest region has n o t been significantly impacted b y anthropogenic sources during the reported sampling period. For example, concentrations of Cr, Ti, V and Ni are very close to natural background levels, suggesting negligible impact from fossil-fueled power plants. Similarly, we may neglect
132 the contribution of automobile emissions because of the very low values of Pb and Ni. Undetectable levels of Na and C1 suggest that the air masses that were sampled were of continental origin with little or no marine component. Therefore, these data may represent background continental aerosol characteristics. An interesting feature of the Northwest aerosol filter analyses is that sulfur concentrations determined by X-ray fluorescence are higher than ionchromatography results. This may indicate that non-water-soluble sulfur may be the major c o m p o n e n t of total sulfur in background air. Since the number of samples is small, however, this may be an untenable conclusion. The situation in the Southeast is quite different. The wide ranges in concentrations of trace metals reflects the varying contributions of anthropogenic sources to the sampled region. It should also be noted that, on the average, all the sulfur measured in the Southeast is water soluble; average concentrations of 2 0 8 1 n g / m 3 and 2 0 6 9 n g / m 3 were determined by X-ray fluorescence and ion-chromatography, respectively.
CONCLUSION Aircraft measurements of atmospheric constituents were performed in remote Northwest and rural Southeast regions of the United States for the purpose of evaluating behavior of air masses having widely different origins. In the Northwest, the ranges of concentrations of sulfate, nitrate, ammonium, and ozolle were: 0.04--0.24pg/m 3, 0.01--0.22#g/m 3, 0.01-0.14 pg/m 3, and 28--40 ppb respectively. These values are among the lowest reported in the literature. Based on the trace metal measurements made in this region and used as pollutant source tracers, these concentrations can be considered as representative of background. In the rural Southeast~ the measured pollutant concentrations are much higher than those from the Northwest, and vary by a wide range. For instance, sulfate ranged from 0 . 4 5 - - 1 9 . 8 2 # g / m 3, which indicates that the data contain both background and anthropogenic sources. This is also reflected in the trace metal data. The aerosol acidities, as represented b y the equivalent pH, are averaged to 4.74 and 4.78 for the northwestern and southeastern regions, respectively.
ACKNOWLEDGEMENTS This work was supported by the U.S. Department of Energy and U.S. Environmental Protection Agency under contract DE-AC06-76 RLO 1830 and Interagency Agreement IAG No. EPA-DW930059-01-0.
133 REFERENCES Alkezweeny, A.J. and C.M. Berkowitz, 1981. Visibility and the chemical composition of aerosols in air masses over Lake Michigan. Paper presented at the 74th annual meeting of the Air Pollution Control Association, June 21--22, 1981, Philadelphia, PA. Alkezweeny, A.J. and J.M. Hales, 1981. The impact of non-precipitating clouds on the transport and formation of acid aerosols. Paper presented at the annual meeting of the American Chemical Society, August, 1981, New York, NY. Alkezweeny, A.J. and N.S. Laulainen, 1981. Comparison between polluted and clear air masses over Lake Michigan. J. Appl. Meteorol., 20: 209---212. Alkezweeny, A.J., N.S. Laulainen and J.M. Thorp, 1982. Physical, chemical, and optical characteristics of a clean air mass over Northern Michigan. Atmos. Environ., 16: 2421--2430. Altschuller, A.P., 1973. Atmospheric sulfur dioxide and sulfate distribution of concentration at urban and non-urban sites in United States. Environ. Sci. Technol., 7: 709--712. Appel, B.R. and Y. Tokiwa, 1981. Atmospheric particulate nitrate sampling errors due to reactions with particulate and gaseous strong acids. Atmos. Environ., 15: 1087--1089. Busness, K.M., A.J. Alkezweeny, R.C. Easter, J.M. Hales and R.N. Lee, 1982. Composite design of an advanced airborne monitoring system, Proceedings of the Conference on In-Situ Air Quality Monitoring from Moving Platforms, pp. 119--133. Cunningham, W.C. and W.H. ZoUer, 1981. The