Spatial and Temporal Variations of the Chemical Composition in Acid Deposition in the Peak District, Northern England

Spatial and temporal variations of the chemical composition in acid deposition in the Peak District, Northern England

D. Driejana^, D.W. Rape/, I.L. Gee' & A.F.R. Watson'

'aric, Department of Environmental and Geographical Sciences, Manchester Metropolitan University, United Kingdom * Department of Environmental Engineering, Institute of Technology

Bandung, Indonesia

Abstract

Six monitoring sites operated on a weekly basis have been established to examine the impact of current emission reduction strategies on ionic concentrations and depositions in an acid sensitive sub-area of the Peak

District. Present concentration and deposition values were compared to the monitoring results in the same area in 1988. On average, H* and nss-SO/" ion concentrations have dropped by approximately 60%, compared to their levels 10 years ago, and NCV has decreased by 30%. However, as the present precipitation amount is higher than that of 1988, I-T and nss-SO/" wet depositions have not decreased in similar proportion as their concentrations. Nitrate deposition shows very little decrease. H\ nss-SO/" and NOg" concentrations are generally higher in summer when the precipitation amount is lower. Significant spatial variability with an increasing gradient from north to south is observed for calcium and sulphate concentrations.

1. Introduction

As a response to economic, cultural and environmental factors in the past decade, acidic precursors have been reduced by the introduction of more stringent industrial emission controls and exhaust emission standards. Sulphur dioxide emissions in the U.K. were reduced by 68% in 1997, relative to 1970 [1]. However, emission of nitrogen oxides were only reduced by 24% from 1970 to 1997, despite a gradual decrease in vehicle emissions with the application of catalytic converters for new cars.

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Power stations remain the main sources for sulphur dioxide emissions and contributed about 62% of the total UK emission in 1997. It is likely that emission reductions from electricity generation were offset by increases in

emissions from the growth in the transport sector. However, a substantial drop in the total mass of coal burnt and the installation of FGD at coal-fired power stations have had a direct effect on hydrogen chloride emissions in the UK, which have decreased from 335 ktonnes in 1970 to about 91 ktonnes in

1997 [1]. This paper discusses spatial and temporal variation and the impact of current emission reduction strategies on ionic concentrations and depositions in an acid sensitive sub-area of the Peak District.

2. Methods

The study area in the Peak District is bordered by the major cities of Manchester to the west and Sheffield to the east (see Fig.l). Consequently, it has a long history of acid deposition dating back to the industrial revolution. Landuse within the Peak District is dominated by agricultural activity such as sheep rearing and designated National Trust protected land. Six monitoring sites were located in an area of approximately 20km by

30km. The network comprises bulk collectors fabricated to the UK National Acid Deposition Network standard [2] and co-located meteorological office rain gauges. Although the term wet deposition is used in this paper, the rain water collectors were collecting bulk precipitation which includes some dry deposition. Site locations were chosen as far as possible to have little influence from local sources as well as being secure and accessible. To prevent the effect of seeder-feeder mechanisms which is known to happen in elevated locations, sites were also located at an elevation below 500m.

Sampling Sites 20 Kilometer*

Figure 1. Location of the sampling sites and the two major conurbations

Site 1 is located near the Hurst Reservoir in Glossop. However, due to persistent vandalism, in August 1999 the site was moved to Padfield which is located within 2 km from the original monitoring site. Site 2 Kinder and Site

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3 Lockerbrook Farm are located at an altitude of 320m and 420m, respectively. Geology of the area for the first three sites is characterised by Namurian shales and grits and soil types belong to the Wilcock and Winter Hill Series. Site 4 Oxlow House lies on a distinctively different geological features. The site is less than 1 km from the Namurian shales and grits in the north but is underlain by Monsal Dale Limestone. Site 5 located in Bamford at an elevation of 210m. The Bee Low Limestone underlies the site 6 in Harpur Hill, with Namurian shales and grits bordering the west and south west sides. Table 1. Site Locations

Location Grid Reference Altitude (m) 1 Hurst Reservoir SK 054 938 250 Padfield SK 033 965 230 2 Kinder Reservoir SK 062 881 320 3 Lockerbrook Farm SK 164892 420 4 Oxlow House Farm SKI 27 824 320

5 Bamford SK 214 832 210 6 Harpur Hill SK 057 708 380

Rainwater samples were collected on weekly basis. The chemical species measured include H\ Cl", N and SO ^" ^a^, Mg^, Na*, K* and

NH/. Anions (CT, NCV, and ]>4") were analysed using ion chromatography. Ca^, Mg^, Na ^ were measured using Atomic Absorption Spectrophotometry, while NH/ was analysed in an external laboratory using a continuous flow analysis method. Non sea salt- sulphate (nss-SO/") was calculated by assuming that all Na* is derived from the sea.

