Jan 28, 1985 - or monthly basis (Wisniewski and Kinsman, 1982). ...... John Wiley. ... of the size Wisniewski 1. and Kinsman J. D. (1982) An overview of acid.
CHEMICAL DIFFERENCES BETWEEN EVENT AND WEEKLY PRECIPITATION SAMPLES IN NORTHEASTERN ILLINOIS DOUGLAS
E. WURFEL and
L. SISTERSON, BRENT
BARRY
M. LESHT
Environmental Research Division. Argonne National Laboratory. Argonne, IL 60439. U.S.A. (Firsr receiced 20 SepI~~er
1984 and in~naf~orrn 28 January 1985)
Abstract-We examine the chemical differences between event and weekly samples of precipitation collected in northeastern Illinois from April 1980 to March 1982. Analyses were conducted for H’, Ca’*, Mg”-. NHf, SO:and NO; concentrations as well as for pH and conductivity. In addition, the 1980-1981 samples. were titrated to determine the total, strong and weak acid concentrations. Although seasonaland annual precipitation amounts were different for the two years, the general pattern ofevent and weekly sample ion concentrations were similar, Weekly samples had significantly less [NH; ] and higher laboratory pH in all seasonsand more [SO:- J in every season but summer. Weekly samples had significantly more [Ca’-] and [Mg’+] during seasons with little precipitation. Event and weekly [NO;] were never significantly different. The weekly samples had more total acidity in the spring but less in the summer. The observed differences may be attributed to chemical degradation of the weekly samples while waiting collection and during shipment between the fieid and the laboratory. Key word index: Acid pr~ipitation, pr~ipitation.
pr~ipitation
chemistry, pr~ipitation
INTRODUCTlOS Many
of the problems
associated
with
the inter-
chemistry network data (reviewed by Hakkarinen, 1983) involve factors such as collector type, samphng location, sampling frequency, sample handling, and anaiytical technique. These problems are especially relevant to acid pr~ipitation monitoring, as samples are collected by different networks on an event, weekly, or monthly basis (Wisniewski and Kinsman, 1982). Although some intercomparisons of co-located precipitation collectors have been made by Berry et nl. (1975) and dePena er al. (1980), and the chemical stability of precipitation samples has been studied by Peden and Skowron (1978), no chemical comparisons of event and weekly precipitation samples have been made over long time periods in an attempt to assess annual and seasonal differences that may resuh because the integrated weekly samples may have changed during storage. If such differences exist, the weekly sampfes would be less representative of the actual precipitation chemistry than the event sampies. Small annual differences between weekly and event precipitation samples may be significant in long term wet deposition assessment and trend evaluations. Further, relatively large seasonal differences would lead to inaccurate assessment of wet deposition to crops during short growing seasons if only weekly data were used. In this study we examine the differences in acidity and other chemical properties in event and weekly comparison
and interpretation
of precipitation
integrity, weekly and event
precipitation samples collected at Argonne National Laboratory, approximately 35 km SW of Chicago, Illinois, from April 1980 to March 1982. The event samples were collected as part of Argonne’s efforts in the Multistate Atmospheric Power Production Pollution Study (MAP3S) and the National Acid Precipitation Assessment Program, while the weekty samples were collected as part of the National Atmospheric Deposition Program (NADP) precipitation sampling network. Both sets of samples were analyzed for the typical inorganic ions present in precipitation. During a portion of the study the event and weekly samples were titrated in the field for the strong, weak, and total acid concentrations. In addition to the chemical determinations, field pW and conductivity measurements were performed on al1 samples when they were collected. The event samples were analyzed at Argonne, and the weekly samples were analyzed at the NADP analytical laboratory in Champaign, Illinois. Differences were calculated by combining pr~ipitation-weights individ~i event sample concentrations over a week (composite event) with the corresponding weekly sample concentrations. 2. SANPLE COLLECTION, STORAGE AND ASALYTICAL PROCEDURES
Argonne’s precipitation collation site is an open grassy field free from large trees and obstaclesfor at least SOm in all directions and is located 4OOm from a lightly traveled, paved roadway. Although there are many pollutant sources within 50 km of the collection site, most are associatedwith Chicago.
1453
1354
L. SETERSONer al.
DOCGLAS
which is approximately 35 km northeast and predominantly downwind (Fig. 1). TWO Aerochem Metrics’ automatic wetdry precipitation collectors placed approx~t~ly five meters apart were activated by a single precipitation sensor. Except for the small difference in motor speeds, the collectors opened simultaneously. An event marker recorded the open and closed time periods of both collectors to the nearest minute. Event samples were removed after each precipitation event during the day, where an event was defined as a period of precipitation in which no interruption was longer than f fi; i.e. if two periods of precipitation were separatedby more than one hour, they were considered two events. When precipitation occurred at night, the event samples were removed as soon as possible the following morning. In some cases, precipitation collected during the weekend was not removed from the collector until Monday morning. On several occasions. particularly at night, more than one event was collected in a single sample; for this study, these cases were considered a singleevent. Weekly pr~pi~tion samples were removed each Tuesday morning at #OOCST as specified by the NADP field Observer Insrruction Manual. Both the weekly and event precipitation buckets were changed at this time even if precipitation did not cam, to reduce the possibility of con~mination or exposure to dry deposition. Immediately after removal of event precipitation. the sample bucket was agitated to insure complete mixing and uniformity before withdrawing two 4Oml aliquots with a plastic, sterile syringe. During wintertime when snow occurred or when the collected rain was frozen, the sample was allowed to thaw completely before the aliquots were withdrawn. From April 1980 to March l981, one of the aliquots was frozen for later chemical analysis. After March 1981 and throughout the rest of the experiment, the aliquots were refrigerated at 4 “C until chemical analysis. The sensi-
Argonne~ational z^
e
Labbratory \
tivity ofour analysis to both storage procedures was tested by analyzing split samples. No statistical difference (at the 5s; signnificancelevel) could be found between any anion or cation concentration in the i 5 frozen and refrigerated sample pairs. The second of the two aliquots was used for the determination of field pH and conductivity. The weekly samples were also agitated upon removal and a 4Oml aliquot was withdrawn with a plastic, sterile syringe. This aliquot was used for the determination of geld pH and conductivity. The remaining sample in the bucket was tightly sealed and shipped. usually the same day, to the NADP analytical laboratory for chemical analysis. Typically, the sealed samples arrived at the analytical laboratory within two days. The event samples were analyzed by Argonne’s Analytical Chemistry Laboratory (ACL) which used liquid ion chromatography for Na’, K’, NH;, Cl-, SOi- and NO;, and game atomic absorption for Ca” and Mg”. The frozen samples were allowed to thaw to room temperature overnight prior to analysis and were agitated and filtered, during injection, with a 0.25pm filter. The filters were first purged with a portion of the sample to eliminate trace contamination by the filters themselves, The refrigerated samples were kept refrigerated until the day of analysis and similarly filtered. Samples were stored, either frozen or refrigerated from 4 to 6 weeks before analysis. The weekly samples were analyzed by the Central Analytical Laboratory (CAL) at the Illinois State Water Survey, the NADP analytical laboratory using an autoanalyzer for all ions except Ca”, Mg”, K’ and Na”, which were determined by flame atomic absorption. Usually pH and conductivity measurements were made on the day the samples arrived from Argonne and the samples were then filtered and stored at room temperature. Samples were typically analyzed within 2 weeks but no later than one month after arrival at CAL (Skowron, pers. comm., 1983). Between April 1980 and March 1981. both the event and weekly samples were further analyzed for acidity by titration. After field measurements were made, simple acid-base titrations were performed according to the procedures developed by Gran (1950, 1952). Details of the titration procedures employed are discussed elsewhere (Sisterson and Wurfel, 1984).The strong acid concentration was determined from titration curves and a plot of Gran’s function (Fig 2). The intersection of Gran’s function with the abscissa gives the
II
0)
2 42 .Z ;i; _I
I
\
t
i
-----150
I
ZOkm
!
41 ‘,,,,,,.,i,,,,,i,l,,I 69
87
LongitZie
(W)
Pig. 1. Local source inventory map around the sampling site. The area of the circles is proportional to the annual sulfur emissions from the source; the largest source shown emits c 165,000 tons of Sy-‘.
* No o3icial endorsement of this product or any other, mentioned in this manuscript are implied by Argonne National Laboratory or its sponsoring agencies.
Fig. 2. Plots of a typical base titration curve and Gran’s function (‘I‘). The intersection of the extrapolated linear portion ofY with theab~is~co~esponds to theamount of titrant (Vi,,) needed to neutralize the strong acid component of the precipitation sample. After Sisterson and Wurfel (1984).
