Fog and rainwater composition in rural SE Australia

Journal of Atmospheric Chemistry 13: 83-95, 1991. 83 © 1991 Kluwer Academic Publishers. Printed in the Netherlands.

Fog and Rainwater Composition in Rural SE Australia

D A V I D P O S T * , H O W A R D A. B R I D G M A N Department of Geography, University of Newcastle, New South Wales, Australia

and

G. P. A Y E R S Division of Atmospheric Research, CSIRO, Aspendale, Victoria 3195, Australia

(Received: 26 September 1990)

Abstract. In order to determine if pollutants from the Wollongong-Sydney-Newcastle industrial area in southeastern Australia can be transported northwards, affecting precipitation quality to the north, a preliminary study of fog and rainwater quality was carried out from January to April 1989. Samples were collected from two sites in the state of New South Wales, one in the Barrington Tops and the other near Dorrigo. These samples were analyzed for the cations sodium, calcium, magnesium, potassium, and ammonium, and the anions chloride, nitrate, sulfate, methanesulfonate, formate and acetate, as well as pH and conductivity.

The mean pH of fogwater from the two sites was 5.48, compared to 5.62 for rainwater. Fogwater also had concentrations of ions 2-6 times those in rainwater. For both fog and rain the ions sodium, chloride, magnesium, and methanesulfonate at both sites were essentially entirely sea-salt derived, while only 5-50% of potassium, calcium, and sulfate were derived from sea-salt. The acid-base balance was adequately described (r = 0.76) by a balance between the acidity contributed by sulfuric and nitric acids, neutralized by the alkalinity of ammonia and (soil dust-derived) calcium carbonate. Comparing this study with others, both within Australia and overseas, fog and rainwater at both sites are not polluted, with acidity only slightly greater than background and concentrations of anthropogenic pollutants very low, and with sea-salt influences accounting for the majority of ionic loading.

Key words: Fogwater composition, rainwater composition, rural Australia.

1. Introduction

Acid precipitation occurs in regions where the natural acid-base balance of the atmosphere is perturbed by the inclusion of large quantities of sulfur dioxide (SO2) and oxides of nitrogen (NOx) from urban and industrial activities. In regions such as the north-eastern comer of north America, and much of Europe, these anthropogenic emissions can exceed natural emissions of sulfur and nitrogen species by factors of 10 or more. It is not the primary pollutants (SO2 and NOx) which directly cause acidification of cloud, rain or fogwater, but their atmospheric oxidation products, sulfuric and nitric acids. Contemporary reviews summarising the current

* Present address: Division of Atmospheric Research, CSIRO, Aspendale, Victoria 3195, Australia.

84 DAVID POST, HOWARD A. BRIDGMAN, AND G. P. AYERS

understanding are given by Mohnen (1988), Schwartz (1989), Galloway (1989), Likens (1989), and McDowell (1988).

Acidification of the environment is not just restricted to the regions close to industrial areas. Oxidation and deposition processes at work in the atmosphere occur over characteristic time scales of order 1-3 days (Environmental Resources, 1983), leading to distribution of deposited acids over distance scales up to a thousand or more km from the source (Schwartz, 1989). Thus atmospheric acidification and the possibility of resultant environmental acidification must be viewed as a regional, and not just local scale phenomenon (Schwartz, 1989).

In Australia, the small population (about 17 million), large land area and low sulfur content of indigenous coals have been interpreted as implying that there should be no major atmospheric acidification from anthropogenic emissions, Thus only in the last decade has any investigation into the problem occurred. Research has been carried out in the Northern Territory (Galloway et al., 1982; Keene et al., 1983; Likens et aL, 1987; Noller et aL, 1990; Gillett et aL, 1990), Cape Grim, Tasmania (Ayers and Ivey, 1988, 1990), the Latrobe Valley, Victoria (Fitzgerald, 1986), Sydney, New South Wales (Ayers and Gillett, 1984; Ayers et al., 1987), and the Hunter Valley, New South Wales (Avery, 1984; Pang Way and Rothwell, 1986; Bridgman et al., 1988). A review of acid rain studies in Australia was carried out by Bridgman (1989). Measurable acidification of rainfall in some areas was observed, but at levels several times lower than those of concern in areas of Europe and North America.

