Environ. Sci. Technol. 1987, 21 1219-1224 I
of DMSe and DMSe+-R in oxic groundwater indicate that either DMSe+-R is released from plant detritus and is transformed to DMSe via hydrolysis or that microbial biosynthesis of DMSe occurs via pathways involving DMSe+-R. Analysis of stored water samples, especially those obtained from biologically active regimes, may underestimate the total selenium content. Future analytical efforts will require a level of sophistication beyond total element determination in order to generate a more reliable picture of the environmental mobility and fate of selenium in aquatic systems. The Kesterson Reservoir and the Salton Sea are good examples of systems in which the dominant solution form of selenium is organoselenium compounds. Models based solely on inorganic species will not adequately describe the complexity of the aquatic chemistry of selenium in such systems. Instead, models applied to biologically active aquatic regimes should address both the occurrence and the kinetics associated with the production and removal of organic as well as inorganic selenium species.
Acknowledgments for their comments and and G* We thank J' editorial assistance in preparing the manuscript. Registry No. Se, 7782-49-2; H3CSeCH8,593-79-3; H3CSeSeCH,, 7101-31-7; (H3C)2Se+(CH2)2CH(NH2)C02H, 7728-97-4.
Literature Cited (1) Duce, R. A.; Hoffman, G. L.; Zoller, W. H. Science (Washington, D.C.)1975, 187, 59-61. (2) Mosher, B. W.; Duce, R. A. J. Geophys. Res. 1983,88(11), 6761-6768. (3) Mosher, B. W.; Duce, R. A., University of Rhode Island, personal communication, 1986. (4) Lewis, B. G.; Johnson, C. M.; Broyer, T. C. Plant Soil 1974, 40, 107-118.
( 5 ) Francis, A. J.; Duxbury, J. M.; Alexander, M. Appl. Microbiol. 1974, 28(2), 248-250. (6) Reamer, D. C.; Zoller, W. H. Science (Washington,D.C.) 1980,208, 500-502. (7) Chau, Y . K.; Wong, P. T. S.; Silverberg, B. A.; Luxon, P. L.; Bengart, G. A. Science (Washington,D.C.) 1976, 192, 1130-1131. ( 8 ) Jaing, B. S.; Robberecht, H.; Adams, F. Atmos. Environ. 1983, 17(1), 111-114. (9) Cutter, G. A.; Bruland, K. W. Limnol. Oceanogr. 1984,29(6), 1179-1192. (10) Cutter, G. A. Anal. Chin. Acta 1978, 98, 59-66. (11) Cutter, G. A. Science (Washington, D.C.) 1982, 217, 829-831. (12) Cooke, T. D. M.S. Thesis, University of California at Santa Cruz, 1985. (13) Virupaksha, T. K.; Shrift, A. Biochim. Biophys. Acta 1965, 107,69-80. (14) Andreae, M. 0.; Barnard, W. R. Mar. Chem. 1984, 14, 267-279. (15) Lewis, B. G.; Johnson, C. M.; Broyer, T. C. Biochim. Biophys. Acta 1971,237, 603-605. (16) Lewis, B. G. In Environmental Biogeochemistry;Nriagu, J. O., Ed.; Ann Arbor Science: Ann Arbor, MI, 1976; Vol. 1, Chapter 26. (17) Bottino, N. R.; e t al. Phytochemistry 1984, 23(11), 2445-245 2. (18) Liss, P. S.; Slater, P. G. Nature (London) 1974, 247, 181-184. (19) Pennington, B., U.S. Department of the Interior, personal communication, 1985. (20) Takayanagi, K.; Wong, G. T. F. Geochim. Cosmochim.Acta 1985,49, 539-546. (21) Kiene, R. EOS, Trans. Am. Geophys. Union 1986,67(44), 1043. (22) Oremland, R.; Zehr, J. EOS, Trans. Am. Geophys. Union 1986, 67(44), 940.
Receiued for review January 12, 1987. Accepted July 6, 1987. Early stages of this research were supported by NSF OCE8216672. Later stages were supported by SWRCB Contract 5-247-250-0.
