The chemical composition of bulk precipitation across the mountains of Snowdonia, U.K.

The Science of the Total Environment, 92 (1990) 223-234 Elsevier

223

THE CHEMICAL COMPOSITION OF BULK PRECIPITATION ACROSS THE MOUNTAINS OF SNOWDONIA, U.K.

B. REYNOLDS, T.G. WILLIAMS and P.A. STEVENS Institute of Terrestrial Ecology, Bangor Research Station, Penrhos Road, Bangor, Gwynedd, North Wales (United Kingdom)

(Received February 3rd, 1989; accepted August 22nd, 1989)

ABSTRACT

Bulk precipitation chemistry was monitored monthly for 18 months at 10 mountain sites and one coastal site in north Wales, U.K. The sites ranged in altitude from sea level to 891 m above sea level in an area where annual rainfall ranges from 1300 to 4000ram. The precipitation was acidic (weighted mean H ÷ of 23geq 1-1, pH 4.64), although ionic composition was dominated by sea-salts. However, ~-70% of the SO4was not of sea-salt origin. Concentrations of excess SO4and NO3were small (42 and 11 ~eq 1-1, respectively) relative to polluted areas of Europe, although deposition rates were large (1.62g S m- 2year- 1and 0.54g N m- 2year- 1as NO~)due to the high annual rainfall. There was considerable spatial variability in the solute concentration and deposition data which resulted from the effects of the complex topography of the area. Simple linear relationships between solute deposition and rainfall quantity were not generally observed except for sea-salt deposition on seaward north-west facing slopes. INTRODUCTION In the U n i t e d Kingdom, the h i g h e s t rainfall o c c u r s in the m o u n t a i n a r e a s w h e r e access is poor. F o r this reason, r o u t i n e sampling of p r e c i p i t a t i o n c h e m i s t r y is r a r e l y u n d e r t a k e n in these regions and t h e r e are r e l a t i v e l y few available a t m o s p h e r i c deposition data. In r e c o g n i t i o n of this, the first r e p o r t of the U.K. Review G r o u p on Acid Rain (RGAR, 1983) r e c o m m e n d e d t h a t f u r t h e r studies be initiated in the m o u n t a i n o u s regions of Britain. Twenty-five y e a r s previously, however, G o r h a m (1958) h a d realised the ecological significance of the large a t m o s p h e r i c inputs of both n u t r i e n t s and p o l l u t a n t s to the remote, h i g h rainfall, m o u n t a i n o u s areas of w e s t e r n Britain. R e c e n t studies u n d e r t a k e n in u p l a n d a r e a s of B r i t a i n h a v e s h o w n evidence for and a g a i n s t a significant v a r i a t i o n in the chemical c o m p o s i t i o n of precipit a t i o n with altitude. R e y n o l d s (1984) and Goldsmith (1986) show no evidence for a c h a n g e in p r e c i p i t a t i o n c h e m i s t r y with altitude while, u n d e r some circumstances, v a r i a t i o n s in snow c o m p o s i t i o n with altitude h a v e been observed (Davies et al., 1984). A m a r k e d v a r i a t i o n in rainfall solute c o n c e n t r a t i o n s with altitude has been observed at G r e a t D u n Fell (Fowler et al., 1988) w h e n e v e r the hill s u m m i t is enveloped in o r o g r a p h i c cloud. The m o u n t a i n s of S n o w d o n i a are close to the west c o a s t of n o r t h Wales.