Details of sampling and analytical methodology have been given by Driejana era/[3].

3. Results and Discussion

3.1. Characteristics of precipitation chemistry and rainfall amount

Quality assurance was applied by duplicate analyses in the laboratory and calculation of ionic balance values. On average, cations comprise 51% of the precipitation chemistry. This might suggest the presence of organic acid [4] in the sample, e.g., in the form of acetic and formic acids [5]. The presence of organic-acid was supported by unidentified chromatograph peaks, which occasionally appeared in the ion chromatograph output. The retention time of this unknowns in relation to the other identified analytes, suggests that the unidentified analyte is acetate. The mean absolute-ion balance value for the overall data set is 8.76%. However, a 15% ion balance cut off had been chosen for data screening to take into account the unaccounted acids in the samples. All ions are log-normal transformed to meet the assumption criteria of normal distributions prior to any statistical analysis. Statistical analysis was performed using SPSS software version 9.0.

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A student t-test analysis was applied to compare the rainfall amount collected from both bulk collectors and the rain gauges. The statistical test found that there was no significant difference between the rainfall amount collected in the bulk collectors and the rain gauges in the spring, summer and autumn. However, winter precipitation showed a significant difference in the rainfall volume collected by the two types of rain collectors. It is suggested that higher wind velocities in the winter affect the collection efficiency of rainwater collected above the ground, in agreement with previous findings of other authors [6,7].

3.2. Spatial Variability

The precipitation-weighted mean concentrations and wet depositions were calculated using precipitation volumes in co-located rain gauges, for the whole period of sampling. The results of precipitation-weighted mean concentrations are presented in Table 2. Hydrogen ion concentration ranged between 9.\/jeq ^ and 15.\/jeq /"'. Calcium has the most varied concentration, which ranged from about 8.0//eg /"* to \92/ueq /"', followed by nss-SO/", with a concentration ranging from 25.Kfjeq f* to 39.8 jjeq /"*.

Nitrate is less varied with a concentration range of 20.8/jeg /"* to 29.9jueq /"'. Ammonium had a concentration range of 30.%peq /"* to 43.\fjeq /"'. In general, the concentrations of NCV (Fig.3), nss- SO/" (Fig.4), and Ca^ (Fig.5) seem to have almost similar patterns, with the highest concentrations found in the southern part of the study area. Hydrogen concentration (Fig.6) displays no similarity with the spatial pattern of other ions, but contrary to other ions, it seems to be higher in the northern part of the study area.

Table 2. Precipitation-weighted Mean Concentrations (peq /"^)

Sites H Cl NO, SO4 nss- Ca Mg Na K NH, SO, Hurst/Padfield 10.8 107.6 20.8 36.2 25.8 8.5 19.1 86.6 6.7 38.3 Kinder 12.7 82.7 23.2 36.1 28.1 8.4 14.8 66.5 5.1 37.2

Lockerbrook 15.1 84.4 21.2 35.2 26.9 8.0 15.3 68.8 5.4 30.8 Oxlow House 13.0 71.7 29.9 47.2 39.8 16.0 13.2 60.9 6.1 43.1

Bamford 14.1 90.6 25.4 47.2 38.5 19.2 17.1 72.1 4.2 36.3 Harpur Hill 9.1 73.9 20.9 36.6 29.3 10.7 13.3 60.1 4.2 38.2

Deposition patterns (Fig.8-Fig.13) closely resemble the ionic concentration patterns. The H* deposition (Fig. 12) also shows higher values in the northern region. Beside showing similarity with concentration patterns, non- anthropogenic ions such as Na*, Mg^ (figures are not shown), and Cl" (Fig.8), depositions also exhibit similarity in shape with the precipitation

(figure is not shown). Deposition loading values at each site are presented in Table 3.