An Analysis of Precipitation Chemistry Measurements in Ontario Waiter H. Chan,* AI J. S. Tang, David H. S. Chung, and Neville W. Reid Ontario Ministry of the Environment, Air Resources Branch, 880 Bay Street, 4th Floor, Toronto, Ontario, M5S 128 Canada
Precipitation chemistry in Ontario from 1980 to 1983 has been assessed to understand its monthly variations and the relative importance of sulfuric and nitric acids to precipitation acidity. Except for nitrate, which has its maximum in March, parameter concentrations peaked in the summer and late spring. Nitrate contributes typically half as much acidity as sulfate on an annual basis, but in the winter months (especially for snow), the nitrate contribution is comparable to or greater than that of sulfate. Free hydrogen ion concentrations determined from laboratory pH measurements may be typically 15% lower than those from the field pH measurements. The observed neutralization of strong acids in precipitation, on the basis and NO3-, especially in late spring of laboratory pH, Sod2-, and summer, can be attributed to NH4+ (19-36%), ea2+ (10-21%), and Mg2+(2% or less). There is some indication of the presence of up to 17% free acids that cannot be accounted for by sulfuric and nitric acids but may be attributable to organic acids.
Introduction Precipitation chemistry measurements have been made Published 1987 by the American Chemical Soclety
in Europe since the 1950s (1). In North America, continuous, large-scale measurements in the form of networks began only after the mid 1970s (2). In most cases, commercial wet-only samplers equipped with a moisture-activated sensor are used. The observed results have been used to examine different aspects of the acid rain phenomenon, e.g., spatial and temporal variability of concentration and deposition patterns, precipitation chemistry, and quality of measurements. This paper describes analyses of daily precipitation chemistry results obtained in the Acidic Precipitation in Ontario Study (APIOS) (3). Although some of the results reported here are not derived through new data analysis approaches, they are about topics that are not y e t conclusive. The analysis of the Ontario data was carried out to add to current knowledge so that more difinitive conclusions may be drawn. It is not our intention to give an in-depth discussion on each topic but rather deliberately to attempt to cover as many topics as possible with a large data base. The objectives of the analysis are as follows: (1) to assess the monthly variations of precipitation chemistry Envlron. Sci. Technol., Vol. 21, No. 12, 1987
1219
/I/
LEGEND
11
1. Melbourne 2. Lonowoods 3 N. Easthope 1. WeII.*iey 5. Raven Lake 6. Balsam Lake 1. Nilhgrwe 8. Domet 9. Whitman Creek 10. Rallton 11. Charleston Lake 12. Graham Lake 13. Forbar Township 14. Oustico Centre
Figure 1. Locations of APIOS daily network sites.
(2) to examine the relative importance of sulfuric and nitric acids (in the form of SO:- and NO;) to precipitation acidity (3) to determine the role of alkaline materials, e.g., NH4+, Ca2+,and Mg2+,in determining the measurements (4) to evaluate the appropriateness of using laboratory pH measurements as a surrogate for field pH measurements (5) to examine the influence of precipitation type, e.g., snow and rain, on the observed chemistry
Experimental Section Data used in the analyses were obtained at 16 sampling stations in Ontario from July 1980 to March 1983. During this period, these 16 sites were distributed in four clusters. The site locations are shown in Figure 1. Within each cluster there are two pairs of samplers separated by about 50-100 km. The two samplers comprising each pair are separated by about 10 km. The distance between clusters is on the order of hundreds of kilometers, up to over 1000 km, resulting in a good spatial resolution. Stringent siting criteria were used in the network, taking into account local sources, potential obstacles, and contaminations. Details can be found in the network document (3). Except for the periods from November to April of 1980/1981 and 1981/1982, during which time daily SES bulk samplers were used a t some of the sites because of the better snow collection efficiency, Aerochem Metrics samplers were used throughout the study. Only results of the wet-only Aerochem Metrics samplers have been used in this analysis; all of the bulk sampler results have been excluded because of potential contributions by dry deposition. Daily (0800 to 0800 next day) precipitation samples were collected directly into polyethylene bags inserted into the samplers. The samples were subsequently transferred to polystyrene bottles. Prior to chemic'al analysis, the samples were stored in a refrigerator at 4 O C . Stringent quality assurance procedures are applied in the network, as described by Bardswick et al. ( 4 ) . Measurements of pH were made with an Ingold electrode on all samples in the laboratory, typically within 2 1220
Environ. Sci. Technoi., Vol. 21, No. 12, 1987
weeks and at certain sites immediately after field collection. Sulfate, NO3-, and Cl- were analyzed by ion-exchange chromatography, NH4+was analyzed by the phenate-hypochlorite colorimetric method, and other major ions (K+, Na', Ca2+,and Mg2+were analyzed by atomic absorption spectroscopy. Detection limits for the various parameters (in mg L-') are 0.05 for S042-,0.01 for NO3-, C1-, and Ca2+, 0.002 for NH4+,and 0.005 for Na+, K+, and Mg2+. Details of the analytical techniques may be found elsewhere (5).
