Hydrochloric Acid: An Overlooked Driver of Environmental Change

Feb 2, 2011 - Long-term precipitation, surface water, and soil solution data suggest that the near-disappearance of HCl ..... Todd Pagano , Morgan Bid...
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Hydrochloric Acid: An Overlooked Driver of Environmental Change Chris D. Evans,*,† Don T. Monteith,‡ David Fowler,§ J. Neil Cape,§ and Susan Brayshaw‡ †

Centre for Ecology and Hydrology, Environment Centre Wales, Deiniol Road, Bangor, LL57 2UW, United Kingdom Centre for Ecology and Hydrology, Lancaster University, Lancaster, LA1 4AP, United Kingdom § Centre for Ecology and Hydrology, Bush Estate, Penicuik, Midlothian, EH26 0QB, United Kingdom ‡

bS Supporting Information ABSTRACT: Research on the ecosystem impacts of acidifying pollutants, and measures to control them, has focused almost exclusively on sulfur (S) and nitrogen (N) compounds. Hydrochloric acid (HCl), although emitted by coal burning, has been overlooked as a driver of ecosystem change because most of it was considered to redeposit close to emission sources rather than in remote natural ecosystems. Despite receiving little regulatory attention, measures to reduce S emissions, and changes in energy supply, have led to a 95% reduction in United Kingdom HCl emissions within 20 years. Long-term precipitation, surface water, and soil solution data suggest that the near-disappearance of HCl from deposition could account for 30-40% of chemical recovery from acidification during this time, affecting both near-source and remote areas. Because HCl is highly mobile in reducing environments, it is a more potent acidifier of wetlands than S or N, and HCl may have been the major driver of past peatland acidification. Reduced HCl loadings could therefore have affected the peatland carbon cycle, contributing to increases in dissolved organic carbon leaching to surface waters. With many regions increasingly reliant on coal for power generation, HCl should be recognized as a potentially significant constituent of resulting emissions, with distinctive ecosystem impacts.

’ INTRODUCTION HCl is a potent pollutant of seminatural ecosystems. Compared to other acid anions, such as nitrate (NO3) and sulfate (SO4), Cl is unaffected by redox processes or anion adsorption. Furthermore, although Cl can be cycled within soil organic matter,1 net biological uptake appears minor, especially in areas with high marine Cl deposition.2 Due to the high mobility of Cl, HCl can readily acidify both soils and surface waters. Emission of HCl occurs primarily through combustion of coal, which contains 0.01-0.5% Cl by weight, and to a lesser extent from waste incineration.3,4 In 1990, global anthropogenic HCl emissions were estimated at 6.6 Tg Cl yr-1.4 At this time, UK HCl emissions were 0.27 Tg Cl yr-1, but a subsequent shift away from coal-fired generation to gas, increased use of lower-Cl imported coal, and installation of flue-gas desulphurization (which is >90% effective at removing HCl3), reduced emissions by 95%, to 0.014 Tg in 2007.5 By comparison, UK S deposition decreased over the same period by 70%, from 0.39 to 0.12 Tg yr-1. The remarkable reduction in HCl pollution, which has been mirrored in other industrialized regions of Europe and North America, has received little attention in terms of ecosystem impacts. In part, this is because HCl has a shorter atmospheric lifetime than most S or N compounds,5 and has been considered primarily as a “near field” pollutant.4,6 Additionally, decreases in HCl emissions have coincided with major decreases in S emissions, and observed ecosystem responses have tended to be attributed to the latter. However, some studies that have examined Cl deposition and biogeochemistry in seminatural ecosystems do suggest the presence of an anthropogenic component.7,8 Globally, by far the largest component of the natural atmospheric Cl cycle is sea-salt aerosol, mobilized by wind stress at the ocean surface.9 Most of this is redeposited on the ocean, but r 2011 American Chemical Society

