Atmospheric Deposition Chemistry Research Articles

Atmospheric deposition and soil water chemistry in Swedish forests since 1985 – Effects of reduced emissions of sulphur and nitrogen

Trends for the atmospheric deposition of sulphur (S) and inorganic nitrogen (inorg-N) to forests and changes in the forest soil water chemistry in Sweden have been assessed since 1985, with special focus on the last 25 years, based on measurements within the Swedish Throughfall Monitoring Network (SWETHRO). The reductions in the deposition of S and inorg-N in the southern part of Sweden corresponded relatively well with the pollutant emission reductions for S and inorg-N from both EU27 + UK and Sweden during 1996/97–2021/22. For northern Sweden the deposition of S and inorg-N decreased to a lesser extent than both European and Swedish emissions. The bulk deposition of NO3-N has decreased more than the deposition of NH4-N over the last 25-year period, which is consistent with the much larger emission reductions for NOx compared to NH3 from EU27 + UK and Sweden. The S concentrations in the soil water, at 50 cm below soil surface, have decreased during the last 25 years, however somewhat less than the S deposition. At sites with low ANC and pH in the beginning of the period, the increase in ANC was generally greater and the increase in pH was smaller, but at sites with high pH and ANC above zero, the increase in pH was dominant, in line with the nonlinear relationship between pH and ANC in the soil water. The incidence of elevated concentrations of NO3-N in the soil water was highest in southwest Sweden, ranging between 4 and 19 % of all measuring occasions since 1985/86. The reduced deposition of N over the 35-year period was not reflected in the incidence of elevated concentrations of NO3-N in the soil water over time.

Persistent chemostatic behaviour of stream solutes in a northern hardwood forest under climatic and atmospheric deposition changes

AbstractConcentration – discharge (C – Q) relationships in streamflow can serve a variety of purposes, including elucidating catchment water sources, quantifying chemical weathering, and estimating solute fluxes. Four main C – Q relationships have been identified: dilution (decreasing C with increasing Q), flushing (increasing C with increasing Q), chemostatic (little or no change in C with increasing Q) and chemostochastic (great variation in C relative to that of Q) behaviour. We used a 40‐year data record for 10 headwater forested catchments in central Ontario, Canada, to examine C – Q relationships for a range of geogenic, exogenic and bioactive solutes. The thin overburden cover of this landscape and general absence of deep groundwater contributions to streamflow suggest that streamflow solute concentrations and their relationship with discharge would be sensitive to pronounced changes in climate and atmospheric deposition chemistry over the monitoring period. We explored the temporal and spatial stability of C – Q relationships in response to these changes at annual and seasonal timescales for relatively wet and dry conditions and for periods with relatively high and low atmospheric deposition. Chemostasis dominated C – Q relationships at the annual and seasonal scales for geogenic (e.g., SiO2) and exogenic (e.g., Cl−) solutes, although bioactive solutes (e.g., NO3−, NH4+) exhibited non‐chemostatic behaviour for some catchments, time periods and wetness conditions. C – Q relationships derived at the annual timescale often masked pronounced seasonal shifts in bioactive solute concentration response to changes in streamflow and provide only partial insight into the production and delivery of such solutes in this forest landscape. Relatively subtle changes in overburden thickness across this landscape appeared to explain some of the deviations from widespread chemostasis. Nevertheless, this general chemostatic behaviour was largely temporally stable across catchments and solutes, highlighting its resilience in the face of such non‐stationary influences as climate conditions and atmospheric deposition.

Long‐term hydrological and biogeochemical datasets from a Mediterranean forest site (Montseny, NE Spain)

AbstractWe present here, and make freely available, two long‐term datasets on the hydrology and hydrochemistry of two small catchments totally or partially covered by holm oak (Quercus ilex) forests. Data have been collected over four decades of research on the ecology of Mediterranean evergreen forests. The datasets contain data on water fluxes in precipitation and runoff, and on the ion concentrations and fluxes in precipitation, throughfall and streamwater. Among other applications, these data have been used to obtain catchment budgets of water and elements, to document decadal changes in atmospheric deposition and streamwater chemistry, to assess the role of Saharo‐Sahelian dust on forest nutrient cycling, and to model habitat suitability for an endangered amphibian under future climates. To encourage their further use by the scientific community, in this paper we summarize the data context and methods, and give a nutshell of obtained results. We provide the links to these datasets on the Zenodo platform.