Results and Discussion In the network operation, one would wish to analyze for all parameters of interest. In certain cases, however, the sample volume collected is insufficient for analysis of the complete list of parameters. Such samples are analyzed for as many parameters as possible according to a predetermined priority. As the intent of this paper is to examine the representative, long-term deposition pattern in Ontario, all the available results (even with varying numbers of data points) are used in the calculations. Analyses are made of the combined data set from all 16 sites to yield a more representative description of the observations in Ontario. If the intent were to examine the absolute concentrations, the combination of different site results would not be reasonable. However, the intent of the study is to examine the relative compositions,which are rather consistent from site to site. Therefore, combining these data actually increases the statistical base, while permitting a compact reporting format. Note that there are fewer samples for January and February because sampling did not begin until mid-February 1981 in some sites. In order to examine the monthly variation, both precipitation-weighted mean and arithmetric mean concentrations were calculated. The arithmetic mean is always higher (by as much as 48%) than the precipitationweighted mean. While the absolute values differ, their relative month-to-month patterns are consistent. The weighted mean is considered to be representative, and it further minimizes biases due to outliers, which usually are associated with a very high concentration and low volume. Monthly Variation in Precipitation Chemistry. Table I summarizes results for H+f(lab)(free hydrogen concentration converted from laboratory pH), SO:-, NO3-, NH4+, Ca2+,and Mg2+. The SO:- concentration peaks in the early summer (June) with a maximum concentration of 0.071 mequiv L-' and has a minimum value of 0.030 mequiv L-l in the winter (January). The Htf distribution also has a maximum in June (0.060 mequiv L-l), but the minimum concentration occurs in October (0.040 mequiv L-l). Nitrate peaks in March (0.046 mequiv L-') and has a minimum in October (0.022 mequiv L-I), a pattern quite different from that of
sot-.
All of the alkaline parameters, NH4+(0.030 mequiv L-I), Ca2+(0.022 mequiv L-l), and Mg2+(0.006 mequiv L-'), peak in April; possibly this is related to a change of the ground cover due to snowmelts and higher wind speeds in April. Relative Importance of Nitrate and Sulfate to Acidity. Assuming that precipitation acidity originates primarily from sulfuric and nitric acids, the ratio of NOgto SO:-, expressed as equivalents, would display the relative importance of their contributions to precipitation acidity. The monthly variation of the precipitationweighted mean ratio is shown in Table 11. It should be noted that, because of the different sequence of averaging, the ratios of the means (from Table I) are not necessarily equal to the mean ratios as reported in Table 11. Except for the period from December to March, during which time the NO3- contribution could be equal to or
Table I. Monthly Variation of Precipitation Chemistry in Ontario (July 1980 to March 1983)
precipitation-weighted mean concentration, mequiv L-' NO; "4' Ca2+ concn SGDP N concn SGDP N concn SGDP N concn SGDP N
Htf(lab) month Na concn SGDPb N Jan Feb March April May June July Aug Sep Oct Nov Dec
230 194 330 305 342 344 286 364 431 456 420 419
0.046 0.059 0.057 0.045 0.050 0.060 0.049 0.057 0.056 0.040 0.044 0.042
1397 1418 1770 2276 3199 3058 3296 4017 4492 3941 3728 3304
so42-
251 194 381 372 355 337 272 366 448 460 431 425
0.030 0.040 0.057 0.066 0.064 0.071 0.056 0.069 0.057 0.040 0.040 0.034
1582 1437 1913 2614 3267 3144 3295 4092 4558 3990 3826 3411
249 193 380 377 355 333 281 366 442 457 424 421
0.032 0.038 0.046 0.038 0.028 0.031 0.024 0.027 0.027 0.022 0.029 0.028
1592 1450 1892 2638 3284 3131 3376 4077 4532 3995 3795 3411
204 173 306 318 309 282 242 336 390 396 379 319
0.013 0.016 0.028 0.030 0.026 0.029 0.021 0.027 0.024 0.017 0.017 0.014
1417 1401 1769 2426 3259 2967 3262 4048 4480 3851 3657 3073
180 165 285 288 292 273 237 318 377 383 370 337
0.009 0.008 0.020 0.022 0.012 0.011
0.011 0.011 0.009 0.008 0.008 0.008
1308 1364 1780 2331 3157 2927 3270 3980 4408 3839 3594 3195
177 161 286 288 284 259 228 313 357 357 330 331
Mg2+ concn SGDP 0.002 0.002 0.005 0.006 0.004 0.004 0.003 0.003 0.003 0.002 0.002 0.002
1312 1293 1717 2234 3023 2729 3120 3828 4207 3450 2960 3127
" N = number of samples. *SGDP = sum of precipitation gauge depths (mm) used in weighting.