The potentially greater problem posed by acid fog has not been studied in Australia. Yet in other studies fog has been shown to contribute a higher propor- tion of dissolved ions and acidity than rain in, for example, the Adirondack Moun- tains of New York State (Falconer and Falconer, 1980) and in mountainous areas in Quebec, Canada (Schemenauer, 1986). A considerable body of additional infor- mation and references on various aspects of the 'acidic fog' phenomenon can be found in the book edited by Unsworth and Fowler (1988).

The purpose of this study, the first fogwater study in Australia, was to examine the summertime quality of fogwater and concurring rainwater at two elevated, rural sites in southeast Australia in order to determine if pollutants can be transported northwards from the Wollongong-Sydney-Newcastle industrial area, and deposited on the escarpment of the Great Dividing Range as an 'acid fog'.

2. Location and Methods of Analysis

Fog, and rain occurring during fog events were collected from the two sites shown on the map in Figure 1: one site was in the Barrington Tops (31"55' S, 151035 , E) and the other 15 km west of Dorrigo in the New England Region (30°20 ' S, 152040 ' E). At Dorrigo, the site was located at 900 m ASL on a ridge with an open S-SE aspect. On the Barrington Tops, the site was located at 400 m ASL, with a moderately open aspect to the south.

FOG AND RAINWATER COMPOSITION IN RURAL SE AUSTRALIA 85

~ New South Wales Site II/ 51 Barrington Tops== ) - TN ~ i Study Site - ~" |

~_, Newcast'~ ~'~ '.~.~ Sydney/ / / - -

° 'o , ; , , - - - . . , _ '

"~.. ( 0 1 O0 200 300 400 Ions "',,~ l I I i I

Fig. 1. Map of New South Wales, Australia, showing sampling sites.

Fogwater was collected passively by samplers standing about 1 m off the ground. Each collector consisted of a double layer of teflon fibres strung around 180 grooves in the circular sampler top and base providing a large surface area for the fog to impact upon. The fogwater then ran down the fibres into a funnel to be collected in a polyethylene sample bottle located in the base. This collector type is that used by Schemenauer (1986) in the Chemistry of High Elevation Fog experi- ments in Canada.

To collect rain, standard plastic rain gauges were used. The rain collected in the clean gauges was transferred to a polyethylene sample bottle in the field. After each exposure the fog and rain collectors and samples were shipped from the field to the University of Newcastle by overnight courier. The collectors were cleaned by repeated rinsing with de-ionised water until the rinsewater conductivity decreased to that of the initial conductivity of the de-ionised water (as per Falconer and Falconer, 1980; Galloway etaL, 1982).

At both sites, samplers were exposed on a day when fog was predicted and then retrieved the next day. The sampling protocol required addition of 1 mL of chloroform per 100 mL of retrieved sample in order to prevent biological activity from subsequently altering the concentrations of organic acids and other labile species, as recommended by Galloway et al. (1982) and Keene et al. (1983). However, there is some question as to whether this amount of chloroform was actually added to all samples. Thus in the discussion that follows the formate and acetate values

86 DAVID POST, HOWARD A. BRIDGMAN, AND G. E AYERS

reported may well be lower than those that actually occurred in the fogs sampled, and pH might be correspondingly elevated.

The pH and conductivity of the fog and rainwater samples were recorded at the University, then the samples were shipped to AGAL (Australian Government Analytical Laboratories) in Tasmania for analysis. At all times between shipment samples were stored in the dark at 4 °C. At AGAL pH and conductivity were re- determined, sodium, potassium, magnesium and calcium concentrations were determined by flame AA, ammonium by the indophenol-blue colorimetric method, and the anions methanesulfonate, formate, acetate, chloride, nitrate and sulfate were determined by suppressed ion chromatography (Dionex AS4A columns). Precision for all individual analyses was to better than 10% uncertainty (see Ayers and Ivey, 1988).

A total of 36 samples was collected from the two sites between January and April 1989, 23 from Dorrigo and 13 from Barrington. Of these 36 samples 18 were fogwater and 18 rainwater.

3. Results

Tables I and II contain arithmetic mean values for the fogwater and rainwater data obtained at Barrington and Dorrigo, along with a selection of results from other locations.