A Study of Atmospheric Deposition Chemistry: Wet, Dry and Nitrogen Deposition

A Study of Atmospheric Deposition Chemistry: Wet, Dry and Nitrogen Deposition

Response of atmospheric deposition and surface water chemistry to the COVID-19 lockdown in an alpine area

The effects of the COVID-19 lockdown on deposition and surface water chemistry were investigated in an area south of the Alps. Long-term data provided by the monitoring networks revealed that the deposition of sulfur and nitrogen compounds in this area has stabilized since around 2010; in 2020, however, both concentrations and deposition were significantly below the average values of the previous decade for SO4 and NO3. Less evident changes were observed for NH4 and base cation. The estimated decrease of deposition in 2020 with respect to the previous decade was on average − 54% and − 46% for SO4 and NO3, respectively. The lower deposition of SO4 and NO3 recorded in 2020 was caused by the sharp decrease of SO2 and particularly of NOx air concentrations mainly due to the mobility restrictions consequent to the COVID-19 lockdown. The limited effects on NH4 deposition can be explained by the fact that NH3 emission was not affected by the lockdown, being mainly related to agricultural activities. A widespread response to the decreased deposition of S and N compounds was observed in a group of pristine freshwater sites, with NO3 concentrations in 2020 clearly below the long-term average. The rapid chemical recovery observed at freshwater sites in response to the sharp decrease of deposition put in evidence the high resilience potential of freshwater ecosystems in pristine regions and demonstrated the great potential of emission reduction policy in producing further substantial ameliorations of the water quality at sensitive sites.

Did desmid assemblages in Dutch moorland pools recover from acidification in the past century?

In order to monitor recovery from acidification caused by acid atmospheric deposition, desmids and water chemistry were sampled in three Dutch moorland pools regularly from 1978 to 2014. Reference desmid samples from the early twentieth century were retrieved from old collections. Changes of the desmid assemblages were assessed by analyses of traits, including indicator values for pH and total phosphate, conservation value, cell volume and surface/volume (s/v) ratio. Direct correspondence analysis (DCA) traced relations between desmids and environmental variables. Between 1916 and 2014, species composition altered due to changes in acidifying atmospheric deposition: The change was most pronounced in pools with relatively flat shores exposed to the atmosphere in extremely dry summers. After the dry summer of 1921, changes were slight, but after the dry summer of 1976, changes were dramatic, when the sulphur and nitrogen compounds stored in the water bottom oxidized and acidified the water. The conservation value declined sharply but increased again until the 1990s, partly due to the decrease in acidifying deposition. Although the acid atmospheric deposition continued to decline until the early 21th century, the conservation value declined again, as did the stability of the desmid assemblages. It is likely that internal eutrophication (nutrients), presence of toxic substances (such as hydrogen sulphide), the decline of aquatic macrophytes (substrate), shading by afforestation (light) and/or reduced supply of carbon dioxide (due to decreased local seepage) play a role. The chemical dynamics due to the large stock of sulphur and nitrogen compounds will hamper the development of rare desmids, bound to stable environmental conditions.

Sulfur Constraints on the Carbon Cycle of a Blanket Bog Peatland

AbstractThe reduction of sulfate (SO42−) represents an alternative terminal electron acceptor for the oxidation of organic matter in peat soils. The greenhouse gas budget in peatlands will be constrained by how much a peatland can utilize SO42− reduction as an alternative to methanogenesis. Using records of atmospheric deposition and stream chemistry coupled with elemental analysis of peat soil, vegetation, particulate organic matter (POM) and dissolved organic matter (DOM), this study estimated a 23‐years long sulfur (S) budget for a blanket bog‐covered catchment in the North Pennines, England. The study showed that: (a) Atmospheric deposition of total S significantly declined over the study period from 2.4 to 0.5 t S/km2/yr. (b) Long term accumulation of S into deep peat at 1 m depth averaged 127 kg S/km2/yr. (c) Total S fluvial flux peaked as 4.5 t S/km2/yr with an average of 0.7 t S/km2/yr. (d) On average, over 23 years, 0.25 t S/km2/yr were reduced to either mineral sulphides or hydrogen sulphide; however, in eight out of the 23 years the catchment was a net producer of S to the streams of the catchment. At maximum observed, S reduction capacity the peatland was capable of a net removal of 71% of atmospheric S deposition. Allowing for the efficiency of energy transfer in the redox process and the oxidation state of peat organic matter means that for every mole of SO42− reduced, 1.69 moles of CO2 were produced, and an average of 0.47 t C/km2/yr are diverted from methanogenesis.

