Dry Deposition Inputs Research Articles

The effects of canopy alteration–induced atmospheric deposition changes on stream chemistry in Japanese cedar forest

Changes in atmospheric deposition have long-term effects on forest ecosystems and streamwater chemistry. Atmospheric deposition changes with forest disturbance. Although many studies have investigated the effects of air pollution, the effects of the long-term changes in atmospheric deposition induced by canopy alteration on streamwater chemistry have not been investigated sufficiently. This study evaluated the changes in atmospheric deposition, separated into dry deposition and canopy leaching components, in a regrowth forest and the resultant effects on streamwater chemistry. The chemical budgets of Cl−, NO3−, SO42−, Na+, NH4+, K+, Mg2+, and Ca2+ were investigated in regrowth and mature Japanese cedar forests from 1998 to 2015 in paired catchments in the Fukuroyamasawa Watershed, central Japan. After forest cutting in 1999, the total deposition of all ions to forest soil decreased; the Cl−, NO3−, SO42−, Na+, and Mg2+ were significantly lower in the regrowth forest than in the mature forest at 14– 15 years due to lower dry deposition, although the canopy had closed. In comparison, the total deposition of K+ and Ca2+ increased with canopy regrowth and was higher in the 15-year-old regrowth forest than the mature forest due to increased canopy leaching. Stream outputs of NO3−, SO42−, K+, Mg2+, and Ca2+ were not affected by the changes in atmospheric deposition, but the stream outputs of Cl− and Na+ decreased gradually for 5–10 years and remained lower (70–90%) at 15 years than those in the mature forest, which is consistent with the lower dry deposition input. The results of this study suggest that not only the canopy alteration but also the edge effects of taller trees of adjacent catchments influence the dry deposition input change, and the long-term streamwater chemistry change in Japanese cedar regrowth forest.

Spatial Distribution and Seasonal Variation of Organophosphate Esters in Air above the Bohai and Yellow Seas, China.

Nine organophosphate esters (OPEs) were investigated in air samples collected over the Bohai and Yellow Seas (East Asia) during a research cruise between June 28 and July 13, 2016. These same OPEs were quantified at a research site (North Huangcheng Island, NHI) in the middle of the Bohai Strait from May 16, 2015, to March 21, 2016. The median total OPE (ΣOPE) concentration over the Bohai and Yellow Seas was 280 pg/m3. Tris(1-chloro-2-propyl) (TCPP) was the most abundant OPE, followed by tris(2-chloroethyl) phosphate (TCEP), tri-iso-butyl phosphate (TiBP), and tri-n-butyl phosphate (TnBP). Particle-bound OPEs accounted for 51 ± 21% of the total OPEs. On NHI, the median ΣOPE concentration was 210 pg/m3, and the average particle-bound fraction was 82 ± 17%. For samples collected on NHI, significant negative linear correlations were found between the gaseous OPEs and 1/T (T: temperature (K)) (except TDCP, TPeP, and TCP). Among the 79 investigated samples, significant correlations between the measured OPE gas/particle partitioning coefficients (Kp,m) and subcooled liquid pressure (PL◦) (p < 0.05) were found for only 14 samples, suggesting that OPEs have low potential to achieve equilibrium or ascribe to the artificial sampling. The annual dry deposition input of OPEs into the Bohai and Yellow Seas is estimated to be 12 tons/year.

Seasonal Patterns of Dry Deposition at a High‐Elevation Site in the Colorado Rocky Mountains