*\ ^ \ \Padfiettfx • \«? irider/'^X, \

Fig.2. Cl" cone.

" /Padfield —' Kinder ~~—'"'' /' /f ,-•—Lockefbrbo" .^k / / O^lovy*/ / ' l3arn$f^ "' _K d.

i««/ % / ? tjlomet^s

Fig. 5. Ca^ cone. Fig. 6. H^ cone. (|ieq/l) Fig. 7. NH/ cone. (Jieq/l) (^ieq/1)

Fig. 9. NOs" dep. (kg/ha/yr)

Fig. 11. Ca^ dep. Fig. 12. H+ dep. Fig. 13.NH/dep. (kg/ha/yr) (kg/ha/yr) (kg/ha/yr)

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Table 3. Annual Deposition (kg ha"' yr"*)

Sites mm H Cl NO] SO, nss- Ca Mg Na K NH4 SO, Hurst/Padfield 1333.5 0.14 50.20 188 7.73 5.51 2.27 3.05 26.57 3.48 7.15

Kinder 1296.6 0.16 37.53 4.21 7.49 5.83 2.17 2.30 19.83 2.59 6.76 Lockerbrook 1358.8 0.21 40.15 4.03 7.64 5.85 2.16 2.49 21.50 2.86 5.85 Oxlow House 1108.9 0.14 27.81 4.63 837 7.07 155 1.75 15.54 2.62 6.69

Bamford 1062.4 0.15 33.69 3.78 8.02 6.55 4.07 2.18 17.63 1.74 5.40 Harpur Hill 1285.6 0.12 33.24 3.76 7.52 6.04 2.76 2.05 17.78 2.13 6.88

Table 4. ANOVA Results

df Between Group Within Group Total F Sig. mm 5 234 239 0.758 0.581 H 5 234 239 _ 1.442 0.210 Cl 5 234 239 0.157 0.978 NO3 5 234 239 1.125 0.348 SO4 5 234 239 3.054 0.011 nss-SO4 5 234 239 2.496 0.032 Ca 5 234 239 7.338 0.000 Mg 5 234 239 0.280 0.924 Na 5 232 237 0.324 0.898 K 5 234 239 0.205 0.960 NH4 5 233 238 1.430 0.214

One-way analysis of variance (ANOVA) results (Table 4) shows that concentrations of SO/", nss-SO/" (non sea salt-sulphate), and Ca^ are significantly different between sites, indicating small-scale spatial variability of those particular ions. Calcium has the highest spatial variability, which is indicated by its F-value, followed by SO/" and nss-SO/". Hydrogen, nitrate, and ammonium might have spatial variability between the sites but the differences are not statistically significant. Calcium is a major neutralising agent that may play an important role in the spatial variability of precipitation composition, given the geological characteristics in the study area. It is suggested that reaction of sulphur dioxide and calcium-bearing particles in the atmosphere results in calcium sulphate particles which were then deposited onto the funnel collectors.

3.3. Year to Year Comparison of Concentrations and Depositions

Four sites (Padfield, Kinder, Oxlow and Harpur Hill) within the current network are located in the same area as the previous monitoring exercise [7,8], therefore allowing for a comparison. The annual precipitation amount,

Air Pollution VIII, C.A. Brebbia, H. Power & J.W.S Longhurst (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-822-8 Air Pollution VIII 661 concentrations and depositions from 1987/88 and 1999/00 were compared and the ratios of H\ NOg", nss-SO/" and Ca^ were calculated. The ratio of NH/ is not provided because previous values were based on estimation rather than actual measurement. The calculation does not exactly show trend values because annual variation due to environmental factors such as local meteorology might exist and to determine such trends it would require data from multiple years. Nevertheless, the calculation of ratios describes the proportion of concentrations or depositions from two different years with considerably different emission backgrounds.

