Orographic enhancement of wet deposition in the United Kingdom: Continuous monitoring

Continuous monitoring of cloud and rain samples at three mountain sites in the UK has allowed consideration of the long term impact of the enhancement of the wet deposition of pollutants by orographie effects, specifically the scavenging of cap cloud droplets by rain falling from above (the seeder-feeder effects). The concentration of the major pollutant ions in the cloud water is related to the relative proximity of each site to marine and anthropogenic sources of aerosol. In general, the concentrations of major ions in precipitation at summit sites exceed those in precipitation to low ground nearby by 20% to 50%. Concentrations in orographie cloud exceed those in upwind rain by between a factor of five and ten. The results are consistent with seeder-feeder scavenging of hill cloud by falling precipitation in which the average concentration of ions in scavenged hill cloud exceed those in precipitation upwind by a factor of 1.7 to 2.3 for sulphate and nitrate respectively at Dunslair Heights and 1.5 to 1.8 for sulphate and nitrate at Holme Moss. The results suggest that the parameterisation of this relationship with scavenged feeder cloud water concentrations assumed to exceed those in seeder rain by a factor of two for the production of predictive maps of wet deposition in mountainous regions of the U.K. is satisfactory.     

The application of210Pb inventories in soil to measure long-term average wet deposition of pollutants in complex terrain

The radionuclide²¹⁰Pb derived from gaseous²²²Rn is present in particle form in the atmosphere attached to the same aerosols which contain the bulk of the pollutant sulphur and nitrogen. When scavenged from the atmosphere by precipitation, the²¹⁰Pb is readily attached to organic matter in the surface horizons of soil. The inventory of²¹⁰Pb in soil can be used to measure the spatial variation in wet (or cloud) deposition within a region due to orography or land use, averaged over several decades (half life of²¹⁰Pb is 22.3 years). Measurements of soil²¹⁰Pb inventories along a transect through complex terrain in north Britain were made to quantify the orographic enhancement of wet deposition, at Great Dun Fell in Cumbria. At the hill summit (～800m asl) precipitation of approximately 2000 mm year^?1 exceeds that on the low ground upwind by a factor of 2.0. The inventory of²¹⁰Pb increases along the same transect by a factor of 3.3, due to seeder-feeder scavenging of orographic cloud. The measurements show that the average ratio of concentrations in scavenged orographic cloud to rain upwind of the hills is 2.2. These data are entirely consistent with the studies of the variation in major ion concentration with altitude at Great Dun Fell and elsewhere. The modelling procedures developed to provide estimates of wet deposition throughout the UK uplands are shown to be consistent with these new field data.     

Analysis of summertime cloud water measurements made in a Southern Appalachian spruce forest

This paper presents an analysis of cloud water measurements made during the summers of 1986 and 1987 at Whitetop Mountain, Virginia (36.639° N, 81.605° W). Analysis of cloud water chemistry, cloud type, and air mass origin are made for each cloud event occurring during one 3 to 4 week measurement ‘intensive’ per year. Regional source/receptor relationships are also investigated. Cloud water concentrations of major ions (i.e., H⁺, SO₄ ^2?, NO₃ ?, and NH₄ ⁺) are consistently higher during orographically formed ‘cap’ cloud events. Differences in cloud liquid water content between cap and frontal cloud events explains most, but not all, of the cloud water ion concentration differences. The remaining difference can be explained by greater rainfall associated with frontal cloud events. Most of the cloud water sulfate measured at Whitetop Mountain is apparently due to nucleation of aerosol sulfate within cloud droplets and not to local in-cloud aqueous phase SO, oxidation. No strong source/ receptor relationships were evident from this analysis. Most 72 hr air trajectories arriving at Whitetop Mountain during the cloud events described in this paper originated in the southeastern United States. Few came from the Ohio River Valley or the northeastern United States.     

Contributions of cloud processes to precipitation chemistry in mixed phase clouds

The role clouds play as processors of atmospheric aerosols and trace gases was studied along the slope of Mt. Rigi in central Switzerland. Upon cloud formation many aerosols and trace gases are efficiently scavenged by cloud drops. The cloud drops can enhance removal of pollutants from the atmosphere by transferring them to snow or rain which falls rapidly to the ground. This often occurs through a process known as riming, where falling ice crystals capture cloud drops. When ice crystals are grown primarily via water vapor deposition, without significant capture of cloud drops, however, the cloud drops isolate atmospheric pollutants from the precipitation process, thereby inhibiting their deposition. Increased riming results in increased precipitation ion concentrations. The extent of ice crystal riming at times exhibits spatial inhomogeneities with greater riming apparent near the mountain summit. Variations in cloud chemistry with drop size indicate that bulk cloudwater composition is not an accurate predictor for the composition of cloud drops captured by the ice crystals.     