AbstractIn the Colorado Rocky Mountains, high‐elevation barren soils are deficient in carbon (C) and phosphorus (P) and enriched in nitrogen (N). The seasonal variability of dry deposition and its contributions to alpine elemental budgets is critical to understanding how dry deposition influences biogeochemical cycling in high‐elevation environments. In this 2 year study, we evaluated dry and wet deposition inputs to the Niwot Ridge Long Term Ecological Research (NWT LTER) site in the Colorado Rocky Mountains. The total organic C flux in wet + dry (including soluble and particulate C) deposition was &gt;30 kg C ha−1 yr−1 and represents a substantial input for this C‐limited environment. Our side‐by‐side comparison of dry deposition collectors with and without marble insert indicated that the insert improved retention of dry deposition by ~28%. Annual average dry deposition fluxes of water‐soluble organic carbon (4.25 kg C ha−1 yr−1) and other water‐soluble constituents, including ammonium (0.16 kg NH4+ha−1 yr−1), nitrate (1.99 kg NO3− ha−1 yr−1), phosphate (0.08 kg PO43− ha−1 yr−1), and sulfate (1.20 kg SO42− ha−1 yr−1), were comparable to those in wet deposition, with highest values measured in the summer. Backward trajectory analyses implicate air masses passing through the arid west and Four Corners, USA, as dominant source areas for dry deposition, especially in spring months. Synchronous temporal patterns of deposition observed at the NWT LTER site and a distant Rocky Mountain National Park Clean Air Status and Trends Network site indicate that seasonal dry deposition patterns are regional phenomena with important implications for the larger Rocky Mountain region.

Comparison of mercury mass loading in streams to atmospheric deposition in watersheds of Western North America: Evidence for non-atmospheric mercury sources

Annual stream loads of mercury (Hg) and inputs of wet and dry atmospheric Hg deposition to the landscape were investigated in watersheds of the Western United States and the Canadian-Alaskan Arctic. Mercury concentration and discharge data from flow gauging stations were used to compute annual mass loads with regression models. Measured wet and modeled dry deposition were compared to annual stream loads to compute ratios of Hg stream load to total Hg atmospheric deposition. Watershed land uses or cover included mining, undeveloped, urbanized, and mixed. Of 27 watersheds that were investigated, 15 had some degree of mining, either of Hg or precious metals (gold or silver), where Hg was used in the amalgamation process. Stream loads in excess of annual Hg atmospheric deposition (ratio>1) were observed in watersheds containing Hg mines and in relatively small and medium-sized watersheds with gold or silver mines, however, larger watersheds containing gold or silver mines, some of which also contain large dams that trap sediment, were sometimes associated with lower load ratios (<0.2). In the non-Arctic regions, watersheds with natural vegetation tended to have low ratios of stream load to Hg deposition (<0.1), whereas urbanized areas had higher ratios (0.34–1.0) because of impervious surfaces. This indicated that, in ecosystems with natural vegetation, Hg is retained in the soil and may be transported subsequently to streams as a result of erosion or in association with dissolved organic carbon. Arctic watersheds (Mackenzie and Yukon Rivers) had a relatively elevated ratio of stream load to atmospheric deposition (0.27 and 0.74), possibly because of melting glaciers or permafrost releasing previously stored Hg to the streams. Overall, our research highlights the important role of watershed characteristics in determining whether a landscape is a net source of Hg or a net sink of atmospheric Hg.

Nutrient and mercury deposition and storage in an alpine snowpack of the Sierra Nevada, USA

Abstract. Biweekly snowpack core samples were collected at seven sites along two elevation gradients in the Tahoe Basin during two consecutive snow years to evaluate total wintertime snowpack accumulation of nutrients and pollutants in a high-elevation watershed of the Sierra Nevada. Additional sampling of wet deposition and detailed snow pit profiles were conducted the following year to compare wet deposition to snowpack storage and assess the vertical dynamics of snowpack nitrogen, phosphorus, and mercury. Results show that, on average, organic N comprised 48% of all snowpack N, while nitrate (NO3--N) and TAN (total ammonia nitrogen) made up 25 and 27%, respectively. Snowpack NO3--N concentrations were relatively uniform across sampling sites over the sampling seasons and showed little difference between seasonal wet deposition and integrated snow pit concentrations. These patterns are in agreement with previous studies that identify wet deposition as the dominant source of wintertime NO3--N deposition. However, vertical snow pit profiles showed highly variable concentrations of NO3--N within the snowpack indicative of additional deposition and in-snowpack dynamics. Unlike NO3--N, snowpack TAN doubled towards the end of winter, which we attribute to a strong dry deposition component which was particularly pronounced in late winter and spring. Organic N concentrations in the snowpack were highly variable (from 35 to 70%) and showed no clear temporal, spatial, or vertical trends throughout the season. Integrated snowpack organic N concentrations were up to 2.5 times higher than seasonal wet deposition, likely due to microbial immobilization of inorganic N as evident by coinciding increases in organic N and decreases in inorganic N in deeper, aged snow. Spatial and temporal deposition patterns of snowpack P were consistent with particulate-bound dry deposition inputs and strong impacts from in-basin sources causing up to 6 times greater enrichment at urban locations compared to remote sites. Snowpack Hg showed little temporal variability and was dominated by particulate-bound forms (78% on average). Dissolved Hg concentrations were consistently lower in snowpack than in wet deposition, which we attribute to photochemically driven gaseous re-emission. In agreement with this pattern is a significant positive relationship between snowpack Hg and elevation, attributed to a combination of increased snow accumulation at higher elevations causing limited light penetration and lower photochemical re-emission losses in deeper, higher-elevation snowpack. Finally, estimates of basin-wide loading based on spatially extrapolated concentrations and a satellite-based snow water equivalent reconstruction model identify snowpack chemical loading from atmospheric deposition as a substantial source of nutrients and pollutants to the Lake Tahoe Basin, accounting for 113 t of N, 9.3 t of P, and 1.2 kg of Hg each year.