during periods of high wind a proportion is transported over land and redeposited in near-coastal areas. Deposited marine base cations (mainly sodium, Na, and magnesium, Mg) can displace acid cations (hydrogen and aluminum) from soil cation exchange sites, causing transient runoff acidification via the “sea-salt effect”.10 Additionally, sea-salt aerosol coming into contact with sulfuric acid (H2SO4) and nitric acid (HNO3) in the atmosphere can undergo a process of “dechlorination”, whereby Cl is displaced from aerosol particles by SO4 or NO3, leading to formation of HCl as a secondary pollutant.11,12 Globally, the amount of HCl produced via dechlorination in the 1990s was estimated at 50 Tg Cl yr-1, approximately double the natural baseline, with the highest rates associated with elevated S and N concentrations over the North Atlantic.9,13 In this study, we evaluate the role of HCl deposition as a driver of ecosystem change by analyzing UK deposition, surface water, and soil-water data collected between 1986 and 2007, the period of major change in HCl emissions. We consider the relative roles of anthropogenic and natural (climate-related) changes in atmospheric Cl loadings, and the impact of observed change on terrestrial and freshwater biogeochemistry, including carbon cycling.

’ METHODS Precipitation chemistry data were obtained from 26 continuously operated sites in the UK Acid Deposition Monitoring Network14 which have been sampled fortnightly since 1986. The Received: October 25, 2010 Accepted: January 13, 2011 Revised: January 6, 2011 Published: February 02, 2011 1887

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Figure 1. Location of deposition (D), surface water (W), and terrestrial (T) monitoring sites used in the study (site numbers correspond to those used in Tables S1 to S3). Color coding of sites represents the deposition zone in which each site is located, based on Fowler et al.,12 where red = Zone 1, orange = Z one 2, yellow = Zone 3, and green = Zone 4.

nonmarine component of Cl (xCl) was estimated by subtracting sea-salt Cl (mCl, calculated from the sea-salt Cl to sodium (Na) ratio of 1.15 mol mol-1) from total Cl. Positive xCl values indicate a pollutant component to wet-deposited Cl, negative values indicate the depletion of Cl in deposition relative to expected sea-salt ratios, due either to dechlorination or the presence of nonmarine sodium, xNa (for a discussion of the possible impact of xNa on results, see Supporting Information). Nonmarine sulfate (xSO4) concentrations were calculated similarly, using a sea-salt SO4/Na ratio of 0.117. We calculated volume weighted mean concentrations for the first (19861990) and last (2003-2007) five years, representing average precipitation composition at the beginning and end of the monitoring period. Mean annual changes were calculated by linear regression against time. We also used volume weighted mean Na concentrations for the two periods to infer changes in sea-salt deposition over the period, on the basis that precipitation Na is

predominantly marine-derived.8 To analyze spatial patterns of change in HCl and H2SO4 deposition, we grouped our results into four UK sulfur deposition zones (Figure 1, Table S1), defined previously by hierarchical clustering of observed xSO4 deposition trends.14 Zone 1 is closest to major emission sources (concentrated in North/Central England) and has seen the largest reductions in xSO4 deposition, whereas Zone 4 represents the cleanest areas (remote Northern/Western coastal regions) and shows the smallest (but still significant) reductions. In addition to fortnightly deposition monitoring, daily precipitation chemistry measurements are available for one site, Eskdalemuir in Southern Scotland. Daily meteorological information (based on the Jenkinson Weather Type classification15) was used to assign precipitation to one of eight source directions.16 For each source direction, data were divided into three equal time periods (1986-1992, 1993-2000, 2001-2007) for which volume weighted mean Cl and Na concentrations were 1888

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Figure 2. Mean rate of change in concentrations of (a) rainfall nonmarine chloride, (b) rainfall nonmarine sulfate, (c) surface water total chloride, and (d) surface water total sulfate, for monitoring sites in four UK deposition zones, where 1 represents sites closest to emissions sources, and 4 represents the most remote sites.12 Note that error bars show standard error of estimated change in each deposition zone. Means sharing the same letter are not significantly different from each other (paired t test, p < 0.05).

