Ambient concentrations,scavenging ratios,and source regions of acid related compounds and trace metals during winter in northern michigan

Daily measurements the atmospheric cocnentrations of HNO₃, NO₃^-, NO₂, SO₂, SO₄²⁻, NH₄⁺, and several trace metals were made at the University of Michigan Biological Station over a 124-day period during the 1984–1985 winter. The composition of the daily precipitation was also determined. The relative contributions of scavenged NO₃⁻ and HNO₃ to the precipitation was estimated by assuming that the NO₃⁻ scavenging ratio was the same as that of trace metals with a similar particle size. Similarly, the SO₄²⁻ and SO₂ contributions were based on the scavenging ratios of NH₄⁺ and trace metals. On this basis, it was determined that the event median NO₃⁻ and HNO₃ scavenging ratios were 500 and 3500, respectively. HNO₃ scavenging accounted for 83% of the total scavenged NO₃⁻. Scavenging of SO₄²⁻ accounted for all the snow SO₄²⁻ in 67% of the events. In the remaining events, some SO₂ was scavenged, with a median scavenging ratio of 219. Overall, 67% of the snowfall acidity appeared to be due to HNO₃ scavenging. Backward air-mass trajectories that were calculated for each event were used to determine the general source regions of the acidic species. Snow associated with air masses from the south and west accounted for 81 and 75% of the deposited NO₃⁻ and SO₄²⁻, respectively.     

Elements in the precipitation of S. Paulo City (Brazil)

Rainwater samples in S. Paulo city were collected on an event basis from October 1983 to October 1985 covering two dry and two rainy periods. Bulk samples only were obtained. At the same site and period, fine, coarse and inhalable particles were also collected. Na⁺, Ca²⁺, K⁺, Mg²⁺, NO₃⁻, SO₄²⁻ and NH₄⁺ contents were determined in rainwater samples, while Na, Ca, K, Cl and S concentrations were measured in aerosol samples. Rainwater is slightly acid (mean pH = 5.0), and contains high concentrations of Ca²⁺, NO₃⁻, SO₄²⁻ and NH₄⁺. Dry and wet fluxes and washout ratios were determined for some elements. Results obtained suggest that the atmospheric composition in this city is strongly influenced by anthropogenic sources.     

Chemical characterization of acid precipitation in Albany,New York

Chemical composition of precipitation in Albany, NY from July 1986 to December 1988 has been studied. Mean volume-weighted concentrations (μeq^ℓ−1) were: acidity, 104.0; alkalinity, −63.7; SO₄²⁻, 52.8; NO₃⁻, 29.8; Cl⁻, 5.6; F⁻, 0.50; NH₄⁺, 19.3; Ca²⁺, 6.5; Mg²⁺, 2.8; Na⁺, 3.5; and K⁺, 1.4. Mean pH was 4.2 . Seasonal patterns were pronounced for most species. Concentrations of H⁺, SO₄²⁻, NO₃⁻, NH₄⁺ and Ca²⁺ peaked in the summer and spring. Deposition was related to rainfall amount by a power law relationship in which the exponent of the equation was ∮.6. Wet SO₄²⁻ deposition was 2.35 keq ha⁻¹ over a 30-month period. The SO₄²⁻ and NO₃⁻ deposition rates observed at Albany indicate that transport from midwestern sources have a major influence at this site. On the average, free H⁺ ion concentrations determined from pH measurements accounted for 51% of the measured total acidity. There were unknown species, most likely organic acids, that could contribute to the acidity. Correlation and regression analyses indicated that major anions, SO₄²⁻ and NO₃⁻, were closely associated with H⁺ and NH₄⁺ ions. Factor analysis revealed four common factors which are related to fossil-fuel combustion, sea spray, cement factory and biomass burning.     

Rainwater composition in Athens,Greece

Wet precipitation-only samplers were used to collect wet deposition at two sites in the Athens basin, Greece for the period March 1986–February 1987.Concentrations of major cations (H⁺, NH⁺₄, Na⁺, K⁺, Ca²⁺ and Mg²⁺) and major anions (Cl⁻, NO⁻₃ and SO²⁻₄) were determined for the first time in rainwater samples in Greece. Bicarbonate concentrations were calculated. The relative importance of natural and anthropogenic sources were estimated by a chemical balance. The majority of rain collected has a neutral or alkaline character. Acidity was due to the presence of H₂SO₄ and HNO₃. The statistical analysis of the correlation between the concentration of chemical species confirm the influence of natural and anthropogenic sources. In all samples, SO²⁻₄ concentrations exceed NO⁻₃ concentrations despite the dominance of low S oil burning in the region. The wet flux of S was calculatd to be 0.34 gm⁻²a⁻¹.     

