Rainfall Chemistry Research Articles

Changes in rainfall chemistry and airborne particulates during a period of major local industrial change

Data from a site in central Scotland were used to quantify the changes in rainfall quality from 1989 to 1998. During this period there have been major changes in industrial activities in the area, particularly the decline in local steel-making and steel-processing activities. Many element concentrations in rainfall decreased over time in parallel with the phased reduction in the activity of local pollutant sources. Trend analyses of the rainfall data identified that the most significant responses have been the lower concentrations of Ca, SO 4S and Mn. There was also a dramatic decline in the capture of airborne particulates by the interception rainfall gauges. Particulates were found to contain mainly hematite, magnetite and quartz, that is similar to what would be expected to be derived from the neighbouring steel industries. The eventual disappearance of these particulates and the responses in rainfall quality match the timescale for the decline and closure of some of the potential sources of pollutants.

Rainfall composition in Minnesota: integrating the chemistry, synoptic meteorology and numerical modelling

Clear quantitative differences (particularly with NH 4 + and NO 3 −) were observed in the chemical composition of rain collected by an in situ, refrigerated event sampler versus using a non-refrigerated, composite, weekly sampling protocol, at a site in the upper Midwestern USA. There was a non-linear, parabolic relationship between pH and SO 4 2− or NO 3 − concentrations. Such relationships could be stratified by the occurrences of three different classes of rainfall chemistry, governed by three different types of branching air trajectories preceding them. These overall processes were influenced by source-receptor issues. A Fourier modulated, three-way parabolic spline regression model was used to explain the non-linear relationships between H + and SO 4 2− or NO 3 −. This model performed much better ( R 2: 0.48–0.61) than the linear ( R 2: 0.008–0.31) and non-modulated three-way parabolic spline ( R 2: 0.22–0.36) regression analyses. Overall, the study allowed an integration of rainfall chemistry with synoptic meteorology and numerical modelling. ©

The influence of meteorology and atmospheric transport patterns on the chemical composition of rainfall in south-east England

Rainwater composition was examined at event temporal resolution, over a 6 month period, at a site in southeast England. The data were used to assess the overall levels of acidic deposition at the site, and to identify functional relationships between wet deposition and causal meteorological processes. Rainfall-weighted average concentrations were close to those estimated from the U.K. national acid deposition network, but deposition levels were below those suggested by network data because of the unusually dry summer in 1989. The rainfall chemistry data were related to locally recorded wind direction, and to back trajectories calculated with analysed wind field data from U.K. Meteorological Office numerical weather prediction models. The influence of local wind direction during rainfall was significant in terms of observed concentrations and deposition. However back trajectory analysis was a better indicator of the Lagrangian history and pollutant loading of the air masses reaching the site, with clear differences in rainfall composition noted between different transport patterns. A power-law relationship existed between wet deposition and rainfall amount, although the data exhibited considerable scatter around the functional relationship because of the influence of transport pattern. Proportional ionic composition was also influenced by transport pattern with enhanced chloride levels for maritime events. The nitrate : sulphate ratio was inversely related to the time of travel from major anthropogenic source regions.

Development and operation of a microprocessor based wet deposition monitor

A microprocessor-based system for monitoring wet deposition is described. The system makes real-time measurements of the conductivity and pH of 0.5 mm sequential rain samples. The samples are then stored on an event or sub-event basis for subsequent laboratory analysis. A brief review of some of the problems associated with collection and storage of rainwater for chemical analysis is given in the context of the current instrument. The system was operated for a six month period at a site in southeast England. The event resolution of the device enabled detailed study of acid deposition episodes and associated air mass back trajectories. The sub-event data enabled study of wet deposition scavenging processes and advective concentration changes. Rainfall chemistry and meteorological changes associated with frontal discontinuities were well characterised by the monitor.

Cation ratios in Cladonia portentosa as indices of precipitation acidity in the British Isles.

The relationship between rainfall chemistry and the concentrations of potassium, calcium and magnesium in the apices (top 5 mm) of the cushion-forming; lichen Cladonia portentosa (Dufour) Coem was investigated. Lichen samples, together with underlying topsoil, were collected from heathlands in close proximity to rain gauges in the UK Acid Deposition Monitoring Network, located in rural areas of the British Isles, which provide wet deposition data based on weekly bulk samples. The ratios K(+) : Mg(2+) and extracellular Mg(2+) : intracellular Mg(2+) in the lichen apices were strongly correlated with H(+) concentration in precipitation. It is suggested that shifts in these ratios occur owing to enhanced displacement of extracellular Mg(2+) by elevated H(+) concentration in acid rain. By contrast, there was no indication of any relationship between total acid deposition and lichen chemistry. The concentration of Mg(2+) in the lichen was weakly correlated with that in soil, whereas lichen Ca(2+) content was not correlated with either precipitation or soil chemistry. It is concluded that these ratios describing changes in lichen Mg(2+) content provide good biomarkers for wet-deposited acidity.