chemical composition of remote area aerosols. J. Aerosol Sci., 12: 367--384. Galloway, J.M., G.E. Likens, W.C. Keene and J.M. Miller, 1982. The composition of precipitation in remote areas of the worlcl. J. Geophys. Res., 87 : 8771--8786. Hoffer, T., J. Kliwer and J. Moyer, 1979. Sulfate concentrations in the southeastern desert of the United States. Atmos. Environ., 13: 619---627. Huebert, B.J. and A.L. Lazrus, 1980. Tropospheric gas-phase and particulate nitrate measurements. J. Geophys. Res., 85: 7322--7328. Kelly, N.A., G.T. Wolff and M.A. Ferman, 1982. Background pollutant measurements in air masses affecting the eastern half of the United States -- I. Air masses arriving from the northwest. Atmos. Environ., 16: 1077--1088. McMullen, T.B., R.B. Faoro and G.B. Morgan, 1970. Profile of pollutant fractions in nonurban suspended particulate matter. J. Air Pollut. Control Assoc. 20: 369---372. Sequeira, R., 1972. Acid rain: An assessment based on acid base considerations, J. Air Pollut. Control Assoc., 32: 2 4 1 - - 2 4 5 . Shaw, R.W., Jr., R.K. Stevens, J. Bowermaster, J.W. Teseh and E. Tew, 1982. Measurements of atmospheric nitrate and nitric acid: The Denuder difference experiment. Atmos. Environ., 16: 845--853.
125
OBSERVATIONS OF AEROSOL CHEMICAL COMPOSITION A N D ACIDITY IN NORTHWEST A N D SOUTHEAST REGIONS OF THE UNITED STATES
A.J. ALKEZWEENY and K.M. BUSNESS
Pacific Northwest Laboratory, Richland, WA 99352 (U.S.A.) (Received January 22nd, 1984; accepted March 2nd, 1984)
ABSTRACT Aircraft measurements of atmospheric aerosols and selected trace gases were performed in remote northwestern and rural southeastern regions of the continental United States. In the Northwest, the average concentrations of major constituents were sulfate, 0.15/~g/ m3; nitrate, 0.05/~g/m3; ammonium, 0.05/~g/m3; and ozone, 33 ppb. The average cloud water pH (of clouds that may form over the region) was calculated to be 4.74; 0.5 g/m 3 liquid water content was assumed. Trace metal concentrations were also measured and used in the discussion as pollutant source tracers. It was concluded that the data from the Northwest may be considered representative of background concentrations. In the Southeast, the average concentrations of major constituents were sulfate, 5.20 #g/m 3; nitrate, 0.25 pg/m 3; ammonium, 0.90 pg/m 3; ozone, 63 ppb; and pH 4.78. These large differences in each species concentration reflect the impact of anthropogenic sources in the rural southeastern region of the United States.
INTRODUCTION
During the summer of 1982, aerosol and ozone data were collected over specific northwestern and southeastern regions of the United States {Fig. 1). Detailed chemical analyses were performed and correlations between measured parameters were calculated to infer characteristics of significantly differing ambient air masses over these regions. The northwestern observations were made in remote areas that are sparsely populated and removed from significant pollution sources. These were rural farm, forested, and semi-pristine areas in Oregon, Washington, Idaho and Montana. These measurements, as will be discussed later, m a y be considered representative of background concentrations. On the other hand, observations made in the Southeast, again over rural farmland and forest areas and over water off the Atlantic Coast, can be influenced by local or long-range transport of pollutants originating from upwind sources. The reported Northwest sampling occurred during June and August 1982. The region was dominated b y high pressure off the West Coast, resulting in 0048-9697/84/$03.00
© 1984 Elsevier Science Publishers B.V.
126
NORTHWEST REGION
SOUTHEAST REGION
Fig. 1. Geographic identification of sampling regions.
clear to partly cloudy skies with light winds, moderate temperatures, low humidities, and excellent visibility. Synoptic conditions for the southeastern region during the sampling period in September 1982 were characterized by high pressure in the Northeastern region with resultant northerly flow into the sampling region. Local weather included clear to partly cloudy skies with poor visibility due to haze, high temperatures and humidities.