Occurrence and dry deposition of organophosphate esters in atmospheric particles over the northern South China Sea

Nine organophosphate esters (OPEs) in airborne particles were measured during a cruise campaign over the northern South China Sea (SCS) from September to October 2013. The concentration of the total OPEs (∑OPEs) was 47.1–160.9pgm−3, which are lower than previous measurements in marine atmosphere environments. Higher OPE concentrations were observed in terrestrially influenced samples, suggesting that OPE concentrations were significantly influenced by air mass transport. Chlorinated OPEs were the dominant OPEs, accounting for 65.8–83.7% of the ∑OPEs. Tris-(2-chloroethyl) phosphate (TCEP) was the predominant OPE compound in the samples (45.0±12.1%), followed by tris-(1-chloro-2-propyl) phosphates (TCPPs) (28.8±8.9%). Dry particle-bound deposition fluxes ranged from 8.2 to 27.8ngm−2d−1 for the ∑OPEs. Moreover, the dry deposition input of the ∑OPEs was estimated to be 4.98tony−1 in 2013 in a vast area of northern SCS. About half of the input was found to relate to air masses originating from China.

Half-century nitrogen deposition increase across China: A gridded time-series data set for regional environmental assessments

A wide variety of studies have revealed a substantial increase in nitrogen (N) deposition in China, but the lack of spatially-explicit time-series N deposition data set has long hindered us from assessing the impacts of atmospheric N input on ecosystem services. In this study, we combined site-level monitoring, gridded precipitation data and atmospheric transport modeling results to generate annual N bulk deposition data in China with a spatial resolution of 10 km × 10 km and a time span from 1961 to 2008. It shows that national average N deposition rate had large interannual variation, and it increased by 59%, from 12.64 kg N ha−1 yr−1 in the 1960s to 20.07 kg N ha−1 yr−1 in the recent decade, with the most rapid increase centered in the southeastern China that is already N-enriched. Large spatial variation as well as dry deposition input has to be taken into account when estimating the amount of N deposited onto land surface of China. The spatial and temporal information on N deposition derived from this study could be used by ecosystem, hydrological, and climate modeling as well as by policy makers for assessing the impacts of nitrogen enrichment on regional climate, water resources, and biogeochemical cycles.

Inertia in an ombrotrophic bog ecosystem in response to 9 years' realistic perturbation by wet deposition of nitrogen, separated by form