calculated. These were used to examine change in Cl/Na ratio, by source direction, over the monitoring period. For surface waters, we analyzed data for 1988-2007 from 11 lakes and 11 streams in the UK Acid Waters Monitoring Network (AWMN17) (Figure 1, Table S2). Due to the potential for internal sources and cycling of Na within the catchments, we could not calculate xCl reliably for surface or soil waters. Therefore, we examined trends in total Cl, and evaluated the extent to which these could be explained by climatic factors, i.e., attributable to sea-salt deposition. For comparability, trends in total SO4 were also analyzed, although trends did not deviate appreciably from those for xSO4, reflecting the dominance of the nonmarine component at most sites. Sites were assigned to the four deposition zones to allow comparison with deposition data. Most AWMN sites are located in reasonable proximity to a deposition monitoring site (Figure 1), with the notable exception of the four Northern Ireland sites. The southeastern sites, Bencrom River and Blue Lough, were assigned to Zone 2 on the basis of their high observed SO4 concentrations (Table S1) and proximity to Belfast. The northwestern sites, Beaghs Burn and Coneyglen Burn, were assigned to Zones 3 and 4, respectively, according to their previous classification.14 Data were again compared for the first (1989-1993) and the last (2003-2007) five full years of monitoring, with annual changes calculated by linear regression. Long-term UK upland soil-water chemistry data are sparse; we analyzed three sites in the Environmental Change Network (ECN18) with records extending over a decade. Two of these, Glensaugh and Sourhope, are moorlands with organo-mineral soils. The third, Moor House, is a blanket bog. All are located within deposition Zone 2, with measured data for 1993-2007. Total Cl and SO4 data were analyzed as above, although the later onset of monitoring meant that calculated means for the first five years covered the period 1993-1997.

’ RESULTS AND DISCUSSION Deposition. During 1986-1990, mean xCl concentrations were positive at all sites (mean of all sites 10 μeq L-1, range 0.256 μeq L-1). Regression of annual means against time indicates that xCl decreased at every site between 1986 and 2007, on average by 19 μeq L-1 (range 4.3-83 μeq L-1), sufficient to give negative mean xCl concentrations at all sites during 2003-2007 (mean of all sites -5.2 μeq L-1, range -18 to -0.3 μeq L-1). As expected, larger decreases were observed at sites with higher initial xCl concentrations. Over the same period, precipitation xSO4 concentrations also decreased at all sites (mean change 35 μeq L-1, range -10 to -72 μeq L-1). Na concentrations decreased at 20 sites, but increased at six (mean change -12 μeq L-1, range -74 to þ73 μeq L-1), with the most consistent declines observed in areas with the smallest marine influence (Zone 1). This suggests a small overall decrease in Na deposition, but in western and northern regions with organic soils and poorly buffered freshwaters there were no clear trends (Table S1). The magnitude of xCl decline relative to the overall reduction in [xCl þ xSO4] loading ranged from 14 to 71%, with a mean of 35%. Thus, average reductions in xCl deposition were about half of those in xSO4 deposition, and at seven sites xCl decreases exceeded xSO4 decreases. While there have been some concurrent reductions in total N deposition across the UK, these have been comparatively modest,19 and there have been no clear trends in nitrate concentrations in soil or surface waters since the 1980s.20 Thus, changes in N deposition cannot account for observed soil and surface water recovery.19 We therefore conclude that reductions in HCl emissions could have accounted for around one-third of all recovery from acidification in the UK over the 20 year period analyzed. The spatial pattern of change in xCl follows that previously identified for xSO4, with the largest decreases in Zone 1 and 1889

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Figure 3. Annual mean concentrations of (a) rainfall nonmarine sulfate, (b) rainfall nonmarine chloride, (c) rainfall total sodium, (d) surface water total sulfate, (e) surface water total chloride, and (f) surface water total sodium, by deposition zone.

smallest decreases in Zone 4 (Figure 2a,b). The xCl decreases observed in remote areas are surprising given the short expected atmospheric lifetime of HCl. It has been, however, previously suggested that longer-range transport of HCl can occur within nonprecipitating clouds.4 Time series data (Figure 3a,b) indicate that xSO4 and xCl decreases were sustained through the period, albeit with considerable (but spatially coherent) interannual variation, particularly for xCl. At the lower-deposition Zone 2-4 sites, mean annual xCl concentrations first fell below zero between 1991 and 1994, whereas in Zone 1 this occurred in 2005. It is possible that later fluctuations in xCl deposition were climatically driven, but there is little evidence that this is related to sea-salt deposition as represented by Na concentrations (Figure 3b,c). The predominance of negative xCl values in recent years could represent the re-establishment of a natural baseline, in which deposited sea-salt is slightly depleted in Cl. Such Cldepletion could occur through the dechlorination of sea-salt aerosol by background levels of H2SO4 or HNO3 in the marine boundary layer, provided that the atmospheric lifetime of the residual Na2SO4 or NaNO3 aerosol exceeds that of HCl, and that an excess of Na is thus deposited over land. In recent years there has been an increase in H2SO4 emissions from shipping around the UK, from 27 kT S yr-1 in 1990 to 54 kT S yr-1 in 2008,21 coinciding with the reduction in land-based emissions. Although