SAMPLING AND ANALYSIS PROCEDURES
Measurements and samples were taken within the boundary layer at elevations ranging from 100--1000 m above ground level, using the Pacific Northwest Laboratory (PNL) DC-3 aircraft (Busness et al., 1982). Generally, sampling was conducted from late morning through mid to late afternoon when the mixed layer was well established. Multiple aerosol samples were collected simultaneously on 3-pm pore-size teflon filters mounted in parallel high-volume air samplers equipped with flowmeters that continuously recorded flow rates and total volume flow. Sample volumes ranged from about 20--50 m 3. One group of filters was analyzed for trace metal content by X-ray fluorescence techniques. The companion set of filters was leached in double-distilled water to remove deposited aerosols; the pH of the resultant solutions was measured and concentrations of sulfate, nitrate and ammonium ions were determined by ion-chromatography in the PNL Precipitation Chemistry Laboratory.
127 TABLE 1 RANGES AND AVERAGE CONCENTRATIONS OF MAJOR PARAMETERS Constituent
Number of Samples
Concentration Range
Average
S.D.
Northwest
SO~- (pg/m 3) NO3+(,ug/m3) NH4 (pg/m °) 03 (ppb) Equiv. pH
9 9 9 9 6
0.04--0.24 0.01--0.22 0.01--0.14 28--40 4.59--5.03
0.15 0.05 0.05 33 4.74
0.06 0.07 0.04 4 0.18
0.45--19.82 0.03--0.84 0.07--3.53 45--90 3.75--6.52
5.20 0.25 0.90 63 4.78
4.48 0.25 0.64 -0.84
S o u theast
SO~- (pg/m 3) SO~ (pg/m 3) NH4 (pg/m 3) 03 (ppb) Equiv. pH
22 16 17 22 16
RESULTS AND DISCUSSION
Ranges and average concentrations of SO~', NO~, and NI-Fa ions and ozone for the two regions are shown in Table 1. The average ozone concentrations are time averages over corresponding filter-sampling times, followed by the usual averaging over the number of observations. The table includes equivalent pH, which is defined as the pH of water in clouds t h a t may be formed over a region characterized by aerosols of similar chemical composition; the calculation is based on an assumed liquid water c o n t e n t of 0.5 g/m 3. Equivalent pH values were calculated from the H + concentrations in the leached filter aerosol solutions as determined from laboratory pH measurements. It should be noted that if nitric acid were present during sampling, the equivalent pH will be lower than what we calculated. The reasons are that nitric acid is highly soluble in water and will be incorporated in clouds during their formation. Secondly, nitric acid did n o t contribute to our H ÷ concentration measurement because teflon filters do n o t capture HNO3. The concentrations of sulfate from the Northwest observations, in the range of 0.04--0.24/ag/m 3, are among the lowest values reported in the literature. For example, Alkezweeny and Laulainen (1981) measured values of 0.17--0.87 pg/m 3 in air masses over Lake Michigan. The air masses were very clean as evidenced by very low aerosol light scattering (bseat : 0.05-0.09 × 10-4m - 1). Hoffer et al. (1979) found the sulfate concentration as low as 0.09 pg/m 3 in the southwestern desert region of the United States. In central Antarctica, Cunningham and Zoller (1981) reported average values of 0.09 and 0.23 pg/m 3 for winter and summer, respectively. Nitrate values of
128
0.01--0.22 pg/m 3 for the Northwest are also among the lowest reported for background or remote area aerosols. McMuUen et al. (1970) analyzed National Air Surveillance Network samples taken at remote areas of the United States and obtained a value of 0 . 4 6 p g / m 3 for nitrate ion. Other values are 0.08--0.51 pg/m 3 found over Lake Michigan in clean air masses (Alkezweeny and Laulainen, 1981; Alkezweeny et al., 1982). Similar values were measured by Huebert and Lazrus (1980) in boundary-layer air over remote continental areas of North America. Possible reasons for the low nitzate values are losses of NH4NO3 by volatilization or the reaction with sulfuric acid, leading to the liberation of nitric acid gas (Appel and Tokiwa, 1981; Shaw et al., 1982) that will pass through the teflon filter. Northwest ammonium ion concentrations are in the range of 0.01-0.14~g/m 3, lower than the values reported b y McMuUen et al. (1970) for remote areas of the United States. Ozone concentrations of 28--40 p p b in the Northwest, are typical of values for clean air masses. For example, Kelly et al. (1982) found an average value of 41 p p b in remote areas in South Dakota. Alkezweeny and Laulainen (1981) measured ozone levels of 26 and 30 ppb over Lake Michigan. An interesting feature of the Northwest data is the aerosol acidity as represented by the equivalent pH described earlier. The calculated pH's are in the range of 4.59 to 5.03 (Table 1). These values suggest that clouds which form in clean air masses may have cloud water acidity comparable to that found in precipitation in industrial regions. It should be noted that 8
C" 0
I
I
~
I
15
I
"''/
5
-6 E
3- r -
I
3
Z
i • ~ l l 2
I 3
I 4
L• 5
2[SO,]+[NO,](#mole/m'xlO-')
I 6
J 0
E 3 ¥
10
5
r 1
•
I 2
SO,(/~rnole/m3xlO -s)
Fig. 2. (a)Ammonium ion versus sulfate and nitrate plot for the northwestern sampling
region. (b) Hydrogen ion versus sulfate plot for the northwestern sampling region.