Cloud chemistry at Mt Rigi,Switzerland: Dependence on drop size and relationship to precipitation chemistry

The chemical composition of winter and spring cloud water sampled at 1620 masl elevation on Mt Rigi in central Switzerland was dominated by NO₃⁻, SO₄²⁻, NH₄⁺ and H⁺. A wide range of concentration levels was observed, with maxima of 3700, 1800 and 4600 micronormal for NO₃⁻, SO₄²⁻ and NH₄⁺, respectively. Concentrations at a lower elevation (1030 masl) site on the mountain were higher due to lower cloud liquid water contents and higher pollutant levels at that site. The lowest pH observed was 2.95; large concentrations of NH₃ in the region prevented pH values from falling even lower. A comparison of simultaneously sampled cloud water and precipitation revealed much higher concentrations for most species in the cloud water, except in one case of extreme precipitation riming when the concentrations in the two phases converged. An exception to the pattern was H⁺; at times the precipitation was more acidic than the cloud water. The chemical composition of the cloud drops varied with drop size. Drops smaller than 10 μm diameter were enriched in NO₃⁻, SO₄²⁻ and NH₄⁺ relative to larger drops. Since the larger drops are the ones most effeciently captured by snow crystals, knowledge of their composition is essential to understanding the chemical implications of accretional growth of precipitation.     

Scavenging ratios and deposition of sulphur,nitrogen and chlorine species in eastern England

Measurements of wet deposited NH₄⁺, SO₄²⁻, NO₃⁻ and Cl⁻, as well as airborne concentrations of these species and gaseous HNO₃, HCl and NH₃, have been made at a site in eastern England. Scavenging ratios based solely upon aerosol-associated species and upon aerosol plus gaseous airborne species are presented and compared with literature values. It appears that HCl and HNO₃ have only a rather minor influence upon wet deposition at our site. Gaseous NH₃ influences ground-level air chemistry appreciably, but scavenging ratios for NH₄⁺ are low, even when based upon aerosol NH₄⁺ concentrations alone, presumably due to altitudinal gradients in this species. The problems inherent in interpretation of scavenging ratios are discussed. Deposition of nitrogen in various chemical forms is estimated from rainwater and air composition. If a transport-limited deposition velocity is assumed for ammonia gas, dry deposition of this species accounts for around 40% of total nitrogen deposition to the ground.     

Changing sources of impurities to the Greenland ice sheet over the last 250 years

Because the composition of precipitation reflects the composition of the atmosphere, polar ice cores provide a useful way of investigating past and present atmospheres. We have measured concentrations of major ions in nine sections of a central Greenland ice core and we found that concentrations of both SO₄²⁻ and NO₃⁻ have increased dramatically over the last 250 years, up to three to four times the 18th century levels. Large changes have also occurred in the average concentrations of several other chemical species, such as NH₄⁺, excess Cl, and Ca²⁺. We used a principal-component analysis to characterize variations of the season of maximum deposition rate of HNO₃ and H₂SO₄ to the snow. We found that source fluctuations of H₂SO₄ are faithfully recorded in the Greenland snow and appear to switch their preferential time of deposition in the snow from summer to winter early in the 20th century. On the other hand, HNO₃ is deposited preferentially during summer throughout the core, emphasizing the role of photochemistry in understanding nitrogen cycling in the Arctic. Anthropogenic inputs have clearly modified the behavior of several chemical compounds in the atmosphere.     