Cloudwater chemistry in the subcooled droplet regime at Mount Sonnblick (3106 M A.S.L., Salzburg,Austria)

Cloudwater and wet precipitation (snow) samples were collected at Mount Sonnblick during two field campaigns in May and November 1991. A newly designed active cloud water samples was used. Concentrations of major anions, cations and carboxylic acids were determined. Cloudwater and wet precipitation samples were generally more acidic in the warm season than in the cold season. Average cloudwater pH was 4.2 in May and 4.5 in November, average pH in snow was 4.4 in May and 5.1 in November. Average levels for sulfate (May: 96 μeq L^?1, November: 64 μeq L^?1) and nitrate (May: 27 μeq L^?1, November: 32 μeq L^?1) in cloudwater at SBO (3 km altitude) were considerably lower than at high mountain sites (0.9–2 km altitude) in the Eastern U.S.A. Cold season levels of sulfate in cloud water at SBO were as low as cloud water levels observed in Alaska. Equivalent concentrations of sulfate, nitrate and ammonium in snow precipitation were basically lower or equal compared to cloudwater but showed higher concentrations and stronger acidity in both phases in May than in November. Cloud to snow ratios for major ions were higher in November showing a wider spread than in May. Average cloud to snow ratios for sulfate were 2.4 in May and 3.5 in November. For nitrate the ratio was 1.7 in May and 2.1 in November. The lower cloud to snow ratios for nitrate are explained by the ability of the ice phase to scavenge nitric acid. Cloud to snow ratios were similar to measurements from the Swiss Alps and generally equal or lower than high elevation cloud to rain ratios from the U.S.A. Cloud to snow ratios for sulfate were used to reconstruct the mixing ratio of sublimation grown ice phase and cloud water droplets during the riming process of the ice particles in the seeder-feeder mechanism. The mixing ratio of ice phase and cloud droplets was estimated to be 1.4 in May and 2.5 in November. Sulfate to nitrate ratios were higher in cloud water than in snow and within the range of values found in North America. Generally, sulfate was more concentrated than nitrate at an equivalent basis for both cloudwater and rainwater. Total equivalent concentrations of acetate were generally higher than those of formate which is in contrast to measurements at remote high elevation sites in the U.S.A.     

Cloud Water Deposition to Forest Canopies of Cryptomeria Japonica at Mt.Rokko, Kobe, Japan

Cloud water deposition to canopies of Cryptomeria japonica at Mt.Rokko, Kobe, Japan, was estimated from throughfall measurements and fog water collections carried out during a full year. Annual cloud frequency was 11.5–15.5% and liquid water content (LWC) was 0.059 g/m³. Since cloud water deposition on to forest canopies was significantly correlated with the amount of fog water collected, the former parameter could be quantitatively derived from throughfall measurements. Annual cloud water deposition on to Cryptomeria canopies was 1420–2860 mm (Av. 2140 mm), corresponding to 90–180% (Av. 122%) of annual rainfall. The rate of deposition was higher at the mountain ridge and the forest edge than at the mountain side and the forest interior. Annual deposition of SO₄ ^2?, NO₃ ^?, H⁺ and NH₄ ⁺ from cloud water was estimated as 204, 153, 2.5 and 58 kg/ha, respectively, equivalent to 5.8–11.7 times the corresponding deposition via rain. The values are equal to, or exceed, the maximum deposition reported for Appalachian forests in the eastern United States. Multiple regression analyses indicate that cloud water deposition on to Cryptomeria canopies was significantly correlated with the following three parameters: cloud frequency, LWC, and wind speed. Thus, these three factors apparently control cloud water deposition on to forest canopies.     