129
Galloway et al. (1982) and Sequeira (1982) report pH values in the range of 3.7 to 7.2 for precipitation in remote and unpolluted areas of the world. If background acidity values like these are frequently found, this fact should be considered in determining the impact of industrial emissions on acid deposition. To speculate on the origin and causes of aerosol acidity, we have performed a linear regression analysis of the relationship between [NH~] and 2[SO~-] + [NO~] (molar units) and found a modest correlation (correlation coefficient = 0.60). The slope of the fit {Fig. 2a) is very close to 1.0, indicating that sufficient ammonia existed to neutralize sulfuric and nitric acids. Thus, in this situation, sulfate ions do not appear to contribute to aerosol acidity. This conclusion is supported by the lack of correlation between H + and SO~- concentrations (Fig. 2b). This suggests that other acidic substances such as organics may be responsible for the resultant low pH. In contrast, the slope for the southeastern region data (Fig. 3a) is 0.56, an indication of excess nitrate and/or sulfate ions over ammonium ions. Furthermore, very strong correlation, R = 0.92, is found between H ÷ and SO~- (Fig. 3b). This high correlation suggests that sulfate is the major contributor to aerosol acidity in the Southeast. The range of sulfate concentrations in the southeastern region is 0.45-19.82/~g/m a, which is more than an order of magnitude greater than the range in the Northwest. High sulfate values of similar magnitude, about 20#g/m a, have been measured by us in rural areas of the Northeast when meteorological conditions were favorable for long-range transport. It is interesting to note that Altshuller (1973) examined the SO: and SO~-results ).05
I
] I
100
•
).04
o
/
7 o x
E ::L
2
/..-
o 50
t Z
3.02
0.0
b ae/
0
50
100
150
2[S04]+[NO,](/lmole/m 'xlO-')
200
0.00
•
I
0.05
I
0.10
0.15
SO,(~mole/m')
Fig. 3. (a) A m m o n i u m ion versus sulfate and nitrate plot for the southeastern sampling region. (b) Hydrogen ion versus sulfate plot for the southeastern sampling region.
130 025
I
I
0.20
0.15
::L
r~-0.42
o" U"}
0.10
0.05
/
/ II 0 0,00 25
=
I
I
30
35
40
03(ppb)
0 45
40
I
I
I
i
I
50
60
70
80
90
100
O,(ppb)
Fig. 4. (a) Sulfate versus ozone plot for the northwestern sampling region. (b) Sulfate versus ozone plot for the southeastern sampling region.
