Cloud Water pH Research Articles

Solubilisation of aerosol trace metals by cloud processing: A laboratory study

The atmosphere has now been recognised as a major source of both trace metals and nutrients to the oceans, with wet deposition being a major contributor to the flux. Solution pH has been suggested to be the major control on metal solubility in rainwater, but for many trace metals this relationship is not a simple one. Aerosols are typically exposed to ~10 condensation/evaporation cloud cycles before removal in rain and, as a result of H 2SO 4 and HNO 3 uptake and SO 2 oxidation, cloudwater pH can be very low. Laboratory studies have been conducted, using Saharan and Urban aerosols, to assess the effect of pH on trace metal solubility. The results for the crustal elements Al and Fe in the Saharan aerosol show that metal solubility is a strong function of pH with highest solubilities seen under low pH conditions, comparable to those found in clouds. Increasing the pH to simulate neutralisation of acidity by ammonia and crustal material results in almost complete removal of these elements from the solution phase, suggesting that a simple relationship between pH and solubility should exist in rainwater. For Al and Fe in the Urban aerosol there is evidence that some of the material solubilised at low pH is kept in solution at high pH, perhaps as the result of complexation by organic matter. Manganese shows high solubility after the initial acidification from both the Saharan and Urban materials with only slight removal from solution at increased pH. For this element it appears, therefore, that the pH-dependent dissolution process is not reversible. It is tentatively suggested that Fe in the Urban aerosol, under these experimental conditions, is under solubility control.

The Relative Importance of Oxidation Pathways and Clouds to Atmospheric Ambient Sulfate Production as Predicted by the Regional Acid Deposition Model

The development and use of a version of the Regional Acid Deposition Model/Engineering Model (RADM/FM) called the Comprehensive Sulfate Tracking Model (COMSTM) is reported. The COMSTM is used to diagnose the relative contributions of each sulfate production pathway to the total atmospheric ambient sulfate predicted by RADM. Thirty meteorological cases are used to aggregate the results into annual estimates. For the operational RADM (denoted RADM 2.6), nonprecipitating cloud production of ambient sulfate dominates over precipitating cloud production, and the hydrogen peroxide pathway dominates over four other aqueous formation routes. Gas-phase production of sulfate contributes ten than 40% of the ambient budget. Further, the COMSTM is used to explore the sensitivity of the RADM's cloud water and rainwater pH's and ambient sulfate predictions to uncertainties in the ammonia emissions inventory. By developing correction floors to improve in-cloud and deposited ammonia, and utilizing them in the COMSTM, it is shown that the uncertainties should have a minimal effect on predicted cloud water and rainwater pH's and on the overall ambient sulfate budget in the operational PADM 2.6.

A Ten-Winter Record of Cloud-Droplet Physical and Chemical Properties at a Mountaintop Site in Colorado

Abstract Cloud-droplet spectra and coincident cloud water pH measurements have been made for a portion of ten consecutive winters (1983/84–1992/93) from clouds that enveloped Storm Peak Laboratory in northwestern Colorado; cloud water ion measurements were made for eight of the winters. To determine if the physical and chemical properties are related, the data were stratified into three populations: pH ≤3.6, 3.6 < pH < 4.6, pH ≥4.6. It was found that clouds with the smallest pH values (3.4) had the largest droplet concentrations (N 329 cm−3), smallest mean droplet diameters (Dbar = 6.4 µm), and largest ion concentrations (e.g., SO44 = 5.7 mg L−1), while clouds with the largest pH values (5.1) had the smallest N values (189 cm−3), largest Dbar values (8.0 µm), and smallest ion concentrations (SO44 = 3.9 mg L−1). Nevertheless, all three populations had similar liquid water contents (LWC ≅ 0.070 g m−3). The equation LWC = π/66Dbar3Nρ where ρ is the density of water, closely describes the relationship between...