ACID DEPOSITION DURING TWO CONTRASTING FRONTAL RAINFALL EVENTS

Analysis of the chemical composition of rain at high temporal resolution provides additional information on wet deposition processes. High resolution data was obtained using a microprocessor-based acid rain monitor at two sites in SW Scotland and SE England. Meteorological details of the transport and wet deposition processes during two frontal rain events were examined and related to rainfall composition. Rapid depletions of ion concentrations during heavy rainfall in the first event were interpreted using a rainfall scavenging model. The sub-event data for the second event showed the influence of frontal discontinuities. Increasing ionic concentrations during this second event were attributed both to the change in air mass, and to diminished upwind precipitation scavenging.     

Accumulation of air pollutants by a Spruce Forest in Eastern Belgium

In the course of a 20 mo sampling campaign, 29 ions and elements were determined in rain water which had been collected under trees and in clearings of a spruce forest. Comparing deposition rates between these two locations gives information as to the filtering capacity of tree foliage towards air pollutants. This filtering capacity depends on the gas reactivity and on the particle diameter while pollutant accumulation rates closely depend upon meteorological conditions, mainly rainfall and duration of dry periods. Therefore, pollutant accumulation by trees should be studied on the basis of well-defined meteorological events rather than on the basis of fixed time sampling intervals, which tend to mask the strong effect of precipitation events.     

Modelling Acidic Deposition in the United Kingdom at a Scale of 5 Km by 5 Km

This paper discusses the use of a long-term Lagrangian receptor-oriented trajectory model for assessing acidic deposition to the United Kingdom at high spatial resolution. Previously, a coarse resolution of 20 km was used. Impact assessments using the critical loads approach now require higher-resolutions of modelled data for future scenarios. In this paper, first results from a higher resolution of 5 km using a revised parameterization of orographic enhancement of the seeder-feeder effect. It was found that country budgets for the two different model resolutions were in good agreement and compared well with observations, except in the far north of Scotland where the straight-line trajectory approach is less valid.     

On processes controlling the distribution and fluxes of boron in natural waters and air along transects across the South Island of New Zealand

The concentrations of boron were measured in rain, air and soil water along transects across the Southern Alps, New Zealand, during southern autumn and early winter 1999. Estimates of wet deposition, dry deposition, and flux of boron leaving the soil in soil water are presented. At the Main Divide of the Southern Alps, strong and constant orographic effects on rain-bearing fronts resulted in low concentrations and little variation of boron in rain. Areas with less orographic influence received varying boron concentrations in the rain depending on the origin and character of the fronts. Concentrations of boron in air were greater near the sea than in the intramontane basin. High mean concentrations in air go together with large variations caused by changes of wind direction and speed during frontal passages. Air reaching the intramontane basin was depleted of boron because of washout over the eastern and western ranges and by dry deposition. After a long dry summer spell, the first autumn flush of soil water contained high concentrations of boron at low soil water flow. Later, as the soil wetted up, the soil water concentration approached that of rain water at all sites. Wet deposition was least at sites away from the coast where there was little rain and less boron in it. High rainfall with low boron concentrations or mean rainfall with high boron concentrations occurred near the eastern and western divides. Dry deposition varied over shorter time intervals and distances than wet deposition. It was high near the coastlines during onshore winds and low in the intramontane basin, especially with weak convective turbulence. The concentrations of boron in springs resembled those in rain and soil water, increasing from the western coastal plain eastwards across the Southern Alps.     

Meteorological characteristics of large acidic deposition events at Kejimkujik,Nova Scotia

The characteristics of acidic deposition at Kejimkujik National Park, a rural site in south-central Nova Scotia, were determined for the period from May 1979 to December 1983. The acidifying pollutants investigated were H⁺, sulphate ion and nitrate ion. Thirty per cent of the total annual amount of acidifying pollutants deposited by precipitation at Kejimkujik was deposited by only about 8% of the precipitation events during the year. These occasions of large deposition are referred to as episodes and occurred, on average, about five times per year, usually between March and November. The precipitation events that produced episodes were almost always rain events. The major meteorological feature producing deposition episodes at Kejimkujik is the sequence of a large high pressure area from mid-continent moving south of Nova Scotia, producing a prolonged southwesterly to westerly flow over the site, and then a weak frontal system, usually from south of the Great Lakes region, bringing moderate amounts of precipitation to the site. Since deposition episodes were almost always preceded by the above sequence of events, the ability to predict episodes was investigated. The results indicate that many “false alarms” could be anticipated because this sequence also occurs for non-episodes.     