Table I. Comparison of fogwater data from a number of studies; all values except pH in geq/L. Figures in parentheses are nonseasalt components. See text for details

Ion Reference

B D 1 2 3 4 5 6

plq 5.6 5.42 3,42 3.6 3.32 3.75 3.5 2.85 Na + 59.8 222 9,8 139 51 36 511 K* 6.1(4.8) 9.0(4.2) 4.8 24 373 130 9 Mg 2+ 13.9(0.3) 51.7(1.2) 5.02 54 78 49 130 Ca 2+ 14.0(11.4) 25.2(15.5) 20.9 168 217 75 49 NH] 19.0 31.4 215 1580 1773 310 781 CI- 86.9(17.2) 269(9.5) 7.5 223 161 6.5 614 NO3 5.6 29.3 87.5 164 1110 1000 690 1850 SO 2 19,2(12.0) 58.6(31.8) 522 255 584 450 520 469 MSA 0.12 0.46 formic 2.2 3.4 acetic 7.8 14.8

(B) Barrington site. (D) Dorrigo site. (1) Schemenauer and Winston (1988), Quebec, Canada. (2) Isaac and Daum (1987), Ontario, Canada. (3) Brewer etal. (1983), Los Angeles, California. (4) Waldman et aL (1983), Pasadena, California. (5) CastiUo and Jiusto (1983), Whiteface Mountain, New York. (6) Jacob et al. (1985), Californian coast.


Table II. Comparison of rainwater data from a number of studies; all values except pH in ~teq/L. Figures in parentheses are nonseasalt components. See text for details

Ion Reference

B D 1 2 3 4 5 6 7 8

pH 5.8 5.5 4.89 5.99 5.8 4.7 4.2 Na ÷ 19 54 3.8 4.3 1167 116 3.5 4.8 85 40 K ÷ 2.2(1.8) 1.9(0.7) 0.9 30.2 16 1.0 1.5 7.0 4.0 Mg 2+ 6.1(1.8) 11(-1.3) 1.2 1.3 247 34 0.65 3.0 30 32 Ca 2+ 11(10.2) 8.2(5.8) 0.6 1.7 78 26 30 6.5 42 34 NH~ 7.1 3.3 1.7 2.9 1.55 12 10.6 48 15 C1- 24(1.9) 65(2.0) 7.5 7.7 1372 137 4.0 11.2 98 60 NO 3 7.3 9.7 3.2 4.1 2.68 11 9.8 23.5 32 32 SO ]- 8.8(6.5) 15(8.5) 5.2 4.0 152 35 30 54 102 58 MSA 0.07 0.16 form 6.8 4.6 6.3 11 ac 5.7 3.8 4.2

(B) Barrington site. (D) Dorrigo site. (1) Gillett etal. (1990), Jabiru, N.T. (2) Likens etal. (1987), Katherine, N.T. (3) Ayers and Ivey (1988), Cape Grim, Tasmania. (4) Fitzgerald (1986), Latrobe Valley, Victoria. (5) Schemenauer and Winston (1988), Quebec, Canada. (6) Likens et al. (1984), Hubbard Brook, New Hampshire, U.S. (7) Hales (1980), Sweden. (8) Hales (1980), France.

3.1. Ion Balance

The prime quality control check employed with the Barrington and Dorrigo data was the calculation of cation and anion sums for each sample to confirm the exis- tence of a charge balance between the cations and anions. Some data elements were not available for samples which had insufficient volume for complete chemical analysis, or were not otherwise reported by AGAL, so that a total of 26 samples, 18 fog and 8 rain, were available for the ion balance check. Figure 2 shows a plot of cation sum versus anion sum for these samples. The excellent correlation coeffi- cient of 0.99 for these data, with the close adherence to the expected unity slope and zero intercept indicate that the analytical data quality is excellent. Two other conclusions evident from Figure 2 are that the fogwater samples had systematically higher ionic loadings than the rainwater samples, and that one extreme event (ion sums almost 1 meq/L) was sampled.

3.2. Seasalt Components

Since both sites are less than 60 km from the coast and onshore flow is common in summer a significant seasalt contribution to dissolved ionic loading was antici-


1

0.9

0.8

0.7

0.6 cation sum, 0.5

meq/L 0.4

0.3

0.2

0.1

0

dl

dw •

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

anion sum, meq/l.

Fig. 2. Cation sum versus anion sum for rain (open squares) and fog (filled squares), I~eq/L.

pated. Following the usual practice (e.g. Keene et aL, 1986), contributions from the major seasalt ions were calculated assuming sodium to be a conservative tracer for seasalt, and a seawater composition as given by Millero (1974). Tables I and II contain mean ion data averaged separately over the fog and rain data from each site, with nonseasalt (nss-) components indicated in parentheses where appropriate. Clearly seasalt, on average, contributed most of the magnesium and chloride in the samples (and sodium, which as noted above we use as the seasalt tracer), as well as 5-50% of the potassium, calcium and sulfate.

