Comparison of Weekly and Daily Wet Deposition Sampling Results

Chapter 20

Comparison of Weekly and Daily Wet Deposition Sampling Results 1

2

L. Ε. Τοpοl , M. Lev-On1, and A. K. Pollack 1

Environmental Monitoring and Services, 4765 Calle Quetzal, Camarillo, CA 93010 Systems Applications, 101 Lucas Valley Road, San Rafael, CA 94903

Downloaded by CORNELL UNIV on August 11, 2016 | http://pubs.acs.org Publication Date: September 3, 1987 | doi: 10.1021/bk-1987-0349.ch020

2

An initial analysis of data from a one-year field study comparing the concentrations of weekly measured samples and weekly values derived from daily samples indicated that the weekly measured ion concentrations were gener ally larger. Although the mean relative bias values ranged from 0 to 34%, most values were less than 10%. In addition, the greatest differences were found in the fall season with biases greater than 10% occurring for ammonium, chloride, sodium, potassium, calcium and magnesium. Samples were collected at three sites in Georgia, Kansas and Vermont to represent the southeast ern, central (west of the Mississippi River) and north eastern regions of the United States. The measurements included precipitation weight, pH, sulfate, nitrate, chloride, ammonium, potassium, sodium, calcium and mag nesium. Other than the two different sampling inter vals, all field and laboratory procedures were identi cal. P r e c i p i t a t i o n s a m p l i n g networks i n the U n i t e d S t a t e s g e n e r a l l y c o l l e c t e i t h e r d a i l y (UAPSP, MAP3S) o r weekly (NADP, NTN) s a m p l e s . I n the c a s e of the weekly s c h e d u l e , samples can remain i n the c o l l e c t o r under ambient c o n d i t i o n s f o r up to seven d a y s , p o s s i b l y r e s u l t i n g i n chemical changes. The o c c u r r e n c e and magnitude of such changes a r e p o t e n t i a l l y i m p o r t a n t i n d e t e r m i n i n g the use of weekly c o m p o s i t i o n d a t a f o r s t u d y i n g wet d e p o s i t i o n e f f e c t s and l o n g - t e r m trends. I n o r d e r to determine the i m p o r t a n c e of c h e m i c a l changes i n t r o d u c e d by the l o n g e r s a m p l i n g p e r i o d , a c o l l o c a t e d s a m p l i n g s t u d y was implemented from O c t o b e r 1983 to O c t o b e r 1984 a t t h r e e s i t e s of the U t i l i t y A c i d P r e c i p i t a t i o n Study Program (UAPSP) n e t work. The s i t e s were s e l e c t e d to r e p r e s e n t the s o u t h e a s t e r n , c e n t r a l (west of the M i s s i s s i p p i R i v e r ) and n o r t h e a s t e r n r e g i o n s of the U n i t e d S t a t e s . The o b j e c t i v e of the s t u d y was to compare the c o n c e n t r a t i o n s o b t a i n e d from weekly samples w i t h t h o s e d e r i v e d from the d a i l y samples. A comparison of p r e c i s i o n of weekly and d a i l y measurements 0097-6156/87/0349-0229$06.00/0 © 1987 A m e r i c a n C h e m i c a l Society

Johnson et al.; The Chemistry of Acid Rain ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