this appears insufficient to cause an overall rise in xSO4 deposition at coastal monitoring sites, it could have led to increased seasalt dechlorination over the ocean, and thus an increasingly negative xCl concentration in wet deposition over land. Negative calculated xCl could also be explained by the presence of some xNa in precipitation (see Supporting Information). Daily precipitation data from Eskdalemuir (Figure 4) show a clear variation in precipitation Cl/Na ratio according to source direction during 1986-1992. Relative to sea-salt, Cl/Na ratios were elevated (indicative of an HCl component), with maximum values in precipitation from the south, southeast, and north. These source directions correspond to industrial areas and coalburning power stations in Northern England and the Scottish Central Lowlands, respectively. Cl/Na ratios were lower (but still above sea-salt ratios) in precipitation from the southwest, west, and northwest, directions associated with the bulk of sea-salt deposition.16 Since 1992, Cl/Na ratios have declined in precipitation from all source directions, and post-2000 means are at or below the sea-salt ratio for most source directions. The general decline in Cl/Na ratio is consistent with observed xCl decreases in the wider ADMN data set, and the spatial pattern of change supports the conclusion that this results from declining anthropogenic HCl emissions. Post-2000 Cl/Na ratios below the seasalt value, particularly in precipitation from the southwest, west, 1890

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Figure 4. Cl/Na ratio in daily precipitation samples collected at Eskdalemuir, by wind direction, during three time periods.

and northwest, support the conclusion that sea-salt dechlorination is occurring in the marine atmosphere, possibly as a result of S emissions from shipping. Surface Waters. Analysis of trends in total Cl concentrations at the AWMN sites suggests reductions over the monitoring period were similar to, or exceeded, those in rainfall xCl; the median decrease in surface water Cl was 52 μeq L-1 over 19 years. Surface water Cl decreases were greatest in near-source Zone 1 sites, and lowest in remote Zone 4 sites (Figure 2c, 3e). Total SO4 concentrations also decreased at all sites (median 30 μeq L-1 over 19 years), again with the largest decreases at Zone 1 sites, but with little differentiation among Zones 2-4 (Figure 2d, 3d). There is evidence of decreasing Na concentrations at Zone 2 and 3 sites, but not at Zone 1 or Zone 4 sites. The interpretation of surface water Cl trends is complicated by several factors. First, both concentrations and calculated rates of change are influenced by evaporation. This will have accentuated changes at the dryer southeastern sites, but should have affected SO4 similarly. Second, total Cl incorporates a dominant marine component, which has been shown to fluctuate on an approximately decadal time scale, as storm frequency varies in relation to the North Atlantic Oscillation (NAO).22 Deposition of marinederived Cl was particularly high in the early 1990s, when the winter NAO Index (December-March mean, NAODJFM) was in a strong positive (i.e., stormy) phase, with a smaller peak in 2007-08. The relationship is demonstrated by a highly significant correlation between median standardized Cl annual concentration across all AWMN sites (ClZ, calculated by subtracting the mean and dividing by the standard deviation for each sample at each site23), and NAODJFM (R2 = 0.42, p < 0.01). Since NAODJFM has declined over the AWMN monitoring period,

(NAODJFM vs year: R2 = 0.24, p = 0.03), climatic factors have likely contributed to the observed decrease in surface water Cl. However a weak negative residual trend (residual of ClZ vs NAODJFM vs year: R2 = 0.18, p = 0.07) might reflect the additional effect of declining xCl deposition. For Na, the decrease in concentrations at many sites, particularly in Zones 2-3, also suggests a climate-related reduction in sea-salt loadings. However, other factors could have contributed to this decrease, particularly reduced leaching of Na from the soil by acid anions as deposition has fallen, a process shown to affect calcium and magnesium leaching in the AWMN catchments and elsewhere.24,25 Overall, it is not possible to unequivocally separate marine and anthropogenic sources of Cl on the basis of available surface water monitoring data. However, were climate-driven fluctuations in sea-salt loadings the only cause of declining Cl concentrations, we would expect changes to have been greatest in marine-influenced northern and western catchments (Zones 3-4), and weakest in the south and east (Zones 1-2). Our analysis shows the opposite, and is therefore consistent with a significant role for reduced anthropogenic xCl loadings, as identified in the deposition monitoring data. Soil Solution. As for surface waters, it is not possible to robustly separate marine and anthropogenic Cl in soil solution. However, clear decreases in total Cl and SO4, were observed at all three ECN sites (Table S3). At Glensaugh, Cl and SO4 concentrations in mineral horizon soil solution fell by an estimated 7.9 and 4.2 μeq L-1 yr-1, respectively, and at Sourhope by 14.1 and 8.8 μeq L-1 yr-1. These rates of change substantially exceed those observed for Zone 2 surface waters. At the Moor House blanket bog site there is near-complete retention of SO4 and consequently no trend, but Cl concentrations have decreased 1891