Chemical differences between event and weekly precipitation samples in northeastern Illinois
equivalences of NaOH required to neutralize the strong acid component of the precipitation sampk. Total acidityMS determined as the quivalent amount of basic titrant needed to t-a& the pH of the sample to 9.5. The d&TereMv between total and strong acid concentrations yielded the c~nc~ntration of weak acid. Conductivity and pH measurements were made at Argonne using a Markson Digita8Eleftromark Analyzer pH meter in conjunction with a %etkman Futura r&liable, glass ~mbination pH electrode and a Ma&son conductivity cell.
3. EVALUATION AND ELIMINATIOX OF DATA
Much of this study addresses fundamental questions that arise when comparing analytical dete~inations made by two laboratories using different analytical techniques. Since it is conceivable that any differences may be small, objective criteria must be developed to determine data outliers so that any apparent differences are not due to a few atypical precipitation samples. For this reason, we developed and used an extensive data screening protocol to eliminate questionable samples. This protocol was based both on collection and analytical criteria and is outlined below. Those individual or weekly samples that were affected by obvious ~cn~mination (e.g. bird excrement) were eliminated. Those eases where collector malfunction resulted in one coflector failing to operate in tandem with the other were also eliminated from the data base. If, however, both collectors operated in tandem and collected sufficiently Iarge samples for anaiysis, the data were included in the data base, even if only part of the precipitation event was sampled. For all acceptable data, composite and weekly sample precipitation amounts rarely differed by more than -. 3qio. ion baIances of the individual event and weekly samples were calculated to evaluate the quality of chemical anaiysis. The ion balance (IB) was defined as IB = [ (anions-~tions)/(anions
fcations)]
x 100~,.
Individual event samples were reanalyzed if the chemical analysis resulted in the ion balance exceeding rt 14%. Approxi~tely loo/, of the individ~l event samples were thus reanalyzed, a reanalysis fraction similar to that (5 8%) resulting from the more complex NADP quality assurance (Peden, 1983) procedure. Throughout the study period individual event sample ion balances rarely exceeded 4 20 ‘&,with most of the balances indicating surplus cations. For those individual event samples accepted afrer reanalysis, the reanatyzed concentration values were used in the data base. On rare occasions, reanalysis showed no significant change in the individual ion concentrations. It was assumed that these una~eptable ion balances were due to incomplete analysis; i.e. the presence of ions, probably organic, not identified. In a separate study, Irving et al. (1979) found weak organic acids in event precipitation sampies at approximately the same frequency as the poor ion balances found here. Since only inorganic species were being investigated in this
1455
study, these samples were included in the data base. During the study period, weekly sample ion balances never exceeded f 10%. Composite samples during the 2-y period usually included more than one precipitation event. Some of the ind~vid~i pr~pitation events, however, were too small for complete field and laboratory analysis. Therefore, there were cases when the chemicaf analyses contributing to the composite event sample were incomptete. In order to evaluate the effect of the unanalyzed individual events on the weekly concentration sum, we estimated pseud~oncentrations for the unanalyzed individual events. Assuming that the w~klyandcom~siteevent~on~ntrations wereequal for each ion
j=I
where fi], is the weekly ion concentration, [i$ is the individual event ion concentration, p, is the indrvidual event pr~ipi~tion amount and n IS the number of individual events in the week, the pseudoconcentration for unanat~ed individ~l event X:was defined as
[ilk =
Pk
If there was more than one unanalyzed individual event in a week, they were combined into one pseudoconcentration. The pseudoconcentrations were used only as a criterion for sample acceptance. We eliminated individual ions from the data base for those weeks in which the pseudoconcentrations were greater than 10 times the highest concentration found for that ion in either individual event or weekly samples during the 2-y study period. Composite samples were based only on the actual analyzed samples; the pseudoconcentrations were not used in the sum. This procedure resulted in incomplete composite event samples being accepted when the pr~~pi~tio~ amount of the unanalyzed individual event sample was less than 2.5 % of the weekly total sample amount, In other words, incomplete composite event samples were classified as acceptable when only very small amounts (in comparison to the weekly amount) were missing. Since ion concentrations tend to be related inversely to precipitation amount, the individual events with very small amounts often have very large ion concentrations. To a first approximation, however, an ion concentration of an individual event sample with a volume of only 2.5 % or less of the total for the week would have to be larger than 10 times the highest individ~l ion concent~tion observed to make a significant contribution to the composite event ion concentration. Hence, based on this empirical analysis of our observations, it appears appropriate to reject composite event samples from the data base if an
DOUGLASL. SWXRSONer al.
1456
unanalyzed individual event sample represents more than 2.5 YQof the total precipitation amount for that week. ‘This may not be true in general. The composite event and weekly individual ionconcentration pairs were further analyzed by the least squares regression technique presented by Prahl (1981a, b) to avoid problems in which a few large differences dominate the results. Each ion pair was plotted (e.g. Fig. 3)and any pair that fell outside of + 3 standard deviations was tentatively considered to be an outlier, This pair was then carefully reviewed for poor ion balances (exceeding f I4 %), for incompleteness in composite event samples (although initially accepted), and for any field note that might indicate a problem in either analysis or storage that was not severe enough to eliminate an individ~l event or weekly sampfe from the data base at an earlier stage. If one or more of these problems was found, the outlier pair for that particular ion was eliminated from the data base. If the sample pair was an outher for several unrelated ions, then the entire composite sample was eliminated from the data base. However, outlier pairs were not eliminated from the data base if no reasonable cause could be found. For the period April 1980 to March 198 1, a total of 83 individual event and 41 weekly samples were collected; for the period April 1981 through March 1982, a total of 84 individual event and 48 weekly sampIes were collected. After data evaluation and elimination, 56 individual event (68 %) and 29 weekly (71%) samples survived for the first year and 70 individual event (83 %) and 44 weekly (92 “/,) samples survived for the second year. However, the actual number of individual event and weekly sampfes used in the intercomparison study varied by ion since we based our comparison on individual ion species rather than on analysis of only those composite event and weekly samples in which no individual ion was eliminated. This made more efficient use of all the data.
ANL SULFAY’E (peq/L)
Fig. 3. Weekly (NADP) vs event (ANL) &fate ion concentrations for pr~~pitatio~ samples collected from April 1980 to March 1981.The heavy dashed line represents a slope of 1, the solid line is the least squares regression, and the dotted line represents f 3~.
4. ANALYTICAL
UNCERTAINTY AND FIELD BLANKS
An estimate of the analytical uncertainty (bias and precision) associated with each laboratory and each parameter determination is necessary to correct raw data. Concentration estimates of both laboratories analyses were corrected for bias. Then, the analytical precision associated with each analysis was determined and combined with other appropriate analyses (weighting by precipitation amount) to estimate the analytical uncertainty associated with the estimates of seasonal, annual and biannual mean values. Differences between the weekly and composite event mean concentrations are considered meaningful here only if they are greater than the combined weighted analytical uncertainties. Both CAL and ACL report analytical results without correction for bias or precision. Bias (8) is defined as the systematic component of analytical uncertainty and is expressed as mean value -certified
B=
value
certified value
1x
lOOo/.