THE CHEMISTRY OF ACID RAIN


230

w i l l be r e p o r t e d s e p a r a t e l y . For t h i s s t u d y a d a i l y sample i s the t o t a l o c c u r r e n c e of p r e c i p i t a t i o n g r e a t e r than 0.51 mm (0.02 i n c h ) i n a 24-hour p e r i o d ( s t a r t i n g a t about 9:00 a.m. l o c a l t i m e ) ; a weekl y sample i s t h e p r e c i p i t a t i o n o c c u r r i n g i n the p e r i o d from 9:00 a.m. of t h e sample p i c k - u p day, g e n e r a l l y Monday, t o the same time and day o f the f o l l o w i n g week. The r e s u l t s were c a l c u l a t e d f o r a l l s i t e s on an a n n u a l and s e a s o n a l b a s i s . A n a l y s e s of the d a t a u s i n g b o t h p a r a m e t r i c and n o n p a r a m e t r i c s t a t i s t i c a l t e s t s were performed t o d e t e c t s i g n i f i c a n t d i f f e r e n c e s i n compared v a l u e s . Four weekly v e r s u s d a i l y (or event) sampling s t u d i e s u s i n g the same t y p e o f samplers as i n t h e p r e s e n t study have been r e p o r t e d . A F l o r i d a s t u d y (1) used t h r e e samplers m o n i t o r i n g on a d a i l y , weekly and b i w e e k l y s c h e d u l e f o r t w e l v e months. A l l a n a l y s e s were performed a t the U n i v e r s i t y of C e n t r a l F l o r i d a w i t h i n t e n days of c o l l e c t i o n . No s t a t i s t i c a l l y s i g n i f i c a n t d i f f e r e n c e s i n p r e c i p i t a t i o n c o m p o s i t i o n w i t h sampling i n t e r v a l were f o u n d . A t P e n n s y l v a n i a S t a t e U n i v e r s i t y (2) a comparison of the i o n c o n c e n t r a t i o n s f o r weekly and d a i l y samples i n d i c a t e d t h a t the weekly samples y i e l d e d lower c o n c e n t r a t i o n s and d e p o s i t i o n v a l u e s f o r a l l the i o n s a n a l y z e d . A study a t N o r t h C a r o l i n a S t a t e U n i v e r s i t y (3) i n v o l v e d f o u r d a i l y and s i x weekly c o l l e c t o r s . The d a t a i n d i c a t e d t h a t g r o s s changes i n pH and s p e c i f i c conductance d i d n o t o c c u r w i t h c o l l e c t i o n p e r i o d s o f one week, b u t s i n c e no o t h e r a n a l y s e s were p e r f o r m e d , t h e d a t a do n o t e l i m i n a t e t h e p o s s i b i l i t y t h a t s e l e c t e d c h e m i c a l changes may have o c c u r r e d . A t Argonne N a t i o n a l L a b o r a t o r y (4) two s a m p l e r s , a c t i v a t e d by a s i n g l e s e n s o r , were used f o r a p p r o x i m a t e l y two y e a r s . T h i s approach a s s u r e d s i m u l t a n e o u s o p e n i n g and c l o s i n g o f b o t h c o l l e c t o r s i n c o n t r a s t t o the o t h e r s t u d i e s . The weekly samples were a n a l y z e d by the I l l i n o i s S t a t e Water Survey whereas the e v e n t samples were a n a l y z e d by Argonne. Weekly samples were found t o have s i g n i f i c a n t l y l e s s NHj and H* i n a l l seasons and more SO4 i n e v e r y season b u t summer. The weekly samples had s i g n i f i c a n t l y more C a and M g d u r i n g seasons w i t h l i t t l e p r e c i p i t a t i o n No s i g n i f i c a n t d i f f e r e n c e s between weekly and event NO 3 were e v i d e n t . The p r e s e n t s t u d y i s unique i n s e v e r a l ways: (a) t h r e e r e g u l a r UAPSP m o n i t o r i n g s i t e s were u t i l i z e d , and each o f t h e s i t e s was equipped w i t h f o u r i d e n t i c a l p r e c i p i t a t i o n s a m p l e r s , two c o l l e c t i n g d a i l y samples and two weekly samples; t h i s a l l o w s f o r d u p l i c a t e d a t a as w e l l as the c a l c u l a t i o n of p r e c i s i o n f o r b o t h s a m p l i n g s c h e d u l e s ; (b) a l l a n a l y s e s were performed i n the same l a b o r a t o r y ; (c) e x c e p t f o r t h e s a m p l i n g s c h e d u l e , a l l p r o c e d u r e s were i d e n t i c a l . + 2