Chemistry of rainfall, throughfall and stemflow in a eucalypt forest and a pine plantation in south-eastern Australia: 2. Throughfall

A rainfall event-based study allowed the examination of factors affecting the amounts and chemical concentrations of throughfall. The amount and frequency of antecedent rainfall was of influence in both forests, in that concentrations were higher with dry antecedent conditions. The debarking season influenced throughfall in the eucalypt forest. Potassium was the dominant cation in the throughfall of both forests, being 50% of the sum of the major cations (Ca, Mg, Na and K) for the pines and 70% for the eucalypts. The largest difference was in sodium, being 29% of the combined cations for the pines and 10% for the eucalypts. There was net uptake of hydrogen ions from rainfall for both forests, i.e. throughfall was less acid than rainfall, but release occurred for a significant proportion of events and was associated with higher rainfall pH. There was a small net input of NH4-N and NO3-N by the eucalypts. A more substantial input of both ions occurred in the pines. However, uptake and release of both ions occurred for different events in both forests. Throughfall samples taken during events showed that the chemical concentration was related to variations in the intensity and continuity of the rainfall, particularly for the major cations. NH4-N and NO3-N did not always follow the cation trends through time.

Chemistry of rainfall, throughfall and stemflow in a eucalypt forest and a pine plantation in south-eastern Australia: 1. Rainfall

Rainfall samples were collected from several hundred rainfall events. Up to nine samples per event were collected from sites 3–500 m apart. They differed substantially in both concentration and composition, even though great care was taken to wash all collectors beforehand. Dryfall, storage and analytical procedures could not explain the variation. When data for rainfall events of similar size (but very different cation inputs) were examined, the major differences were associated with the prevailing wind direction. Events leading to high concentrations were associated with easterly winds and showed the influence of a marine source. Chemical concentration and composition were not altered by the occurrence of a drought year followed by a very wet year. The mean pH was 5·3 and ranged from 4·6 to 5·8. For a given event, a difference of 0·5 often occurred between collection sites. Regular seasonal variations in the concentration of NO3-N and the NO3/NH4 ratio occurred, with the ratio being higher in summer due primarily to bush fires. Organic nitrogen comprised 14% of total nitrogen.

Chemistry of rainfall, throughfall and stemflow in a eucalypt forest and a pine plantation in south‐eastern Australia: 3. Stemflow and total inputs

Chemistry of rainfall, throughfall and stemflow in a eucalypt forest and a pine plantation in south‐eastern Australia: 3. Stemflow and total inputs

Chemistry of rainfall, throughfall and stemflow in a eucalypt forest and a pine plantation in south-eastern Australia: 3. Stemflow and total inputs

Some factors affecting stemflow chemistry were examined. For events of similar size, the amount and frequency of antecedent rainfall were important for both forests. Bark type was important for the eucalypts, as was the debarking season for the smooth-barked species. Correlations for tree basal area versus concentration were established for the pines and three eucalypt species. On average, stemflow was responsible for 10% of the nutrient input for the eucalypts and 11% for the pines. For the pines, the cation composition is dominated by Na and K. Eucalypt stemflow is dominated by K. The relative inputs of inorganic nitrogen and the major cations are presented for the pines and four eucalypt species. There was a net uptake of NH4-N by the pines and a release of NO3-N. For the eucalypts, both were released by all species, except for E. melliodora which absorbed NO3-N. E. rossii dominated nitrogen release. Apart from E. melliodora, the pH of stemflow of all eucalypt species and pines was less than that of rainfall. The pH values for E. melliodora and rainfall were similar. The chemical concentration of stemflow samples taken during events was greatly influenced by the intensity and continuity of the rainfall, and different eucalypt species were influenced in different ways. The relative chemical inputs of rainfall, throughfall and stemflow of the principal cations, Ca, Mg, Na, and K, were very variable, even between events of similar total rainfall. The total cation average inputs in g ha−1 mm−1 of rainfall were: for the eucalypts, rainfall 10·7 (34%), throughfall 17·4 (55%) and stemflow 3·4 (11%); and for the pines, rainfall 10·7 (32%), throughfall 18·4 (55%) and stemflow 4·7 (14%). Thus, on average, throughfall makes the major contribution, but in individual events of similar size, rainfall input can be anything from 25 to 78% of the total for the eucalypts and 10 to 54% for the pines. There is also a substantial variation in the cation composition of all three components. It was possible to make a reasonable assessment of the probable throughfall and stemflow inputs by considering antecedent conditions, season, and certain event characteristics such as the intensity and rain angle. For the eucalypt forest, the summer debarking of the smooth-barked species affected the concentration and volume of both throughfall and stemflow. For rainfall however, assessment of likely inputs was not possible except for some marine-influenced easterly events.