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Figure 5. Concentrations of chloride, sulfate, and pH of soil solutions at 10 and 50 cm within a blanket peat at the Moor House Environmental Change Network site, Northern England.

almost monotonically, by 49 μeq L-1 at 10 cm and 40 μeq L-1 at 50 cm (Figure 5). The retention of SO4 results from reduction processes and subsequent accumulation of S in organic matter in the waterlogged peat, with infrequent reoxidation episodes during drought years26 (Figure 5b). Nitrate similarly undergoes near-total retention. As the only mineral acid anion in peat porewaters, falling Cl concentrations thus appear to be the sole driver of the ca. 0.3 unit pH increase observed at this site (Figure 5c). This progressive change in pH cannot be explained by a change in sea-salt loadings over such a prolonged period, because the sea-salt effect occurs via transient shifts in ion exchange equilibria. It therefore appears to be attributable to the reduction in xCl concentrations observed both generally across the UK, and specifically at the adjacent (Cow Green) deposition monitoring site (Table S1). Environmental Implications. Our results suggest a need to reassess both the mechanisms of ecosystem acidification and recovery, and their wider environmental implications. In terms of terrestrial and freshwater impacts, higher than anticipated levels of historic HCl deposition to remote sites could explain several incompletely resolved phenomena. First, both monitoring and soil resurvey data (Figure 5c27,28) show that peatlands subject to atmospheric pollution did acidify, and are now recovering,

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despite the capacity of peats to retain most or all pollutant SO4 and NO3. This appears potentially attributable to HCl which, because Cl is highly mobile, can effectively transfer acidity through soils and into surface waters. Second, previous work showed that high (total) Cl deposition to Northwest Europe during the early 1990s was strongly acidifying, and attributed this to a climate-driven peak in sea-salt inputs.29 A similar climate-driven Cl deposition peak today might therefore be expected to cause a repetition of the acidic conditions of that period, with negative consequences for ecosystem recovery. Our data suggest, however, that xCl made a significant contribution to total Cl deposition in the past, and therefore that a given annual Cl deposition would have been more acidifying in the early 1990s than the same Cl input today or in future. Because xCl was mainly deposited in precipitation derived from the south or east, whereas sea-salt deposition largely derives from the west or southwest,16,22 the two forms of Cl input would mostly have occurred separately. Nevertheless, the observed small reduction in deposition Cl/Na ratios associated with westerly airflows at Eskdalemuir (Figure 4) does suggest some reduction in the anthropogenic component of Cl even in marine-dominated precipitation. Therefore, the acidifying effect of sea-salt deposition events may be somewhat smaller in the future than in the past. Finally, we argue that the impacts of HCl deposition may extend beyond acidification to include a wider range of ecosystem processes, notably in relation to the carbon cycle. Many recent studies have shown rising concentrations of dissolved organic carbon (DOC) in European and Northeast North American surface waters,28-31 with major implications for aquatic ecosystems, drinking water treatment costs and health risks, and possible effects on terrestrial carbon balances.33 In an assessment of 522 European and North American long-term data sets, Monteith et al.29 proposed a link between organic matter solubility, and soil acidity and ionic strength, based on an inverse relationship between surface water DOC trends, and those in SO4 and Cl concentrations. SO4 decreases were associated with reductions in S deposition, whereas Cl changes were linked to fluctuating sea-salt driven acidification. Our analysis now suggests that the role of Cl may have been twofold. Most strikingly, the “global” relationship derived by Monteith et al. gave strong positive residuals for most UK sites, implying that Cl changes had a greater impact on DOC in the UK than elsewhere. This could be explained if a proportion of past UK Cl deposition occurred as HCl, rather than NaCl, as the biogeochemical effect per unit reduction in Cl may have been underestimated. As well as DOC leaching, many other key elements of the carbon cycle are sensitive to acidity, particularly decomposition processes33-36 and methanogenesis.37,38 The carbon balance of peatlands and other key ecosystems may therefore have been significantly impacted by HCl deposition. Recognition of the role of HCl deposition fills a gap in our understanding of the mechanisms by which fossil fuel burning impacts natural ecosystems. In some areas and ecosystems it appears to have been a major agent of change, with impacts extending to terrestrial biodiversity, water quality, and carbon cycling. Remarkably, HCl has largely been overlooked in scientific assessments of ecosystem air pollution impacts, and by international legislative programmes such as the UNECE Convention on Long Range Transboundary Air Pollution.39 This contrasts with the concerted research and legislative focus on the potential acidifying impact of nitrogen emissions, even though most nitrogen is retained in terrestrial ecosystems. Although 1892