Techniques for pollution monitoring in remote sites: III. Near real-time monitoring of cloud water conductivity and pH

A semi-portable flow-through cloud water monitoring system was developed for measurements of cloud water conductivity and pH in remote sites lacking AC line power. This system was tested from May to September on Camels Hump mountain, Vermont. High temporal resolution data from seven cloud events were collected during the 1991 growing season. Mean cloud water conductivity and pH for all events was 467 μmhos cm−1 and 3.2, respectively. The highest conductivity was 997 (μmhos cm−1 recorded on 19 September 1991 and the lowest pH of 2.9 was recorded during several events over the summer. Data from this system may be used to achieve a better understanding of the chemical environment in areas experiencing forest decline.

Chemical Dynamics of Clouds at Mt. Mitchell, North Carolina

The role of clouds as the primary pathway for deposition of air pollutants into ecosystems has recently acquired much attention. Moreover, the acidity of clouds is highly variable over short periods of time. Cloud water collections were made at Mt. Mitchell State Park, North Carolina, using a real-time cloud and rain acidity/ conductivity (CRAC) analyzer during May to September 1987, 1988 and 1989 in an effort to explore extremes of chemical exposure. On the average, the mountain peak was exposed to cloud episodes about 70 percent of experimental days. The lowest pH of cloud water in nearly real-time (∼10 min.) samples was 2.4, while that in hourly integrated samples was 2.6. The cloud pH during short cloud events (mean pH 3.1), whjch results from the orographic lifting mechanism, was lower than that during long cloud events (mean pH 3.5), which are associated with mesoscale or synoptic atmospheric disturbances. On the average, the pH values in nonprecipitating cloud events were about 0.4 pH unit lower th...

Regional analysis of cloud chemistry at high elevations in the eastern United States

Results from the collection and chemical analysis of cloudwater samples collected from May to October 1986–1988 from the five high-elevation ( ⪆950 m MSL ) Mountain Cloud Chemistry Program (MCCP) sites (Whiteface Mountain, NY; Mt Moosilauke, NH; Shenandoah Park, VA; Whitetop Mountain, VA; Mt Mitchell, NC) in the eastern United States are summarized. The resulting database documents the regional chemical climatology of high-elevation forest ecosystems in the eastern U.S. Clouds occured at these sites on 32–77% of the days during the sample collection period. More than 90% of cloud samples were acidic ( prmpH<5.0). The lowest cloudwater pH (2.29 integrated 1-h collection period) was recorded at Mt Mitchell, NC. At all sites sulfate and nitrate were the dominant anions and hydrogen and ammonium were the dominant cations in cloudwater samples. Mount Mitchell received the most acidic clouds and highest chemical exposures, while the Whiteface summit site received the least acidic and lowest chemical exposures compared to other MCCP high-elevation sites. Cloud pH and major chemical components exhibited a seasonal trend with the maxima during the summer months, and correlated well with temperature and ozone concentrations. The mean equivalent ratios of SO 4 2− to NO 3 − were found to be 1.9–3.9 at these sites. It is noted that SO 4 2− correlated highly with hydrogen ion, suggesting that contribution to cloud acidity by sulfate and/or its precursors may be significant.

The Characterization of Extreme Episodes of Wet and Dry Deposition of Pollutants on an Above Cloud-Base Forest during its Growing Season

Abstract An analysis of a 3-yr database (1986–88) acquired new Mount Mitchell (35°44′05″N, 82°17′15″W, 2038 m MSL) where the forest consists primarily of Fraser fir and some red spruce stands is presented. The site was immersed in clouds for 28%–41% of the time during each of the three growing seasons (15 May–15 September). This study only investigated extreme episodes of wet (cloud-water pH% ≤.3:1)and dry (eg., an ozone concentration ≥ 70 ppb) acidic deposition. Extreme wet events occasionally relieved periods of high ozone (≥ 70 ppb) exposures during the final field intensive. Extreme wet and dry events could activate the decline mechanism in any above cloud-base forest, especially if the trees are exposed to such events during very early or very late stages of their Lives. The exposure of the forest to natural climatic stress, such as drought condition wintertime temperatures during the growing season, snow storm during early spring, etc., would also subject the forest to a stressful period during whic...