Precision (P) is defined as the random component of analytical uncertainty and is expressed as P I
c L
2 x standard deviation mean
1 +100%. I” J
Bias and precision for NAL3P sample analyses were taken from the CAL Qualiry Assurance Reportfor 1979 (Stensland et al., 1980). The reported values that best bracketed the actual concentrations of individual ions analyzed in weekly samples collected during the 2-y study were used. The 1979 values were used because no other published reports were available when the present study began; however, these values did not change significantly in 1980-1982 (Peden, pers. comm., 1982). Bias and precision for the individual event samples were determined by blind analysis of standards prepared by chemists not involved with the precipitation study. These standards were not certified by an independent laboratory. The values of the prepared standards corresponded as closely as possible to those used by CAL in their determination of bias and precision. The estimates of individ~l event bias and precision were determined from 10 analyses of each ion six separate times during the 2-y study. The average of the six trials was used for each of the ion concentrations. The individual values did not vary significantly. Uncertainty determinations for precipitation amounts were done by weighing both event and weekly empty buckets at Argonne. Weekly empty buckets were preweighed at CAL; hence, reweighing the weekly buckets at Argonne (taken to be the certif% value) provided bias and precision estimates for sample amounts. Tables 1 and 2 show the analytical uncertainty, expressed as bias and precision for each parameter in the intercomparison study. The individual sample amounts and analytically determined ion concentrations were first corrected for
Chemical differences between event and weekly precipitation samples in northeastern Illinois
1457
TabIe 1. Bias for individ~l parameters in the intercom~~son study determined from the Analytical Chemistry Laboratory (ACL) for event sample analysis and the Central Analytical Laboratory (CAL) for weekly sample analysis. Identical pH electrodes were used by both laboratories, therefore the NADP statistics are given for both ACL and CAL for H’. Although weekly buckets were preweighedat CAL, they were reweighed at Argonne to generate the statistics
Parameter Rain amount H’
ACL (event) CAL (weekly) Concentration Bias (9,) Concentration Bias (93
3QOg 13oOg 83.2@q /- 1 871.Oreqf-r
car+
Mg’+
K’
6.84peq F-’ 15.97peq/*’ 68.86 +teqI - ’ 4.69 j.teq I-’ 13.98 req (- I 32.90 peq P-l 3.35 req i - L 9.97 fieq (- ’
0.0
0.0 +4.5 +3.4
3mg 13oog 83.2fieq t- ’ 871.0yeqT’
+2.0 +I.S +4.s + 3.4
3.34 peq /-I 17.91 peq (-’ 72.36 peq (-’
- 10.4 $4.4 +3.1
I.97 peq F’ 9.38 peq (- ’ 24.68 peq (- ’
- 14.3 + 2.0 -0.2
+4.3
0.84 geq ( - ’ 6.90 peq (- ’
- 7.4 +OS
+ 9.6
- 15.8 -12.7 + 14.0 +13.3
0.0 + 15s
Na’
2.74 peq I - ’ 5.66 geq t-t 34.45 peq f - ’
$41.9 3.0 - 12.2
3.05 fleq !- ’ 6.83peq(-’ 21.?.Speqf-’
- 14.3 + 0.4 +O.i
NH:
0.55 beq T ’ Sl.OOneq /-I
- 5.3 -3.2
0.55 peq i-r 1.66j&Ji-’ 113.65fieq f - ’
+ 8.0 + 1.1
1.42 peq t- ’ 19.36peq /-t 61.29 geq P-r
-4.2
- 2.4
7.90 peq I- ’ 27,IOpeq /-’
NO;
SO:-
Cl‘
13.21 peq f-r 20.41 fieq /-’ 163.93lreqT’ 4.80 Meq(- t 28.21 peq r’-’
bias by Iinear intonation between the values given for each parameter in Tabfe 1. The bias for the lowest ion concentrations were assumed constant to the detection limit; bias for concentrations greater than the largest ~on~nt~tions given in Tabfe 1 were assumed to be the same as for the hugest concentrations. Precision was then determined by linear interpolation of individual samples (corrected for bias) for the values given in Table 2. The detection limit is defined as twice the standard deviation of the baseline noise; therefore, the precision is f: 100% at the detection limit. Precision for concentrations greater than the largest concentrations given in Table 2 were assumed to be the same as for the largest concentrations. Numerous blanks (deionized water rinses) were analyzed for the event buckets, the glassware used in the field analyses, and the storage bottles. The deionized water used for rinses was also analyze& Similar tests were performed for weekly samples and reported by Stensland et al. (1980). Both ACt and CAL participated in blind analyses sponsored by the U.S. Environmental Protection Agency and U.S. Geological Survey. The results indicated that both
+a.! + 1.6 -0.5
-3.5 + 6.6 -0.2 -0.2 - 5.6 +0.3
15.OO~eq8-r 106.65fleq (-’ 213.3O~eqI-’ 9.87 I.teq I-’ 39.78 peq f - ’ 79.27 fleq I- ’
-2.2 +0.3 -0.2 + 10.4 f 4.6 + 4.4
laboratories agreed well with cert&d ion coneentrations of prepared simuiants used as the blind (Sisterson, 1980, 1981). Results of event bucket rinses indicated detectable but not q~nt~bIe amounts of K” in some of the blanks. Although the [K’] was smatf, event (and weekly) concentrations of K+ were aiso small (typically O.iOmg/-’ or less). The K’ concentration analyses was therefore eliminated from the intercomparison study since the small amounts of [K’] in the event buckets could si~~~ntiy affect the amount of [K’] in the collected precipitation sampie. The Clpeak was frequently clipped by IO-30% or missed entirely when its concentration was low due to its close proximity to the water dip (Williams, pers comm., 1981). Since nearly all event and weekly Cl- concentrations were small (typically less than 0.35mg(- ‘f, cl- was also eliminated from the intercomparison study. The Na’ concentrations were also very low in both the individual event and weekly sampies (typically less than 0.15 mgt- ‘) and were poorly determined by ion chromatography in the event samples. From the limited number of composite event and
1458
DOUGLASL. SISERSON et uf.
Table 2. Same as Table 1 except for precision ACL (event)
Parameter
Concentration
Rain amount
Precision ( %)
+ 100.0
2e
k1.9 +0.5
3mg 13oog H’
83.2 peq f - ’ 871.0jieq f-l
+ 10.1 * 3.0
+ loo.0
2g
* 1.9 +0.5
3mg 13oOg 83.2 peq f- 1 871.0peqr1
+ 10.1 + 3.0
0.50 fieq (- ' 6.84peq r - 1 15.97peq (-’ 68.84 peq f- ’
+ 100.0 & 17.4 * I.6 k6.8
72.36 peq !- I
i: 100.0 26.7 14.0 +2.6
0.25 jdeq /- ’ 4.69 peq f- ’ 13.98 peq 6- ’ 32.90 peq I-’
+ 100.0 k4.5 + 0.0 * 0.0
0.17peq 1“ 1.97geq/-’ 9.38 peq t ‘ ’ 24.68 peq / _ ’
It loo.0 + 3.0 F3.l i_ 2.0
K’
1.28 peq f-’ 3.35 fieq /-I 9.97 geq f-t
f 100.0 F38.6 + 5.3
O.lOfieq i-’ 0.84 peq /- ' 6.90geq /- ’
* loo.0 + 10.7 i3.1
Na’
0.87 peq f - ’ 2.74 peq f - ’ 3.48 peq (- ' f9.58peq (-”
+ loo.0 * 16.3 k4.2 & 8.0
0.17fleqk-’ 3.05 fieq I - ' 6.83,ueq 1c-' 21.75jfeq t- ’
* 100.0 k6.7 4 1.4 + I.5
NH;
1.1i peq i-’ 5.54 Meq(-’ 78.12 peq /- ’
f 100.0 +2.1 +3.5
I.11 jieq(“ 5.54 peq /- I 16.63peq/-L 113.65peq/-’
* 100.0 + 37.0 +10.1 + 3.3
NO;
0.16peq 1.42 peq 23.71 peq 63.39 peq
(I’ (- i /-’ C-l
* 100.0 + 12.3 + 7.6 &-5.0
0.32 peq / - 1 7.90peq P- ’ 27.10peq L-’
k 100.0 rf:5. I + 3.6
so:-
1.04fleq 13.21 peq 73.82 jieq 126.35 peq
C-l f-’ C-’
f 100.0 & 12.3 24.6 24.1
0.29 yeq f _ ’ 15.OO~eq 1”-’ 106.65 peq t*‘- ’ 213.3Opeq f-’
1.13 geq (-’ 2.99 fleq /- ’ 6.86 geq i- ’
i 100.0 f. 17.0 k8.7
Ca”
Mg’-
a-
f -t
weekly cases where Na’ and Cl - concentrations could be determined, equivalence concentrations between them were nearly equal. This is not surprising since maritime tropical airmasses (the assumed source of Na * and Cl-) have low concentrations of sea salt once distant from oceans. Therefore, elimination of both Na+ and Cl- from the ~ter~~~son study did not sign&antIy alter ion baiancts. A&o, conductivity measurements were performed but not included in this study. 5. DATA
CAL (weekly) Concentration Precision ( “/,j
ANALYSIS
AND OBSERVATIONS
Annual and seasonal weighted means of ion concentration and acidity and the associated analytical uncertainties were computed for the individual study years as well as for both years combined. The differences between the composite event (hereafter event unless otherwise noted) and weekly means were first tested for significance using the Wilcoxon match-pairs signed-ranks test (Wilcoxon, 1949)and then compared
1.00geq I-’ 3.34 jieq /- ’ 17.47peqf-’
0.56 peq 9.87 peq 39.78peq 79.27 eeq
t(” ft-
’ t
L t
+ 1OQ.O + 13.5 * 1.7 21.2 * 100.0 2 11.6 +1.4 f 2.1
to the associated analytical uncertainties to evaluate the magnitude of those differences If the differences were larger than theassociated analytical uncertainties, then they were considered meaningful. The averaged data are listed in Appendix 1 and include the number of weeks of data used in the dete~ination, the event and weekly mean concentrations of icing ions and their differences expressed in peq /- ‘, thecoefkient of variation of the difftrence, and the two-tailed probability significance level obtained from the Wilcoxon test. If WCand Wu are the composite and actual weekly sample concentrations, respectively, and c and a are the associated analytical uncertainties, respectfully, then the coefficient of variation of the difference in concentrations is (2 + 2)1’2 Wc-Wa ’ where a value less than 1.00 indicates a ‘m~ningfui non-zero difference in this study.