The Robinson Forest environmental monitoring network: Long‐term evaluation of streamflow and precipitation quantity and stream‐water and bulk deposition chemistry in eastern Kentucky watersheds

AbstractThe University of Kentucky (U KY) has owned Robinson Forest (37.460723° N, 83.158598° W) since 1923, conducting experiments crucial to understanding the environmental effects of land management in the region. Part of the management of Robinson Forest has been collection of environmental data, including precipitation quantity, bulk‐deposition chemistry, streamflow, stream‐water chemistry, and air and stream temperature. Over the years, these data have been collected and archived using various technologies and have been mostly inaccessible for research use – unedited and uncompiled, scattered across several spreadsheets and paper records. Through a partnership between the U.S. Geological Survey (USGS) and U KY, daily precipitation data for six stations and stream data from four watersheds in Robinson Forest have been compiled for 1971–2018, checked for transcription errors, and annotated for changes in methodologies. These data are available as a USGS data release at https://doi.org/10.5066/P9FPLG1O. Improved accessibility of this data set provides an important research resource for understanding water quality in minimally effected forests in the region. Preliminary results indicate that these data present a valuable opportunity to evaluate linkages among atmospheric deposition and stream chemistry, the effects of environmental policy, such as the Clean Air Act, and effects from nearby land disturbance in the form of surface mining. Furthermore, these data fill a geographic and physiographic gap in what is available to examine deposition and streamflow patterns over the last 45 years, supplementing those long‐term records of research sites in northern (e.g., Hubbard Brook Experimental Forest), central (e.g., Fernow Experimental Forest) and southern Appalachia (e.g., Coweeta Hydrologic Laboratory). As an oasis in the midst of significant surface mining activity, Robinson Forest presents a unique opportunity to understand environmental conditions characteristic of minimally disturbed forests similar to pre‐mining conditions in the Central Appalachian region.

Atmospheric deposition chemistry in a Brazilian rural area: alkaline species behavior and agricultural inputs.

Since 2000s, several studies have been reporting an increase of alkaline species in atmospheric deposition worldwide. This study aims to evaluate and give a better understanding about atmospheric deposition chemistry collected in Lavras, a Brazilian city with rural background. Bulk atmospheric deposition samples were collected from March 2018 to February 2019 and major ionic species were quantified. The pH values ranged from 5.52 to 8.29, with an average of 5.92 and most deposition samples (~ 94%) were alkaline (pH > 5.60). For the whole sampling campaign, the ions profile in volume weighted mean (VWM) was described as follows: Ca2+ (35.02) > NH4+ (11.26) > Cl- (11.19) > Mg2+ (9.04) > NO3- (8.57) > Na+ (5.65) > K+ (2.61) > SO42- (2.43) > H+ (0.94) μmol L-1. We identified Ca2+ and NH4+ as the most predominant species accounting for 53% of the total ionic species distribution. In addition, all samples showed neutralization factor (NF) index greater than 1, with mean value of 6.4. Regarding regression analysis, acidity neutralization precursors such as calcium and ammonia accounted for 50% and 4%, respectively. In addition, samples alkaline pattern was mainly due to agricultural sources, including fertilizer production and application, and cement manufacturing inside the county air basin.