Overall therefore, seasalt is the major contributor to ionic loading in both the fogwater and rainwater sampled. A qualitative, but nonetheless convincing confir- mation of this conclusion is provided by factor analysis with varimax rotation, which when applied to the composite fog/rainwater dataset yields the results given in Table III. While there have been criticisms of factor analysis when used as a quantitative tool for assessing relative source contributions to sampled rainwater or aerosol composition (Henry, 1987), in the present application it is used simply as a means of identifying variables which exhibit associations in variance. Only factor loadings likely to have significance (those with values above 0.4) are included in Table III, and the extreme event noted above was excluded.

The groupings of variance revealed in Table HI reflect clearly the role of seasalt as the major contributor to variance in the data. Factor 1 is clearly identified as the 'marine' or 'seasalt' factor by the high loadings for sodium, magnesium, chloride


Table ILl. Varimax-rotated factor weights and variance explained

Factor 1 Factor 2 Factor 3

58.9 32.3 7.6

0.78 0.44 0.88 0.55 0.89 0.52

% var. explained

H +

Na + K + Mg 2+ Ca 2+

NH~ C1- NO~ SO~- MSA HCOO- CH3COO-

0.93

0.69 0.89

0.9t

0.75

0.74 0.91

0.87 0.69

0.97

and MSA, as these species are all expected to be dominated by the seasalt in coastal environments (Keene et al., 1986). The MSA tracer is especially unam- biguous, as MSA derives from the oxidation of dimethylsulfide emitted essentially only from the ocean surface (Andreae, 1990). The large contribution of seasalt towards the concentrations of potassium, calcium and sulfate in these samples is also evident in the significant loadings for these ions in Factor 1, but at levels lower than for sodium, magnesium, chloride, and MSA, indicating that at least one other source of potassium, calcium and sulfate are also significant contributors.

That other source is clearly identified as a 'continental' influence (Factor 2) which has high loadings for the dust-derived ions potassium and calcium, and ammonium, which has a much greater continental than marine source (Ayers and Gras, 1983), and nitrate and sulfate which have large continental sources in the form of anthropogenic emissions of NOx and SO 2.

These two sources seem quite obvious from Table III. Less so are the relation- ships of hydrogen ion, formate, and acetate with the remaining ionic species. Factor 3 consists only of hydrogen ion and formate, which seems reasonable as a 'formic acid' source, presumably of biological origin (Keene and Galloway, 1988). How- ever the presence of hydrogen ion and especially acetate in the seasalt factor is difficult to explain, though in the case of acetate the cautionary note regarding the organic acids must be borne in mind here. The situation for hydrogen ion may well be too complicated to interpret from this Table, as the absolute values for [I-I ÷] were typically only a few ~teq/L, whereas the acid anions and alkaline cations were at much larger concentrations. Thus the observed IH+I comprise the small difference yielded by the neutralisation of a much larger acidic component by a similarly large alkaline component, making IH +] a nonconservative variable, nonlinearly related to a wide range of other species.


3.3. Between-Site Differences

The mean concentrations of the characteristic seasalt ions, sodium, chloride, magnesium, methanesulfonate and sulfate were all considerably higher at the Dorrigo site than at the Barrington site (see Tables I and II) in both the rainwater and fogwater data. This is consistent with the fact that Don-igo is closer to the ocean. However, when seasalt corrections are made for the major seasalt-derived ions (data in parentheses in Tables I and II) and the Dorrigo and Barrington data are re-compared the composition of rain and fog at each site is rather similar, with just a few exceptions in the case of fog. These exceptions are ammonium, nitrate, nss-sulfate and acetate, for which the Dorrigo mean concentrations exceed those from Barrington by about a factor of 2, or more. Much of these differences can be attributed to the single 'extreme event' at Dorrigo mentioned earlier, which is dis- cussed separately later. Thus we conclude that while the seasalt component of both rain and fog was much larger at Dorrigo than at Barrington, the nonseasalt components were generally quite similar, except for the single 'extreme event' encountered at Dorrigo.