+ 2

Procedures F o r t h i s s t u d y t h r e e s i t e s from the UAPSP network t h a t t y p i f i e d d i f f e r e n t r e g i o n s and c l i m a t e s of t h e e a s t e r n U n i t e d S t a t e s were selected. These s i t e s were U v a l d a , GA, i n the S o u t h e a s t , L a n c a s t e r , KS, west of the M i s s i s s i p p i R i v e r , and U n d e r b i l l , VT, i n the N o r t h east. C r i t e r i a used f o r s i t e s e l e c t i o n f o r p r e c i p i t a t i o n m o n i t o r i n g a r e o u t l i n e d i n the U.S. EPA Q u a l i t y A s s u r a n c e Manual f o r P r e c i p i t a t i o n Measurement Systems ( 5 ) . C o l l o c a t e d sampling was performed a t t h e t h r e e s i t e s f o r approxi m a t e l y one y e a r , from O c t o b e r 9, 1983 f o r G e o r g i a and Vermont and O c t o b e r 12, 1983 f o r Kansas t o August 15, O c t o b e r 10, and O c t o b e r 17, 1984 f o r G e o r g i a , Vermont and K a n s a s , r e s p e c t i v e l y .



20.

TOPOL ET AL.

Weekly and Daily

Wet Deposition

Sampling

Results

231

F i e l d Operations. Each s i ^ e had f o u r a u t o m a t i c p r e c i p i t a t i o n c o l l e c t o r s (Aerochem M e t r i c s ) and a U n i v e r s a l R e c o r d i n g W e i g h i n g Bucket R a i n Gauge ( B e l f o r t ) w i t h an e i g h t - d a y s p r i n g powered c l o c k and s t r i p c h a r t r e c o r d e r . An event pen marker was i n t e r f a c e d w i t h the samplers and n o t e d the sampler l i d open and c l o s e t i m e s on t h e r a i n gauge s t r i p c h a r t . A l l o p e r a t o r s were t r a i n e d i n the s i t e o p e r a t i o n s based on the U.S. EPA " O p e r a t i o n and M a i n t e n a n c e Manual f o r P r e c i p i t a t i o n Measure ment Systems" (6) and the UAPSP " F i e l d O p e r a t o r I n s t r u c t i o n Manual" ( 7 ) . D a i l y s i t e v i s i t s were g e n e r a l l y made a t about 9:00 a.m. local time t o check the equipment and t o remove any p r e c i p i t a t i o n c o l l e c t ed by t h e d a i l y s a m p l e r s . Weekly samples were removed on Monday from the G e o r g i a and Vermont s i t e s and on Tuesday a t the Kansas site. I f no e v e n t o c c u r r e d f o r a week, t h e b u c k e t s were r i n s e d w i t h d e i o n i z e d water t o remove any d u s t t h a t may have d e p o s i t e d and t h e y were t h e n r e u s e d . A n a l y s i s o f the r i n s e water s e r v e d as a dynamic blank. F o r t h e samples g r e a t e r t h a n 0.51 mm, a maximal volume of 500ml was t r a n s f e r r e d t o a c l e a n , l a b e l e d , p l a s t i c b o t t l e and s e a l e d ; the r e s t o f the sample was d i s c a r d e d . I f l e s s t h a n 0.51 mm, t h e sample was a l s o d i s c a r d e d and was l o s t f r o m the weekly composite of daily results. T h i s w i l l produce some e r r o r s i n t h e comparisons w i t h the weekly samples. The d a i l y sample was s t o r e d i n a r e f r i g e r a t o r a t about 4°C t o m i n i m i z e d e g r a d a t i o n . The same p r o c e d u r e was f o l l o w e d f o r the weekly samples e x c e p t t h a t t h e s e were s h i p p e d t h e same day t h a t t h e y were c o l l e c t e d and were n o t r e f r i g e r a t e d . F o r s h i p p i n g t o the l a b o r a t o r y , t h e d a i l y samples were packed d i r e c t l y from t h e r e f r i g e r a t o r i n t o an i n s u l a t e d S t y r o f o a m box w i t h f o u r f r o z e n g e l p a c k s . Weekly samples were packed t o g e t h e r w i t h t h e c o r r e s p o n d i n g d a i l y samples and the c a r t o n was s h i p p e d by a i r f r e i g h t each week. Q u a l i t y c o n t r o l p r o c e d u r e s (5-7) i n the f i e l d i n c l u d e d semi annual a u d i t v i s i t s . These p r o c e d u r e s were employed t o maximize c a p t u r e o f uncontaminated samples, t o i d e n t i f y and document them, t o p r e s e r v e t h e i r i n t e g r i t y u n t i l t h e i r a r r i v a l a t the l a b o r a t o r y , t o o b t a i n dynamic b l a n k s and to d e t e r m i n e each s i t e s p r e c i s i o n and a c c u r a c y of pH and c o n d u c t i v i t y measurement. (The f i e l d measure ments were used as a q u a l i t y c o n t r o l t o d e t e r m i n e i f p r e c i p i t a t i o n samples had degraded between t h e f i e l d and l a b o r a t o r y measurements.) 1