SO2 oxidation in an entraining cloud model with explicit microphysics

A model of the chemical evolution of the droplets in a hill-cap cloud is presented. The chemistry of individual droplets forming on cloud condensation nuclei of differing size and chemical composition is considered, and the take-up of species from the gas phase by the droplets is treated explicitly for the droplet population. Oxidation of S(IV) dissolved in cloud droplets is assumed to be dominated by hydrogen peroxide and ozone. Hydrogen peroxide is normally found to be the dominant oxidant for the oxidation of sulphur dioxide (except in the presence of substantial concentrations of ammonia gas, which increases droplet pH and the contribution made by the oxidant ozone). The entrainment of hydrogen peroxide from above the cloud top increases the amount of sulphate produced in conditions where the reaction is otherwise oxidant limited by the availability hydrogen peroxide. These conditions occur when there are high concentrations of sulphur dioxide accompanied by low cloudwater pH values. Within droplets formed on sodium chloride aerosol, reduced levels of acidity lead to an increase in sulphate production as a result of an enhanced reaction between SO2 and the oxidant ozone. This results in an overall higher increase in cloudwater sulphate than would be expected assuming an even distribution of all reactants amongst the droplets. In addition, concentrations of the hydrogen sulphite ion predicted to occur in the cloudwater can be substantially in excess of those predicted from the bulk cloudwater pH. This is consistent with recent observations.

A field study of the oxidation of SO2 in cloud

This paper describes the execution and analysis of one of a series of field experiments performed at Great Dun Fell in the northern Pennines of England designed to study the oxidation of SO2 in cloud. The experiments are designed to isolate the effect on cloud chemistry by artificially releasing SO2 gas into the air entering cloud base. Observations made in cloud on April 30, 1986, clearly and unequivocally demonstrated the production of sulphate ions in cloudwater. A simultaneous decrease in H2O2 concentration demonstrated that about 40% of this arises from rapid reaction between H2O2 and SO2. The results of a comprehensive model of the cloud physical and chemical development at the site show good agreement with the observations, and indicate that the remaining fraction of sulphate produced resulted from the reaction of O3 with SO2 in cloudwater, promoted by the relatively high pH found in the cloud. Extrapolation of the results to other input gaseous concentrations by means of the model demonstrates the importance of gaseous ammonia in controlling cloudwater pH and hence the oxidation of SO2 by O3, in that it is found that over a wide range of SO2 concentrations, the total sulphate produced by the time air reaches a given point in the cloud is virtually independent of the initial SO2 concentration, and roughly proportional to the ambient ammonia concentration.

Temporal and Vertical Distribution of Acidity and Ionic Composition in Clouds: Comparison between Modeling Results and Observations

Cyclic temporal variations of pH and ionic concentration in sampled clouds which traversed the Mt. Mitchell State Park site (35°44′05″N, 82°17′15″W, 2006 m MSL) during the summers of 1986, 1987 and 1988 are reported. These clouds typically had a measured pH minimum during their initial and final stages. The cause of this basic cyclic pattern is attributed to sampling at different vertical levels of the cloud. This is substantiated by visual observations made while sampling. Our results also suggest that the measured pH patterns do not always exhibit minima during the formative and dissipative stages of the cloud, apparently in response to the underlying dynamical processes. The relationship between temporal pH measurements made at a stationary site to vertical cloud levels provides insights into the physical processes (e.g., nucleation scavenging near cloudbase, dry air entrainment near cloudtop) influencing the observed cloudwater chemistry on a real-time basis and would improve cloud chemistry models. The determination of the vertical profiles of acidity and of the position within a cloud is feasible but sometimes impractical. Hence, due to its relatively more cost effectiveness and versatility, a simple cloud chemistry model which explicitly estimated the vertical acidity profile of a cloud was sought to simulate the temporal acidity measurements at our site. A few of our event-averaged cloudwater pH values are compared with the results of such a cloud chemistry model and the two are found in excellent agreement. For the cloud event of 19 August 1987, the model predicted an average pH value of 3.03 compared with the average measured value of 3.09. The results of such comparisons between the observed and computed pH are discussed, and it is pointed out that the model could be used on an operational basis to predict the pH of cloudwater that would impact upon the montane forest. Such estimates could be strategically used to improve the long-term forest health either by protecting the new plant growth from such acidic deposition, or by cloud deacidification through cloud seeding.