from the U.S. National Air Surveillance Network and concluded that the regional background due to transport of urban SO: and SO~- is 5.0 pg/m a. It is possible that the southeastern region was under the influence of long-range transport, as reflected by the wide range in concentrations of ionic species, ozone, and equivalent pH. Since ozone is formed b y a photochemical reaction, a correlation of ozone with sulfate may lead to interesting speculation a b o u t the origin of the measured sulfate. For the Northwest, such a correlation is insignificant (Fig. 4a). If the sulfate formation by photochemical reaction is ruled out, then it may have been either directly emitted or the result of aqueous sulfur oxidation in nonprecipitating clouds. This kind of cloud is very c o m m o n in this region during the summertime. Furthermore, high-sulfate formation in such clouds has been indicated by the measurements of Alkezweeny and Hales (1981) made in the vicinity of fair weather cumulus clouds. On the other hand, the calculated correlation between ozone and sulfate from the southeastern region data is 0.55 (Fig. 4b). Alkezweeny and Berkowitz (1981) found a similar value for data collected over Lake Michigan in air masses of different origins. However, when these data were classified as being of urban and rural origin, they found the correlation coefficient to be 0.90 for urban air and close to zero for rural air. The rural air, moving over Lake Michigan, more likely traversed clean, remote regions which were not significantly influenced by pollutant sources. Therefore, this modest correlation in the southeastern data may be the result of anthropogenic and background sources.
131
TABLE 2 AVERAGE
R E G I O N A L E L E M E N T A L C O M P O S I T I O N (ng/m 3)
Element
Northwest
Southeast
A1 Si S C1 K Ca Ti V Cr Mn Fe Ni Cu Zn Br Pb Sa Na a C1 a
0 -+ 1 3 2 1 2 1 + 57 1 1 7 + 15 0+8 14+3 27 + 3 5 -+2 0-+0.5 0 1 48 + 2 0 0-+2 2-+1 0 0-+1 5 2 -+ 9 0 0
16 298 2081 260 55 64 17 2 8 4 109 7
Ka
NH4 a
0
42
+ 16 + 274 + 1548 + 283 + 28 -+ 3 3 + 14 + 1 + 21 + 5 + 99 + 12
8+3 2+1 1 5 + 11 2069 + 1712 1 7 5 -+ 2 1 4 115 + 171 15 + 3 0 1227 + 940
aConcentrationsacquiredvia ion-chromatography.
Mass ratios of sulfate to nitrate are quite different in the two regions, the Northwest ratio of a b o u t 3 is substantially smaller than the ratio of 21 in the Southeast. It is possible that our values for nitrate, at least in the Southeast, are underestimated because of ammonium nitrate losses from the filter as discussed earlier. Our nitrate of 0 . 0 3 - - 0 . 8 4 # g / m 3 for the Southeast are much lower than the values reported by Shaw et al. (1982) (0.6--1.2 #g/m 3 ) for surface samples obtained in rural areas in North Carolina. The results of X-ray fluorescence analyses of filter samples acquired in the two regions are shown in Table 2. The uncertainty shown for the Northwest data (a small data set) is the c o m p o u n d error of both analytical results and sampling. Standard deviations are indicated for the larger Southeast data set. The Northwest filters were exposed in eastern Oregon. The lower section of Table 2 shows the results of analyses of companion filters by ion-chromatography, indicating the water-soluble portion of the aerosol collections. If we consider the trace metals as tracers for pollutant sources, it appears that the Northwest region has n o t been significantly impacted b y anthropogenic sources during the reported sampling period. For example, concentrations of Cr, Ti, V and Ni are very close to natural background levels, suggesting negligible impact from fossil-fueled power plants. Similarly, we may neglect
132 the contribution of automobile emissions because of the very low values of Pb and Ni. Undetectable levels of Na and C1 suggest that the air masses that were sampled were of continental origin with little or no marine component. Therefore, these data may represent background continental aerosol characteristics. An interesting feature of the Northwest aerosol filter analyses is that sulfur concentrations determined by X-ray fluorescence are higher than ionchromatography results. This may indicate that non-water-soluble sulfur may be the major c o m p o n e n t of total sulfur in background air. Since the number of samples is small, however, this may be an untenable conclusion. The situation in the Southeast is quite different. The wide ranges in concentrations of trace metals reflects the varying contributions of anthropogenic sources to the sampled region. It should also be noted that, on the average, all the sulfur measured in the Southeast is water soluble; average concentrations of 2 0 8 1 n g / m 3 and 2 0 6 9 n g / m 3 were determined by X-ray fluorescence and ion-chromatography, respectively.