Mechanisms controlling the water chemistry of small lakes in Northern Ireland

Gibbs' model of world water chemistry has been adopted as a standard. In the present study, we demonstrate that its applicability to upland lakes is limited due to the modification of water chemistry by catchment processes, in particular, the contribution of rock weathering to sodium ion concentration. In Northern Ireland, 615 lakes between 1 and 100 hectares surface area were sampled and analysed in the summers of 1988–1990. Data from these lakes are used to derive a modified version of Gibbs' model, which accounts for local rainfall chemistry and the contribution of rock weathering or other sources to sodium ion concentration.

Soilwater chemistry in a holm oak ( Quercus ilex) forest: inferences on biogeochemical processes for a montane-Mediterranean area

Soil solution and free-flowing soilwater were sampled at various depths for 3 years in a plot of holm oak ( Quercus ilex L.) in the Montseny mountains (NE Spain). The soil solution retained at −65M kPa in the mineral soil at depths of 20 and 40 cm had a different chemistry from that of throughflow under the humic layer (H-layer throughflow) and, to a lesser extent, from deep subsurface flow. The dominant mobile anion in the soil solution and the deep flow was SO 4 2−, whereas in the H-layer, SO 4 2− was overridden by alkalinity. H-Layer throughflow chemistry was extremely variable, in consequence of the large number of biogeochemical processes affecting it (e.g. rainfall chemistry, dry deposition, leaching from canopy and litter, decomposition), and of the quantity of water available for transport of solutes. The chemistry of the soil solution was more predictable, as it was governed mainly by nutrient uptake, cation exchange reactions and the seasonal wetting and drying cycles. The chemistry of the deep subsurface flow was often intermediate between that of the soil solution and that of the H-layer throughflow; this suggested a mixture of displaced pre-event soil solution and of H-layer throughflow circulating through preferential flow paths. With humid antecedent conditions, the chemistry of the deep subsurface flow approached that of the soil solution. The cation concentration relationships in the soil solution were strongly linear. In a homogeneous soil, cation exchange theory predicts this to be expected only for cations of the same charge, whereas cations of different charges should follow power relationships. Our results, however, are consistent with a theoretical approach involving cation exchange reactions in a highly heterogeneous environment. Indeed, our study illustrates the heterogeneous nature of the soils, as the power relationship has been obscured completely. Soilwater chemistries were markedly different from those of streamwater, particularly under dry conditions. During baseflow, the stream is fed by groundwater, and the soils are probably disconnected from the stream. During humid periods, the chemistry of the streamwater tends toward that of the deep subsurface flow. Chemical hydrograph separation indicates that, on average, stormflow water comprises an approximately one to one mixture of groundwater and deep subsurface flow.

Linking variations in short- and medium-term stream chemistry to rainfall inputs — some observations at Plynlimon, Mid-Wales

Continuously monitored stream water conductivity and pH from the Hafren and Hore streams at Plynlimon, Mid-Wales, is examined. Large inputs of rainwater of high conductivity can be traced to the stream. For events producing low flows (less than about 1 mm h −1), the chemistry of the rainfall is not observed in the stream; for high flow events the rainfall chemistry influences not only the immediate stream response but also the conductivity-flow relationship in subsequent smaller storms. The data indicate that a strong differentiation of water types is present, even after the catchment has been inundated, and that evapotranspiration and dry deposition are important factors in the maintenance of conductivity gradients within the catchment. Comparison of the acidity (pH/Acid Neutralisation Capacity) with the conductivity shows that the acidity in the rainfall is removed by the catchment soils: whereas the rainfall conductivity may affect the stream conductivity directly there is no corresponding relationship for acidity. Rapid chemical reactions, such as ion exchange, control stream acidity but do not affect the conductivity of the stream waters to the same extent.