Chemical differences between event and weekly precipitation samples in
Figures 4(a) and (b) show the di&ences in per cent between seasonal means of all event and weekly individual ion concentrations for the entire study period. However, only those ions whose differences have been determined to be significant by the Wilcoxon test are discussed in the text. Tables 3(a)-S(a) show those differences determined to be sign&ant at the 0.05 level for seasonal and annual subsets of the data. Tables 3(b)-S(b) show the corresponding actual differences between event and weekly mean concentrations as well as the coeE&ient of variation (in parenthesis) described in the preceeding paragraph. Generally, NADP sampIes have greater [Mg’+ J, [SO:-], but less [H’] as determined by laboratory pH @Ho. Concentrations of Ca*+ are greater in weekly samples only when weekly samples also have greater [Mg”+]. However, at other times, event samples have greater [Ca”]; Ca’+ is the only ion in the intercomparison study that does not show consistent behavior. There is never any statistically significant difference between event and weekly [NO;]. The largest differences in acidity as determined from pH!, tend to occur during those seasons that have larger [Ca’ ‘1 and [Mg”], but differences also occur during other sasons. Concentrations of SOi- are greatest in weekly samples except in summer for 1980 and in spring and summer for 1981, while [NH:] is always greater in event samples regardless of season.
northeastern Illinois
1459
Percent Difference Between Event and Weekly Mean Seasonal CoJcentrat!ons of ,w, , Cay,Mg:‘,H flandl;l f , ,
l
a
s
F
W
.S
S
F
1981
1980
Fig. 4,(a) The differences in percent between seasonal means of composite event and weekly [H* r’], [H’ f]. [Ca*+] and [Mg”] for the entire study period. Solid symbols indicate the coefficient of variation of the difGrence was less than 1.00 indicating a m~ningful non-zero difference.
6. DISCUSSION OF OBSERVATIONS It is not surprising that weekly samples which have larger [Ca*+] and [Mg’+] also have less [H+]. Generally, the most important mineral components of natural windblown dust are quartz, carbonates, feldspars and clay (Winker, 1973).The calcareous soils of northeastern Illinois would therefore be expected to contain a large amount of calcium and magnesium carbonates. Carbonates can readily react with the strong inorganic acids present in precipitation twinkler, 1976) by the following displacement reaction example for Ca2*
Percent Difference Between Event and Weekly Mean Seasonal Concentrations of +,ool , sop-. gOi, a:d Nt(t , i
+eo +@I
1
CaCO, + H,SO, = CaSO, + Hz0 + COz, and CaC03 + 2HN01 = Ca(NO&
+ Hz0 + CO1.
Similar reactions occur with Mg’*. These reactions can decrease the pH of precipitation. Partial neutralization of acidity can also take place in precipitation before it is collected. This was observed during the multilaboratory Oxidation and Scavenging Characterimtion of April Rain (OSCAR) experiment in April 1981. A sequential precipitation collection device was operated by Argonne as part of the intermediate network of OSCAR (Raynor, 1982). Precipitation samples were collected in plastic bottles that were changed after an accumulation of ap proximately 100 ml, corresponding to 0.06 mm (0.024
-20
t
-&I ~
-60
\
cl so: 0
NO;
Q NH;
t -60 t 4001
s
f
s
I
1980F
I
I
i
t
w
s
S
F
f88f
Fig. 4(b). Same as Fig. 4(a) except for [Sot-]. [NO;] and [NH:].
DOUGLAS L. SISTERSON rt ai.
1460
Table 3a. Two-tailed probability significance levels less than 0.05 (5 “,) for the differences between mean weekly and composite event precipitation sample ion ~ncentratjons for April 1980 to March 1982.The ( +) indicates weekly samples have larger concentration, the ( -) indicates composite event samples ion concentrations are larger Parameter
All data
Spring
Summet
Fall
Winter
H’f H‘/ Ca” ‘Mg:’ NH; NO; soi -
-0.0018 - 0.m
- 0.0280
- 0.~80
- 0.~8 + 0.0038 + o.ooo2 - o.ooo2
-o.Mx)z
+ o.ooo2
+ 0.0002
+ o.ooOG -o.oooo -0.ooo2
+0.0010 - o.OcQO
+O.OOGG +0.0098
+0.0114 -0.0012
Table 3b. The percent difference between mean weekly and event precipitation sample ion concentrations for April 1980to March 1982.Thecoefficient of variation of thedifference isgiven in parenthesis. A value less than 1.OO indicates the absolute difference is greater than the analytical uncertainty. The ( +) indicates weekly samples have larger concentrations, the ( -) indicates composite event samples ion concentrations are larger Ail data
Parameter H-f H’C &?’
Spring
Summer
- 4.6(0.43) - 11.9(0.17)
- 5.3(7.47)
- lO.l(O.33)
+ 1.710.02) - 7.8(0.04)
- 9.5(0.07)
f 2. I(O.03) - 6.9(0.08,
+ 5.7(0.IO)
+6.8(0.13)
Mg’* NH; NO; SO:-
Fall
Winter
- 14.0(0.26) + 3.4(0.10) + 4. I(O.02) -6.6(O.l0)
-23.1(0.16)
+ ~4.8(0.07}
+ l3.2(0.08)
+ I b(O.05) - 20.5(0.@4)
Table 4a. Same as Table (3(a) except for April 1980 to March 1981
Parameter H’f w+c Ca2+
2+
All data 198@-1981
Spring 1980
Summer 1980
- O.oGO8 - 0.0030
- 0.0278
-0.01 I8
+ 0.0278 - 0.0278
NO; NM;3I-
$ o.ooOo - 0.m
so:-
+0.0168 +O.OOOO to.0432
Fall 1980
Winter 198c-1981
- 0.0208
-0.0180 + 0.0280
-0.0180 +0.0180
- 0.0076
+O.Ol%O -0.0180
+0.0180 - 0.0280
+ 0.0180
+0.0180
Table 4b. Same as Table 3(b) except far April 1980 to March 1981 Spring 1980
Summer 1980
Fall 1980
Parameter
All data 1980-1981
H’f H’L ca2+ Mg” NH: NO; so:-
- 1l.S(O.26) - 12.5(0.41) - 17.6(0.291 -5.4(1.10) - LO(O.69) -2.2(0.19) +3.2(0.17) + 3.1(0.05) + 2.0(0.03) + 3.0(0.04) - S.l(O.20) -8.3(0.10) - 7.6(0.07) -8.3(0.12) f Z.S(O.19) 17.5(O.oP1 + 6.6(0.14) + 6.2(0.23)
inches) of rain. The pH and conductivity of each sequential sample were determined immediately after colleCtion. Their patterns for a typical convective storm are given in Fig. 5 and show a decrease in precipitation acidity (increasing pH) and cond~tivity (ionic strength) as the rain rate increases. The contrast-
Winter 1980-1981 - 9.4(0.54) + 5.8(0.07) + 3.4(0.03) -4.7(0.19) f 11.3(0.13~
ing pH and conductivity patterns of the first OSCkR event which occurred after a prolonged dry period are shown in Fig. 6. The National Weather Service reported dust aloft over northeastern Illinois during this time and the skies were noticeably turbid before the onset of precipitation. In this event, precipitation
1461
Chemical ditTerences between event and weekly precipitation samples in northeastern Illinois Table Sa. Same as Table 3(a) except for April 1981 to March 1982 All data 1981-1982
Parameter H’f Hf! Ca” z+ NO: Et&
Spring 1981
-0sloOO -0.0218 + o.OOQO -0.00@3 -0.0058
sot-
Summer 1981
Fall 1981
Winter 1981-1982
- 0.0022 + 0.0230 i 0.0022 - 0.0022
-0.0100
- 0.0076 - 0.0050 - 0.0208
+0.OOUO
-i-0.0034 - 0.0034
- 0.0070
+ 0.0034
+ 0.0050
Table 5b. Same as Table 3(b) except for April 1981 to March 1982 Parameter
Ail Data 1981-82
Fall 1981
Summer 1981
Spring
1981
Winter 1981-82 -5.0(1.89)
H’f H’f Caz+
- 17.6(0.24) - 20.0(0.24) - 28.7(0.17)
- 16.4(0.IS) -.2(0.12)
2+
1.6(0.03) - 8.6(0.05)
+
z? NU4 so3-
- 9.8(0.~~
-t-2.6(0,12) + 3.2(0.03) - 5.2(0.15)
-2.2(0.16) + 4.8(0.02) -7.5(0.12) -16.1jO.06) +13.0(0.11) t14.2(0.01)
+5.1(0.14)
OSCAR
4/c%-14181
ANL
CONDUCTIVITY
0
300
600
900
I200
I500
Ia00 mm OF RAIN
2100
2400
2700
3000
3300
Fig. 5. Determination ofpH and conductivity for sequential samples collected at Argonne on 13-14 April 1981.Collection time in minutes is given above the pH graph; storm type is given below the graph. Praipitation samples were collected in bottles that were changed after an aaumulation of _ 100 g (- 100 ml), corresponding to 0.6 mm (0.024 inches) of ram. acidity increased (decreasing pH) as rain rate increased.