Source contributions in precipitation chemistry and analysis of atmospheric nitrogen deposition in a Sahelian dry savanna site in West Africa

Experimental data on precipitation chemistry were collected at a semi-arid savanna in Senegal (Dahra) in 2013, 2014 and 2017. The chemical composition of precipitation was analyzed for inorganic and organic ions, using ionic chromatography. The pH values of precipitation range from 4.50 to 8.50 with 89% of the samples having basic pH. The composition of precipitation was controlled by four source contributions: marine, terrigenous, biogenic, and organic acids emissions from vegetation. The terrigenous contribution was the highest accounting for 42% of the total annual Volume Weighted Mean ionic concentrations, due to the proximity of the Saharan desert, followed by the marine source representing 36%, due to the location of Dahra close to the Atlantic Ocean. Nitrogenous (N) contribution represents 16% of the mean annual total ionic charge, from biogenic sources and livestock crossing the site all year round. Finally, the lowest contribution is from organic acidity (5%), due to the low density of vegetation especially during the dry season. Wet deposition fluxes in Dahra for all compounds show larger values than at other Sahelian savanna sites. Dry N deposition in Dahra was also estimated by inferential method using gas concentration measurements and modeled dry deposition velocities. The total N deposition fluxes (wet plus dry) range from 3.80 to 4.81 kgN ha−1 yr−1, comparable to fluxes at other semi-arid savannas in Niger and Mali. Wet deposition contributed with 37–53% of the total N flux, suggesting that wet N deposition is equally important to dry deposition fluxes for direct N loading to savanna ecosystems in the Sahel, with a large contribution of reduced compounds. This study shows that Dahra presents a precipitation chemistry composition with characteristics close to those from other Sahelian sites, with however the specificity of being more influenced by the proximity of the Atlantic ocean and the presence of livestock year round.

Bulk Deposition and Main Ionic Composition in a Remote Tropical Region: Nevado de Toluca, Mexico

The Nevado de Toluca Natural Protected Area (Zona de Proteccion de Flora y Fauna Nevado de Toluca, ZPFFNT), Central Mexico, encompasses one of the four highest (> 4000 m a.s.l.) volcanoes in Mexico, Nevado de Toluca; an extended area of woodland surrounds this volcano. Although identified as a remote area based on its high altitude, the ZPFFNT is not far from the urban and industrial zones of Toluca (~20 km) and Mexico (~72 km) cities, which potentially threatens the environmental health of the ZPFFNT by emitting SO2 and NOX. Acid precipitation falling on areas with low alkaline reserve leads to environmental acidification and land degradation. To provide reliable data on the air pollution reaching the ZPFFNT and the related potential risks, our study analyzed the bulk atmospheric deposition chemistry and its temporal dynamics throughout an annual cycle. There are two well-defined seasons: (a) cold/dry with SW–NE wind direction and (b) warm/rainy with NE–SW wind direction. The pH, electric conductivity (K25), cations, Cl−, and HCO3− were statistically higher in the cold/dry period. Differently, NO3− and SO42− showed steadier behavior. Bulk deposition pH remained acidic at all times but reached extreme low values from July to September during the warm/rainy season. In the cold/dry season, alkaline cations were important in partially neutralizing the acidic compounds from the urban and industrial zones of Toluca and Mexico cities. Previously assumed to be safe based on its remoteness, the ZPFFNT is threatened by acid precipitation, which demand the implementation of preventative and mitigating actions as part of a management plan to avoid environmental deterioration.

Chemistry of dry and wet atmospheric deposition over the Balearic Islands, NW Mediterranean: Source apportionment and African dust areas

Wet and dry aerosol deposition samples were collected from September 2010 to August 2012 at a remote background site in the Mallorca Isle (Western Mediterranean). Ions and major and trace elements were determined in soluble and insoluble fractions. Temporal variations of chemical components are discussed and interpreted. The overall pattern associated to long-range-transport air masses is studied: Dry/Wet deposition ratios, charges and composition depend clearly on the meteorological scenario. E.g. Dry/Wet ratio is 1:1 when air comes from North Africa, in contrast to a 1:9 ratio under the mainland Europe influence.Moreover, an innovating source apportionment study was conducted integrating both dry and wet deposition samples. Six sources were revealed, including marine aerosols (32%); two different mineral factors, African dust (15%) and regional dust (12%); two anthropogenic factors, one related to road traffic (8%) and another to regional sources (17%); and a mixed factor having biomass burning emissions and others sources (17%). Temporal variations and influence from long-range-transport air masses are also investigated.Fertilization deposition trends have also been explored, observing nutrients settling, as well as nitrate and sulphate, due to their agricultural interest. An important peak during January–February 2012 is studied in depth.Having in mind the strong impact of African dust on the global deposition budget, the analysis of elemental ratios between key dust components was investigated in order to identify major source areas affecting Western Mediterranean: Western Sahara, Algeria-Hoggar Massif and Tunisia-Libya. Differences among these regions are evident. E.g. the impact of industrial emissions is well-detected under outbreaks from Tunisia-Libya, with relatively high content of Ni and Pb.