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224 They contain several summits over 950 m above sea level and receive a high annual rainfall, often in excess of 4000mm on the higher peaks. The area is, however, typical of the mountain regions of western Britain. The study reported here monitored the composition of bulk precipitation over an 18 month period at 10 sites, ranging in altitude from 90 to 891 m above sea level, in the Snowdonia mountains. STUDY AREA AND METHODS The study area stretches from the south western tip of the Isle of Anglesey, adjacent to the Menai Straits, to the mainland and includes some of the highest peaks in nor t h Wales (Fig. 1). Between these two extremes of altitude is a relatively low lying plateau of land (ca 100 m above sea level) across which, within a distance of -~ 10km, the annual rainfall increases from 1200 to 2400 mm y ear -I. On and around Snowdon and neighbouring peaks, this figure exceeds 4000 mm ye a r -1 . The area has a mild maritime climate with prevailing south westerly winds. Snowfall may occur on the mountains anytime between October and May. Bulk precipitation collectors were located at 11 sites within the area. These formed two transects from the Anglesey coast at Newborough (Site 1) across the main m ount ai n blocks of the Glyders (Sites 2, 4 and 9) and Snowdon (Sites 3 and 5-10) with an additional collector adjacent to Beddgelert forest (Site 11) (Fig. 1 and Table 1). The precipitation collector comprised a 150mm diameter polypropylene funnel assembly connected to a 101 collecting bottle enclosed in black

A n g l e s e y ~

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Km J~] Landabove400m Rainfal~mm • Summitover s 950m

Fig. 1. Location map of the bulk precipitation collectors in Snowdonia, North Wales.

225 TABLE 1 Details of bulk precipitation collection sites across Snowdonia Site

U.K. grid reference

Name

No.

Newborough Moel y Ci Pen y Bwlch Cwm Idwal Ffynnon y Gwas Clogwyn Crib Goch Llydaw Dyffryn Mymbyr Cwm Dyli Beddgelert

1 2 3 4 5 6 7 8 9 10 11

SH 422 641 SH 589 659 SH 547 597 SH 647 596 SH 584 456 SH 598 554 SH 619 551 SH 639 549 SH 672 565 SH 654 542 SH 556 518

Height above sea-level (m) 10 366 335 419 497 777 891 472 290 91 343

Rainfall (mm) Study period

30 year annual average

1307 869 945 3816 3805 2684 3503 3702 3867 4207 4025

900 1500 2200 3200 3200 3800 4000 3600 3300 3200 2800

p o l y t h e n e to exclude l i g h t f r o m t h e sample. T h e funnel w a s l o c a t e d 1.5 m a b o v e the g r o u n d s u r f a c e a n d a r i n g of p l a s t i c spikes was fitted to the r i m of t h e funnel to d e t e r birds f r o m p e r c h i n g . A p l u g of 500 #m n y l o n m e s h was p l a c e d into the n e c k of e a c h funnel to exclude i n s e c t s a n d p l a n t debris f r o m t h e s a m p l e bottle. T h e funnels a n d the n y l o n m e s h plugs w e r e c h a n g e d at e a c h collection. A n y snow collected by the funnel was i n c l u d e d in the b u l k p r e c i p i t a t i o n sample. S a m p l e s w e r e collected at 4 w e e k i n t e r v a l s b e t w e e n D e c e m b e r 1984 a n d April 1986. On r e t u r n to the l a b o r a t o r y , the v o l u m e a n d pH of e a c h s a m p l e w e r e d e t e r m i n e d p r i o r to f i l t r a t i o n t h r o u g h 0.45 pm p o r e size m e m b r a n e s . S o d i u m a n d K w e r e d e t e r m i n e d by flame e m i s s i o n a n d Ca a n d M g by flame a t o m i c a b s o r p t i o n s p e c t r o p h o t o m e t r y . N i t r a t e , SO4, C1, PO4 a n d NH4 w e r e d e t e r m i n e d by ion c h r o m a t o g r a p h y . Sodium, K, Ca a n d M g w e r e m e a s u r e d w i t h a p r e c i s i o n of _+5%, NH4 a n d t h e a n i o n s to _+2% a n d pH to + 0.05 pH units. RESULTS AND DISCUSSION