The chemical analysis of these samples showed that the first few sequential samples c~ilcctcd were cloudy and contained larger [Cat+] and [Mg”J than in later ~ucnt~~ sampics. Because these particles are large compared to particies contributing to acidity in precipitation, they are efficiently scavenged and removed earlier by precipitation. The higher concentration of Ca2 * and Mg’+ results from the partial neutralization of the acidity early in the rain event. As the rain continued, most of the neutralizing particles had been removed and the rain samples became more acidic. The same reactions that partially neutralize acidity
during precipitation may also continue within the sampic while in the bucket prior to collection or during storage awaiting analysis. P&n and Skowron (1978) have shown that soil-derived ions (Ca2*, Mg’“, I(*) in collected precipitation show dramatic increases in concentration with time when not stored properly. They hypothesized that soil particles not yet in cquilibrium with the aqueous phase slowly dissolve. An ion exchange mcehanism is probably involved, with the soil particles acting as a cation exchange medium. These effects are most pronounced when the undissolved particulate co~entration in collected precipitation samples is highest. Obviously, the longer the
14611
DOCGLASL. SISTERSOH et af,
570. rs_
OSCAR d/8/81
:1 SM.24
“,
-__I
ANL
49v _-l_
2~. -2..
450-
5 __
____.. ITI
--.
--..
_-
ie_, nv-
50.
--.-
Fig.
6.
.
_.-__
__.
-.
--~
20’ ,o *-_._ 0
1l[ 2.
CCMWTt’llTY
5 40. ;i x 30
3
hmp
-300
900 550 mm OF RAIN
-’ I200
Same as Fig. 5 except collecred on 8 April 198I.
precipitation sample remains in the bucket, the longer soil-derived particles have to dissolve and reduce precipitation acidity. This would be expected to be a dominant feature of weekly samples particularly when precipitation occurs early in the week during periods of increased atmospheric particle loading typical of prolonged periods with little precipitation. Table 6(a) shows the seasonal precipitation totals from rain gage data during the entire study period. The spring of 1980 was very dry compared to 1981. Table 6(b) shows monthly precipitation totals for the same time period, indicating that during April-June 1980 rainfall amounts were low compared to the corresponding period in 1981, Thus the observed seasonal difference is not due to a single dry month. Occasionaily, farm activities (tilling and pIanting) which begin in early April, contribute significantly to atmospheric soil particle loading. During 1980-1981,a moderate drought in the southern third of Illinois led to blowing dust in northeastern Ihinois in early spring and summer (Changnon, 1982). Indeed, Ca” * concentrations, for example, were the largest ofany season for both weekly and event samples in the spring of 1980 (see Appendix 1). Yet, there was no significant difference between weekly and event (Ca”“] (Table 4a) as might be expected, while there was a significant difference in pr~ipitation acidity determined from pHf for spring and summer of 1980. Usually, weekly and event acidity differences occur for pH(, not the field pH (pHf). We believe that the partial neutralization of precipitation took place rapidly due to the larger concentrations of soil particles so that the acidity changed rapidly both within the weekly bucket before collection and in the event samples after colleetion but before chemical analysis. When particle concentrations are not unusually large, the change in acidity may occur more slowly. In these cases, the only significant differences in acidity are seen in pHI since the changes occur slowly but steadily after collection in the weekly samples while the event samples are preserved sooner. Weekly Ca2+ and MgZf concentrations (Appendix
1) are larger in the fall and winter of 1980 and in the summer and fall of 198 1 than for any other season. Fall is typically dry and harvesting and tilling soil activities are greatest. Precipitation amounts were low in fall for both 1980 and 1981 (Table 6a) and may explain the observed behavior in [Ca” ‘] and [Mg* ‘1. The winter of 1980-1981 was also very dry in comparison to that of 1981-1982. With increased mean wind speeds typical of winter and little snow cover, increased soil particle resuspension would be expected. The summer of 1981 was drier than summer 1980 and probably accounts for the increased weekly [Mg’+]. In each of these cases, acidity, determined by pHt’ was less in weekly samples. Event [Ca’“] are significantly larger than weekly concentrations in summer 1980 (Table 4a), and spring 1981 and winter 1981-1982 (Table 5a), and these differences are meaningful. This is a reversal of the expected trend and shows no obvious relation to precipitation acidity. Event Ca2’ concentrations are greater during seasons which received the largest pr~ipitation amount (see Table 6a) but no physical or chemical explanation can be offered. Weekly and event concentrations of Mg2 * are not significandy different during these seasons. Even though the soil-drived particles may affect precipitation acidity, they do not entirely explain those differences in terms of equivalencies (see Appendix 1). As stated earlier, to maximize the amount of usable data, the number of weeks of precipitation data for each ion differ and will at&et ion balances in seasonal and annual weekly and event values reported. However, other factors may be affecting precipitation acidity as well and cannot be discounted. The most sign&ant difference between weekly and event samples is [NH:) (event samples having more). This could indicate volatilization of NH: to NH3 in weekly samples. Although this can take place, it is dependent upon the pH of the precipitation and usually occurs for pH values greater than 5.0. Nearly all of the weekly samples collected were between pH 4 and 5; volatilization would therefore not be expected. Nitrification processes by biological conversion of NHf to NO; or N, by bacteria (Elsden, 1962; Mortenson, 1962; Aleem et al., 1964; Hugcs, 1972) present in precipitation is possible but there are no m~ningful differences in weekly and event [NO;] to support this hypothesis. It may be possible that any resulting increases in nitrogen are, therefore, Ns or organic nitrogen complexes, neither of which were analyzed for in this study. Even so, bacteria in precipitation would be expected to be seasonal with the greatest concentrations in summer; yet the data clearly do not show any seasonal dependence. Sulfate particles generally are quite small with the majority of the mass distributed between 0.2 and 1.0 pm aerodynamic diameter (Heard and WitTen, 1969; M&.Sros, 1970; Kadowaki, 1976; Roberts and Friedlander, 1976; Whitby, 1978). Furthermore, much of the atmospheric aerosot in this size range is
1463
Chemical differences between event and weekly precipitation samples in northeastern Illinois
composed of (NH&SO, (Rodhe er al., 1972; Charlson et al., 1974; Waggoner et al., 1976). This size range corresponds to the smaller end of the 0.1-40 pm diameter size range of the effective cloud condensation nuclei (Junge, 1963). but at typical cloud supersaturations can preferentially act as cloud condensation nuclei (Scott, 1978). Furthermore, in the higher ambient humidities associated with precipitation, (NH&SO4 aerosol grow rapidly to a size that allow them to beefficiently scavenged by failing precipitation (Sisterson ef al., 1985). Although NH: and SO:- are expected to be highly correlated in precipitation, SO:can also come from the liquid phase oxidation of SO* (Penkett et ol., 19’79;Adamowicx, 1979; Fisher, 1982). The oxidation rate of SOr increases with increasing temperature (Hales, 1982). During precipitation, SO2 may be in equilibrium with cloud droplets or falling precipitation but not oxidized to SO:- due to the higher acidities of precipitation yet unaffected by any partial neutralization. As the weekly sample acidity decreases with time, soil-derived metal catalysts (Hegg and Hobbs, 1978) may allow SOr to be oxidized. Much of the observed larger weekly [SO:-] may therefore result from the longer storage time between deposition and preservation in the laboratory. Individual event samples, as mentioned before, are immediately refrigerated or frozen after collation while weekly samples are filtered only after they arrive at CAL. In wintertime, both individ~l event and weekly samples are thawed and agitated to insure a representative sample. Individual event samples are immediately refrigerated or frozen but weekly sampIes usualiy remain thawed and at room temperature during shipment and while waiting processing at CAL. Long periods at room temperatures could allow weekly winter samples to undergo chemical change, in&ding the oxidation of SO2 to SO:-. This may explain the differences that occur in fall, winter and early spring when cool or cold ambient temperatures greatly retard chemical reactions of the collected precipitation while waiting collection. In summer, warm temperatures allow the
oxidation of SO2 to SO:- to occur rapidly in both individual event and weekly samples. The data sup ports this hypothesis since SO:- differences were not significant during seasons with no snow. Note that a si~~~nt difference between weekly and composite event SO:- concentrations occurred in the spring of 1980 when there were occurrences of snow (Tables 6a and bf but no significant difference when there were no snow occurrences in the spring of 1981. Although individual event samples were frozen or refrigerated, chemical analysis never showed the SO: that would be expected if SO1 were not fully oxidized to so:-. However, individ~l event samples were allowed to come to room temperature for several hours before analysis and, as in weekly samples, SO:- may have converted to SO:- &fore analysis. Individual event samples had little or no head space in the stored samples while weekly samples were shipped in the bucket in which the sample was collected (approximately 13 /) and had much head space allowing exchange between sample and any trapped ambient so*. The fact that the weekly samples were allowed to come to room temperature for extended periods may also explain the decrease in observed [NH: J. Bacteria present in individual event precipitation would be minimized in samples collected in wintertime because of the cold temperatures. Thus, NH: is much better preserved in individual event samples, particularly in colder months.