Cloud chemistry measurements and estimates of acidic deposition on an above cloudbase coniferous forest

The wet, dry and cloud water deposition of acidic substances on the forest canopy are considered as major mechanisms for pollutant induced forest decline at high elevations. Direct cloud capture plays a predominant role of intercepting acidic substances in above cloud-base forests. We conducted a field study at Mt. Mitchell, North Carolina (35°44′05″N, 82°17′15″W; 2038 m MSL)—the highest peak in the eastern U.S.—during May–September 1986 and 1987 in order to analyze the chemistry of clouds in which the red spruce and Fraser fir stands stay immersed. It was found that Mt. Mitchell was exposed to cloud episodes 71% of summer days, the cloud immersion time being 28% for 1986 (a record drought summer in southeastern U.S.) and 41% for 1987. Sulfate, NO 3 − , NH 4 + and H + ions were found to be the major constituents of the cloud water, which was collected atop a 16.5 m tall meteorological tower situated among 6–7 m tall Fraser fir trees. The initiation of precipitation in clouds invariably diluted the cloud water acidity. The cloud water pH during short episodes (8 h duration or less), which resulted from the orographic lifting mechanisms, was substantially lower than that during long episodes, which were associated with meso-scale and synoptic-scale disturbances. Sulfate accounted for 65% acidity in cloud water, on the average, and contributed 2–3 times more than the NO 3 − . Inferential micrometeorological models were used to determine deposition of SO 4 2− and NO 3 − on the forest canopy and the hydrological input due to direct cloud capture mechanism. The cloud water deposition ranged between 32 and 55 cm a −1 in contrast to the bulk precipitation which was about 130 cm a −1 as measured by an on-site NADP (National Atmospheric Deposition Program) collector. For S compounds, wet, dry and cloud water deposition accounted for 19%, 11% and 70%, respectively for 1986, and 16%, 8% and 76%, respectively for 1987. For N compounds, dry deposition contributed 35% and 23% for 1986 and 1987, respectively, whereas, cloud water deposition contributed 50% and 65% for 1986 and 1987, respectively. Our estimates are compared with the reported literature values for the other sites.

The chemical composition of intercepted cloudwater in the Sierra Nevada

The chemical composition of cloudwater in the Sierra Nevada is dominated by NO 3 −, SO 4 2−, and NH 4 +. Cloudwater pH is determined largely by the balance between the concentrations of these three species, although inputs of formic and acetic acid also are believed to be important, particularly when anthropogenic inputs are small. Cloudwater samples collected in Sequoia National Park (SNP) exhibited pH values ranging from 3.9 to 6.5; Yosemite National Park (YNP) cloudwater samples had pH values ranging from 3.8 to 5.2. Samples collected at YNP were more acidic than those collected at SNP. The difference in pH between the two regions appears to be due to relatively small differences in inputs of NO 3 −, SO 4 2−, and NH 4 +. In the absence of inputs of NH 3, cloudwater pH values in the Sierra may fall below 3. Over 250 h of cloud interception were observed during a 12 month period at a cloud monitoring site at 1856 m elevaton in SNP. Estimates of cloudwater deposition of NO 3 −, SO 4 2−, and NH 4 + indicate that cloud interception contributes significantly to regional acid deposition for closed forest canopies. Cloud interception may be the dominant deposition mechanism for isolated conifers and ridgetop canopies, where wind speeds are higher and cloudy air parcels can impact directly on foliar surfaces.

The effects of microphysical parameterization on model predictions of sulfate production in clouds

Model predictions of sulfate production by an explicit cloud chemistry parameterization are compared with corresponding predictions by a bulk chemistry model. Under conditions of high SO2 and H2O2, the various model predictions are in reasonable agreement. For conditions of low H2O2, the explicit microphysical model predicts sulfate production as much as 30 times higher than the bulk model, though more commonly the difference is of the order of a factor of 3. The differences arise because of the size-dependent distribution of pH in the droplets, which is in turn a consequence of the size-dependent distributions of sulfate and ammonium ions. Experimental evidence for differing size-dependent distributions of ammonium and sulfate ions is reviewed and compared with model predictions. The results of the sulfate production comparison suggest that bulk cloud water pH is not always a reliable indicator of aqueous reaction conditions in clouds. Related to this, the ozone oxidation of aqueous S(IV) appears to be of more significance than would be suggested by pH measurements of bulk cloud water.