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Environmental Science & Technology measures to reduce S emissions have also effectively controlled HCl emissions in the UK, it may remain (or become) important in areas of the world increasingly reliant on coal burning for power generation. In general, the role of changing atmospheric pollutant inputs should not be ignored in studies of the environmental change, which are often focused on other drivers such as climate change.

’ ASSOCIATED CONTENT

bS

Supporting Information. Tables of observed precipitation (S1), surface water (S2) and soil water chemistry (S3), and a discussion of the potential impact on nonmarine Na deposition on results. This information is available free of charge via the Internet at http://pubs.acs.org/.

’ AUTHOR INFORMATION Corresponding Author

*Tel: þ44 1248 374500; e-mail: [email protected].

’ ACKNOWLEDGMENT The AWMN and ADMN are supported by the Department of the Environment, Food and Rural Affairs (Defra) and the UK Devolved Administrations. The ECN is supported by Defra, NERC and 12 other governmental organizations and institutes. The Moor House ECN site is operated and supported by the Centre for Ecology and Hydrology, Glensaugh and Sourhope by the Macaulay Institute. We are grateful for the comments of three reviewers. ’ REFERENCES

€ (1) Oberg, G.; Sanden, P. Retention of chloride in soil and cycling of organic matter-bound chlorine. Hydrol. Process 2005, 19, 2123–2136. (2) Svensson, T.; Lovett, G. M.; Likens, G. E. Is chloride a conservative ion in forest ecosystems? Biogeochemistry 2010 No. 10.1007/ s10533-010-9538-y. (3) McCulloch, A.; Aucott, M. L.; Bencovitz, C. M.; Graedel, T. E.; Kleiman, G.; Midgley, P. M.; Li, Y.-F. Global emissions of hydrogen chloride and chloromethane from coal combustion, incineration and industrial activities: Reactive Chlorine Emissions Inventory. J. Geophys. Res. 1999, 104, 8391–8403. (4) Lightowlers, P. J.; Cape, J. N. Sources and fate of atmospheric HCl in the U.K. and western Europe. Atmos. Environ. 1988, 22, 7–15. (5) Dore, C. J.; Murrells, T. P.; Passant, N. R.; Hobson, M. M.; Thistlethwaite, G.; Wagner, A.; Li, Y.; Bush, T.; King, K. R.; Norris, J.; Coleman, P. J.; Walker, C.; Stewart, R. A.; Tsagatakis, I.; Conolly, C.; Brophy, N. C. J.; Hann, M. R. UK emissions of air pollutants 1970 to 2006; Report to the Department for Environment, Food and Rural Affairs; AEA Technology plc., 2008. (6) Gorham, E. Atmospheric pollution by hydrochloric acid. Q. J. R Meteorol. Soc. 1958, 84, 274–276. (7) Lovett, G. M.; Likens, G. E.; Buso, D. C.; Driscoll, C. T.; Bailey, S. W. The biogeochemistry of chlorine at Hubbard Brook, New Hampshire, USA. Biogeochemistry 2005, 72, 191–232. (8) Shapiro, J. B.; Simpson, H. J.; Griffin, K. J.; Schuster, W. S. F. Precipitation chloride at West Point, NY: Seasonal patterns and possible contributions from non-seawater sources. Atmos. Environ. 2007, 41, 2240–2254. (9) Keene, W. C.; Khalil, M. A. K.; Erickson, D. J.; McCulloch, A.; Graedel, T. E.; Lobert, J. M.; Aucott, M. L.; Gong, S. L.; Harper, D. B.; Kleiman, G.; Midgley, P.; Moore, R. M.; Seuzaret, C.; Sturges, W. T.; Benkovitz, C. M.; Koropalov, V.; Barrie, L. A.; Li, Y. F. Composite global