Interactions of aerosols (ammonium sulfate,ammonium nitrate and ammonium chloride) and of gases (HCl,HNO3) with fogwater

The concentrations of aerosols (NH₄NO₃, (NH₄)₂SO₄ and NH₄Cl) and of gases (HCl_(g), HNO_3(g), NH_3(g) were determined by denuder methods under different conditions (in the absence of fog, before, during and after fog events). At this site situated in an urban region, high concentrations of the gaseous strong acids HCl_(g) and HNO_3(g) are observed. NH₄Cl and NH₄NO₃ aerosols represent a major fraction of the Cl⁻ and NO₃⁻ aerosols (<2.4 μm)collected by denuders. During a fog event, very high concentrations of SO₄²⁻ were found in small aerosols, which are attributed to the aqueous phase oxidation of SO₂ under the influence of high pH due to the presence of NH₃. Differences in SO₄²⁻ concentrations measured in aerosols (<2.4 μm) and in fog droplets were probably due to mass-transport limitations of the SO₂ oxidation. Ammonium sulfate aerosols represent in some cases a significant fraction of the total S present (SO_2(g) + SO₄²⁻. Soluble aerosols and gases contribute to the composition of fogwater and are released again after fog dissipation.     

Numerical modeling of long-range transport of acidic species in association with mesoβ-convective-clouds across the Japan sea resulting in acid snow over coastal Japan—II. Results and discussion

The acid snow/rain model [describedin Part I, Kitada et al., Atmospheric Environment27A, 1061–1076, 1993] was applied to investigate transport/transformation/deposition of acidic species in association with snow-precipitating cloud over the Japan Sea in winter. The model results showed: (1) The snow-precipitating clouds generated by relatively weak convective motions tend to trap aerosols of sulfate and nitrate and soluble gases such as SO₂ and HNO₃ below cloud levels, thus keeping their concentrations at higher levels than those for no-cloud situations. The mechanisms involved are: transfer of gas- and aerosol-phase species to cloud-phase through absorption and nucleation scavenging, then their transfer from cloud to snow through riming, and subsequent release from sublimating snow back to gas- and aerosol-phases below cloud base. (2) In-cloud oxidation enhanced the overall conversion of SO₂ to SO₄²⁻ by some 25% with respect to no-cloud situation after 12 h. Furthermore, contributions to the oxidation were 77.4%, 21.1% and 1.5% for S(IV)H₂O₂, S(IV)O₂ with catalysts of Fe³⁺ + Mn²⁺ and S(IV)O₃ reactions, respectively. (3) The sulfate wet deposited by precipitating snow for 12 h was due mostly to in-cloud scavenging and in-cloud oxidation, i.e. 66% by nucleation scavenging and the remaining by in-cloud oxidation of S(IV), while the contribution of below-cloud scavenging was negligible. (4) The adsorption process of HNO₃ onto the surface of falling snow was found to account for major below-cloud scavenging of snow, and thus in contrast to SO₄²⁻, the below-cloud scavenging contributed very significantly to the nitrate wet deposition. Throughout the stimulation, below-cloud scavenging was responsible for 75% of the snow-NO₃⁻ formation. Therefore, taking account of this process in acid snow models is important.     

Source-receptor relationships for wet SO42− and NO3− production

Precipitation chemistry data for the years 1982–1985 from 110 stations distributed across the continental U.S. and southern Ontario Province are used to describe the geographic distributions of SO₄²⁻ and NO₃⁻ in precipitation. Volume-weighted, wet SO₄²⁻ and NO₃⁻ concentrations, averaged over the 4 years of observation by season and annullly, show coherent patterns with maxima in the northeastern U.S. and southeastern Canada about ten times greater than the minima observed in the Intermountain and Pacific Northwest regions.Tests for empirical source-receptor relationships indicate that, in land areas with relatively low emissions of SO₂ and NO_x, the associations between wet SO₄²⁻ concentrations and SO₂ emissions and between wet NO₃⁻ concentrations and NO_x emissions within 560 km of each precipitation chemistry station are weak or nonexistent (r²⩽0.42). The remaining land areas show moderate to strong associations between SO₂ and SO₄²⁻ and NO_x and NO₃⁻ during the spring and summer, but only weak to nonexistent associations during the winter. The associations between emissions and concentrations, e.g. SO₂ and SO₄²⁻, are equally well represented by either a linear or a power law function. However, at the level of aggregation employed, the data do not substantiate a linear-proportional relationship between concentrations and anthropogenic emissions. Furthermore, emissions of SO₂ and NO_x are highly correlated, as are the emissions of RHC and NO_x.     