Rainfall and runoff chemistry of Florida pine flatwoods

Rainfall chemistry was monitored for 10 yr and correlated with runoff chemistry of an undisturbed 140 ha pine-cypress flatwoods watershed in Florida. Volume-weighted rainfall averaged 4.8 pH units and decreased by 0.013 units yr−1, which was not statistically significant. The seasonal variation of pH showed a minimum during the summer months. Regression analyses demonstrated that rainfall acidity was weakly correlated with nitrate-N and sulphate-S. Significant but slight annual decreases were found for Na, Ca and K levels in rainfall; these were due to large fluctuations occurring during the early years of the study. The average volume-weighted monthly runoff acidity was nearly one pH unit less than in rainfall and remained constant over time. Levels of nitrate-N and phosphate-P showed a significant decrease while Ca showed a significant increase in runoff over time. The pine-cypress flatwoods ecosystem appeared to absorb most of the H+, N and P, while losing only a little of the bases.

Studies on the effects of air pollution on limestone degradation in Great Britain

The CEGB and the Cathedrals Advisory Commission for England formed a Joint Working Party in 1985 to promote a research programme aimed at improving the understanding of the relationships between stone decay, atmospheric pollution and other factors. The programme has included exposure of limestone samples at York Minster and eight other sites in England and Scotland selected to give a mix of urban, marine and rural locations. All of the sites have comprehensive air pollution and meteorological monitoring and measurement of rainfall chemistry. At two sites samples have been fumigated with controlled levels of sulphur dioxide. Over all sites, there was a significant trend to increased weight loss with increase in average sulphur dioxide concentration, but a negative trend with total nitrogen oxides and with nitrogen dioxide. For sample exposures longer than 200 days, the sulphur dioxide dependence at the inland Liphook fumigation site was about half that found near the coast at Littlehampton. There was no significant trend to increase weight loss with total rainfall amount for the complete data set, but the analysis was dominated by the very wet Scottish site, which experienced the lowest average concentrations of air pollutants. A theoretical model for the chemical dissolution of rainwashed limestone has been derived from consideration of the ion and mass balances between the incident rain water and run-off water. The model has been fitted to the measured loss rates from the stonework field trials. With the exception of the very wet Scottish site, the difference between the stone loss rate, calculated from the model, and the mean measured loss rate for any particular exposure was generally smaller than the variation between the triplicate samples. Variation in the dry deposition velocity between sites and exposure periods does not appear to have been a very significant factor, and no residual effect due to the concentrations of nitrogen oxides was found. The natural solubility of limestone in water was the dominant term in describing the stone loss, and neutralization of the rainfall acidity the least significant. The volume of the intercepted rainfall and the variation in the pH of the run-off water with rainfall intensity have been identified as the two most significant terms which require more precise quantification. The data from the inland fumigation site used in the model predict a stone loss due to sulphur dioxide in the air of less than 1 μm yr −1 surface recession per ppb SO 2.

Three-year growth responses of Pinus taeda L. to simulated rain chemistry, soil magnesium status, and ozone

Height, diameter, and biomass were measured for loblolly pine (Pinus taeda L) seedlings grown in soil containing 15 or 35 Μg Mg g−1 and exposed from May to October in 1987, 1988, and 1989 to three O3 concentrations (sub-ambient, ambient, or twice-ambient) and to rain pH levels of 3.8 or 5.2. Reduction in biomass accumulation in seedlings exposed to twice-ambient O3vs sub-ambient O3 was 14% (P = 0.03) in 1987, 11.4% (P = 0.002) by 1988, and 8% (P = 0.15) by 1989. The greatest height growth occurred in seedlings exposed to twice-ambient O3, and the greatest stem diameter growth occurred in seedlings exposed to sub-ambient O3. A comparison of stem volume (d2h) with stem biomass suggested that tissue density was reduced by elevated O3. Biomass accumulation response to rainfall chemistry was small (5.5% reduction in the low pH treatment in 1989) and not statistically significant for most plant tissues. Growth response to soil Mg status was not significant. Hoewever, in 1989 treatment interactions between rainfall chemistry and soil Mg status were observed. Height was 5% greater (P = 0.02) and biomass was 6% greater (P = 0.10) in seedlings grown in higher-Mg soil and receiving higher-pH rainfall than seedlings grown in any of the other pH-Mg treatment combinations. The data suggest direct adverse effects of near ambient O3 and indirect, slower acting and interacting adverse effects of rainfall chemistry and soil nutrient status on growth of loblolly pine.