Table 6a. Seasonal and annual pr~ipitatjon totals from rain gage data during the intercomparison study period. Snow values are water equivalents TOtal
Season
Rain
Snow
1980
Rain
Snow
Total 1981
Spring Summer Fall Winter
8.34 17.95 6.01 1.46
0.15 0 0.12 1.49
8.49 17.95 6.19 2.95
15.11 13.29 3.43 5.32
0 0 0.87 3.26
15.11 13.29 4.30 8.58
Table 6b. Monthly precipitation totals from rain gage data during the intercomparison study period. Snow values are water equivalents Month Jan. Feb. Mar. Apr. May
June July
Aug. Sep. Oct. Nov. Dee.
Rain
Snow
1.13 0.66 2.35 0.15 2.81 0 3.18 0 3.02 0 8.42 0 6.51 0 3.45 Trace 0.58 0.13 2.04 0.59
Total 1980
Rain
Snow
Total 1981
1.79 2.50 2.81 3.18 3.02 8.42 6.51 3.45 0.71 2.63
0 0.17 0.98 1.21 0.48 0.11 5.65 0 3.38 0 6.08 0 3.75 0 6.94 0 2.60 0 1.21 Trace 1.73 0.32 0.49 0.55
0.17 2.19 0.59 5.65 3.38 6.08 3.75 6.94 2.60 1.21 2.05 1.04
TOtal
Rain
Snow
1982
1.47 0.32 3.53 2.08
1.42 0.66 1.18 0.60
2.89 0.98 4.71 2.68
1464
~OUCXAS
7. liTRATiON
L. SISTERSONet ai.
RESULTS
As mentioned earlier, many individual event and weekly samples in 198C-1981 were titrated according to Gran’s procedure as discussed earlier. The annuaf and seasonal total (TAC), strong (SAC) and weak (WAC) acid concentrations of weekly and event samples and their differences in geq P- ‘, coefficient of variation of the difference (as discussed before), the two-tailed probability significance levels, and the number of weeks of data used are all given in Appendix 2. Table 7(a) shows those differences exceeding the two-tailed significance level of 0.05 (5%) for TAC, SAC and WAC. Only SAC showed a significant difference (event samples having more acidity) in the annual mean. Seasonally, weekly samples had larger TAC and WAC in spring, but less TAC and SAC in summer. No significant differences in acidity occurred in fall or winter. Table 7(b) shows the actual difference between weekly and event acidity which correspond to Table 7(a). The uncertainty estimates for H’ determined by titration are approxi~teIy S-10% as deduced from error analysis given by Sisterson and Wurfel(1984). In al1 cases, differences between weekly and event sample acidity are greater than the analytical uncertainty. Although event samples had more SAC annually, only summer values are significantly larger. Because WAC is determined from (TAC-SAC), weekly spring samples apparently had greater TAC due to increased WAC but less TAC due to less SAC in summer. Significant acidity changes are observed between weekly and event samples titrated immediately after collection only in spring and summer and indicate that these differences ~~orres~nding to samples in Tables 4(a) and (b) resulted from changes in chemistry after precipitation stopped but before cotiection. Again, it must be cautioned that not all precipitation samples used in Tables 7(a) and (b) correspond to those in
Tables 4(a) and (b), and make cause and etfect detcrminations difficuk. Since event samples are usually collected as soon as possible after a precipitation event. the observed differences in acidity areconsidered to be due to changes in the weekly sample chemistry. The SAC from the titration is converted to a calculated pH and compared to field pH electrode measurements; the results are shown in Table 8. Sisterson and Wurfef (1984) showed that an uncertainty between field pH and caictdated pH from titration using this data was - 0.03 + 0.10 pH units. We concfude that there is a negligibie difference between these independent measurements of strong acids in the weekly and event samples. Thus, there is little or no contribution to the electrode pH measurement from weak organic acids. Therefore, any differences in precipitation acidity determined in the field do not appear to be due to the presence of weak organic acids. This is not surprising since from the analysis of all proton sources in precipitation in the northeastern U.S., weak (and Bronstead) acids, except for small and irregular contributions from organic acids, contribute only to the total acidity (TAC), not the free acidity (Galloway et al., 1976).
8. SUMMARY
AND CONCLCSIONS
Event and weekly precipitation samples were collected at Argonne in northeastern Illinois from April 1980 to March 1982. Event samples were combined and weighted by precipitation amount over the corresponding weekly sample period and compared annuafly and seasonally for both years as well as separately. Although precipitation patterns were different for individual years, genera1 trends between weekly and composite event sample ion concentrations were similar. For the two year mean annual concentration differences, only NO; dtierences were not significant. Weekly sampies had tess [NH:] ( - 30.1 “/,I, @I*]
Table 7(a). Same as Table 3(a)except for April 1980through March 1981 for total (TAC), strong (SAC) and weak (WAC) acid concentrations
Parameter TAC SAC
1980-1981
Spring
Summer
1980
1980
f 0.0278
-0.0180 - 0.0280
- 0.01146
WAC
Winter Fall 1980 1980-1981
i- 0.0278
Table 7(b). Same as Table 3(b) except for April 1980to March 1981for total (TAC), strong (SAC) and weak (WAC) acid concentrations Parameter TAC SAC WAC
1980-81
Spring 1980 +21(0.33)
- 13(0.25) f 3NO.26)
Summer 1980 -38(0.17) - 19(0.28)
Winter Fall 1980 1980-1981
Chemical differences between event and weekly precipitation samples in northeastern Illinois Table 8. Annual and seasonal comparison of held pH (by electrode) to calculated pH (from titration strong acid concentration) for weekly and event samples for April 1980 to &March 1981 Field pH Period
NADP
All data 1980-1981 4.32 4.34 Spring 1980 Summer 1980 4.30 Fall 1980 4.39 Winter 1980-1981 4.26
ANL 4.22 4.24 4.17 4.42 4.28
Calculated PH ANL NADP 4.28 4.2-l 4.27 4.36 4.27
4.19 4.18 4.14 4.38 4.25
from both field pH (- 8.1%) and laboratory pH (- 19.3 %). but more [Mg’“] (+ 36.7 %) and [SO:-] (+ 8.0%). Each of these differences were greater than the associated analytical uncertainty. Seasonally, weekly samples had less [NH:] in all seasons (spring - 35.6%. summer -26.8%, fall - 26.2 % and winter - 68.4 %) and those differences always exceeded the anaiytical un~rtainty. Acidity determined from the laboratory pH was significantly less in all seasons for weekly samples but only greater than the analytical uncertainty for summer ( - 13.5 %), fall (- 26.1 %) and winter (- 39.1 %). Weekly [SO:-] were significantly greater in every season but summer (spring $11.3 %, fall + 22.7 % and winter + 21.3 %). Although weekly [Mg”] was significantly greater in all seasons but spring (summer f 47.4%, fall $73.6 % and winter + 40.2 %), weekly [Ca”] was significantly greater only in the fall (+ 22.5 %). These [Mg2+] and [Caf +] differences were greater than the associated analytical un~r~inty. Differences between event and weekly [NO;] were never sign&ant for any season. Titration data for the period April 1980 to March 1981 showed that weekly strong acid concentration was significantly less annually (- 21.8 %). Seasonally, weekly samples had greater total and weak acidity in spring (+ 9.2 y0 and + 23.0 %, respectively) but had less strong and total acidity in summer (- 30.6 % and - 21.1x, respectively). These differences were greater than the associated analytical uncertainty. In all cases, the differences in event and weekly ion concentrations were due to chemical changes that continue after the collection of precipitation. Since the magnitude of the differences will depend on the various ion concentrations in the precipitation, which can be expected to be a function of geographic location, as well as the time a sample remains ‘unpreserved’ while waiting analysis, these results may differ from site to site. For the samples collected at Argonne, biological conversion of NH: to NO; may be responsible for the observed decrease of (NH: J in weekly samples although no corresponding increase in weekly [NO;] was observed. It may be possible that NH: was converted to Nz or organic nitrogen and were not resolvable in this study. If the loss of NH: was due to biological conversion, it would be expected to be seasonal (greatest in the warm months), but this determined
1465
was not observed. Individual event samples were either refrigerated or frozen immediately after collection, while weekly samples were preserved by filtering but only after arrival at the analytical laboratory which may be several days to a week after collection. In cold seasons when samples were cold or frozen at the time ofcoltection, both individual event and weekly samples were thawed for field analysis to insure a representative sample is withdrawn from the collected pr~ipitation for analysis. However, individ~l event samples were refrigerated or frozen immediately while weekly samples were stored and shipped at room temperature. Since the colder ambient temperatures preserve individual event and weekly samples in the field, they would be expected to be chemically similar when thawed. This is supported by the fact that event and weekly pHf values were usually not significantly different while pHP values were almost always significantiy different. In the interim period between field and laboratory analysis, however, event samples were preserved by refrigeration while weekly samples were not. Therefore chemical reactions probably took place within the weekly sample while these same reactions were retarded by immediate preservation of the event samples. The biological conversion of NH: to NO; could, therefore, take place in weekly samples throughout the year. The increased weekly [SO:-] can also be explained in the same way. Unoxidized SO, in equilibrium with winter time precipitation may be oxidized in the thawed weekly samples during storage and shipping, while in summer, such reactions take place as quickly in the field in both event and weekly so that there is no observed difference in summer [SO:-]. Therefore, as observed the largest overall differences between event and weekly samples occur during cold months. extend our appreciation to Florence Williams of the Analytical Chemistry Laboratory at Argonne National Laboratory for the chemical analysis of prtipitation samples, to Mark Pedcn and Loretta Skowron of the Central Analytical Laboratory at the Illinois State Water Survey for their many hours of helpful conversation, and to Don Nelson of the Environmental Research Division at Argonne National Laboratory for his review of the manuscript, and to Tim Martin and Michael Pezzimentti of the Acknowledgemenrs-We
Environmental Research Division at Argonne National Laboratory for their assistance with computer data analysis. This research has been funded as part of the National Acid Precipitation Assessment Program by the U.S. Environmental Protection Agency and the U.S. Department of Energy. Although the research described in this article has been funded wholly or in part by the U.S. Environmental Protection Agency’s Multistate Power Production Pollution Study, it has not been subjected to the Agency’s required peer or policy review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. REFERENCES Adamowicx R. F. (1979) A model for the reversible washout of sulfur dioxide, ammonia and carbon dioxide from a polluted atmosphere and the production of sulfate in raindrops. Atmospheric Encironment 13, 105-121.