Radfield Kinder site Ox low Harpur Hill n H cone. rj NO3 cone. QN03-Ndep. jnssSO4 cone. ^nssSCM-Sdep. • Ca cone. m Ca dep. g precipitation

Figure 14. Ratios of annual precipitation-weighted mean concentrations and depositions between 1999/2000 and 1987/1988

Ratio values for the 4 major ions at the 4 comparable sites are presented in Fig. 14. It can be seen that across the 4 sites, H* and nss-SO/" concentrations decrease considerably to about 60-80% and 50-60%, respectively, but NOg" is only reduced by 20-30%. The decline in oxidised N and S concentrations in precipitation might reflect the emission levels which have been reduced nationally by approximately the same magnitudes. Concentration of Ca^ has also substantially decreased. This work is in agreement with Lee et al [9] who described 40% to 60% calcium decline in the long-term trend base cations in the UK. The cause of the decline is uncertain, it is hypothesised that the decline was due to the decrease in fly-ash from coal and fuel oil combustion and other industrial emissions (e.g. cement manufacturing, open limestone quarrying, metal refining), as a consequence of better emission controls and changes in energy sources for space heating [9]. The same level of improvement has not particularly been shown in terms of deposition, especially for oxidised N and S. In general, the ratios of depositions are higher than concentration ratios, due to the 1999/2000 precipitation amount being about 20% to 50% higher. Because present FT concentrations are much lower, its deposition loadings are also lower than during the previous measurement. The same situation appears for Ca^ deposition. Oxidised N deposition levels in 1999/2000 only decreased by 20% of the levels in 1987/88. Due to the precipitation factor, there is a non- linear relationship between the decrease in emissions and in deposition

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loadings. This implies that in terms of deposition, ecosystems in a region with higher precipitation amount (as in the northwest England) might receive a lower overall benefit from current emission reductions.

3.4. Temporal Variation

Temporal variation is examined by comparing seasonal variations between sites and between historical and present data sets. Site codes of 1, 2, 4 and 6 are used to represent Padfield, Kinder, Oxlow House and Harpur Hill, respectively. The precipitation amount (Fig.20) is highest in the winter and lowest in the summer. Summer and spring precipitation in both years were of similar magnitude, but in the winter of 1999/2000 precipitation was approximately twice that in 1987/1988. The trends in observed non- anthropogenic ions as Na^ (Fig. 19), K% and Mg^ follow the precipitation temporal pattern. The same seasonal pattern was also observed for Cl", suggesting that the source of present-day Cl" might be more dominated by natural (marine) sources rather than combustion processes, as was also found from a principal component analysis [3]. Unfortunately meteorological data was not available for a detailed investigation of the relation between higher rain events in the winter and the origin of those ions in precipitation chemistry. Different seasonal variations appear for H* (Fig. 15), NO]" (Fig. 16) and nss-SO/"(Fig.l7), which are highest in the summer and lowest in the winter. This situation also appeared for present-day measurements of Ca^ (Fig. 18), while previous measurements showed the highest Ca**" concentration in the autumn. Although it showed a similar seasonal pattern with acid precursors, such as nss-SO/", Ca^ has the highest deposition loading in the winter, similar to other natural-source-origin ions (see Fig.24). This suggests the complexity of Ca*" sources, e.g. soil dusts, marine aerosols, and industries, and its atmospheric physico-chemical reaction. Part of the Ca^ in the samples might have resulted from dry deposition of CaSC>4 deposited onto the funnel collectors as it might also affect SO/" spatial variability. However, as the sea was also identified as one of the sources of Ca\ some of the bulk of

Ca^ might be related to higher rain events in the winter. The impact of precipitation on the amount of ion deposited is demonstrated clearly by observing NO]" at the first site (Padfield), which had the highest concentration but the lowest deposition loading in the summer, due to its lowest precipitation level compared to the three other sites.

Temporal variation also shows that in both concentration and deposition, a decrease in H* and nss-SO/" is apparent, while a similar picture has not appeared for NO]", supporting the analysis in Section 3.3 that during the last decade there was only a slight improvement in N oxidised deposition. As one expected, there was no marked difference in concentration of natural-source origin ions between the two observation years, their depositions temporal variability are closely dependent on that of precipitation.