Wet Deposition to an Upland Area of England in 1988 and 1999: Measurements and Modelling

Fine spatial-scale measurements of acidic deposition have been made over two time frames: 1987/1988 and 1999/2000. Over this period, dramatic reductions in precursor emissions have occurred. The two sets of data are compared in order to quantify deposition reductions. Orographic enhancement of wet deposition from the seeder-feeder mechanism is important in the UK at high-elevation sites such as these and the measurements were corrected for this process using a standard methodology. The data were also modelled using a simple long-term Lagrangian acid deposition model and comparisons made with both sets of observations at a resolution of 5 km_5 km. Future scenarios (2010) of emissions were modelled to examine the impact of UK and other European sources in order to determine optimal protection of ecosystems in this region.     

Quantifying Fine-Scale Variability in Pollutant Deposition in Complex Terrain using 210Pb Inventories in Soil

The accumulation of 210Pb in organic material within the surface (0 to 20 cm depth) horizons of soil is used to quantify local variability in the atmospheric inputs through wet deposition, cloud droplet deposition and dry deposition of aerosols. The method has been applied to quantify the long-term (~50 yr) average enhancement in deposition as a consequence of orographic effects on a 800 m mountain in southwest Scotland. The 210Pb inventory increases by a factor of 2.5 up the hillslope and is comparable to the modeled increase in wet deposition of major ions, and larger than the increase in rainfall with altitude by a factor of two. A second study site examined the increase in deposition beneath a Norway spruce canopy relative to open grassland at an elevation of 450 m in the Scottish Borders. The inventory of 210Pb under the forest canopy exceeded that in the grassland by approximately 35%, in good agreement with deposition estimates obtained from a continuous record of cloud frequency and meteorological variables.     

Wet and dry deposition of nutrients in Central Alberta

Dry and wet deposition rates of various forms of phosphate and N and of Fe, sulphate, Na, K, and silica ions are reported for a 1 yr period in central Alberta. The results are extrapolated from event samples of rain and snow, and from dry deposition samples in distilled water collectors and snowpack. Following corrections for contamination and evaporation the most reliable dry deposition rates are found for orthophosphate, organic and nitrate N, and Fe, sulphate, and K ions. Ion concentrations in snow are significantly lower than those in rain for ammonia, organic, and total N and for sulphate and silica ions. More than 50% of the concentrations of total phosphate and total N in wet deposition samples is dissolved or in fine particles (less than 0.45 μ) but only about one third of dry deposition sample concentrations is in such form. Dry-to-wet deposition ratios for the year exceed one for filtered total N, filtered total phosphate, Fe, and sulphate. The largest dry-to-wet deposition ratios are about 5 for sulphate and unfiltered total N.     

Preliminary analysis of atmospheric scavenging processes in the industrial region of Cubatao,Southeastern Brazil

During a German-Brazilian research project on vegetation damage, atmospheric scavenging processes were investigated, considering transport and local emissions. For this goal, radar precipitation data was evaluated together with analyses of fractionated rain samples of one stratiform and one convective rain event. The temporal variation of pollutant concentration in rain water and the meteorological conditions were compared for both rain events at two stations: one at the top of the Serra do Mar mountain ridge ( 900 m) with minor influence of local emissions and one at sea level, close to the high indutrialized area of Cubatao. At the station close to the industrial plants, major influence of the local sources on wet deposition was determined. During the stratiform rain event, an additional influence of transport could be observed. On the top of the Serra do Mar, major influence of the local sources could only be observed during the convective rain event, while transport is mainly responsible for wet deposition during the stratiform event.     

Nutrient Input Through Occult and Wet Deposition into a Subtropical Montane Cloud Forest

Chemical composition of fog and rain water was studied during a 47-day experimental period. The differences between the fog and rain water were found to be significantly for most analyzed ions. H⁺, NH₄ ⁺, NO₃ ^?, and SO₄ ^2? made up 85% of the total median ion concentration in fog and 84% in rain water. The total mean equivalent concentration was 15 times higher in the fog than in the rain water. The fog water samples were classified according to their air mass history. The analysis of the 120 h backward trajectory led to the identification of three advection regimes. Significant differences of ion concentrations between the respective classes were found. Air masses of class I travelled exclusively over the Pacific Ocean, class II were carried over the Philippines, and class III were advected from mainland China. The turbulent fog water deposition was determined by the means of the eddy covariance method. The total (turbulent plus gravitational) fog water fluxes ranged between +31.7 mg m^?2 s^?1 and ?56.6 mg m^?2 s^?1. Fog water droplets with mean diameters between 15 μm and 25 μm contributed most to the liquid water flux. The sample based nutrient input was calculated on the basis of the occult and wet deposition, and the concentrations of the simultaneously collected fog and rainwater samples, respectively. The nutrient input through wet deposition was about 13 times higher than through occult deposition.     