Long-term changes (1990–2015) in the atmospheric deposition and runoff water chemistry of sulphate, inorganic nitrogen and acidity for forested catchments in Europe in relation to changes in emissions and hydrometeorological conditions

The international Long-Term Ecological Research Network (ILTER) encompasses hundreds of long-term research/monitoring sites located in a wide array of ecosystems that can help us understand environmental change across the globe. We evaluated long-term trends (1990–2015) for bulk deposition, throughfall and runoff water chemistry and fluxes, and climatic variables in 25 forested catchments in Europe belonging to the UNECE International Cooperative Programme on Integrated Monitoring of Air Pollution Effects on Ecosystems (ICP IM). Many of the IM sites form part of the monitoring infrastructures of this larger ILTER network. Trends were evaluated for monthly concentrations of non-marine (anthropogenic fraction, denoted as x) sulphate (xSO4) and base cations x(Ca+Mg), hydrogen ion (H+), inorganic N (NO3 and NH4) and ANC (Acid Neutralising Capacity) and their respective fluxes into and out of the catchments and for monthly precipitation, runoff and air temperature. A significant decrease of xSO4 deposition resulted in decreases in concentrations and fluxes of xSO4 in runoff, being significant at 90% and 60% of the sites, respectively. Bulk deposition of NO3 and NH4 decreased significantly at 60–80% (concentrations) and 40–60% (fluxes) of the sites. Concentrations and fluxes of NO3 in runoff decreased at 73% and 63% of the sites, respectively, and NO3 concentrations decreased significantly at 50% of the sites. Thus, the LTER/ICP IM network confirms the positive effects of the emission reductions in Europe. Air temperature increased significantly at 61% of the sites, while trends for precipitation and runoff were rarely significant. The site-specific variation of xSO4 concentrations in runoff was most strongly explained by deposition. Climatic variables and deposition explained the variation of inorganic N concentrations in runoff at single sites poorly, and as yet there are no clear signs of a consistent deposition-driven or climate-driven increase in inorganic N exports in the catchments.

An assessment of the Ca weathering sources to surface waters on the Precambrian Shield in central Ontario

There is increasing concern over the negative ecological impacts caused by falling calcium (Ca) concentrations in lakes, particularly in central Ontario, Canada. Forecasting regional changes in lake Ca concentrations relies on accurate estimates of mineral weathering rates that are not widely available. In this study, bulk atmospheric deposition, surface water and soil chemistry along with 87Sr/86Sr isotope measurements were used to provide regional insight into weathering controls on Ca concentrations in lakes. Regionally, Ca concentrations in 90% of 129 lakes sampled in central Ontario were <0.1 mmol L−1 and the Ca/Sr ratio in lakes increased and the K/Sr ratio decreased with increasing Sr concentration, which is indicative of greater Ca sources from calcite or apatite in the higher Ca lakes. Significant relationships between 87Sr/86Sr ratios and Ca/Sr rations in dilute acid (0.1 M HCl) soil extracts are also indicative of the presence of trace amounts of calcite or apatite in surficial soils. Within the low (<0.7 mmol L−1) Ca lakes, defined in this study that are considered most at risk from falling Ca concentrations, 87Sr/86Sr ratios fell within the range observed in weak acid soil extracts and were also significantly related to Ca/Na and K/Sr ratios in surface waters. There were large inconsistencies however, between Ca/Na ratios and Ca/Sr in surface waters and soil acid extracts that suggest differences in 87Sr/86Sr ratios in surface waters of the low Ca lakes do not simply reflect differences in Ca derived from non-silicate minerals in surficial soils and that that Ca sources from deeper soil or bedrock are also important contributors to surface water Ca in these low Ca lakes.