Solute concentrations M e a n solute c o n c e n t r a t i o n s , w e i g h t e d by r a i n f a l l a m o u n t , for the 18 m o n t h period a r e s h o w n in T a b l e s 2 and 3. S e v e n m o n t h s w i t h s i g n i f i c a n t snowfall w e r e e x c l u d e d f r o m t h e a n a l y s i s for s e v e r a l reasons. S n o w f l a k e s a n d r a i n drops differ in t h e i r r e m o v a l of p o l l u t a n t s f r o m t h e a t m o s p h e r e a n d t h e r e is e v i d e n c e t h a t s n o w a n d r a i n m a y h a v e different c h e m i c a l c o m p o s i t i o n s e v e n w h e n collected u n d e r v e r y s i m i l a r e n v i r o n m e n t a l c o n d i t i o n s ( R a y n o r a n d H a y e s , 1983; Reynolds, 1983). C o l l e c t i o n a n d m e a s u r e m e n t of snowfall is also e x t r e m e l y difficult u n d e r t h e w i n d y c o n d i t i o n s w h i c h u s u a l l y a c c o m p a n y snow in the m o u n t a i n s . F r e e z i n g of a c c u m u l a t e d snow in c o l l e c t o r funnels m a y h a v e r e s u l t e d in u n d e r - c o l l e c t i o n of r a i n d u r i n g s u b s e q u e n t events. S i g n i f i c a n t

226 TABLE 2 Rainfall weighted mean and range (in parentheses) of pH and cation concentrations Oeq 1-1) in bulk precipitation across Snowdonia Site

Na

Mg

K

Ca

NH4

H

pH

Cation sum

1 2 4 9 3

11 5 6 7 8 10

209 (70-378) 113 (30-300) 70 (13-131) 83 (35-174) 109 (30-231) 104 (44-204) 100 (39-231) 96 (48-231) 70 (17-239) 65 (9-139) 83 (35-196)

49 (18~84) 29 (8-76) 17 (1-31) 20 (10-41) 26 (10-54) 25 (8-51) 24 (10-51) 22 (13-53) 16 (8-58) 16 (7 35) 21 (7-46)

7 (2 15) 4 (1-10) 1 ( < 1-3) 2 ( < 1-5) 2 (1-6) 2 (1-13) 3 ( < 1-5) 2 ( < 1-6) 1 ( < 1-4) 1 ( < 1-6) 2 ( < 1-4)

20 (14-33) 19 (11-49) 8 (5-13) 8 (6-11) 15 (6-42) 11 (6-18) 10 (4-17) 10 (5-19) 7 (4 17) 9 (5-23) 9 (5-12)

28 (13-68) 40 ( < 5-107) 11 ( < 5-37) 14 ( < 5-43) 27 (7-78) 15 ( < 5-85) 13 ( < 5-39) 23 (7-52) 19 ( < 5-72) 13 ( < 5-38) 13 ( < 5-47)

25 (< 1-105) 35 (12-91) 18 (5-107) 18 (2-50) 32 (13-62) 21 (12-43) 22 (6-40) 27 (9-54) 19 (8-53) 24 (8-54) 27 (9-56)

4.60 (3.9~6.40) 4.45 (4.04-4.93) 4.74 (3.97-5.32) 4.74 (4.30-5.74) 4.49 (4.21-4.90) 4.68 (4.37-4.94) 4.66 (4.40-5.23) 4.57 (4.27-5.04) 4.73 (4.28-5.12) 4.63 (4.27-5.12) 4.57 (4.25-5.05)

338 240 125 145 211

178 172 180 132 128 155

snowfall also occurred at the higher altitude sites when rain was falling in the valleys. Exclusion of data in this manner assumes that the chemistry of rainfall during the omitted months is adequately represented by the remaining data. However, as there are pronounced seasonal cycles in the concentrations of major ions in rainfall, the values presented will not be the same as precipitation weighted annual averages, although the difference between them is uncertain. The proportion of individual solutes derived from sea-spray has been estimated by assuming that all the Na in precipitation is of marine origin and that the ratio of Na to other ions in sea-spray is the same as that in bulk seawater. The "excess" concentration is the difference between the measured concentration and the estimated sea-spray or sea-salt contribution (RGAR, 1987). There has been considerable debate over the relationship between the chemical composition of marine aerosols and that of bulk seawater. It has often been stated that differences in composition arise from fractionation processes during particle formation at the air-sea interface (e.g., Chesselet et al., 1972; Wilkniss and Bressan, 1972). However, following an extensive review of the literature, Duce and Hoffman (1976) concluded that for Ca, Mg and SOt, apparent enrichment relative to Na in aerosols probably results, in most cases,