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Galloway J. N., Likens G. E. and Edgcrton E. S. (1976) Acid precipitation in the northeastern United States: pH and acidity. Science. 1Cash.194, 7X-724. Gran G. (1950) Determination of the equivalent point in potentiometric titrations. Acra Gem. Stand. 4, 559-577. Gran G. (1952) ~te~inatio~ of the equivafence point in potentiometric titrations-II. Analyst 77, 661-67 1. Hakkarinen C. S. (1983) Past and ongoing network intercomparison studies. Proc. Advisory Workshop on Methods for Comparing Precipitarion Chemisrry Data. Utility Acid Precipitation Study Program. Rensselaerville, New York. 10-l: August 1982,4-25 to 4-44. Hales J. M. (1982) Chapter A-6: The acidic deposition phenomenon: Precipitation scavenging processes. Critical Assessment Document: The _ A&c Deposition Phenomenon and its Effects. Preoared for the U.S. EPA through North Carolina S&e University, Acid Precipitation Program, 10 May 1982 (draft copy), Chapter A-6. Heard M. J. and Wiffen R. D. (1969) Electron microscopy of natural aerosols and the ide~t~~tio~ of particulate ammonium sulfate. Armospfreric Environment 3, 337-340. Hegg D. A. and Hobbs P. V. (1978) Oxidation of sulfur dioxide in aqueous systems with particular reference to the atmosphere. Atmospheric En&onmenr 12, 241-253. Huges M. N. (1972) 77re fnarganic Chemisrrp of Biological Processes pp. 200-229. John Wiley. Irving P. M., Sisterson D. L. and Marmell S. (1979) Strong and weak acids in precipitation at Argonne, 1979. Argonne Nationai Laboratory Radiological and Environmental Research Division Annual Report ANL-79-65, Part III, 47-48. Junge C. E. (1963) Air Chemistry and Radioacticity. Academic Press. New York. Kadowaki S. (1976) Size distribution of atmospheric total aerosols, sulfate. ~monium and nitrate particles in the Nagoya area. Atmospheric Environment IO, 39-43. Mtstiros E. (1970) Seasonaland diurnal variations of the size distribution of atmospheric sulfate particles. Tellus 22, 235-238.
Mortenson L. E. (1962) Inorganic nitrogen assimilation and ammonia incorporation. The Bucferiu. 0 Trearise on Structure and Function, Volume III: Bios~nr~esis (edited by Gunsatus 1. C. and Stanier R. Y.j_pp. 119-162. Academic Press, New York. Peden M. and Skowron L. (1978)ionic stability of precipitation samples. Atmospheric Encironmenr 12, 2343-2349. Peden M. (1983) Sampling. analytical, and quality assurance protocols for the National Atmospheric Deposition Program. Sampling and analysis of rain. ASTM STP 823, pp. 72-83. Penkett S. A., Jones B. M. R., &ice K. A. and Egglcton A. E. J. (1979)The importance of atmospheric ozone and hydrogen peroxide in oxidizing suiphur dioxide in cloud and rainwater. Atmospheric ~~~iro~nr 13, 1X-137. Prahl W. H. (1981a) Fitting linear equations to sets of experimental data. Chem. Engng 10 Ayust 1981, 85-88. Prahl W. H. (1981b) Further note on fitting linear equations to sets ofex~rimen~l~ta.C~m. Engng 5 Ocrober 1981,5. Raynor G. S. (1982) Design and preliminary results of the intermediate density precipitation chemistry experiment. Preprint, Third Joint Conference on Applications of Air Pollution Meteorology, I I-IS January 1982.San Antonio, Texas, pp. 47-49. Roberts P. ‘I. and Friedlander S. K. (1976) Analysis of sulfur in deposited aerosol particles by varporization and Rame photometric detection. Atmospheric Environment 10, 403-408.
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1467
Chemical differences between event and weekly precipitation samples in northeastern Illinois
Appendix 1. Weekly and composite event precipitation chemistry and statistical analyses results for all data used in the intercomparison study. Data presented are weekly and composite event concentration means and their difference in peq!-’ for the various time periods indicated, that difference (weekly-event/mean) in per cent. the coefficient of variation of the difference (where a value less than 1.00 indicates the absolute difference is greater than the analytical uncertainty), the two-tailed signilicance level determined from the Wilcoxon matched-pairs signed-ranks test, and the number of weeks of data used for computations for individual ion concentrations. fleq/-i Year
Weeks
Parameter
Weekly
Event
H’f H’/ Ca”
54.7 55.6 14.6 5.6 22.2 30.0 73.9
59.3 67.5 14.8 3.9
Absolute diff.
Percent diff.
cov
2-tail probability
29.2 68.2
-4.6 - 11.9 - 0.2 + 1.7 -7.8 +0.8 + 5.7
-8.1 - 19.3 - 1.4 + 36.7 -30.1 +2.5 +s.o
0.43 0.17 0.76 0.02 0.04 0.41 0.10
-0.0018 -o.oooo + 0.0898 + o.oooo - 0.0000 +0.2108 +o.OOcO
Mg” NH; NO; so:-
46.8 40.4 15.4 5.4 21.8 27.9 63.9
49.3 45.7 17.7 5.2 31.3 27.7 57.1
- 2.5 -5.3 -2.3 +0.2 -9.5 +0.2 +6.8
- 5.4 - 12.3 - 14.4 + 4.0 - 35.6 +0.7 +11.3
1.37 7.47 0.13 0.34 0.07 2.48 0.13
- 0.0594 -0.0280 - 0.0980 + 0.0980 - 0.0002 + 0.6530 + 0.0098
Summer
H’f H’P Car+ Mg” NH: NO; so:-
64.3 69.5 13.6 5.4 22.4 31.5 83.2
72.1 79.6 13.6 3.3 29.3 30.4 83.0
- 7.8 -10.1 0.0 +2.1 -6.9 + 1.1 +0.2
-11.5 - 13.5 0.0 + 47.4 -26.8 + 3.6 +0.3
0.46 0.33 232.0 0.03 0.08 0.49 4.61
- 0.0884 - 0.0080 + 0.4762 +0.0010 -o.OcQa - 0.8484 +0.3052
13 18 18 18 18 17 18
Fall
H’f H’( Car+ Mgr+ NH; NO; so:-
50.3 46.8 16.8 7.7 21.8 30.6 72.6
51.3 60.8 13.4 3.6 28.4 30.1 57.8
-1.0 - 14.0 + 3.4 +4.1 -6.6 +0.5 + 14.8
- 1.9 -26.1 + 22.5 + 73.6 -26.2 + 1.7 + 22.7
4.16 0.26 0.10 0.02 0.10 1.14 0.07
-0.1158 -0.OC08 + 0.0038 + O.ooo! - 0.0002 +0.3318 + O.OcO?