Laboratory Operations. I f s u f f i c i e n t sample was p r e s e n t , t h e f o l l o w i n g o r d e r o f a n a l y s i s was t a k e n t o m i n i m i z e chance o f d e g r a d a t i o n : pH, NH , S O 4 , NO], C l ~ , £a , Mg , Na , and Κ . The pH was measured e l e c t r o m e t r i c a l l y , N H 4 by automated c o l o r i m e t r y , t h e a n i o n s by i o n chromatography, and the m e t a l l i c c a t i o n s by f l a m e a t o m i c absorption. I f i n s u f f i c i e n t sample was a v a i l a b l e f o r complete a n a l y s i s , t h e a n i o n s were measured b e f o r e ammonium. A l l samples were a n a l y z e d w i t h o u t f i l t r a t i o n . However, t h e samples were d e c a n t ed t o e l i m i n a t e sedimented p a r t i c l e s . A l l a n a l y t i c a l i n s t r u m e n t s were c a l i b r a t e d a t l e a s t once a day. I n a d d i t i o n , r e a g e n t b l a n k s , s p i k e s and s p l i t samples were r u n d a i l y . A f u l l d e s c r i p t i o n of the q u a l i t y c o n t r o l program implemented f o r UAPSP by EMSI i s p r o v i d e d e l s e w h e r e ( 8 - 9 ) . 4