On the interpretation of event and sub-event rainfall chemistry

Variations in precipitation chemistry between and within rain events have been examined in order to identify possible relationships with synoptic, mesoscale and micrometeorological processes. A microprocessor-based acid rain monitor was used to provide high resolution meteorological and rain chemistry data from which two case study events have been selected to illustrate event and sub-event rainfall chemistry characteristics. Event rainfall chemistry is strongly influenced by the history of the prevailing air mass and the synoptic situation. From back trajectories calculated at the 950 mbar level it is clear that air mass history can change markedly within a few hours. These observations emphasise the value of high resolution rainfall chemistry measurements. Pollutant concentrations in rainwater have been shown to fluctuate markedly within the course of individual events as a result of both advective and scavenging processes. Advective effects may result from: (a) air mass discontinuities at frontal zones; and/or (b) variable rainfall interception of the air mass prior to arrival at the site. A simple mathematical model has been developed to describe the scavenging mechanisms and it shows good agreement with field observations. Theoretical considerations suggest that in-cloud processes give rise to most of the observed decline in concentrations.

Anthropogenic and natural contributions to the rainfall chemistry of a mountainous area in the Cevennes National Park (Mont-Lozere, southern France)

Nine years of bulk precipitation chemistry data for a mountainous site in southern France are presented. The mean rainwater is of moderate acidity (pH 4.63, alkalinity −4 μEql −1) even though there is high, non-marine, deposition of sulphur (near 20 kg ha −1 year −1). Based on the statistical characteristics of the data set, three major sources of elements are identified: long range pollution, Mediterranean sea salt aerosols and aeolian erosion dust, mainly from the Sahara. Most of the sulphur in the rainfall is present as neutral sulphate salts, owing to the latter being rich in alkaline minerals; this counteracts the strong acid input.

Microbiology and chemistry of acid lakes in Florida: II. Seasonal relationships

To determine relationships of chemistry and microbiology in lakes exposed to acid precipitation, three clear and two dark acid lakes on the Katharine Ordway Preserve near Gainesville, Florida were monitored from February 1986 to July 1987. Chemical measurements included Dissolved Organic Carbon (DOC), pH, acidity, temperature, dissolved oxygen, and nutrients. Bacterial densities, bacterial biomass, bacterial growth, bacterial assimilation of glucose, protozoan densities, and chlorophyll a were also measured. Rainfall and rainfall chemistry data were obtained from the Bradford Forest Acid Deposition station approximately 34 kilometers NNW of the preserve.

Atmospheric Deposition and Foliar Leaching in a Regenerating Southern Appalachian Forest Canopy

SUMMARY (1) Incident precipitation, throughfall and stemflow were collected to examine the importance of factors potentially determining net canopy element fluxes, and to quantify canopy exchange and dry deposition rates in a regenerating southern Appalachian forest. (2) Net throughfall fluxes (throughfall minus precipitation transfers) showed consistent canopy effects on rainfall chemistry, with SO42~, PO43~, Cl~, K+, Ca2+ and Mg2+ added to rainfall by foliage, whereas NO3~-N, NH4+-N and H+ ions were absorbed from precipitation. Storm characteristics (event amount and duration) accounted for the largest portion of the variability in growing season net throughfall fluxes, suggesting that canopy exchange was the major mechanism of throughfall transfer. (3) Stemflow fluxes increased canopy exchange rates of SO42~, PO4?~, Cl~, K+, and Mg2+ by greater than 20% in a regression model of total below-canopy element fluxes. (4) Cation leaching fluxes were highly variable (C.V.>50%) over spatial scales of several m2, but could be explained largely by heterogeneity in canopy cover. (5) Foliar cation leaching losses in the early successional forest accounted for 4—13% of leaf nutrient reserves. As cation throughfall transfers were highest during storms with the greatest hydrogen ion uptake from rainwater, it is hypothesized that acid precipitation is causing accelerated foliar nutrient leaching in south-eastern hardwood forests.

Assessment of wet deposition mechanisms in an upland Scottish catchment

A network of collectors were installed at various altitudes and degrees of exposure in the Allt a Mharcaidh catchment, northeast Scotland, in an attempt to obtain an accurate assessment of wet deposition loading. Results indicate that the quantity and quality of bulk deposition is constant over the whole catchment. “Enhancement deposition” as measured by a filter-gauge interception collector indicated that there was greatest deposition at altitude. The concentrations of all elements, except for hydrogen, were greater than that of catchment bulk deposition at the higher altitudes; at lower altitude enrichment was only appreciable for sodium and chloride. Input/output chloride budgets were used to assess catchment evapotranspiration rates and the relative proportions of enhancement deposition within different altitudinal ranges. The calculation gives a catchment evapotranspiration of 18.5% and a chloride enhancement deposition 2.5 times greater at higher altitudes than at lower altitudes. Rainfall chemistry in this high-level Cairngorm catchment appears independent of the positioning of the rainfall collectors. Different altitudes within the catchment receive an additional loading due to enhancement deposition, dependent upon the frequency of cloud/mist cover. This additional loading must be included in the assessment of total catchment loadings and in the calculation of evapotranspiration.