15 17 16 17 16 17 17
Winter
H’f H’/ Ca*+ Mg’+ NH: NO; so:-
50.5 47.6 13.4 4.9 19.1 29.5 68.9
52.3 70.7 13.3 3.2 39.6 28.2 55.7
- 1.8 -23.1 f0.1 + 1.6 -20.5 + 1.3 + 13.2
-3.4 -39.1 + 0.9 c40.2 -68.4 +4.5 +21.3
2.16 0.16 2.40 0.05 0.04 0.17 0.08
- 0.0842 - 0.0002 + 0.4380 +0.0114 -0.0012 +0.2096 + o.cQO2
‘6 24 29 29 29 29 28
All data 1980-1981
H’f H’C Ca2’ Mg2+ NH: NO; so:-
46.8 58.8 17.4 6.2 22.4 34.5 76.3
58.6 63.8 17.6 3::: 31.9 69.7
-11.8 - 5.0 -0.2 + 2.0 - 7.6 + 2.6 + 6.6
- 22.5 -8.2 - 1.2 + 38.7 - 29.0 + 7.9 + 9.0
0.26 0.69 1.36 0.03 0.07 0.19 0.14
- O.OCQ8 - 0.0030 + 0.0504 +o.OOOO -o.OGOO +0.0168 + o.oooo
Spring 1980
H’f H’C Car+ Mg2+ NH; NO; so:-
45.3 30.9 24.0 7.6 25.9 39.0 71.9
57.8 45.8 23.8 4.6 34.2 37.1 71.7
-12.5 - 14.9 +0.2 + 3.0 -8.3 + 1.9 +6.2
-24.2 -38.7 + 0.8 + 48.9 - 27.6 f4.9 + 8.3
0.41 0.54 2.64 0.04 0.12 0.41 0.23
- 0.0279 -0.1798 +0.7532 +0.027s - 0.0278 + 0.3454 + 0.0432
Summer
H’f H’/ Ca” Mg2 * NH: NO; so:-
48.8 70.1 16.1 5.5 23.1 35.9 83.0
66.4 12.9 18.3 4.5 31.4 32.2 79.6
-17.6 - 2.8 - 2.2 +1.0 -8.3 +3.7 + 3.4
- 30.6 -4.0 - 12.9 +21.4 - 30.7 + 10.7 + 4.2
0.29 1.71 0.19 0.10 0.10 0.23 0.43
-0.011s - 0.6744 - 0.0208 +0.1098 -0.0076 +0.1386 +o.l386
60 67 71 72 72 72 72
All data 1980-1982
16 I2 16 16 17 17 16
Spring
15 20 21 21 21 21 21
Mg” NH: NO; so:H’f l
EL
1980
1368
DOUGLASL. S~STERSON et ai.
Appendix 1. lContdl Jleq/-L Weeks
Year Fall 1980
Winter 1980-1981
Parameter
Weekly
Event
H”f H‘t ch2+ Mg’” NH; NO; so: -
33.4 35.9 13.9 7.1 17.0 23.8 61.0
36.4 41.3 20.7 4.0 22.1 22.5 43.5
H’f H’i Cal+ Mg’ + NH: NOT SO:-
52.5 47.2 13.7 5.5 18.5 31.2 57.7
Absolute diff. +0.2 - 5.4
Percent diff.
+ 5.4
COV
Z-tail probability
43.2 +3.1 - 5.1 + 1.3 + 17.5
- 14.0 + 26.5 + 56.4 - 26.2 + 5.8 + 33.4
2.93 1.10 0.17 0.05 0.20 0.57 0.09
-0.2365 -O.OlSO 4 0.0280 +o.oI%a -0.0180 r0.31w 40.0180
52.3 56.6 7.9 2.1 23.2 30.2 46.4
-l-O.2 -9.4 “t 5.s c3.4 -4.7 + 1.0 +!I.3
+0.4 - 18.3 s53.5 + 90.5 -22.3 + 3.5 +21.5
22.70 0.54 0.07 0.03 0.19 OS7 0.13
+ 0.892% -0.OlSO +0.01%0 CO.0180 - 0.0280 to.3980 CO.Oi%O
32 43 42 43 43 43 43
All data 1981-1982
H’f H”r” Ca*+ MgZ” NH; NO; so: -
60.6 53.2 12.6 5.2 21.4 26.9 72.3
59.9 69.6 12.S 3.7 30.0 27.4 67.2
i-o.7 - 16.4 -0.2 f 1.6 -9.6 -0,s + 5.1
+ 1.3 - 26.8 -1.3 +35.2 - 33.4 - 18.6 i- 7.3
3.7 0.15 1.24 0.03 0.05 0.72 0.14
- 0.4774 - o.twO + 0.6302 f o.oooo -o.oOW - 0.663% i”O.O@Xl
10 11 10 10 11 11 II
Spring 1981
H’f H’L Ca” Mg”’ NH; NO; so:-
47.4 41.3 11.7 4.4 20.i 23.2 58.8
45.0 45.6 14.9 5.5 29.9 23.3 51.0
+ 2.4 -4.3 - 3.2 -1.0 -9.S -0.1 + 7.8
f 5.3 - 10.0 - 23.5 - 20.9 -39.1 -0.3 + 14.1
1.90 I .07 0.11 0.09 0.0s 8.40 0.15
f 0.87Y-I -0.0594 -0.021% + 0.5950 - 0.005s - 0.5938 +0.061%
7 12 12 I2 I2 12 I2
Summer 1981
H”f
82.5 69.0 to.9 5.2 21.6 26.7 83.6
78.8 86.6 8.3 2.0 26.8 28.2 56.8
+3.7 - 17.6 + 2.6 + 3.2 - 5.2 - I.5 - 3.2
+4.6 - 22.6 + 27.0 -+87.5 -21.2 -5.3 - 3.8
1.32 0.24 0.1’ 0.03 0.15 0.49 0.46
+ 0.4990 - 0.002’ + 0.0230 f oGO22 - 0.00’2 -0.1824 ho.7536
6 10 II 11 II 11 I1
Fall 1981
Mg’ * NH: NO; so: -
60.3 54.5 18.8 8.I 25.3 35.6 80.8
63.4 74.5 15.3 3.3 32.8 35.6 67.8
-3.1 - 20.0 1-3.5 f 4.8 -7.5 0.0 + 13.0
- s.2 -31.0 + 20.5 + 85.3 - 26.0 -0.1 + 17.5
1.6% 0.24 0. I 3 0.0’ 0.111 23.50 0.1 I
- 0.248% -0.0100 +0.061s + 0.0031 - 0.0034 4 0.6463 + 0.0034
P IO 9 10 9 10 10
Winier
H*f H’/ Ca2’ M$” NH: NO; so:-
49.7 47.8 13.3 4.6 19.9 28.8 73.7
52.2 76.5 15.5 3.7 36.0 27.4 59.5
-2.5 -28.7 -2.2 + 0.9 - 16.1 + 1.4 + 14.2
-5.0 -46.2 -15.5 +21.4 - 57.5 +4.9 +2l.4
1.89 O.I? 0.16 0.12 0.06 0.56 0.01
- 0.0076 -0.0050 - 0.020% + 0.3862 - 0.0070 + 0.4446 i- 0.0050
ZL *+ !& NO; so: -
1981-1982
H’f H’( 0””
1469
Chemical differences between event and weekly precipitation samples in northeastern Itlinois
Appendix 2. Same as Appendix 1except for titration dete~ination of total (TACf, strong (SAC). and weak (WAC) and acid concentrations fieq(-’ Weeks
Year
Parameter
Weekiy
Event
Absolute diff.
Percent diff.
2-tail cov
-0.7610 -0.0046 -0.3458
Ail data 1980-1981
TAC SAC WAC
178 52 126
193 65 126
-. IS -13 -0
-8.1 -21.8 -0.5
: 6
Spring 1980
TAC SAC WAC
241 55 186
220 I::
-11 i-21 +38
- +9.2 18.2 + 23.0
0.46 0.33 0.26
+ - 0.0278 0.0748 +0.0278
7 7 I
Summer 1980
TAC SAC WAC
162 54 109
200 73 128
-38 -19 -19
-21.1 - 30.6 - 16.0
0.17 0.28 0.81
-0.0180 - 0.0280 -0.3104
5 5 5
Fat1 1980
TAC SAC WAC
I48 43 105
147 42 105
+1 +1 +o
+1.0 +2.5 40.5
4.70 5.74 319.00
+ 0.2250 -0.8928 + 0.2250
5 5 5
Winter 1980-1981
TAC SAC WAC
132 54 79
145
-13 -3 -10
-9.1 -3.4 - 12.5
0.56 3.26 I 64
- 0.5002 -0.5002 -0.6858
23 23
0.27 0.25 151.00
p