Importance of Alkaline Species in Determining Acidity. Table I11 contains results designed to illustrate the influence of factors other than the concentrations of sulfuric and nitric acids in determining the acidity of precipitation. The first column displays the ratio H+f/ (S042- + NO3-) by month, together with the number of data points and the precipitation gauge depth used in the averaging. Provided that all the precipitation SO?- and NO3- are associated with H2S04and HNO, [a reasonable assumption in the light of current knowledge of atmospheric chemistry (9)], the less than unity Htf/(SO?- + NOs-) ratios indicate that neutralization must have occurred. The two acids account for a larger fraction of the free hydrogen ion concentration in the colder months with a maximum of 79% in February. In the warmer months, the fractions are lower with a minimum of 47% in April, reflecting the higher concentrations of alkaline materials, e.g., NH4+,Ca2+,and MgZt (Table I). The relative contributions of NH4+,Ca2+,and Mg2+are shown in subsequent columns of Table 111. There is definite evidence for the neutralizing effect of NH4+,accounting for 19% (February) to 36% (August and September) of the total possible acid contents attributable to sulfuric acid (as S042-)and nitric acid (as NO,-). Except in September when the additional NH4+brings about a value of unity in the (H+f+ NH4+)/(S0?- NO3-) ratio, in all other months, the ratio is less than unity, suggesting the presence of other alkaline materials in the precipitation samples, These are most likely to be associated with Ca2+and Mg2+. The inclusion of Ca2+in the ratio accounts for an additional 10% (February) to 21% (March) of the missing
Table 11. Monthly Variation of Precipitation Nitrate to Sulfate Equivalence Ratios in Precipitation in Ontario (July 1980 to March 1983)
precipitation-weighted mean, NOa-/S042-(R) N" R SGDPb
month Jan Feb March April May June July Aug SeP Oct Nov Dec
244 190 374 367 351 330 271 359 434 454 424 419
1.27 1.32 Q.96 0.60 0.46 0.47 0.53 0.45 0.50 0.59 0.78 1.06
1571 1427 1883 2588 3246 3124 3293 4038 4471 3977 3795 3405
" N = number of samples. SGDP = sum of precipitation gauge depths (mm) used in weighting. greater than that of S042-,the NO3- contribution is typically half that of sulfate, indicating that HN03 and H2S04 contribute one-third and two-thirds, respectively, to free precipitation acidity. The reversed pattern in winter is consistent with the lower sulfate concentration and the slightly higher NO3- concentration during those months. This observation also reflects possibly the lower oxidation of SO2 to S042-in the winter (6) and the negligible scavenging of SOzby snow (7,8). The lowest N03-/S02- ratio is observed in August (0.45) and the highest in February (1.32).
+
Table 111. Summary of Neutralization Capacity Results (Equivalence Ratios)
(H+f+ ",I'/
H+f/ (S042- + NO3-) month Jan Feb March April May June July Aug Sep Oct Nov Dec
(S042+ NOc)
Na
R
SGDPb
N
R
SGDP
208 182 312 290 322 311 254 338 401 427 400 381
0.76 0.79 0.60 0.47 0.52 0.59 0.56 0:54 0.64 0.63 0.71 0.70
1354 1366 1726 2222 3129 3010 3212 3882 4322 3880 3681 3202
169 164 260 261 283 269 224 310 348 367 359 298
0.96 0.98 0.88 0.74 0.83 0.91 0.89 0.90 1.00 0.91 0.96 0.92
1217 1331 1642 2121 3049 2904 3141 3816 4206 3723 3555 2908
N = number of samples.
(H+f+ NH4+ + Ca2+)/ (S042+ NOs-) N R SGDP 153 150 238 250 263 258 212 294 325 353 335 280
1.11 1.08 1.09 0.94 0.98 1.03 1.04 1.05 1.13 1.07 1.08 1.04
1140 1270 1601 2076 2930 2825 3102 3747 4084 3661 3414 2833
+
(H'f + NH4+ + Ca2+ Mg2+)/(SO4*- + NOc) N R SGDP 147 139 229 243 254 244 202 285 303 325 297 274
1.13 1.09 1.08 0.96 0.99 1.03 1.05 1.05 1.11
1.08 1.07 1.06
1099 1152 1539 1993 2792 2628 2959 3595 3837 3241 2779 2741
SGDP = sum of precipitation gauge depths (mm) used in weighting. Environ. Sci. Technol., Vol. 21, No. 12, 1987
1221
Table IV. Comparison of Field and Laboratory Hydrogen Ion Measurements
month
precipitation-weighted mean concentration, mequiv L-l Htf(lab)/ Htf(field) - Ca2++ Nn SGDPb Htf(field) H'f(1ab) Mg2+
Jan Feb March April May June July Aug Sep Oct Nov Dec
88 131 124 129 68 99 80 112 178 142 155 164
930.2 1427.8 1005.0 1021.1 813.3 1394.9 1133.4 1614.1 2651.2 1638.6 1813.1 1715.4
1.02 0.87 0.85 0.68 0.79 0.82 0.84 0.83 0.91 1.03 1.03 0.93
0.002 0.017 0.004 0.042 0.018 0.015 0.009 0.018 0.040 -0.003 0.012 0.008
0.008 0.012 0.024 0.041 0.012 0.013 0.012 0.012 0.010 0.010 0.010 0.009
N = number of samples. SGDP = sum of precipitation gauge depths (mm) used in weighting.