Wet deposition of nitrogen (N) occurs in oxidized (nitrate) and reduced (ammonium) forms. Whether one form drives vegetation change more than the other is widely debated, as field evidence has been lacking. We are manipulating N form in wet deposition to an ombrotrophic bog, Whim (Scottish Borders), and here report nine years of results. Ammonium and nitrate were provided in rainwater spray as NH4 Cl or NaNO3 at 8, 24 or 56kgNha(-1) yr(-1) , plus a rainwater only control, via an automated system coupled to site meteorology. Detrimental N effects were observed in sensitive nonvascular plant species, with higher cumulative N loads leading to more damage at lower annual doses. Cover responses to N addition, both in relation to form and dose, were species specific and mostly dependent on N dose. Some species were generally indifferent to N form and dose, while others were dose sensitive. Calluna vulgaris showed a preference for higher N doses as ammonium N and Hypnum jutlandicum for nitrate N. However, after 9years, the magnitude of change from wet deposited N on overall species cover is small, indicating only a slow decline in key species. Nitrogen treatment effects on soil N availability were likewise small and rarely correlated with species cover. Ammonium caused most N accumulation and damage to sensitive species at lower N loads, but toxic effects also occurred with nitrate. However, because different species respond differently to N form, setting of ecosystem level critical loads by N form is challenging. We recommend implementing the lowest value of the critical load range where communities include sensitive nonvascular plants and where ammonium dominates wet deposition chemistry. In the context of parallel assessment at the same site, N treatments for wet deposition showed overall much smaller effects than corresponding inputs of dry deposition as ammonia.

Tracing industrial sulfur contributions to atmospheric sulfate deposition in the Athabasca oil sands region, Alberta, Canada

Anthropogenic S emissions in the Athabasca oil sands region (AOSR) in Alberta, Canada, affect SO4 deposition in close vicinity of industrial emitters. Between May 2008 and May 2009, SO4-S deposition was monitored using open field bulk collectors at 15 sites and throughfall collectors at 14 sites at distances between 3 and 113 km from one of the major emission stacks in the AOSR. At forested plots >90 km from the operations, SO4 deposition was ∼1.4 kg SO4-S ha−1 yr−1 for bulk deposition and ∼3.3 kg SO4-S ha−1 yr−1 for throughfall deposition. Throughfall SO4 deposition rates in the AOSR exceeded bulk deposition rates at all sites by a factor of 2–3, indicating significant inputs of dry deposition especially under forest canopies. Both bulk and throughfall SO4 deposition rates were elevated within 29 km distance of the industrial operations with deposition rates as high as 11.7 kg SO4-S ha−1 yr−1 for bulk deposition and 39.2 kg SO4-S ha−1 yr−1 for throughfall at industrial sites. Sulfur isotope ratio measurements of atmospheric SO4 deposited in the AOSR revealed that at a few selected locations 34S-depleted SO4, likely derived from H2S emissions from tailing ponds contributes to local atmospheric SO4 deposition. In general, however, δ34S values of SO4 deposition at distant forested plots (>74 km) with low deposition rates were not isotopically different from δ34S values at sites with high deposition rates in the AOSR and are, therefore, not suitable to determine industrial S contributions. However, O isotope ratios of atmospheric SO4 in bulk and throughfall deposition in the AOSR showed a distinct trend of decreasing δ18O-SO4 values with increasing SO4 deposition rates allowing quantification of industrial contributions to atmospheric SO4 deposition. Two-end-member mixing calculations revealed that open field bulk SO4 deposition especially at industrial sites in close proximity (<29 km) to the operations is significantly (17–59%) affected by industrial S emissions and that throughfall generally contained 49–100% SO4 of industrial origin. Hence, it is suggested that δ18O values of SO4 may constitute a suitable tracer for quantifying industrial contributions to atmospheric SO4 deposition in the AOSR.

Sources of gaseous oxidized mercury and mercury dry deposition at two southeastern U.S. sites