3.4. Comparison Between Fog and Rain

For all but one of the ions fog had a greater range of values and a higher mean concentration than rain: this systematic difference is readily apparent if the mean fogwater ion concentrations at each site (Table I) are compared with those for rainwater (Table II). In general, rainwater was much like fogwater diluted by a factor of about 3.

One notable exception was the case of formate. For rain, formate ranged from 0-40 Ixeq/L with a mean of 5.4 ~teq/L. However, for fog, the range was only 0-10 ~teq/L with a mean of 3.0 ~teq/L. This difference in formate concentrations is unusual given observations in other parts of the world (see Keene and Galloway, 1988), and given our earlier comments regarding the organic acid results must be treated with caution.

The occurrence of higher ionic concentrations generally in fogwater compared with rainwater is a well-known phenomenon, as can readily be seen from the data from other locations also tabulated in Tables I and II. It is readily attributed to a dilution effect: liquid water contents in raining clouds are much greater than those in fog. The case of the organic acids may not be so simple, however, as it is known that fast organic acid production pathways can exist in cloud or fog, and that these are nonlinear with hydrogen ion and possibly other variables (Keene and Galloway, 1988). Since the production and loss mechanisms for atmospheric formic and acetic acids are at this time rather poorly understood, it is impossible to draw firm conclusions using our limited data.

F O G A N D R A I N W A T E R C O M P O S I T I O N IN R U R A L SE A U S T R A L I A 91

3.5. Sources of Acidity

To identify the ions that contribute to the acid-base balance of the collected water, a series of correlations was calculated between the major cations, hydrogen, ammonium, and nss-calcium, and the anions nss-sulfate and nitrate. As for the factor analysis, the extreme event was excluded from the calculations. Results are given in Table IV and Figure 3.

160

140

120

100 H + N H 4 +

nss-Ca, 80

ueq/L 60

40

20

0

nn I i

• I I

I

0 20

[]

I I I I I

40 60 80 100 120

nss-S04 + NO3, ueq/L Fig. 3. Partial cat ion sum (H + + NH~ + nss -Ca 2®) vs partial anion sum (nss-SO~- + NO~) for rain

(open squares) and fog (filled squares), ~teq/L.

Table IV. Acid-base balances and correlation coefficients (N = 25)

Ions Correlat ion coeff. Significance

H ÷ vs. n s s -SO]- 0.13 nil H ÷ vs. nss-SO~- + N O ] -0 .01 nil H + + NH~ vs nss-SO 2- + N O 3 0.72 p < 0.001 above + nss -Ca 2+ 0.76 p < 0.001 above + organics 0.76 p < 0.001

A significant relationship between the cations and anions involved only occurred when ammonium was added to hydrogen ion ( r - 0.72, p < 0.001). The inclusion of nss-calcium to the partial cation sum improved the relationship slightly. The implication from Table IV is that the free acidity in the samples was controlled


by nitrate and sulfate acidity, neutralised to a small extent by nss-calcium but mostly by ammonia. This influence of ammonia is a reasonable result for a region which is primarily agricultural pasture and forest, without major areas of open soil which could be a source of soil dust, and which is subjected predominantly to onshore air flows over the verdant coastal strip. It is interesting to note that further south in the more heavily populated and industrialised Hunter Valley, where several large coal- fired power stations are located, calcium was the dominant acid-neutralising cation in rain (Pang Way and Rothwell, 1986).

3.6. The Extreme Acid Event

The fog collection at the Dorrigo site on 31 January was a unique event, with much higher acidity and total ion concentrations than any other collection. The amount of fog collected was small, equalling just 1.48 mm of precipitation. This low volume probably contributes to the high ionic strength observed, however this alone may not be sufficient to explain the acidity and high concentrations of ions, as two other fog events were equally small but neither was as acidic. This extreme event had a pH of 4.08, compared to the next lowest pH of 4.64.

This event had the highest concentrations of sulfate, nitrate, sodium, chloride and MSA recorded during the study. This suggests a major seasait influence as all of these ions apart from nitrate are seasalt derived. However, some other influences are also likely, based on the fact that the nss-sulfate (146 ~teq/L) and nitrate (126 ~teq/L) concentrations were more than double the next highest concentrations in the other samples. This source of sulfate and nitrate must also be the source of the extreme acidity of this event, suggesting that the source is anthropogenic.