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emissions of reactive chlorine from anthropogenic and natural sources: Reactive Chlorine Emissions Inventory. J. Geophys Res. 1999, 104, 8429–8440. (10) Wright, R. F.; Norton, S. A.; Brakke, D. F.; Frogner, T. Experimental verification of episodic acidification of freshwaters by sea salts. Nature 1988, 334, 422–424. (11) Yue, G. K.; Mohnen, V. A.; Kiang, C. S. A mechanism for hydrochloric acid production in cloud. Water Air Soil Pollut. 1976, 6, 277–294. (12) Roth, B; Okada, K. On the modification of sea-salt particles in the coastal atmosphere. Atmos. Environ. 1998, 32, 1555–1569. (13) Graedel, T. E.; Keene, W. C. The budget and cycle of Earth’s natural chlorine. Pure Appl. Chem. 1996, 68, 1689–1697. (14) Fowler, D.; Smith, R. I.; Muller, J. B. A.; Hayman, G.; Vincent, K. J. Changes in the atmospheric deposition of acidifying compounds in the UK between 1986 and 2001. Environ. Pollut. 2005, 137, 15–25. (15) Jenkinson, A. F.; Collinson, B. P. An initial climatology of gales over the North Sea; Synoptic Climatology Branch Memorandum No. 62; Bracknell Meteorological Office, 1977 (16) Brayshaw, S. A. Changes to chloride precipitation at Moor House Environmental Change Network station since 1992. MSc Dissertation, University of Lancaster, 2010. (17) Monteith, D. T.; Evans, C. D. The UK Acid Waters Monitoring Network: A review of the first 15 years and introduction to the special issue. Environ. Pollut. 2005, 137, 3–13. (18) Morecroft, M. D.; Bealey, C. E.; Beaumont, D. A.; Benham, S.; Brooks, D. R.; Burt, T. P.; Critchley, C. N. R.; Dick, J.; Littlewood, N. A.; Monteith, D. T.; Scott, W. A.; Smith, R. I.; Walmsley, C.; Watson, H. The UK Environmental Change Network: Emerging trends in the composition of plant and animal communities and the physical environment. Biol. Cons. 2009, 142, 2814–2832. (19) RoTAP. Review of Transboundary Air Pollution; Report to the Department of Environment, Food and Rural Affairs. Centre for Ecology and Hydrology: Edinburgh, 2011. (20) Curtis, C. J.; Evans, C. D.; Helliwell, R. C.; Monteith, D. T. Nitrate leaching as a confounding factor in chemical recovery from acidification in UK upland waters. Environ. Pollut. 2005, 137, 73–82. (21) Dore, C., AEA Technology plc. Personal communication. (22) Evans, C. D.; Monteith, D. T.; Harriman, R. Long-term variability in the deposition of marine ions at west coast sites in the UK Acid Waters Monitoring Network: Impacts on surface water chemistry and significance for trend determination. Sci. Total Environ. 2001, 265, 115–129. (23) Evans, C. D.; Cooper, D. M.; Monteith, D. T.; Helliwell, R. C.; Moldan, F.; Hall, J.; Rowe, E. C.; Cosby, B. J. Linking monitoring and modelling: Can long-term datasets be used more effectively as a basis for large-scale prediction? Biogeochemistry 2010, 101, 211–227. (24) Evans, C. D.; Cullen, J. M.; Alewell, C.; Kopacek, J.; Marchetto, A.; Moldan, F.; Prechtel, A; Rogora, M.; Vesely , J.; Wright, R. Recovery from acidification in European surface waters. Hydrol. Earth Syst. Sci. 2001, 5, 283–298. (25) Evans, C. D.; Monteith, D. T.; Reynolds, B.; Clark, J. M. Buffering of recovery from acidification by organic acids. Sci. Total Environ. 2008, 404, 316–325. (26) Adamson, J. K.; Scott, W. A.; Rowland, A. P.; Beard, G. R. Ionic concentrations in a blanket peat bog in northern England and correlations with deposition and climate variables. Eur. J.Soil Sci. 2001, 52, 69–79. (27) Emmett, B. A.; Reynolds, B.; Chamberlain, P. M.; Rowe, E.; Spurgeon, D.; Brittain, S. A.; Frogbrook, Z.; Hughes, S.; Lawlor, A. J.; Poskitt, J.; Potter, E.; Robinson; D. A.; Scott, A.; Wood, C.; Woods, C. Countryside Survey: Soils Report from 2007; CS Technical Report No. 9/ 07; Centre for Ecology and Hydrology, 2010. Available at http:// www.countrysidesurvey.org.uk/pdf/reports2007/CS_UK_2007_TR9revised.pdf. (28) Kirk, G. J. D.; Bellamy, P. H.; Lark, R. M. Changes in soil pH across England and Wales in response to decreased acid 1893