232


D a t a Management. D a t a were i n v a l i d a t e d f o r the f o l l o w i n g r e a s o n s : o b v i o u s p h y s i c a l c o n t a m i n a t i o n o f a sample, i n s t r u m e n t m a l f u n c t i o n , e v i d e n c e of e r r o n e o u s d a t a e n t r y , and o u t l y i n g o b s e r v a t i o n s . L a b o r a t o r y c h e m i s t s m a n u a l l y checked a p p r o x i m a t e l y 5% o f a l l d a t a , c o n c e n t r a t i n g on extreme o b s e r v a t i o n s of c o n c e n t r a t i o n s and i o n b a l a n c e d a t a , thus v e r i f y i n g i n s t r u m e n t c a l i b r a t i o n and i n t e g r i t y o f the a n a l y t i c a l p r o c e s s . Based on t h e s e c h e c k s some samples were d e s i g n a t e d f o r r e a n a l y s i s f o l l o w e d by r e e n t r y of r e s u l t s i n t o the d a t a b a s e . No extreme o b s e r v a t i o n s were d e l e t e d from the main d a t a b a s e i f they p a s s e d a l l the c h e c k s d e s c r i b e d h e r e . Belowd e t e c t i o n - l e v e l v a l u e s were s e t e q u a l t o z e r o . The d i s t r i b u t i o n o f c o l l o c a t e d d i f f e r e n c e s f o r the d a i l y and weekly measurements were used t o d e t e c t o u t l i e r s . The d i s t r i b u t i o n s f o r the t h r e e p r e c i p i t a t i o n t y p e s a t each s i t e were symmet r i c a l with long t a i l s . I n t h i s s t u d y , the t a i l s were t r u n c a t e d a t ±3σ from the mean f o r each o b s e r v a b l e , and p a i r s of samples w i t h the extreme ( i . e . minimum and maximum) d i f f e r e n c e s were r e j e c t e d from p a r a m e t r i c s t a t i s t i c a l a n a l y s e s a c c o r d i n g t o the f o l l o w i n g criteria: (a) one of t h e f o u r major i o n d i f f e r e n c e s (H , SO^, N O 3 and N H 4 ) o f t h a t sample p a i r was o u t s i d e t h e 3σ a c c e p t a n c e l i m i t s . (b) a t l e a s t t h r e e o f the minor i o n d i f f e r e n c e s of t h a t sample p a i r were o u t s i d e the 3σ a c c e p t a n c e l i m i t s , or (c) one o f the minor i o n d i f f e r e n c e s of t h a t sample p a i r was o u t s i d e the 3σ a c c e p t a n c e l i m i t s and t h r e e out of the f o u r p r i m a r y i o n d i f f e r e n c e s f o r the same sample p a i r were a t e i t h e r extreme o f the c o l l o c a t e d p a i r d i s t r i b u t i o n . The s c r e e n i n g s t e p s o u t l i n e d above reduced the number o f d a i l y samples from 67, 86 and 142 to 63, 72 and 133, and t h e c o r r e s p o n d i n g weekly samples from 35, 38 and 52 t o 33, 37 and 47 f o r t h e G e o r g i a , Kansas and Vermont s i t e s , r e s p e c t i v e l y . These s c r e e n e d d a t a r e p r e s e n t 94%, 84% and 94% o f the t o t a l d a i l y samples and 94%, 97% and 91% of the t o t a l weekly samples from t h e t h r e e r e s p e c t i v e s i t e s . The s m a l l f r a c t i o n o f d a t a i n the t a i l s a r e p r o b a b l y due t o c o n t a m i n a t i o n and t h e i r e l i m i n a t i o n removes t h e i r d i s p r o p o r t i o n a t e l y l a r g e i n f l u e n c e on h y p o t h e s i s t e s t i n g by p a r a m e t r i c methods. Nonp a r a m e t r i c t e c h n i q u e s a r e r e l a t i v e l y i n s e n s i t i v e t o the d a t a i n the t a i l s o f the d i s t r i b u t i o n , and u n s c r e e n e d d a t a were used f o r t h e s e analyses. The c o l l o c a t i o n d a t a were used to d e t e r m i n e c o n c e n t r a t i o n means and medians of d a i l y and weekly sampling and t o o b t a i n the b i a s between the measured weekly and the d e r i v e d weekly v a l u e s , compos i t e d f r o m the c o r r e s p o n d i n g d a i l y r e s u l t s . A l l d e r i v e d weekly c o n c e n t r a t i o n s were c a l c u l a t e d s e p a r a t e l y f o r each c o l l e c t o r as t h e p r e c i p i t a t i o n - w e i g h t e d mean, C = i i i / i i > i i d a i l y c o n c e n t r a t i o n and sample w e i g h t . F o r d e r i v e d weekly p r e c i p i t a t i o n , the sum o f the d a i l y volumes f o r the week was taken f o r e a c h precipitation collector. The d a t a were c l a s s i f i e d by sampling s i t e , and by m e t e o r o l o g i c a l s e a s o n ( s p r i n g = March - May, summer = June - A u g u s t , f a l l = September - November, and w i n t e r = December - F e b r u a r y ) . +

c

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20.

TOPOL ET AL.

Weekly and Daily

Wet Deposition

Sampling

Results

233

O n l y events g r e a t e r than 0.51 mm (0.02 i n c h ) r a i n gauge d e p t h were i n c l u d e d i n t h i s study to e n s u r e t h a t s u f f i c i e n t sample was a v a i l a b l e f o r a n a l y s i s . Thus i f a week had o n l y s m a l l d a i l y e v e n t s no c o r r e s p o n d i n g d a i l y and d e r i v e d weekly d a t a were a v a i l a b l e ; howe v e r , e v e n t s g r e a t e r than 0.51 mm a l l o w e d p a r t i a l a n a l y s i s and were included. When an o b s e r v a b l e v a l u e was m i s s i n g , no c o l l o c a t e d d i f f e r e n c e was o b t a i n e d f o r t h a t o b s e r v a b l e . However, t o c a l c u l a t e a mean v a l u e i f one o f the o b s e r v a b l e s was m i s s i n g , the v a l i d c o l l o c a t e d measurement was t a k e n as the mean. F o r a week w h i c h c o n t a i n e d one or more m i s s i n g d a i l y samples the d e r i v e d weekly and t h e c o r r e s p o n d i n g measured weekly samples were d i s c a r d e d from the comparison o n l y i f the p r e c i p i t a t i o n o f the m i s s i n g d a i l y sample(s) a c c o u n t e d f o r more t h a n 20% o f the week's t o t a l . T h i s was done t o maximize the number of d a t a p o i n t s and the power o f the s t a t i s t i c a l t e s t s .