Numerical study of droplet size dependent chemistry in oceanic, wintertime stratus cloud at southern mid-latitudes

Cloud droplet chemistry is modelled for the first 150 m of rise in a wintertime, mid-latitude, marine stratus cloud using observations made at and near the Cape Grim Baseline Station as a source of input parameters. The emphasis in this work was to study the variation in droplet chemistry as a function of both droplet size and nucleus composition, with a particular focus on the way in which oxidation of dissolved sulfur dioxide varied. At 150 m above the condensation level, solute concentration as a function of droplet size was found to increase by as much as 2 to 3 orders of magnitude for only a factor of 2 increase in droplet radius, primarily as a consequence of the 1/r dependence in the droplet growth equation. This type of size dependence exists at all levels in the model cloud, and has a significant influence on oxidation rate of sulfur dioxide in droplets growing on ‘sulfate’ nuclei, oxidation by ozone being favoured in the smallest droplets, but oxidation by hydrogen peroxide being favoured in the larger droplets. Oxidation by ozone is favoured at all sizes in droplets formed on sea-salt nuclei as a result of the initially high alkalinity of these droplets, and in the cloud overall is calculated to be the more important oxidation pathway. Although based on a simplified chemical scheme, these results suggest that both size-dependent and nucleus-dependent chemistry of cloud droplets may need to be considered explicitly in cloud modelling work. Volume-weighted mean pH values in the range 5 to 6 were predicted from sensitivity studies in which input variables were varied over reasonable ranges, in agreement with two sets of bulk cloud-water pH data obtained by aircraft near Cape Grim.

Field studies of the SO2/aqueous S(IV) equilibrium in clouds

Concentrations of S(IV) were measured in cloudwater at Great Dun Fell and compared with theoretical HSO3− assuming equilibrium between aqueous and gaseous phases in cloud. Detectable concentrations of S(IV) in the range of 1 × 10−6 to 17.2 × 10−6 mol dm−3 were observed only in samples which contained low H2O2 concentrations, generally <1 × 10−6 mol dm−3. Concentrations of S(IV) were below the detection limit of 1 × 10−6 mol dm−3 in samples which contained hign H2O2 levels (1 × 10−6−80 × 10−6 mol dm−3) confirming that either SO2 or H2O2 acts as the limiting reagent in the oxidation of SO2 in cloudwater. Equilibrium HSO3− concentrations were estimated from the measured cloudwater pH, the gas phase SO2 concentration and the ambient temperature and found to be on average about 5 times lower than the measured S(IV) concentrations. The possible role of formaldehyde in stabilizing S(IV) in cloudwater is discussed. The kinetic data available in the literature suggest that the complexation reaction between S(IV) and HCHO is too slow to account for the observed difference between measured and calculated S(IV) concentrations over the typical lifetime of clouds in our study. S(IV) accounted for up to 10% of the SO42− measured in stored cloudwater samples.