CONCLUSION Aircraft measurements of atmospheric constituents were performed in remote Northwest and rural Southeast regions of the United States for the purpose of evaluating behavior of air masses having widely different origins. In the Northwest, the ranges of concentrations of sulfate, nitrate, ammonium, and ozolle were: 0.04--0.24pg/m 3, 0.01--0.22#g/m 3, 0.01-0.14 pg/m 3, and 28--40 ppb respectively. These values are among the lowest reported in the literature. Based on the trace metal measurements made in this region and used as pollutant source tracers, these concentrations can be considered as representative of background. In the rural Southeast~ the measured pollutant concentrations are much higher than those from the Northwest, and vary by a wide range. For instance, sulfate ranged from 0 . 4 5 - - 1 9 . 8 2 # g / m 3, which indicates that the data contain both background and anthropogenic sources. This is also reflected in the trace metal data. The aerosol acidities, as represented b y the equivalent pH, are averaged to 4.74 and 4.78 for the northwestern and southeastern regions, respectively.
ACKNOWLEDGEMENTS This work was supported by the U.S. Department of Energy and U.S. Environmental Protection Agency under contract DE-AC06-76 RLO 1830 and Interagency Agreement IAG No. EPA-DW930059-01-0.
133 REFERENCES Alkezweeny, A.J. and C.M. Berkowitz, 1981. Visibility and the chemical composition of aerosols in air masses over Lake Michigan. Paper presented at the 74th annual meeting of the Air Pollution Control Association, June 21--22, 1981, Philadelphia, PA. Alkezweeny, A.J. and J.M. Hales, 1981. The impact of non-precipitating clouds on the transport and formation of acid aerosols. Paper presented at the annual meeting of the American Chemical Society, August, 1981, New York, NY. Alkezweeny, A.J. and N.S. Laulainen, 1981. Comparison between polluted and clear air masses over Lake Michigan. J. Appl. Meteorol., 20: 209---212. Alkezweeny, A.J., N.S. Laulainen and J.M. Thorp, 1982. Physical, chemical, and optical characteristics of a clean air mass over Northern Michigan. Atmos. Environ., 16: 2421--2430. Altschuller, A.P., 1973. Atmospheric sulfur dioxide and sulfate distribution of concentration at urban and non-urban sites in United States. Environ. Sci. Technol., 7: 709--712. Appel, B.R. and Y. Tokiwa, 1981. Atmospheric particulate nitrate sampling errors due to reactions with particulate and gaseous strong acids. Atmos. Environ., 15: 1087--1089. Busness, K.M., A.J. Alkezweeny, R.C. Easter, J.M. Hales and R.N. Lee, 1982. Composite design of an advanced airborne monitoring system, Proceedings of the Conference on In-Situ Air Quality Monitoring from Moving Platforms, pp. 119--133. Cunningham, W.C. and W.H. ZoUer, 1981. The chemical composition of remote area aerosols. J. Aerosol Sci., 12: 367--384. Galloway, J.M., G.E. Likens, W.C. Keene and J.M. Miller, 1982. The composition of precipitation in remote areas of the worlcl. J. Geophys. Res., 87 : 8771--8786. Hoffer, T., J. Kliwer and J. Moyer, 1979. Sulfate concentrations in the southeastern desert of the United States. Atmos. Environ., 13: 619---627. Huebert, B.J. and A.L. Lazrus, 1980. Tropospheric gas-phase and particulate nitrate measurements. J. Geophys. Res., 85: 7322--7328. Kelly, N.A., G.T. Wolff and M.A. Ferman, 1982. Background pollutant measurements in air masses affecting the eastern half of the United States -- I. Air masses arriving from the northwest. Atmos. Environ., 16: 1077--1088. McMullen, T.B., R.B. Faoro and G.B. Morgan, 1970. Profile of pollutant fractions in nonurban suspended particulate matter. J. Air Pollut. Control Assoc. 20: 369---372. Sequeira, R., 1972. Acid rain: An assessment based on acid base considerations, J. Air Pollut. Control Assoc., 32: 2 4 1 - - 2 4 5 . Shaw, R.W., Jr., R.K. Stevens, J. Bowermaster, J.W. Teseh and E. Tew, 1982. Measurements of atmospheric nitrate and nitric acid: The Denuder difference experiment. Atmos. Environ., 16: 845--853.