Wet deposition measurements have shown that relative to other parts of the US, the southeastern region has the highest mercury (Hg) inputs. The source of this Hg has been investigated by multiple researchers and is suggested to be derived from local, regional and global sources. Here we focus on trying to understand potential sources of Hg to this area during periods dominated by dry gaseous oxidized mercury (GOM) deposition. Dry deposition of GOM to a surrogate surface was measured in conjunction with speciated atmospheric Hg, and ancillary parameters from September 2007 through September 2008 at two sites located within 25 km of coal-fired power plants (CFPPs) near Yorkville, GA, and Pensacola, FL. Mean weekly GOM dry deposition, daily GOM, and daily sulfur dioxide (SO2) concentrations were significantly (P < 0.01) higher at the Yorkville site by factors of 1.5, 2.0, and 1.8, respectively. At both sites, GOM and SO2 concentrations were significantly correlated (P < 0.05) in every season on hourly and daily time scales. Wind rose diagrams showed significantly enhanced GOM and SO2 concentrations when air moved to the sites from the direction of the nearest CFPPs. Most periods of enhanced GOM concentrations ([GOM] > 98th percentile), were also associated with NOy/SO2 ratios that were within 25% of that reported for the local CFPPs (N = 27 of 33 at Yorkville, N = 18 of 26 at Pensacola). During these events, termed Category 1, mean GOM/SO2 enhancement ratios were 2.4 ± 0.1 and 2.3 ± 0.1 pg m−3 ppb−1 for Yorkville and Pensacola, respectively (range = 0.5 to 5.5). The remaining events at both sites (termed Category 2) displayed significantly lower SO2 concentrations, yet GOM concentrations were not significantly different compared to Category 1 events. The potential sources of GOM during the Category 2 events at OLF were investigated using gridded frequency distributions (GFD) of 72-h atmospheric back trajectories. During these periods there was a greater component of air mass transport from the free troposphere, and less precipitation along the trajectory paths compared to GFDs for Category 1 events. GFDs developed for the weeks when GOM dry deposition was in the upper quartile at both sites simultaneously revealed a similar pattern to the GFDs of Category 2 GOM concentration events, that is, greater free tropospheric transport and relatively little precipitation. Although dry deposition inputs are thought to represent <15% of total annual Hg deposition in this region, we suggest that a significant portion of this Hg could be derived from sources outside the local area.

Comparison between pure-water- and seawater-soluble nutrient concentrations of aerosols from the Gulf of Aqaba

Seawater-soluble nutrient fractions of aerosols better represent the contribution of aerosol dry deposition to the nutrient load from this source to the ocean than any estimates based on aerosol nutrients leached in pure water or acidic solutions. To understand the solubility difference between seawater and pure water, 31 pairs of aerosol samples collected from the Gulf of Aqaba were extracted in Sargasso seawater and pure water under consistent experimental conditions and procedures. Major inorganic N species (NO 3 − and NH 4 +) in the aerosols show similar solubilities in Sargasso seawater (average 44 and 23 nmol m − 3 ) and pure water (average 41 and 23 nmol m − 3 ). Seawater-soluble PO 4 3− concentrations (average 0.4 nmol m − 3 ) are slightly lower than the purewater concentrations (average 0.5 nmol m − 3 ). Total soluble N and P, which include dissolved organic compounds, extracted from the aerosols into the seawater (average 65 and 0.4 nmol m − 3 ) are significantly lower than those extracted by pure water (average 75 and 0.7 nmol m − 3 ). It was found that the dissolution of crustal-dominated trace metals (e.g. Fe and Al) strongly decrease in the seawater compared to that in pure water, while similar amounts of aerosol Zn are leached in both seawater and pure water. The percentage solubilities of non-crustal trace metals (Cu, Ni and Zn) are about one or two orders of magnitude higher than those of Fe and Al in the seawater. Our comparison experiments suggest that some previous reports may have overestimated the dry deposition inputs of aerosol P, Fe, Al, Cu, and Ni to the ocean as a result of the use of solubility estimates obtained from pure water extractions. The estimated dry deposition fluxes of soluble nutrients showed that the atmospheric nutrient input could increase the possibility of P limitation in the Gulf of Aqaba and also contribute a significant fraction of dissolved nutrients to the euphotic zone during stratification period (April to October).