acidity on the basis of mineral acids. The contribution due to Mg2+is small, typically 2% or less, indicating that it is not a major neutralizing agent in the Ontario precipitation samples. With the inclusion of Ca2+and Mg2+, except in April and May, an overcompensation of up to 13% is noted. Significant neutralization of acid anions in North American precipitation has been reported by Munger and Eisenreich (10). The major contrast to our findings is that their observed effect due to major cations (e.g., Ca2+and Mg2+)was a factor of 2-3 higher than that of NH4+. Field versus Laboratory pH Measurements. Some networks measure and report laboratory pH measurements alone. In these networks the pH of samples is not measured in the field immediately after collection, but rather samples are sent to a central laboratory for pH determinations, which are made typically within 2 weeks after sample collection. Table IV summarizes H+f(lab)/H+f(field) ratios for the APIOS network. The data are limited because small-
volume samples are not subject to field pH measurement. The ratio is significantly less than unity in most months but slightly exceeds unity in January, October, and November. It is lowest in April (0.68) and is typically around 0.85. This systematic loss of acidity during the interval prior to laboratory analysis has been interpreted as a consequence of the microbial consumption of organic acids (11). However, it could also be due to slow neutralization by soil or dust particles. The latter process would lead to an increase in the concentrations of Ca2+and Mg2+in the sample, since these are the cations normally associated with the neutralizing capacity of such particles. If the loss of acidity is due to neutralization, it would then follow that the total amount of calcium and magnesium in the sample must equal or exceed the amount of hydrogen ion lost. The quantities [H+f(field)- H+f(lab)] and (Ca2++ Mg2+)have, therefore, also been included in Table IV. Only for a few months (e.g., February, May, August, and September) is it clear that neutralization alone falls significantly short of accounting for the loss of acidity. It is worth commenting on an apparent anomaly in the results reported in Table IV. For the month of November, the ratio of laboratory hydrogen ion concentration to that measured in the field is greater than unity, but this is contradicted by the positive difference between field and laboratory values given in the next column. This effect arises from the occurrence of a few cases where the field hydrogen ion concentration is substantially higher than the laboratory value. These cases affect the depthweighted mean difference much more than they do the depth-weighted mean ratio. The presence of organic acids in precipitation has been demonstrated at other locations and at all times of the year ( I I , 1 2 ) . It is likely that both effects occur in the samples collected in the APIOS network. I t is clear from the results presented in Table IV that when the pH of precipitation is an important parameter, it should be measured as soon as possible after sample collection. Rain versus Snow Chemistry. Because pollutant
Table V. Monthly Variation of Precipitation Chemistry in Ontario ( J u l y 1980 to March 1983) precipitation-weighted mean concentration, mequiv L-' H+Alab)
50:-
rain month Nn concn SGDPb N
snow concn SGDP N
rain concn SGDP N
snow concn SGDP
Jan Feb March April Oct Nov Dec
0.024 0.026 0.037 0.034 0.018 0.021 0.022
0.024 0.035 0.047 0.042 0.023 0.031 0.026
0.035 0.039 0.041 0.024 0.014 0.018 0.030
32 57 130 274 407 262 147
0.033 0.048 0.070 0.075 0.042 0.047 0.043
351 544 830 1983 3624 2321 1324
174 98 177 48 16 71 215
"4'
month
N
Jan Feb March April Oct Nov Dec
28 49 102 235 348 233 126
rain concn SGDP 0.012 0.017 0.030 0.034 0.017 0.018 0.015
316 524 780 1867 3490 2241 1278
N = number of samples. 1222
N 139 86 147 37 13 53 138
891 631 706 267 88 539 1250
31 56 128 280 404 258 144
353 558 824 2014 3626 2314 1322
175 99 176 47 16 68 213
906 630 689 260 88 515 1247
N 29 56 121 222 401 257 149
rain concn SGDP N
snow . ~ ~ . concn SGDP
0.041 0.062 0.056 0.052 0.041 0.052 0.052
0.045 0.052 0.052 0.023 0.018 0.025 0.030
300 510 810 1706 3569 2273 1331
15 100 145 37 16 68 211
precipitation-weighted mean concentration, mequiv L-' Ca2+ Mg2" snow rain snow rain concn SGDP N concn SGDP N concn SGDP N concn SGDP N 0.011 0.011 0.023 0.020 0.007 0,009 0.013
797 611 642 199 80 467 1007
26 48 105 211 338 231 125
0.013 0.007 0.024 0.025 0.008 0.008 0.007
288 510 788 1773 3480 2231 1249
118 85 126 33 13 52 154
0.008 0.008 0.015 0.010 0.007 0.005 0.009
715 618 642 199 80 467 1122
27 47 109 212 312 199 127
* SGDP = sum of precipitation gauge depths (mm) used in weighting.