Long Term Trends and Dynamics of Dissolved Organic Carbon (DOC) in a Subtropical Reservoir Basin

This study evaluates the long term trends and seasonal patterns of dissolved organic carbon (DOC) concentration in the Fei-Tsui Reservoir basin in Northern Taiwan during the period of 2000 to 2015. The non-parametric seasonal Mann-Kendall test was conducted to identify the trends of DOC and its potential drivers (e.g., temperature, runoff, atmospheric acid deposition and stream water chemistry). The monthly tributaries and water surface DOC concentrations in Fei-Tsui Reservoir had showed strong temporal and seasonal variability. The sulfate (SO4) concentration had exhibited statistically significant decreasing trend over a period of 16 years. The decreasing trends of anions (SO4 and NO3) and base cations (Ca and Mg) as well as increasing trends of pH and acidification index (ACI) in Fei-Tsui Reservoir and streams indicated recovery from acidification. However, there was no significant annual trend in DOC concentration of Fei-Tsui Reservoir and streams. Significant positive correlation was obtained between DOC and trophic state index of Fei-Tsui Reservoir. The results suggested that DOC concentration at the water surface of Fei-Tsui Reservoir was mainly driven by the re-oligotrophication and temperature effects rather than a decrease in atmospheric sulfur deposition. Stream DOC concentration was largely determined by the temporal variability in temperature and rainfall. We concluded that climatic and hydrological factors are the dominant drivers for stream DOC dynamics in the study.

Response to “Commentary on ‘A possible trade-off between clean air and clean water’ by Smith et al. (2017)”

We appreciate the commentary by Robarge et al. (2017) regarding our recent feature article on the potential trade-offs between clean air and clean water (Smith et al. 2017). The purpose of our article was to further the discussion on the causes of the divergent soluble phosphorus (SP) and total phosphorus (TP) loading trends that have been observed in Western Lake Erie Basin tributaries (Baker et al. 2014; Jarvie et al. 2017). In 2014, shortly after the Toledo, Ohio, water crisis, we put together a list of 25 “potential causes” of increased SP loading to Lake Erie, with changing atmospheric deposition chemistry as one of the potential causes listed (Smith et al. 2015). Since the writing of that article, numerous studies investigating several of the “potential causes,” including tile drainage, soil test P levels, nutrient management, and tillage practices, have been conducted. While we acknowledge that many factors have likely contributed to changing SP loading trends, examining the potential impact of individual and combinations of factors, including the effects of changes in atmospheric deposition, will be fundamental to improving P management and equipping producers with the necessary tools to decrease P losses to Lake Erie and other sensitive waterbodies.

Marine Primary Productivity as a Potential Indirect Source of Selenium and Other Trace Elements in Atmospheric Deposition.

Atmospheric processes play an important role in the supply of the trace element selenium (Se) as well as other essential trace elements to terrestrial environments, mainly via wet deposition. Here we investigate whether the marine biosphere can be identified as a source of Se and of other trace elements in precipitation samples. We used artificial neural network (ANN) modeling and other statistical methods to analyze relationships between a high-resolution atmospheric deposition chemistry time series (March 2007-January 2009) from Plynlimon (UK) and exposure of air masses to marine chlorophyll a and to other source proxies. Using ANN sensitivity analyses, we found that higher air mass exposure to marine productivity leads to higher concentrations of dissolved organic carbon (DOC) in rainfall. Furthermore, marine productivity was found to be an important but indirect factor in controlling Se as well as vanadium (V), cobalt (Co), nickel (Ni), zinc (Zn), and aluminum (Al) concentrations in atmospheric deposition, likely via scavenging by organic compounds derived from marine organisms. Marine organisms may thus play an indirect but important role in the delivery of trace elements to terrestrial environments and food chains.

Alkalinization and acidification of stream water with changes in atmospheric deposition in a tropical dry evergreen forest of northeastern Thailand

Field surveys on atmospheric deposition and stream water chemistry were conducted in an evergreen forest in northeastern Thailand characterized by a tropical savanna climate with distinct dry and wet seasons. Atmospheric deposition of ion constituents by throughfall and stemflow was shown to increase in the beginning and end of the wet season, reflecting the precipitation pattern. The pH and electrical conductivity of stream water increased with alkalinity and base cation concentrations due to mineralization of organic matter by the first rain and retention of anions in soil during the start of the wet season. After initial alkalinization, the pH and alkalinity declined rapidly with the highest SO42− concentration displayed in the middle towards the end of the wet season. The magnitude of peaks in SO42− concentration (13.5–60.6 μmolc/L) reflects deposition during the first 2 months of the wet season (March and April) in respective years (60.8–170 molc/ha). Release of SO42− with H+, which is retained in soil during the early wet season, may cause acidification later in the season. The deposition and concentration of SO42− declined over 6 years. However, the pH of stream water declined with increasing concentrations of SO42− and other major ions. The release of materials accumulated in the ecosystem was facilitated by the decrease in SO42− concentration/deposition and increased precipitation in the middle–late wet season. The retention-release cycle of SO42− largely contributed to both seasonal and interannual variations in stream water chemistry in the tropical savanna climate studied.