NITROGEN FLUXES IN THROUGHFALL AND LITTERFALL IN TWO NOTHOFAGUS FORESTS IN SOUTHERN CHILE

Nitrogen return by leaf litterfall was compared with atmospheric nitrogen deposition for two deciduous Nothofagus stands in southern Chile, located in areas with contrasting land use. The litterfall return of nitrogen in a Nothofagus alpina (Poepp. et Endl.) Oerst. stand at San Pablo de Tregua, in the Cordillera de los Andes, was similar to the litterfall return in a Nothofagus obliqua (Mirb.) Oerst. stand at Paillaco, in the Central Depression. In contrast, the net throughfall and stemflow deposition differed significantly between the two stands. At San Pablo de Tregua, the annual bulk deposition of NH4+ and NO3- was significantly higher than the throughfall and stemflow deposition reaching the forest floor. This demonstrates an uptake of inorganic nitrogen by the aboveground biomass, which was correlated with month-to-month variation in the bulk inorganic nitrogen deposition by precipitation (r2 > 0.73). At Paillaco, the canopy uptake of nitrogen was not directly detectable due to an estimated dry deposition input of 4-8 kg inorganic N ha-1 y-1. In both stands, nitrogen return through leaf litterfall (~ 50 kg N ha-1 y-1) was much higher than the atmospheric N deposition load (< 10 kg N ha-1 y-1). The results consequently confirm the low level of atmospheric pollution in the forests of this region. However, the clear difference in net throughfall deposition of inorganic nitrogen between the two stands suggests that the external N input by dry deposition may be considerably higher in the Central depression than in Cordillera de los Andes. More emphasis on monitoring dry atmospheric N deposition is a necessary tool for evaluating the consequences of future emission changes

Dry deposition of trace elements to the western North Atlantic

Data for Al, Fe, Na, Sb, Se V, and Zn in aerosol particles from Bermuda are used in combination with parameterized dry deposition velocities to investigate trace element recycling on atmospheric sea salt. During July and August, which are in the high dust season, &gt;10% of the monthly mean aerosol Na can be ascribed to crustal sources rather than sea salt, but in general the contribution of mineral dust to Na dry deposition is small (∼1%) relative to that from sea salt. In fall and winter, ∼10 to 20% of the aerosol Al, a commonly used indicator of mineral dust, is attributable to recycled sea salt, but the dust fluxes then are small, and longer‐term averages show a recycling effect of only 1 to 5% on dry deposition. The percentage of recycled Fe appears to be even less than for Al. Noncrustal/non‐sea‐salt (NC/NSS) sources account for &gt;70% of the aerosol Sb, Se, V, and Zn, but differences in the dry deposition velocities for three particle types strongly affect their air/sea exchange. Dust can account for more than half of the new dry deposition inputs of V and Zn while the amounts of Sb and Se recycled on sea spray approach or exceed their new inputs from dust and the NSS/NC sources.

Nitrogen transformations in a forested catchment in southern Norway subjected to elevated temperature and CO 2

Model predictions on the response of soil processes to global warming are mostly inferred from small-scale laboratory studies. In this study, a forested catchment in southern Norway was enclosed by a greenhouse and experimentally manipulated by increasing CO 2 (+200 μl l −1 above ambient) and temperature (+3–5°C). This paper reports on the effects of the climate manipulation on N mineralization and nitrification. We measured net N mineralization and nitrification in a control and treated part of the greenhouse as well as in an uncovered reference catchment in plots dominated by Calluna vulgaris (L.) Hull or Vaccinium myrtillus L. Net N mineralization in the 0–10 cm soil layer significantly increased, most likely as a result of increased temperature. The effect was largest in plots dominated by Calluna. Nitrification did not significantly increase. Soil moisture inside the incubated cores was not affected by the climate change treatment. Pre-treatment mineralization was similar inside and outside the enclosure whereas nitrification was higher inside the enclosure. The NH 4 + content was significantly lower inside the chamber due to removal of acidifying components from the precipitation and lower inputs of dry deposition. We found however no differences in pH, %C and %N of the LF and H layer and total C and N in the soil cores between the two catchments. Mineralization was generally higher under Vaccinium than under Calluna even though measured soil chemical and physical characteristics were similar. Nitrification was higher under Calluna than under Vaccinium.