Geochemistry of Trace Elements in the Ground Water of Cuttack City,India

The concentration of some trace metals, like Fe, Mn, Cu, Pb, Zn, Co, Cr and Ni, was measured in the potable water of Cuttack City,India during winter, summer and rain events in the course of a two year period. Apart from Fe, all other trace metals fell well within the maximum permissible limit set by WHO for drinking water. On the basis of spatial/temporal variations, weathering of rocks and anthropogenic input were found to be main sources of trace elements in the ground water. The considerable temporal changes indicate that a majority of trace metals is entering the aquifer during rain fall.Factor analysis of the trace element data suggest that Fe, Mn and Cr are interrelated with each other and derived significantly from the aquifer, whereas input of Pb and Co may be due to atmospheric deposition in the study area.     

Monitoring of Acid Rain over Korean Peninsula

In order to investigate the temporal and spacial deposition characteristics of acid rain, five monitoring sites were set up in the central part of Korean Peninsula. Rainwater samples were collected by wet-only sampler from 1992 to 1997. The pH and conductivity values were measured and the major water soluble ionic components were also analyzed. Ion balance between anion and cation sum was calculated to check out the data quality of samples. The acidity of rainwater was influenced by the local sources and meteorological factors such as surface wind, moving path of storm, precipitation process, and so on. Backward trajectories were depicted to investigate the effect due to moving path of weather system. The rainwater shows strong acidity in winter and weak in summer. The pH and ion concentrations were highly dependent on synoptic weather system. In addition, we sampled fog/cloud at Mt. Sobaek (BAPMoN Station) from June to August 1995 and characterized the fog/cloud chemistry according to surface wind system of sampling site. The volume-weighted mean pH of fog/cloud samples is 4.39 at Mt. Sobaek during the sampling period.     

Quantification of In Situ Denitrification Rates in Groundwater Below an Arable and a Grassland System

Growing season rain and cloud events were sampled between 1984–2010 at Lakes of the Clouds (LOC) (1,540 m above sea level (ASL)) which is 1.6 km SW of the summit of Mount Washington, NH (1,914 m ASL). Mount Washington's summit is in the clouds ca. 51 % of the time. All samples were measured for pH, while cations and anions were measured consistently from 1996 to 2010. Annual mean cloud and rain water hydrogen ion concentrations declined significantly from 1984–2010. Nighttime cloud and rain hydrogen, sulfate, ammonium, and nitrate ion concentrations were significantly greater compared to daytime. Ion mean concentrations declined over the 1996–2010 timeframe and more rapidly since 2005. Co-located filter-based aerosol measurement (PM_2.5) at LOC had higher ratios of ammonium to sulfate in summer daytime samples post (1995–2010) full implementation of the 1990 Clean Air Act Amendments. This suggests a shift towards more neutralized sulfate aerosol dissolution into clouds with relatively more ammonium and declines in acidity. The origin of cloud water sampled, which ranges from regional fronts to orographic lower elevation air mass uplift, along with the diurnally shifting nocturnal boundary layer that often puts the LOC site in and out of the mixed layer, likely contributes to the diurnal and inter-annual variability observed.     

Carbon Stock, Afforestation and Acidic Deposition: An Analysis of Inter-Relation with Reference to Arid Areas

Recent advances in desert afforestation underlines its viability and importance in combating global warming and acidification. In this paper, the inter-relation between afforestation, global warming and acid rain has been analyzed. Numerical simulations indicate that afforestation of deserts has distinct advantage as carbon sink and as an important factor for changing microclimate of the region rather than a source of energy. Acidic deposition may well be utilised as fertiliser in nutrient deficit soil of tropical arid areas. However, past trends and projections of acidic deposition in arid areas adjacent to Thar deserts indicate an early efforts are required to cap the opportunity. Delays may contribute towards more incidences of failures.