Exploring lag times between monthly atmospheric deposition and stream chemistry in Appalachian forests using cross-correlation

Forecasts of ecosystem changes due to variations in atmospheric emissions policies require a fundamental understanding of lag times between changes in chemical inputs and watershed response. Impacts of changes in atmospheric deposition in the United States have been documented using national and regional long-term environmental monitoring programs beginning several decades ago. Consequently, time series of weekly NADP atmospheric wet deposition and monthly EPA-Long Term Monitoring stream chemistry now exist for much of the Northeast which may provide insights into lag times. In this study of Appalachian forest basins, we estimated lag times for S, N and Cl by cross-correlating monthly data from four pairs of stream and deposition monitoring sites during the period from 1978 to 2012. A systems or impulse response function approach to cross-correlation was used to estimate lag times where the input deposition time series was pre-whitened using regression modeling and the stream response time series was filtered using the deposition regression model prior to cross-correlation. Cross-correlations for S were greatest at annual intervals over a relatively well-defined range of lags with the maximum correlations occurring at mean lags of 48 months. Chloride results were similar but more erratic with a mean lag of 57 months. Few high-correlation lags for N were indicated. Given the growing availability of atmospheric deposition and surface water chemistry monitoring data and our results for four Appalachian basins, further testing of cross-correlation as a method of estimating lag times on other basins appears justified.

Temporal and spatial patterns in the chemistry of wet deposition in Southern Alps

In the last decades, in Europe a large effort was carried out to reduce sulphur and nitrogen emission in the atmosphere, in order to improve air quality and reduce the acidity of atmospheric deposition and the amount of nitrogen compounds it carries to terrestrial and aquatic ecosystems. This resulted in a sharp decrease in the deposition of sulphate and acidity, while until recently a decrease of the atmospheric load of nitrogen compounds was not evident.In this paper, we focus on the subalpine and alpine areas in North-Western Italy and Southern Switzerland (Canton Ticino), receiving high deposition of atmospheric pollutants transported from emission sources in the Po Valley, one of the most urbanised and industrialised areas of Europe. Long-term studies, covering a 30-year period (1984–2014), were carried out on the chemistry of atmospheric deposition in this area and its effects on surface water bodies through a cooperation between Swiss and Italian research institutions.A total of 14 atmospheric deposition wet-only sampling sites operate in this area, covering a wide latitudinal and altitudinal range (about 200–1900 m a.s.l.).A spatial gradient in the deposition of sulphate and nitrogen compounds was evident both in the 1990s and in recent times (2008–2012), with highest values in the south-eastern part of the area, close to the major emission sources. Deposition also varied depending on local topography.The analysis of long-term trends revealed a large proportion of significant decreasing trends in the concentration of both sulphate and nitrogen compounds. Deposition changes were less evident, due to the high interannual variability in the data, caused by the highly variable precipitation amount, ranging from 1200–1300 mm in dry years to 3000 mm in wet years.Sulphate concentrations and deposition decreased steadily since the 1980s, while ammonium and nitrate showed a widespread decrease only in the most recent period (after 2006). However, nitrogen wet deposition is still high with respect to critical loads: inorganic N deposition ranges from 60–70 meq m−2 y−1 (as the sum of ammonium and nitrate) at the alpine sites to 120–140 meq m−2 y−1 at the southern lowland sites. Deposition of ammonium has acquired an increasing importance in time, especially at the southern, more polluted sites: the relative contribution of reduced N to wet N deposition passed from about 50% in the early 1990s to 56–57% in recent years.The observed temporal changes in nitrogen deposition followed the emission decrease but with a delay. This aspect may be relevant in monitoring the effects of emission reductions as a result of the enforcements of international and national regulations.