227 TABLE 3 Rainfall weighted mean and range (in parentheses) of anion concentrations (peq1-1) in bulk precipitation across Snowdonia Site

C1

SO4

NO~

Anion sum

1 2 4 9 3 11 5 6 7 8 10

206 (28-536) 127 (34--367) 79 (25-203) 93 (45-183) 116 (31-248) 107 (37 214) 99 (31-262) 102 (56-243) 76 (23-254) 79 (31-226) 87 (28-226)

80 (4(~181) 76 (39-150) 31 (19-62) 42 (21-75) 74 (46-131) 43 (26~7) 35 (19-75) 54 (26-94) 33 (19-75) 39 (19-75) 35 (19-81)

23 (949) 32 (12~6) 10 (6-22) 11 (6-29) 20 (10-66) 11 (4-34) 11 (6-23) 15 (6-37) 9 (4-28) 9 ( < 1-27) 10 (5-28)

309 235 120 146 210 161 145 171 118 127 132

f r o m i n c l u s i o n of n o n - s e a p a r t i c l e s in the e n v i r o n m e n t a l samples, w i t h fract i o n a t i o n c o n t r i b u t i n g no m o r e t h a n a few percent. T h e y also c o n c l u d e d t h a t t h e r e w a s little s t r o n g e v i d e n c e for significant f r a c t i o n a t i o n of C1. P o t a s s i u m m a y be f r a c t i o n a t e d due to its a s s o c i a t i o n w i t h o r g a n i c m a t e r i a l in the seas u r f a c e m i c r o l a y e r (Duce a n d H o f f m a n 1976), w h i c h is often c h a r a c t e r i s e d by a n o m a l o u s l y h i g h c o n c e n t r a t i o n s of h e a v y m e t a l s a n d a c t i n i d e s a s s o c i a t e d w i t h p a r t i c u l a t e a n d o r g a n i c m a t e r i a l (Bacon a n d E l z e r m a n , 1980; W a l k e r et al., 1986). T h e sea is also a s o u r c e of s u l p h u r gases s u c h as d i m e t h y l sulphide ( B a r n a r d et al., 1982; T u r n e r a n d Liss, 1985), a n d s u l p h a t e a r i s i n g f r o m t h e s e e m i s s i o n s is i n c l u d e d in t h e e s t i m a t i o n of excess SO4. Sea-salt ions (Na, Mg, C1, K, sea-salt Ca a n d sea-salt SO4) d o m i n a t e d t h e p r e c i p i t a t i o n c h e m i s t r y , a c c o u n t i n g for b e t w e e n 61 a n d 78% of the b u l k ionic c o m p o s i t i o n (Table 4). I n d i v i d u a l l y , b e t w e e n 26 a n d 44% of t h e Ca and b e t w e e n 18 a n d 35% of the SO4 w a s c o n t r i b u t e d by sea-salts (Table 4). S i g n i f i c a n t excess K (30%) was f o u n d o n l y at Sites 1 a n d 2. T h e r e was a n o v e r a l l d e c r e a s e in the c o n c e n t r a t i o n of sea-salt ions a c r o s s t h e m o u n t a i n s in a n a p p r o x i m a t e l y N W - S E d i r e c t i o n (Fig. 2). T h e t r e n d w a s i n d e p e n d e n t of a l t i t u d e a n d p r o b a b l y reflected t h e influence of the t o p o g r a p h y on t h e d e p o s i t i o n of l a r g e sea-salt

228 TABLE 4 Rainfall weighted m e a n c o n c e n t r a t i o n s and percentage contribution of sea-salt Ca and SO 4 and total sea-salts in bulk precipitation across Snowdonia Site

1 2 4 9 3 11 5 6 7 8 10

Sea-salt Ca"

Sea-salt SO~

Total sea-salts

#eq1-1

%

geq1-1

%

geq1-1

%

9 5 3 4 5 4 4 4 3 3 4

45 26 38 50 33 36 40 40 43 33 33

25 14 8 10 13 12 12 11 8 8 10

31 18 26 24 17 28 34 20 24 21 29

505 292 178 212 271 254 242 237 174 172 207

78 61 73 73 64 75 76 68 70 67 72

a Based on Na.