Chemistry of a polluted cloudy boundary layer

A one‐dimensional photochemical model for cloud‐topped boundary layers is developed which includes detailed descriptions of gas‐phase and aqueous‐phase chemistry, and of the radiation field in and below cloud. The model is used to interpret the accumulation of pollutants observed over Bakersfield, California, during a wintertime stagnation episode with low stratus. The main features of the observations are well simulated; in particular, sulfate accumulates progressively over the course of the episode due to sustained aqueous‐phase oxidation of SO2 in the stratus cloud. The major source of sulfate is the reaction S(IV) + Fe(III), provided that this reaction proceeds by a non radical mechanism in which Fe(III) is not reduced. A radical mechanism with SO3− and Fe(II) as immediate products would quench sulfate production because of depletion of Fe(III). The model results suggest that the non radical mechanism is more consistent with observations, although this result follows from the absence of a rapid Fe(II) oxidation pathway in the model. Even with the non‐radical mechanism, most of the soluble iron is present as Fe(II) because Fe(III) is rapidly reduced by O2−. The S(IV) + Fe(III) reaction provides the principal source of H2O2 in the model; photochemical production of H2O2 from HO2 or O2(−I) is slow because HO2 is depleted by high levels of NOx. The aqueous‐phase reaction S(IV) + OH initiates a radical‐assisted S(IV) oxidation chain but we find that the chain is not propagated due to efficient termination by SO4− + Cl− followed by Cl + H2O. A major uncertainty attached to that result is that the reactivities of S(IV)‐carbonyl adducts with radical oxidants are unknown. The chain could be efficiently propagated, with high sulfate yields, if the S(IV)‐carbonyl adducts were involved in chain propagation. A remarkable feature of the observations, which is well reproduced by the model, is the close balance between total atmospheric concentrations of acids and bases. We argue that this balance reflects the control of sulfate production by NH3, which follows from the pH dependence of the S(IV) + Fe(III) reaction. Such a balance should be a general characteristic of polluted environments where aqueous‐phase oxidation of SO2 is the main source of acidity. At night, the acidity of the cloud approaches a steady state between NH3 emissions and H2SO4 production by the S(IV) + Fe(III) reaction. A steady state analysis suggests that [H+] at night should be proportional to (ESO2/ENH3)1/2 where ESO2 and ENH3 are emission rates of SO2 and NH3, respectively. From this analysis it appears that cloud water pH values below 3 are unlikely to occur in the Bakersfield atmosphere during the nighttime hours. Very high acidities could, however, be achieved in the daytime because of photochemical acid production by the gas‐phase reactions NO2 + OH and SO2 + OH.

Monitoring the chemical climate of the Mt. Mitchell State Park for evaluation of its impact on forest decline

During the last decade, a new type of forest decline has become increasingly apparent, especially at high elevations, in north America and western Europe. One of the causes for this decline could be anthropogenic air pollutants, the deposition of which is facilitated by natural causes at high elevations such as strong windfield, direct cloud capture by the forest canopy, high frequency of frosting and riming etc. At Mt. Mitchell, North Carolina (35°44′05″N, 82°17′15″W) which is the highest peak (2,038 m MSL) in the eastern United States, the decline of red spruce and Fraser fir forest is noticeable above the cloud-base which is frequently observed around 1585 m MSL. In order to quantitatively assess the chemical climate of Mt. Mitchell State Park, we erected a 16.5 m tall aluminium walk-up tower and instrumented it with an electronic weather station, cloud water collectors and ozone analyzers. Our observations during summer 1986 confirm that Mt. Mitchell has a high frequency of being immersed in clouds. During early morning hours, cloudiness is maximum in frequency and results in short duration cloud events ranging from 2-8 h. Although the average pH of precipitation was about 4.4, the pH of cloud water ranged between 2.2–5.4. Short-duration cloud events (8 h or less) were more acidic, in general, than the long duration ones during which the cloud water was most acidic at the onset and dissipative stages. Both types of event were characterized by highly variable liquid water content (LWC) which was lower for the short events. The pH of the cloud water was demonstrated to be a function of wind direction at the site. For the average wind direction of 210°, the pH was 3.4. During early summer episodes, neutralization of the acidic anions (SO‾‾ 4 and NO‾ 3 ) was not as effective as in the later summer episodes. During all cloud episodes, ozone concentration registered a dramatic decrease. The average minimum ozone concentration of about 63 ppb during the whole summer of 1986 was above the background level (50 ppb) in the United States. Diurnal ozone variation revealed a nocturnal maximum of about 75 ppb after sunset. This is contrary to conventional midday maximum in ozone concentration reported in the literature. Episodic excursions in ozone concentration equal to or exceeding 100 ppb level were found on 5 occasions. The maximum cloud water deposition rate, which is estimated using a micrometeorological model, is found to be 1.30 mm h -1 . Ionic deposition on the forest canopy is found to be similar to that reported for the Mt. Moosilauke (New Hampshire) forest. Ionic deposition due to direct cloud capture is found to be 2 to 5 times the deposition due to precipitation, and the evaporation associated with wind speeds as high as 15-18 m s -1 realized during some episodes is pointed out to be of great significance regarding the canopy exposure. DOI: 10.1111/j.1600-0889.1989.tb00128.x