Environ. Sci. Technol., Vol. 21, No. 12, 1987
0.003 0.003 0.006 0.007 0.002 0.002 0.002
315 454 791 1699 3091 1766 1289
113 84 121 33 13 46 149
766 644 612 221 88 514 1221
snow concn SGDP 0.002 0.002 0.004 0.004 0.001 0.001 0.002
687 618 571 188 80 331 1053
Table VII. Variation in NO- Equivalence Ratios in Precipitation in Ontario (July 1980 to March 1983)
Table VI. Site-to-Site Variability in Precipitation Chemistry (mequiv L-l) precipitation-weighted concentration all" rain snow N b concn N concn N concn
site Longwoods SO?NO,
H+f(lab) NH4+ Ca2+ Mg2+ Dorset SO?NO8Htf(l+ab) NH4 Ca2+ MgZ+ Charleston S042Lake NO, H+f(lab) NH4+ Ca2+ Mg2+ all sites SO4" NOs-
Htf(lab) NH4+ Ca2+ Mg2+
72 71 70 70 67 68 131 130 131 125 127 121 74 74 74 73 73 72 2514 2501 2354 2095 2008 1930
0.054 0.034 0.051 0.022 0.016 0.005 0.037 0.030 0.048 0.016 0.007 0.002 0.033 0.026 0.040 0.013 0.006 0.002 0.043 0.031 0.046 0.019 0,011 0.003
53 53 52 51 50 50 55 55 55 55 54 51 39 39 39 38 38 38 904 899 835 778 751 726
0.060 0.034 0.052 0.023 0.018 0.005 0.054 0.031 0.058 0.020 0.010 0.003 0.044 0.028 0.046 0.016 0.007 0.003 0.056 0.035 0.052 0.023 0.014 0.004
10 10 10 10 10 10 52 '51 52 48 52 50 18 18 18 18 18 17 783 778 715 600 568 546
" Including rain, snow, and mixed-precipitation types. number of samdes.
0.020 0.032 0.035 0.010 0.014 0.004 0.019 0.028 0.038 0.012 0.004 0.001 0.018 0.025 0.035 0.007 0.005 0.001 0.026 0.032 0.039 0.014 0.009 0.002
*N
month
N"
Jan Feb March April Oct Nov Dec
30 55 128 270 402 258 144
N03-/S0?- (R) rain snow R SGDPb N R SGDP 0.71 0.84 0.68 0.57 0.59 0.70 0.61
350 537 824 1964 3613 2314 1322
172 97 172 47 16 68 212
1.65 1.96 1.40 0.75 1.00 0.96 1.69
889 628 683 260 88 515 1246
" N = number of samples. SGDP = sum of precipitation gauge deDths (mm) used in weighting.