particles (Peirson et al., 1973). Sea-salt concentrations at Sites 9 and 10 on the eastern edge of the main mountain area were higher than those at some sites within the mountains (4, 7 and 8). This may have resulted from the deposition of large sea-salt particles carried inland along the south west facing Glaslyn valley, which provides a relatively uninterrupted corridor for south westerly winds off the sea. Local topography may have affected the efficiency of gauges at some of the sites, although the extent of this effect is not known. Concentrations at Site 6, which was on a south west facing slope, were greater than the higher altitude Site 7, which was close to an exposed ridge. High wind speeds and turbulence may have caused under-capture of small droplets with high solute concentrations at this site. The spatial variation in the proportions of sea-salts did not completely reflect the pattern in the concentrations. The highest proportion of sea-salts was found at the coastal site (1), but declined significantly inland at Sites 2 and 3, although sea-salt concentrations were high at these sites (Table 4). The proportions of sea-salt SO 4 and total sea-salts at Site 6 were similar to those at Sites 2 and 3. The acidity of the precipitation and the concentration of pollutant-related ions varied, but not systematically, across the mountain sites. The most acid rainfall occurred at Sites 2 and 3 and coincided with some of the highest concentrations of excess SOt (Fig. 3), NO3 and NH 4 (Tables 2 and 3). The concentrations and proportions of excess Ca were also high at these sites, which were situated on hill slopes overlooking the comparatively flat low-lying land between the Menai Straits and the mountains. This area supports intensive agriculture and the fields periodically receive lime and fertilizer additions and occasional applications of sewage sludge.

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Fig. 3, Hydrogen ion and excess SO 4 concentrations in bulk precipitation at sites along two N W - S E transects across the Snowdonia mountains

231 P r e c i p i t a t i o n at Site 6 w a s m o r e acid a n d c o n c e n t r a t i o n s of excess S04, NO~ and NH4 w e r e significantly g r e a t e r t h a n at Site 5, w h i c h w a s l o c a t e d a t a l o w e r a l t i t u d e on the s a m e u n i f o r m hill slope. This g e n e r a l l y a c c o r d s w i t h observ a t i o n s at G r e a t D u n Fell (GDF), w h e r e solute c o n c e n t r a t i o n s in r a i n f a l l i n c r e a s e d w i t h a l t i t u d e up a u n i f o r m hill slope ( F o w l e r et al., 1988). S c a v e n g i n g of cap cloud d r o p l e t s by falling r a i n h a s b e e n p r o p o s e d as the m e c h a n i s m r e s p o n s i b l e for this at G D F a n d m a y also a p p l y to t h e s e two sites on Snowdon. At the o t h e r sites, the complex t o p o g r a p h y i n t r o d u c e s l a r g e v a r i a b i l i t y into the results, m a k i n g simple a l t i t u d e - c o n c e n t r a t i o n a n a l y s e s i n a p p r o p r i a t e .