Of considerable interest is the fact that the organic ions formate and acetate dis- played their lowest concentrations during this extreme event. This adds to the implication that the event involved some sort of transport of pollutants from an urban/industrial source, since the presumably locally produced (biological source; see Keene and Galloway, 1988) organic ions had very low concentrations. Also, this one observation of extreme acidity had a singular meteorology consisting of an intense low pressure system off the coast of New South Wales, immediately adjacent to the study areas. This would tend to produce rapid transport of air from the southern source regions of Sydney and Newcastle.

4. Comparison with Other Studies

4.1. Fogwater

The lack of any previous fogwater data from Australia precludes comparison with other regions in this country. However the data in Table I show that the pH of fogwater measured in this study is much higher than that measured in most overseas


fogwater studies: fog is much more acidic in Quebec, Ontario, New York, and Los Angeles than at the rural Barfington and Dorrigo sites in Australia. This difference is obviously due to very strong anthropogenic influences in the more polluted northern hemisphere locations. Even the most acidic fog event in the present study (pH = 4.08) was not as acidic as is recorded regularly overseas (Table I). Concen- trations of the major anthropogenic pollutants, sulfate and nitrate, are also much higher for all of the overseas studies. Concentrations of ocean-derived ions however are similar.

4.2. Rainwater

Table II shows the mean pH and ion concentrations for the rain samples at both Australian sites compared to those obtained in other rainwater studies, both within Australia and overseas.

Compared with other Australian studies, rainfall acidity at both study sites was approximately the same as has been found in the Latrobe Valley, Victoria, but less than that found in Katherine, Northern Territory, the Newcastle/Hunter Region and Sydney, New South Wales (Bridgman, 1989). Acidity at Dorrigo and Barring- ton was also less than at Quebec, Canada; and most studies carried out in the United States (Likens et aL, 1984; see also MAP3S/RAINE, 1982). The higher acidity in tropical Australia is due to higher concentrations of organic acids (Likens et al., 1987; Ayers and Gillett, 1988), which more than equal the greater anthropogenic influences at Barrington and Dorrigo. In other locations however (Newcastle, Sydney, Canada and the U.S.), the higher acidity compared with the Barrington and Dorrigo sites is readily attributed to greater anthropogenic influences, as evidenced by the higher mean concentrations of nss-sulfate and nitrate at the more polluted locations (see Table II).

MSA has only been reported in rain from Cape Grim, Tasmania (Ayers and Ivey, 1990). Concentrations at Cape Grim between 1983 and 1987 ranged from values of 0.02-0.05 ~teq/L in midwinter, to 1 ~teq/L in midsummer. The seasonal cycle of MSA at the latitude of Dorrigo and Barrington is unknown, but the concentrations observed at these sites sit comfortably within the range shown at Cape Grim.

Concentrations of formate at the study sites were about half of those found in tropical Australia, however as emphasised earlier no extended discussion of the present data is warranted.

5. Conclusions

In general, at both sites, the fog and rainwater sampled was not polluted, with pH averaging >5. Concentrations of anthropogenic pollutants were mostly low, with seasalt ions accounting for the majority of total ionic loading. This situation contrasts very strongly with fogwater studies from the more polluted regions of the northern hemisphere where pH values below 4 are common.


At both sites studied the ions sodium, chloride, magnesium, and methanesulfonate were virtually entirely seasalt derived. The Dorrigo site showed a signifi- cantly greater seasalt influence than the Barrington site, consistent with it being closer to the coast.

The most important alkaline species affecting both fogwater and rainwater acidity at Dorrigo and Barrington was ammonia, while the most important acid anions were sulfate and nitrate.

On average, fogwater had a higher concentration of ions than rainwater. For the inorganic ions, fogwater had ion concentrations typically 2-6 times those in rainwater. Mean pH of fogwater was 5.48, while that of rainwater was 5.62.

Finally, the one occasion of high fogwater acidity (pH 4.08) encountered in this study was consistent with rapid transport from the strong anthropogenic source regions of Sydney and Newcastle to the south. Meteorological control thus appears to be the critical factor in whether the rural Dorrigo and Barrington sites experi- ence isolated episodes of acidic fog.

Acknowledgements

The authors gratefully acknowledge the assistance with field sampling given by Mr Mylie Sawtell at the Dorrigo site and Forestry Commission staff at the Gloucester office. This project was supported financially by a CSIRO-University of Newcastle Co-operative Research Grant.

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Fog and rainwater composition in rural SE Australia

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