=
scavenging efficiencies are different for rain and snow, it is important to consider the effect of precipitation type on chemical species concentration. These data are summarized in Table V for the months when both rain and snow are possible, i.e., October to April. Since precipitation in October is predominantly in the form of rain, the snow data for that month are limited. Except for NO3- concentrations, which may be higher in either precipitation type depending on the month, all other parameters have higher (or comparable) concentrations in rain than in snow. The greatest percentage differences between rain and snow concentrations occur in October for SO4%,H+f,and NH4+and in April for Ca2+and Mg2+. The data from all months with both rain and snow samples are pooled together to show the overall statistics of rain vs snow concentrations. Table VI shows data from three different sites separated by a distance up to 500 km. Even though the absolute values differ, the relative patterns remain the same. Table VI also summarizes the overall results from all 16 sampling sites in Ontario. Clearly, NO3- concentrations in rain and snow are comparable. All the other parameters decrease in concentration in snow. Table VI1 lisb the ratio of NO, to S042-to indicate their relative importance to acidity in rain and snow. For all months, snow has a higher ratio of N03-/S042-than rain, typically twofold or more (except April and November). In rain, the range is from 0.57 to 0.84, showing the NO3is 16% to more than 40% lower than S042-. However, in snow, except in April, NO3- is higher than or comparable to SO?-. It is interesting to note the effect of temperature on this ratio; during the very cold months, December to March, NO< is from 40% to 96% higher than SO:-. The preferential scavenging of nitrate (relative to sulfate) by snow is now well established (7,13,-15) and has important implications for the control of acidic deposition in regions of high snowfall.
Conclusions Precipitation samples collected at 16 sites in Ontario over 1980-1983 were analyzed to assess the monthly variations and relative contributions to precipitation acidity. Major conclusions are summarized as follows: (1)Monthly variations in precipitation chemistry are observed. Sulfate peaks in the summer and bottoms out in the winter. Free hydrogen concentration also peaks in the summer, indicating predominance of SO-: contribution to acidity. (2) Nitrate concentration peaks in March and is lowest in October. Alkaline parameters (e.g., NH4+,Ca2+,and Mg2+)peak in April, which may be related to ground cover change and higher wind speeds during that month. (3) For precipitation types (rain and snow) considered together, NO, contribution to acidity is typically half that of S042-except for December to March (especially for snow) at which time NO3- could be comparable to or greater than Sod2-. (4) Free hydrogen concentrations from laboratory pH measurements are underestimates of the acidity of the incident precipitation. A typical difference of 15% between laboratory and field measurements is observed, suggesting the need for immediate pH (or acidity) measurement after collection. (5) The major neutralizing cations for sulfuric and nitric acids are NH4+(19-36%), Ca2+(10-21%), and Mg2+(2% or less). (6) There is some indication that a fraction of the true acidity (up to 17%) is not accounted for by H2S04and HN03 and alkaline neutralization. The difference may be related to the presence of unmeasured organic acids. (7) Nitrate concentrations are higher in snow than in rain. Other parameters, H+f,SO,-: NH4+,Ca2+,and Mg2+, in rain are either higher than or comparable to those in snow. Acknowledgments We thank the APIOS technical staff for sample collection, William Chang for his assistance in computer programming, Maris Lusis and Frank Tomassini for their review of this paper, and the referees for their suggestions. Registry No. H2S04, 7664-93-9; HNO,, 7697-37-2; H+, 12408-02-5; NH4+,14798-03-9; Ca, 7440-70-2; Mg, 7439-95-4.
Literature Cited (1) Granat, L. Atmos. Enuiron. 1978, 12, 413-424. (2) Wisniewski, J.; Kinsman, J. D. Bull. Am. Meteorol. SOC. 1982, 63, 598-618. (3) Chan, W. H.; Orr, D. B.; Bardswick, W. S.; Vet, R. J. Acidic
Precipitation in Ontario Study, the Event Wetlory Deposition Network, 1st revised ed.; Ontario Ministry of the Environ. Sci. Technol., Voi. 21, No. 12, 1987
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Environment: Toronto, Ontario, 1985; Report ARB-14185-AQM. Bardswick, W. S.; Chan, W. H.; Orr, D. B. Water, Air, Soil Pollut. 1986, 30, 981-990. Handbook of Analytical Methods for Environmental Samples; Ontario Ministry of the Environment,Laboratory Services and Applied Research Branch Toronto, Ontario, 1983; Vol. 1 and 2. Anlauf, K. G.; Bottenheim, J. W.; Brice, K. A.; Wiebe, H. A. Water, Air, Soil Pollut. 1986, 30, 153-160. Chan, W. H.; Chung, D. H. S. Atmos. Environ. 1986,20,
1397-1402. Summers,P. W. In Precipitation Scavenging;Semonin,R. G., Beadle, R. W., Eds.; Technical Information Centre, ERDA Springfield, VA, 1977; pp 88-94.
(9) Finlayson-Pitts,B. J.; Pitts, J. N., Jr. Atmospheric Chemistry; Wiley-Interscience: New York, 1986. (10) Munger, J. W.; Eisenreich,S. J. Environ. Sci. Technol. 1983, 17, 32A-42A. (11) Keene, W. C.; Galloway, J. N. Atmos. Environ. 1984, 18, 2491-2497. (12) Guiang, S. F.; Krupa, S. V.; Pratt, G. C. Atmos. Environ. 1984,18, 1677-1682. (13) Summers,P. W.; Barrie, L. A. Water,Air, Soil PoElut. 1986, 31, 523-535. (14) Topol, L. E. Atmos. Enuiron. 1986, 20, 347-356. (15) Dasen, J. M. Atmos. Enuiron. 1987, 21, 137-141.