Solute deposition Solute d e p o s i t i o n at e a c h site (Table 5) w a s e s t i m a t e d u s i n g the M e t e o r o l o g i cal Office 30 y e a r (1941-1970) a v e r a g e a n n u a l r a i n f a l l figure, i n t e r p o l a t e d f r o m m a p data, c o m b i n e d w i t h the r a i n f a l l w e i g h t e d m e a n solute c o n c e n t r a t i o n d e t e r m i n e d d u r i n g t h e study. F o r t h e m a j o r i t y of sites (2, 3, 11, 5, 6) on t h e n o r t h west side of t h e m a i n m o u n t a i n s u m m i t s (i.e. t h o s e > 950 m), sea-salt d e p o s i t i o n i n c r e a s e d l i n e a r l y w i t h r a i n f a l l a m o u n t to a m a x i m u m a t Site 6 (Fig. 4). T h e N W - S E d e c r e a s e in sea-salt c o n c e n t r a t i o n (Fig. 2) was offset by the p r o p o r t i o n ally l a r g e r i n c r e a s e in r a i n f a l l a m o u n t . At the r e m a i n i n g sites, t h e r e l a t i o n s h i p b e t w e e n d e p o s i t i o n a n d r a i n f a l l was less consistent. H y d r o g e n ion d e p o s i t i o n also i n c r e a s e d , t h o u g h n o t in a simple l i n e a r m a n n e r , w i t h r a i n f a l l a m o u n t . M a x i m u m d e p o s i t i o n was o b s e r v e d a t Site 6, w h e r e h i g h c o n c e n t r a t i o n s coincided w i t h h i g h a n n u a l rainfall. At Sites 2 a n d 3 the r e l a t i v e l y h i g h H ion c o n c e n t r a t i o n s w e r e offset by low r a i n f a l l a m o u n t s (Fig. 3). F o r t h e r e m a i n i n g ions, no c o n s i s t e n t r e l a t i o n s h i p b e t w e e n d e p o s i t i o n a n d q u a n t i t y of r a i n f a l l w a s observed. H i g h e s t d e p o s i t i o n s of excess SO4 a n d NO~ w e r e found a t Site 6. T h e l a r g e v a r i a b i l i t y in the c h e m i c a l c o n c e n t r a t i o n d a t a was p r o b a b l y t h e TABLE 5 Estimated annual wet deposition (g m -2 year- 1) across Snowdonia Site

Na

Mg

K

Ca

NH,-N

H

C1

Total SO4 S

Excess SO4-S

NO3 N

1 2 4 9 3 11 5 6 7 8 10

4.32 3.90 5.12 6.27 5.50 6.72 7.36 8.36 6.40 5.40 6.08

0.54 0.53 0.67 0.79 0.70 0.84 0.93 1.03 0.80 0.72 0.80

0.23 0.21 0.13 0.23 0.18 0.25 0.32 0.27 0.12 0.14 0.22

0.37 0.57 0.54 0.56 0.68 0.62 0.67 0.80 0.56 0.65 0.61

0.35 0.84 0.50 0.64 0.82 0.59 0.60 1.24 1.06 0.64 0.57

0.023 0.053 0.059 0.060 0.070 0.058 0.069 0.103 0.075 0.085 0.087

6.57 6.75 8.96 10.89 9.02 10.64 11.20 13.68 10.80 10.08 9.92

1.16 1.82 1.61 2.21 2.60 1.94 1.78 3.26 2.09 2.26 1.77

0.79 1.49 1.18 1.68 2.14 1.38 1.16 2.56 1.56 1.80 1.26

0.29 0.67 0.47 0.53 0.62 0.43 0.50 0.80 0.48 0.46 0.43

232 6 •

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0.5

500

1500

2500 Rainfall

3500

4500

mm

Fig. 4. Sea-salt deposition and rainfall quantity at mainland sample sites; (m) sites on the NW side of main m o u n t a i n summits; and (e) sites on SE side of main summits.

main factor contributing to the non-linear relationship generally observed between deposition and rainfall amount. The average annual deposition of excess SO4, NO3 and H ion across the mountain area as a whole (i.e. excluding Site 1) is high compared with much of Britain. The average wet deposited excess SO4 was 1.62 g S m -2 year- 1, NO3 was 0 . 5 4 g N m - 2 y e a r -1 and H ion was 0.072gm-2year -1. These values are comparable to data for north Wales collected during a survey of atmospheric deposition throughout the Principality in 1984 (Donald and Stoner, 1989). However, the high depositions observed at Site 6 (2.56gSm-2year -1, 0 . 8 0 g N m - 2 y e a r -1 and 0 . 1 0 3 g H ÷ m - 2 y e a r -1) exceed any reported for that survey. Within the Snowdonia mountains, the average annual wet deposited acidity, excess SO4 and NO~ fall into the highest deposition categories reported by the U.K. Review Group on Acid Rain (RGAR, 1987), although the concentrations of these ions are relatively low compared with central and eastern Britain. For example, at Bottesford in eastern England, mean annual concentrations of excess SO4 were 121 peq 1-1, NO3 50 #eq 1-1 and H ion 66 #eq 1-1 (RGAR, 1987) compared with 42, 11 and 23peql 1, respectively, for the mountain sites in Snowdonia. Thus, in common with southwest Scotland, high deposition rates in Snowdonia result from the high annual rainfall.