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deposition. Global Change Biol. 2010, No. doi 10.1111/j.1365-2486. 2009.02135.x. (29) Monteith, D. T.; Stoddard, J. L.; Evans, C. D.; de Wit, H.;   Forsius, M.; Høgasen, T.; Wilander, A.; Skjelkvale, B. L.; Jeffries, D. S.; Vuorenmaa, J.; Keller, B.; Kopacek, J.; Vesely, J. Rising freshwater dissolved organic carbon driven by changes in atmospheric deposition. Nature 2007, 450, 537–540. (30) Erlandsson, M.; Buffam, I.; Folster, J.; Laudon, H.; Temnerud, J.; Weyhenmeyer, G. A.; Bishop, K. Thirty-five years of synchrony in the organic matter concentrations of Swedish rivers explained by variation in flow and sulphate. Global Change Biol. 2008, 14, 1191–1198. (31) Oulehle, F.; Hruska, J. Rising trends of dissolved organic matter in drinking-water reservoirs as a result of recovery from acidification in the Ore Mts., Czech Republic. Environ. Pollut. 2010, 157, 3433–3439. (32) Bellamy, P. H.; Loveland, P. J.; Bradley, R. I.; Lark, R. M.; Kirk, G. J. D. Carbon losses from all soils across England and Wales 19782003. Nature 2005, 437, 245–248. (33) Reth, S.; Riechstein, M.; Falge, E. The effect of soil water content, soil temperature, soil pH-value and the root mass on soil CO2 efflux - a modified model. Plant Soil 2005, 268, 21–33. (34) Sinsabaugh, R. L. Phenol oxidase, peroxidase and organic matter dynamics of soil. Soil Biol. Biochem. 2010, 42, 391–404. (35) Mulder, J.; De Wit, H. A.; Boonen, H. W. J.; Bakken, L. R. Increased levels of aluminium in forest soils: Effects on the stores of soil organic carbon. Water, Air Soil Pollut. 2000, 130, 989–994. (36) Andersson, S.; Nilsson, S. I. Influence of pH and temperature on microbial activity, substrate availability of soil-solution bacteria and leaching of dissolved organic carbon in a mor humus. Soil Biol. Biochem. 2001, 33, 1181–1191. (37) Wang, Z. P.; Lindau, C. W.; Delaune, R. D.; Patrick, W. H. Methane emissions and entrapment in flooded rice soils as affected by soil properties. Biol. Fertil. Soils 1993, 16, 163–168. (38) Inubushi, K.; Otake, S.; Furukawa, Y.; Shibasaki, N.; Ali, M.; Itang, A. M.; Tsuruta, H. Factors influencing methane emission from peat soils: Comparison of tropical and temperate wetlands. Nutr. Cycling Agroecosyst. 2005, 71, 93–99. (39) Sliggers, J.; Kakebeeke, W. Clearing the air: 25 years of the Convention on Long-Range Transboundary Air Pollution; United Nations Publication E.04.11.E.20, 2004. Available at http://www.unece.org/ env/lrtap/conv/conclusi.htm.

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dx.doi.org/10.1021/es103574u |Environ. Sci. Technol. 2011, 45, 1887–1894