Hydrological and chemical input to fir trees from rain and clouds during a 1-month study at Clingmans Peak, NC

Abstract Wet deposition inputs were measured at Clingmans Peak, NC, during August–September of 1986. Rain, cloud water and throughfall were collected in an attempt to determine total wet inputs to this high elevation spruce-fir ecosystem. Previous reports have suggested that cloud interception provides as much moisture to the canopy as rain. In this study the cloud interception rate was determined based on the throughfall volume collected beneath the trees, after accounting for losses in the canopy. The cloud interception rate in the canopy was only 0.5 mm water day −1 compared to a rainfall rate of 6.8 mm water day −1 . The pH of cloud water ranged from 3.1 to 5.1 compared to a range of 4.1–5.2 for rain water. Other species were 6–33 times more concentrated in clouds than rain water. Throughfall had higher levels of Ca 2+ , Mg 2+ , K + and SO 4 2− than the sum of cloud and rain inputs, but lower levels of NH 4 + and NO 3 − , indicating N uptake by the canopy. It has been suggested that the trees on Clingmans Peak are receiving large doses of pollutants due to heavy rainfall, high cloud interception and high ion concentrations in the clouds. However, the wet inputs to the canopy during this study were comparable to those measured at low elevation sites throughout the northeastern U.S.A. In contrast, a single exposed tree on the summit intercepted 10 times more cloud water and received 6 times higher wet deposits from rain and clouds than trees in the canopy. It is unclear whether the atmospheric loadings received by exposed trees, such as trees standing alone, trees on the edge of the canopy, or trees on the summit, are sufficient to result in forest decline.

Studies on non-precipitating cumulus cloud acidification

A one-dimensional cumulus cloud model is used to study the chemical transformations occurring during the formation, growth and dissipation of a cloud. A parabolic vertical velocity profile is introduced to create an updraft which entrains and detrains air laterally. The cloud forms in the upper half of the updraft structure where lateral detrainment occurs, all entrainment being from below the cloud base. As the updraft velocity increases in magnitude, the cloud grows and deepens. Downdraft is introduced into the fully developed cloud so that lateral entrainment into the parcel is from regions above cloud top. The cloud dissipates and eventually disappears as the downdraft penetrates and reaches the cloud base. Predicted liquid water contents and cloud structure are consistent with field observations. Detailed gas- and aqueous-phase chemistries are included in the cloud model. First-order rate constants for the oxidation of S(IV) to S(VI) increase with increasing concentrations of H 2O 2 in the background and with increasing cloudwater content. The cloudwater pH depends primarily on the degree of oxidation of S(IV) to S(VI) by H 2O 2 which is the most important oxidant of S(IV) in the aqueous phase. HO 2, OH, Cl 2 − and the intermediate SO 4 − also make significant contributions to the oxidation, while for these runs, O 3contributes less than 2% to the overall oxidation. Cumulus clouds are shown to behave like large variable volume reactors with entrainment of background air from below-cloud regions and release of processed air from in-cloud regions. The first-order rate constant for the oxidation of S(IV) to S(VI) ranges from 1 % min −1 to 33% min −1, depending on cloud properties and trace species concentrations.

Acidic cloud water and cation loss from red spruce foliage

Declines of red and Norway spruce in North America and Europe are occurring at high elevations where cloud interception is a major source of water and chemical deposition and the mean pH of cloud water is markedly lower than that of rainfall. At the summit of Whitetop Mountain (1700 m) in the southern Appalachians the chemistry of ambient cloud water was compared on a cloud event basis to that of cloud water-generated throughfall from red spruce saplings. Large shifts occurred in the relative importance of all the cations measured as cloud water became throughfall, whereas the relative importance of the major anions (SO42−, NO3−, Cl−) remained relatively constant. Hydrogen ion and NH4+ percentage contributions were reduced to two-thirds and one-third of their original percentages respectively, whereas the other four major cations (K+, Na+, Mg2+, Ca2+) all at least doubled in importance. Losses of Ca and Mg from native red spruce foliage were observed to intensify markedly with increases in the acidity of cloud water. On sites where the available pools of certain cations are already marginal, losses of cations caused by acidic cloud water may contribute to nutrient deficiencies.