s

S t a t i s t i c a l T e s t i n g . P r e l i m i n a r y e v a l u a t i o n o f the d a t a f o r c o l l o c a t e d c o n c e n t r a t i o n d i f f e r e n c e s f o r e a c h o f the o b s e r v a b l e s showed t h a t the d i s t r i b u t i o n s were s y m m e t r i c a l b u t n o t G a u s s i a n . Similar d i s t r i b u t i o n s were o b t a i n e d f o r measurement b i a s (weekly d e r i v e d minus weekly measured c o n c e n t r a t i o n s ) as shown i n the R e s u l t s s e c t i o n below. T h e r e f o r e , b o t h a r o b u s t p a r a m e t r i c t e s t ( p a i r e d t - t e s t (10)) and a n o n p a r a m e t r i c t e s t (Wilcoxon s i g n e d r a n k t e s t (11) were p e r f o r m e d to d e t e r m i n e the s t a t i s t i c a l s i g n i f i c a n c e o f t h e d i f f e r e n c e s . The advantages o f t h e W i l c o x o n t e s t a r e t h a t i t a p p l i e s under more g e n e r a l c o n d i t i o n s than the t - t e s t and i t i s a f f e c t e d o n l y m i n i m a l l y by extreme v a l u e s ; the d i s a d v a n t a g e i s t h a t the t e s t compares r e l a t i v e r a n k s of the d a t a and n o t the a c t u a l data values. Although the s t a t i s t i c a l s i g n i f i c a n c e of parametric t e s t r e s u l t s a r e s e n s i t i v e t o t h e o u t l i e r r e j e c t i o n scheme c h o s e n , t h e p a r a m e t r i c t e s t s r e q u i r e fewer d a t a p o i n t s t o d e t e c t p o p u l a t i o n d i f f e r e n c e s a t the same s i g n i f i c a n c e l e v e l . To t e s t the d i f f e r e n c e s among s i t e s and among s e a s o n s , a n a l y s i s o f v a r i a n c e (ANOVA) t e c h n i q u e s were u t i l i z e d . F o r the one-way ANOVA the s t a n d a r d F - t e s t was augmented by t h e Welch and Browne-Forsythe t e s t s ( 1 2 ) , i n o r d e r t o a c c o u n t f o r the u n e q u a l v a r i a n c e s o f the compared groups ( s i t e s o r s e a s o n s ) . Throughout t h i s s t u d y a s i g n i f i c a n c e l e v e l o f 0.05 was u t i l i z e d . A l l the t e s t s were p e r f o r m e d w i t h the BMDP s t a t i s t i c a l r o u t i n e s ( 1 3 ) . R e s u l t s and

Discussion

A c a l e n d a r of weekly e v e n t s showing t h e number o f samples f o r each week a f t e r o u t l i e r r e j e c t i o n i s p r e s e n t e d f o r each s i t e i n F i g u r e 1. The g r e a t e s t number of b o t h d a i l y and weekly e v e n t s o c c u r r e d i n Vermont and the s m a l l e s t number i n G e o r g i a . The l a r g e s t number o f e v e n t s per month o c c u r r e d i n J u l y f o r G e o r g i a , and i n May and June f o r Kansas and Vermont. S i t e Method B i a s . Method b i a s , d e f i n e d as d e r i v e d weekly minus measured weekly v a l u e s , and r e l a t i v e mean b i a s , the b i a s per mean d e r i v e d weekly v a l u e , were c a l c u l a t e d f o r each s i t e . The a v e r a g e of the c o l l o c a t e d p a i r r e s u l t s f o r e a c h d a i l y and weekly sample was u s e d . F i g u r e 2 shows the d i s t r i b u t i o n o f the method b i a s f o r hydrogen i o n and s u l f a t e f o r each s i t e . The median, mean, and


234


GEORGIA

ι

ι

ι

r

ι

1

1

1

1

1

1

1

1

1


SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT

KANSAS

—

γ

j

1

1

f " ι " " f " ι " " i " " î " — I " " Γ " Ί"—I 1

SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT VERMONT

"1

1

I

1

Γ

' ' Τ

1

1

1

f

' ' 'f

·'·]"·'

1 ' •• τ

1 ' ' Ί 1

SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT

F i g u r e 1. Number of samples p e r week f o r t h e G e o r g i a , Kansas and Vermont s i t e s f o r 1983-1984.