233 CONCLUSIONS P r e c i p i t a t i o n c h e m i s t r y across the m o u n t a i n s of S n o w d o n i a is d o m i n a t e d by sea-salt derived solutes, a l t h o u g h a d e c r e a s i n g c o n c e n t r a t i o n g r a d i e n t with d i s t a n c e from the sea is observed. T h e rainfall is acidic (rainfall weighted m e a n pH 4.64) and c o n t a i n s a high p r o p o r t i o n of excess SO4 (-~ 75%) derived from i n d u s t r i a l emissions, supplemented, to some extent, by s u l p h u r gases released from the sea. C o n c e n t r a t i o n s of excess SO4 and NO3 are r e l a t i v e l y low c o m p a r e d with c e n t r a l and e a s t e r n Britain, a l t h o u g h high deposition r a t e s r e s u l t from the high a n n u a l rainfall. F o r one u n i f o r m hill slope exposed to the west, solute c o n c e n t r a t i o n s i n c r e a s e d with altitude. At the o t h e r sites, simple a l t i t u d e - c o n c e n t r a t i o n a n a l y s e s were i n a p p r o p r i a t e due to the large v a r i a b i l i t y in the d a t a arising from the complex t o p o g r a p h y . This has implications for the regional applic a t i o n of o r o g r a p h i c e n h a n c e m e n t models (e.g., Hill et al., 1988) to solute deposition in m o u n t a i n o u s areas. Sea-salt deposition on the seaward, n o r t h west facing, slopes of the m o u n t a i n s increased l i n e a r l y with rainfall amount, b u t this simple relationship was not observed on the l a n d w a r d side. H y d r o g e n ion deposition also increased, t h o u g h not systematically, with rainfall a m o u n t across all the sites. F o r the o t h e r ions, t h e r e was no c o n s i s t e n t r e l a t i o n s h i p b e t w e e n deposition and rainfall q u a n t i t y . Thus, w i t h i n a m o u n t a i n o u s area, it would be e r r o n e o u s to simply e x t r a p o l a t e deposition from a low a l t i t u d e site to h i g h e r altitudes on the basis of i n c r e a s i n g rainfall. ACKNOWLEDGEMENTS The a u t h o r s would like to t h a n k D.G. H e w e t t (ITE Bangor), J.H. Williams (UCNW Bangor) and I. Ellis-Williams (formerly NCC Warden, Cwm Idwal) for assistance with sample collection, and S. H u g h e s (ITE Bangor) and D. Roberts and colleagues at ITE M e r l e w o o d for p e r f o r m i n g the chemical analyses. The m a n y helpful comments on the m a n u s c r i p t received from D. F o w l e r (ITE Bush) and M. H o r n u n g (ITE Merlewood) are also g r a t e f u l l y acknowledged. The study was p a r t funded by the Welsh Office. REFERENCES Bacon, M.P. and A.W. Elzerman, 1980. Enrichment of 21°Pband 21°poin the sea-surface microlayer. Nature, 284: 332-334. Barnard, W.R., M.O. Andreae, W.E. Watkins, H. Bingemer and H.-W. Georgi, 1982. The flux of dimethylsulfide from the oceans to the atmosphere. J. Geophys. Res., 87: 8787~8793. Chesselet, R., J. Morelli and P. Buat-Menard, 1972.Variations in ionic ratios between reference sea water and marine aerosols. J. Geophys. Res., 77: 5116-5131. Davies, T.D., P.W. Abrahams, M. Tranter, I. Blackwood, P. Brimblecombe and C.E. Vincent, 1984. Black acidic snow in the remote Scottish Highlands. Nature, 312: 58~1. Donald, A.P. and J.H. Stoner, 1989.The quality of atmospheric deposition in Wales: Arch. Environ. Contam. Toxicol., 18: 109-119. Duce, R.A. and E.J. Hoffman, 1976. Chemical fractionation at the air/sea interface. A. Rev. Earth

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