Seasonal sulfate deposition and export patterns for a small Appalachian watershed

Sulfate deposition and exports from 1988–92 were analyzed for a small headwater catchment in north-central West Virginia. Annual sulfate inputs, estimated by applying throughfall-adjusted ratios to bulk deposition values, and outputs were approximately equal for the five years. Annual mean throughfall-adjusted deposition and export loads were 55.78 and 55.48 kg ha-1, respectively. While these results indicate the watershed has reached sulfate equilibrium relative to current deposition levels, seasonal sulfate accumulations and deficits were evident. Deposition and exports averaged 5.61 and 2.49 kg ha-1 mo-1, respectively, during the growing season, and 3.69 and 5.22 kg ha-1 mo-1 during the dormant season. Sulfur accumulated within the soil during the growing season because inputs of wet and dry sulfur deposition were high while outputs were negligible. The latter was due largely to the lack of runoff resulting from high evapotranspirational demands. By contrast, net sulfate losses occurred during dormant seasons, primarily due to high runoff, even though inputs declined during this season. Researchers working on other watersheds have interpreted similar input/output patterns to mean that sulfate accumulated during the growing season is the source of sulfate exported during the dormant season. However, radioisotopic evidence from a companion study on this watershed showed that some labeled sulfate applied to the watershed more than a year earlier was still present in the organic and mineral soil layers at the point of application (with some as soluble sulfate), and in soil water dispersed throughout the watershed. Its presence indicates that dormant season exports can originate from sulfate deposited over longer periods than just the previous growing season or even previous year. Volume-weighted concentrations in soil leachate suggest that dormant-season sulfate losses resulted from progressive depletion of the anion through the soil profile. During the fall and early winter, soluble sulfate was depleted in the upper soil horizons; in later winter, depletion occurred in the lower horizons.

Polychlorinated biphenyls in a rural watershed in the southern prairie region of Canada

Polychlorinated biphenyl (PCB) concentration patterns were determined in air, precipitation and surface water in a small agricultural watershed (70 km 2) in the Canadian prairies, with no known local source of these compounds. Air temperature was related to PCB concentrations for lower chlorinated congeners and air mass back-trajectories indicated that elevated levels in ambient air were observed when air moved into the region from the south. Dissolved phase ΣPCB concentrations ranged from 20 to 6350 pg/liter, air concentrations ranged from 1.4 to 751 pg/m 3 and concentrations in precipitation ranged from 88 to 9703 pg/liter. The stagnant layer (two-film) model and the surface renewal models were used to estimate PCB volatilization during both 1994 and 1995. The surface renewal model predicted greater volatilization losses than the two-film model, although the surface renewal model has been shown to under-predict gas exchange in small turbulent streams. This suggests that for a small stream, the two-film model drastically under-predicts gas exchange estimates. There was overall net deposition of PCBs to the South Tobacco Creek when precipitation and dry deposition inputs to the entire watershed were included. Similar levels of PCBs are exported from this watershed by both volatilization and stream discharge.

Regional mass budgets of oxidized and reduced nitrogen and their relative contribution to the nitrogen inputs of sensitive ecosystems

Wet deposition of nitrogen is reasonably well monitored throughout Europe, whereas the dry deposition inputs are provided largely by models. Recent long-term measurements of NO 2 and NH 3 fluxes to semi-natural vegetation have shown that rates of NH 3 deposition exceed those of NO 2, typically by an order of magnitude. Incorporating the results of these dry deposition measurements in regional deposition budgets shows that the inputs of reduced nitrogen contribute the dominant fraction of the total nitrogen inputs in most regions of the UK. The results are illustrated by comparing the atmospheric mass-budget for oxidized nitrogen over the UK. Of the annual UK emissions of NO x, amounting to 780 kt N (Salway et al., 1997), only 5% is dry deposited to terrestrial surfaces within the country while 15% is wet deposited, whereas for the reduced nitrogen, 42% of emissions (of the 260 kt N, Salway et al., 1997) are dry deposited and 46% are wet deposited. Even more striking are the relative contributions of oxidized and reduced nitrogen to semi-natural vegetation, which is a particularly efficient sink for NH 3. The species composition of semi-natural vegetation is also regarded as very sensitive to nitrogen inputs. The distribution of nitrogen deposition among different land uses shows that the average input to forest in the UK is 33 kg N ha −1 annually of which 78% is reduced nitrogen. The other land uses receive about 15 kg N ha −1 of nitrogen of which between 55% and 65% is NH x. Critical loads for nutrient nitrogen are exceeded primarily in forested and moorland areas as a consequence of NH 3 dry deposition and wet NH 4 + deposition. For forests the area in exceedance of 20 kg N ha −1 year −1 critical load represents 70% of the forest area (1.4 × 10 6 ha −1) while for moorland the area in exceedance is 13% of the moorland area and occupies 1.04 × 10 6 ha −1).