1

TOPOL ET AL.

Weekly and Daily Wet Deposition

Sampling

Results

SULFATE

HYDROGFN

Georgia


Georgia

-0.500

40

-

35

-

-0.250

0.000

0250

0.500

Kansas

30 25

-

20

-

15

LOD:

JUL 0.016

-0.008

0.000

0.008

0.016

-0.500

-0.250

0.000

0250

Vermont

Vermont

0.016

-0.008

0.000

0.008

DIFFERENCE (MG/L)

0.016

0.500

-0.500

-0250

0.000

0250

0.500

DIFFERENCE (MG/L)

F i g u r e 2. (Weekly d e r i v e d minus weekly measured) d a t a f o r hydrogen i o n and s u l f a t e a t t h e G e o r g i a , Kansas and Vermont sites. (The number o f d a t a p o i n t s i n c l u d e d i n each b a r i s l i s t e d above t h e b a r , and a l l extreme v a l u e s a r e i n c l u d e d i n the extreme b a r s . The c u r v e r e p r e s e n t s the G a u s s i a n d i s t r i b u t i o n c a l c u l a t e d from t h e mean and s t a n d a r d d e v i a t i o n o f t h e data.)




236

r e l a t i v e mean v a l u e s of the r e s u l t i n g b i a s f o r a l l o b s e r v a b l e s f o r a l l the p r e c i p i t a t i o n t y p e s combined a r e l i s t e d f o r the t h r e e s i t e s i n T a b l e I . The median and mean b i a s e s t h a t t e s t e d t o be s i g n i f i c a n t by t h e W i l c o x o n and p a i r e d t - t e s t s , r e s p e c t i v e l y , a r e n o t e d i n the t a b l e . D i f f e r e n c e s i n the s i g n i f i c a n t r e s u l t s between t h e two t e s t s may be due t o the s l i g h t l y d i f f e r e n t d i s t r i b u t i o n s used because o f o u t l i e r d e l e t i o n . The c h o i c e of o u t l i e r s w i l l e f f e c t the t - t e s t r e s u l t s , which depend on the mean and s t a n d a r d d e v i a t i o n , but w i l l n o t a f f e c t the W i l c o x o n t e s t r e s u l t s w h i c h do n o t depend on t h e spread of the d i s t r i b u t i o n . Most o f the b i a s v a l u e s a r e n e g a t i v e , i n d i c a t i n g t h a t t h e weekl y measured c o n c e n t r a t i o n s a r e l a r g e r t h a n the weekly d e r i v e d v a l u e s . F o r the G e o r g i a s i t e , the weekly measured v a l u e s a r e l a r g e r t h a n , o r e q u a l t o , the ones d e r i v e d from the d a i l y d a t a f o r a l l t h e o b s e r v a b l e s except potassium. The G e o r g i a s i t e a l s o shows the l a r g e s t number o f a n a l y t e s w i t h s t a t i s t i c a l l y s i g n i f i c a n t b i a s between the two c o l l e c t i o n methods. The l a r g e r p r e c i p i t a t i o n amounts, as w e l l as i o n c o n c e n t r a t i o n s , f o r t h e weekly c o l l e c t e d samples i n d i c a t e t h a t e v a p o r a t i o n i s n o t the cause o f t h e l a r g e r o b s e r v e d c o n c e n t r a tions. P o s s i b l e e x p l a n a t i o n s a r e : (a) s m a l l p r e c i p i t a t i o n samples ( χ

n

—KQXX ο

x+

x

+

+

X

o

ο ο

-40· -50

ι

I

I

I

I

,ox ,X

Ι

Γ

SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT 1983 - 1984

F i g u r e 3. V a r i a t i o n o f p e r c e n t r e l a t i v e b i a s (weekly d e r i v e d measured/weekly d e r i v e d ) w i t h time f o r hydrogen i o n and s u l f a t e a t t h e G e o r g i a , Kansas and Vermont s i t e s .


20.

TOPOL ET AL.