Received for review March 21,1986. Revised manuscript received June 26, 1987. Accepted August 25, 1987.
Aerosol Formation and Growth in Atmospheric Aromatic Hydrocarbon Photooxidation Jennlfer E. Stern, Rlchard C. Flagan, Danlel Grosjean, and John H. Selnfeld" Department of Chemical Engineering, California Institute of Technology, Pasadena, California 9 1125
An experimental study of aerosol formation in aromatic hydrocarbon/NO, systems has been conducted in an outdoor smog chamber. Aerosol size distributions were measured as a function of time in toluene, m-xylene, ethylbenzene, or 1,3,5-trimethylbenzene photooxidations to determine the rates of new particle formation and the effects of initial particles on aerosol formation and growth. Aerosol yields from the aromatic gas-phase photooxidations were found to be approximately 2-5% by mass of the starting aromatic species. Simulations of the aerosol behavior in these experiments have been carried out using an integral model that includes a vapor source, homogeneous nucleation, condensational growth, and particle loss by deposition. Predictions from the model are in relatively good agreement with the experimental observations. Results indicate that the nucleation mechanism in these systems is still not completely understood.
Introduction Aromatic hydrocarbons are important components of anthropogenic atmospheric emissions (1). Earlier studies have shown that aromatics are important precursors of organic aerosols in the atmosphere (2-7). Organic aerosol formation occurs via gas-phase degradation routes that produce a condensable vapor that is converted to aerosol via homogeneous nucleation and heterogeneous condensation. These mechanisms have not been studied previously in the aromatic hydrocarbon/NO, system. In a system with a source of condensable vapor and no initial particles, the partial pressure in the vapor phase can build up to supersaturated levels, at which point homogeneous nucleation can occur and aerosol particles are formed. Subsequent condensation onto these particles will help relieve the vapor-phase supersaturation, causing nucleation eventually to cease. After this point, subsequent gas-to-particle conversion will occur by condensation. However, in the presence of initial particles, condensational growth can begin as soon as a low vapor pressure species is generated. With a sufficiently large number of initial particles it is possible that the vapor will never achieve the supersaturation needed for nucleation, as condensational growth will always be the dominant mechanism for gasto-particle conversion. We have studied experimentally in an outdoor smog chamber the effect of preexisting 1224
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particles on the subsequent aerosol behavior in aromatic photooxidations in order to observe the nucleation suppression occurring with varying levels of initial particles. The analysis of the experimental data tests our ability to simulate simultaneous nucleation and condensation in such photooxidation systems. The principal unknown quantity in the description of aerosol formation and growth is the rate of homogeneous nucleation of the aerosol precursors. Indeed, predicting the rate of homogeneous nucleation of a substance is one of the long-standing challenges in condensed matter physics. The two most popular nucleation theories, the so-called classical theory and the Lothe-Pound theory, differ in their predicted rates by something approaching 20 orders of magnitude (8). Thus, the data obtained from the smog chamber experiments on the rates of particle formation and growth will aid in assessing our ability to describe nucleation of organic aerosol constituents. Moreover, studying how those rates are modified in the presence of foreign particles provides additional measures of the nucleation rate as the condensing species concentration is altered by condensation onto the preexisting particles. In the next section we present a description of our experimental facility. Next we discuss the inversion of the raw aerosol data, followed by a summary of the measured aerosol yields. Finally, we describe the aerosol model we have used and present the results from the simulations.
Experimental Description The essential nature of the experimental system is described in Leone et al. (9). The Teflon chambers used for this study were constructed of 10 Teflon panels of 1.2 m X 10 m each. Gas-phase sampling was done from Teflon tubing that extended approximately 30 cm into the chamber through Teflon ports. Aerosol-phase sampling was carried out through separate copper lines extending about 15 cm into the chamber. These copper lines minimized depositional losses of the aerosol. Most of these experiments were conducted in dualchamber mode. In dual-chamber mode, a poly(viny1 chloride) (PVC) pipe was placed across the chamber at its midpoint to divide the reactor into two equal sides. Air leaks across the divider were undetectable. The volume of each side of the divided chamber, as measured by the injection of known amounts of NO or NOz,was approxi-
0013-936X/87/0921-1224$01.50/0
0 1987 American Chemical Society