Deposition of nitrogen and other major inorganic compounds in Norway, 1992–1996

Representative measurements of air and precipitation chemistry have been used to estimate atmospheric deposition for the period 1992–1996. In coastal areas of SW Norway, large precipitation amounts with high concentrations of nitrate and ammonium resulted in annual nitrogen (N) inputs of more than 2 g m −2year −1 (20 kg ha −1year −1). Dry deposition is relatively unimportant in the areas with high precipitation intensities, whereas in some areas with less precipitation, forested areas may receive up to 30–50% of the total deposition in the form of dry deposition. Inputs of dry deposition to non-forested areas are much smaller, particularly during the period of snow cover. Comparison with similar estimates for the periods 1983–1987 and 1988–1992 show that there has been a decrease in the deposition of reduced N, by 2% and 14%, and in the deposition of oxidized N, by 6% and by 23%, relative to the two periods. The trend for oxidized N is somewhat smaller than expected based on reported emission reductions in Northern Europe. Changes in the emission rates from off-shore activities in the North Sea may partly explain the observed discrepancies. For ammonium, the trend may be influenced by both reduced emissions and by the reductions of sulphur dioxide and nitrogen oxides, which may have reduced the rate of conversion from ammonia to ammonium aerosol, thereby decreasing the atmospheric residence time and the transport across the North Sea. Inter annual variations in the meteorological conditions may also have influenced the amounts of deposition.

Sulfur and nitrogen budgets for five forested appalachian plateau basins

Sulfur and nitrogen input‐output budgets were estimated for five forested Appalachian Plateau basins in Pennsylvania for the period October 1988 to March 1990. Wet and dry deposition inputs were determined on a weekly basis from data collected at atmospheric deposition monitoring stations located near the study sites. Stream export was estimated from intensively sampled stream chemistry and continuous discharge data collected on all five basins. On four of the five basins, deposited sulfur was essentially in balance with stream flow export of sulfur (92‐120% exported) for the 1989 water year. The fifth basin had net retention of deposited sulfur, with only 42% exported. All five basins retained the vast majority of deposited nitrogen (only 3‐18% exported). The fraction of atmospherically deposited sulfur exported in stream flow was greater by a mean factor of 14 versus nitrogen, implying that sulfur dominates base cation leaching processes on these non-carbonate-based catchments. Although basins in the study were relatively homogeneous in terms of topography, climate, geology and land use, local basin conditions caused significant diAerences in input‐output budgets, pointing to the need for replicated basin studies in a region. #1997 by John Wiley & Sons, Ltd.

Throughfall deposition of ammonium and sulphate during ammonia fumigation of a scots pine forest

The estimation of the dry deposition of sulphur dioxide to forests is confounded by the possibility of codeposition of SO 2 with NH 3 on leaf surfaces. A sector of Scots pine forest was selectively fumigated with NH 3 to give average concentrations up to 15 ppbV (nL L -1 ) above ambient, in order to test the hypothesis that increased air concentrations of NH 3 would enhance the dry deposition of SO 2 , and the consequent amounts of SO 4 2- measured in throughfall below the forest canopy. Ammonia gas, generated by evaporation of concentrated aqueous solution, was released above the canopy in proportion to wind speed when the wind direction was between south and west. Concentrations of NH 3 at canopy height were measured using passive diffusion tubes; throughfall was preserved with thymol and measured weekly. Meteorological data and SO 2 concentrations were recorded continuously, to permit the estimation of dry deposition input. Deposition of NH 4 + in throughfall over 8 months was increased by up to 40 meq m -2 relative to 'control' sites upwind of the NH3 release point, with largest values closest to the release point. Deposition of SO 4 2- in throughfall was also enhanced in the fumigated area, by up to 20 meq m -2 , even though average ambient SO 2 concentrations were 2.3 ppbV. The results are discussed in terms of the factors controlling SO 2 deposition on forest surfaces, the development of appropriate deposition models, and their relevance to using throughfall as an estimate of total S deposition.