Weekly and Daily

Wet Deposition

Sampling

239

Results


g e n e r a l l y y i e l d s h i g h e r a n n u a l mean c o n c e n t r a t i o n s t h a n d a i l y sampling, b u t the d i f f e r e n c e s a r e s m a l l , a r e assumed t o be due t o method and n o t s i t e t o s i t e d i f f e r e n c e , and a r e n o t c o n s i d e r e d practically significant. However, a network t h a t changes i t s samp l i n g s c h e d u l e w i l l see t h e b i a s e f f e c t and, a l t h o u g h s m a l l , i t can i n t e r f e r e with trend a n a l y s i s . E f f e c t o f Season on Method B i a s . The e f f e c t o f season on t h e conc e n t r a t i o n b i a s was a l s o examined. F o r t h e f a l l q u a r t e r , t h e 1983 and 1984 d a t a were combined. The median, mean and r e l a t i v e mean b i a s e s f o r t h e weekly d e r i v e d minus weekly measured c o n c e n t r a t i o n s ( f o r a l l s i t e s grouped t o g e t h e r ) and t h e r e s p e c t i v e W i l c o x o n and paired t - t e s t r e s u l t s are presented i n Table I I . S i g n i f i c a n t b i a s i s seen from e i t h e r t h e W i l c o x o n o r t - t e s t f o r n i t r a t e and c a l c i u m i n t h e s p r i n g , and f o r hydrogen i n t h e summer. However,_the f a l l season r e v e a l s s i g n i f i c a n t b i a s by b o t h t e s t s f o r H , SO^, C I , Na , Ca and Mg and f o r K by t h e W i l c o x o n t e s t o n l y . The measured weekly c o n c e n t r a t i o n s a r e l a r g e r t h a n t h e d e r i v e d ones i n a l l t h e s e s i g n i f i c a n t b i a s cases except f o r H . Higher C a and M g concent r a t i o n s , w h i c h g e n e r a l l y a r e from b a s i c s o i l d u s t , a r e e x p e c t e d t o y i e l d lower a c i d i t y ( H ) i n a l l samples. A one-way ANOVA d e t e c t e d s i g n i f i c a n t d i f f e r e n c e s i n b i a s among t h e seasons f o r N O 3 , C I and Mg* o n l y , as shown i n T a b l e I I . The r e s u l t s i n d i c a t e t h a t a m a j o r i t y o f t h e a n a l y t e s show s i g n i f i c a n t b i a s i n t h e f a l l , and t h e l a r g est r e l a t i v e b i a s f o r a l l observables a l s o occurs i n the f a l l . The s i g n i f i c a n t f a l l season b i a s e s a r e d i f f i c u l t t o e x p l a i n . F o r t h e f o u r major i o n s , t h e r e l a t i v e b i a s e s a r e under 13%. The l a r g e b i a s f o r p o t a s s i u m i n d i c a t e s t h a t t h e r e a r e sampling p r o b l e m s , p o s s i b l y due t o a d s o r p t i o n - d e s o r p t i o n from t h e p l a s t i c b u c k e t . These r e s u l t s i n d i c a t e t h a t t h e b i a s i n c o n c e n t r a t i o n between weekly and d a i l y samples i s s i m i l a r f o r most of t h e c o n s t i t u e n t s i n a l l seasons b u t the f a l l . +

+

+

+ 2

+ 2

+

2

Summary and The * *

*

*

Conclusions

r e s u l t s of t h i s study i n d i c a t e that C o n c e n t r a t i o n s o f weekly c o l l e c t e d samples a r e g e n e r a l l y h i g h e r t h a n t h o s e of t h e d e r i v e d weekly samples. No c o r r e s p o n d e n c e o c c u r s between c o n c e n t r a t i o n b i a s f o r t h e major i o n s and t h e b i a s i n p r e c i p i t a t i o n volume c o l l e c t e d . Therefore, the o b s e r v e d b i a s e s cannot be a t t r i b u t e d t o e v a p o r a t i o n . A l t h o u g h many i o n i c c o n c e n t r a t i o n b i a s e s between t h e weekly measured and d e r i v e d samples a r e s t a t i s t i c a l l y s i g n i f i c a n t , t h e i r magnitude i s g e n e r a l l y

Comparison of Weekly and Daily Wet Deposition Sampling Results

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