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Legends used in Fig.15-Fig.25 : --0--1 87/88 •— 1 99/00 --A--2 87/88 A 299/00 - - 0 - - 4 87/88 # 4 99/00 __.*._ 6 87/88 + 6 99/00

Spring Summer Autumn Venter

Figure 15. Temporal variation of H* concentration

I Spring Summer Autumn Venter j Spring Summer Autumn Venter Figure 16. Temporal variation of N(V Figure 17. Temporal variation of nss- concentration SO/" concentration

Spring Summer Autumn venter Spring Summer Autumn Winter

Figure 18. Temporal variation of Ca^ Figure 19. Temporal variation of Na* concentration concentration

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600

Spring Summer Autumn VMnter Spring Summer Autumn Winter

Figure 20. Temporal variation of Figure 21. Temporal variation of FT precipitation deposition

0.20 0.50 0.18 0.45 0.16 | 0.40 0.14 ; 0.35 . 0.12 i _=T0.30 0.10 I 0.25 » 0.08 JT0.20 0.06 I 0.15 0.04 0.10 0.02 0.05 0.00 ' 0.00 I Spring Summer Autumn Winter Spring Summer Autumn Venter Figure 22. Temporal variation of Figure 23. Temporal variation of oxidised N deposition oxidised nss-S deposition

1.60 1.40 1.20 J.OO

0.40 0.20 0.00 I Spring Summer Autumn Venter Spring Summer Autumn VMnter

Figure 24. Temporal variation of Ca^ Figure 25. Temporal variation deposition deposition

4. Conclusion

This paper has shown significant spatial and temporal variability of precipitation chemistry and deposition in a sensitive upland area of Northern England. During the two study periods, namely 1987/1988 and 1999/2000, the spatial patterns of the precipitation-weighted mean concentrations and

Air Pollution VIII 665 depositions were found to be broadly similar. Higher concentrations and depositions of nss-SO/" and Ca^ were observed in the southern part of the study area, while the opposite is demonstrated by H* patterns.

Temporal analyses on a site-specific basis show a major reduction of the magnitudes of the precipitation-weighted mean concentrations, and to a lesser extent depositions of nss-SO/", Ca^, and H\ from 1987/1988 to 1999/2000. However, NCV shows little change in precipitation-weighted concentration and deposition. This reflects national trends in sulphur and nitrogen emissions.

References

[1] AEATech., National Atmospheric Emission Inventory, Annual Report 1997, http://www.aeat.co.uk/netcen/airqual/naei/annreport/chap5.html. [2] Hall, D.J., The precipitation collector for use in the secondary national

acid deposition network, Warren Spring Laboratory, Stevenage, Report LR561 (AP), 1986. [3] Driejana, Raper, D.W., Gee, I.L., Spatial Variation and Source Attribution of the Chemical Composition in Acid Deposition in the Peak

District, Northern England, Proc. of the Air & Waste Management Association's 2000 Annual Conference & Exhibition, Salt Lake City, 2000 (to appear). [4] Keene, W.C., Galloway, J.N., and Holden, J.D.Jr, Measurement of weak organic acidity in precipitation from remote areas around the world., J.

Gcop/zyj. #&?earc/;, 88, pp. 5122-5130, 1983. [5] Morales, J.A., deGraterol, L.S., Velasquez, H., Nava, M.G., deBorrego, B.S., Determination by ion chromatography of selected organic and inorganic acids in rainwater at Maracaibo, Venezuela, J. C/z/"om^ogA"op/7_y W, 804, pp. 289-294, 1998.

[6] Rodda, J.C. and Smith, S.W., The significance of the systematic error in rainfall measurement for assessing wet deposition, Atmos. Environ. 20(5), pp. 1059-1064, 1986. [7] Raper, D.W.; Longhurst, J.W.S.; Gunn, J., Evidence for small scale variation in acid deposition: A study from the Derbyshire High Peak

District, Acid Deposition: Sources, Effects, and Controls, ed. Longhurst J.W.S., British Library, London and Technical Communication, Letchworth, pp. 25-65, 1989. [8] Raper, D.W and Lee, D.S., Wet Deposition at the sub-20 km Scale in a rural upland area of England, Atmos.Environ., 30(8), 1193-1207, 1996.

[9] Lee, D.S., Espenhahn, S.E., Baker, S., Evidence for long-term changes in base cations in the atmospheric aerosol, J. Geophys. Research, 103, pp. 21955-21966, 1998.