A chemical characterization of atmospheric aerosol in Sapporo

Monthly mean chemical composition of aerosol with diameter less than 8 μm was identified in Sapporo in 1982. The mass of aerosol was made up of nine components: elemental C, organics, SO₄²⁻, NO₃⁻, NH₄⁺, Cl⁻, Na⁺, soil particles and water. The concentrations of carbonaceous particles (elemental C and organics) was relatively high (12.7–16.0μ m⁻³) in autumn and winter (October–February) due to emission from domestic heating and comprised 36–41% of total aerosol mass. Higher concentration of soil particles was observed in spring (March–May) (9.7–13.1 μg m⁻³) and comprised 22–29% of total aerosol mass due to suspension by strong wind. On the other hand, the concentration of excess SO₄²⁻ (non-sea salt SO₄²⁻), which ranged from 2.6–5.2 μg m⁻³, did not change remarkably with season, and the fraction of excess sulfate increased to 21% in summer (July–August) probably due to photochemical transformation from SO₂. Nitrate concentration was far less than that of SO₄²⁻ throughout the year in Sapporo.     

Fogwater chemistry at Riverside,California

Fog, aerosol, and gas samples were collected during the winter of 1986 at Riverside, California. The dominant components of the aerosol were NH₄⁺, NO₃⁻, and SO₄₂₋. Gaseous NH₃ was frequently present at levels equal to or exceeding the aerosol NH₄⁺. Maximum level were 3800, 3100, 690 and 4540 neq m⁻³ for NH₄⁺, NO₃²⁻ and NH_3(g), respectively. The fogwater collected at Riverside had very high concentrations, particularly of the major aerosol components. Maximum concentrations were 26,000 29,000 and 6200 μM for NH₄⁺, NO₃⁻ and SO₄²⁻, respectively. pH values in fogwater ranged from 2.3 to 5.7. Formate and acetate concentrations as high as 1500 and 580 μM, respectively, were measured. The maximum CH₂O concentration was 380 μM. Glyoxal and methylglyoxal were found in all the samples; their maximum concentrations were 280 and 120 μM, respectively. Comparison of fogwater and aerosol concentrations indicates that scavenging of precursor aerosol by fog droplets under the conditions at Riverside is less than 100% efficient.The chemistry at Riverside is controlled by the balance between HNO₃ production from NO_x emitted throughout the Los Angeles basin and NH₃ emitted from dairy cattle feedlots just west of Riverside. The balance is controlled by local mixing. Acid fogs result at Riverside when drainage flows from the surrounding mountains isolate the site from the NH₃ source. Continued formation of HNO_3(g) in this air mass eventually depletes the residual NH_3(g). A simple box model that includes deposition, fog scavenging, and dilution is used to assess the effect of curtailing the dairy cattle feedlot operations. The calculations suggest that the resulting reduction of NH₃ levels would decrease the total NO₃⁻ in the atmosphere, but nearly all remaining NO₃⁻ would exist as HNO₃. Fogwater in the basin would be uniformly acidic.     

Spatial variability of urban precipitation chemistry and deposition: Statistical associations between constituents and potential removal processes of precursor species

The precipitation chemistry of Greater Manchester, a Metropolitan County in the northwest of England, has been examined for small scale spatial variability using a network of 18 bulk precipitation collectors. Significant spatial variability was found for concentrations of non-marine SO₄²⁻, NO₃⁻, NH₄⁺, Ca²⁺ and H⁺ ions. The statistical associations between the data were investigated using correlation, partial correlation and principal components analyses. It was found that zero-order correlation coefficients were inadequate for the interpretation of the data and that the computation of first, and higher order partial correlation coefficients was necessary in order to explain the interrelationships between the data and their spatial variability. The statistical associations between the data suggest relationships between Ca²⁺ and non-marine SO₄²⁻, and NO₃⁺ in precipitation which are discussed in terms of their possible precursor species. Potential source effects were examined in conjunction with atmospheric removal processes. The dry deposition of SO₄ particles, rather than the dry deposition of SO₂, may explain the spatial variability of non-marine SO₄²⁻. The erosion of CaSO₄ formed from the reaction of SO₂ with CaCO₃ on urban surfaces with subsequent resuspension is thought to be the basis of the relationship between Ca²⁺ and non-marine SO₄²⁻ concentrations in precipitation. The wet and dry deposition of CaCO₃ particles from local sources may be partially responsible for the spatial variability of H⁺, and dry deposition and scavenging of NH₃, in conjunction with the predominant wind direction may explain the spatial variability of NO₃⁻ and NH₄⁺ ions. Ammonia is thought to originate from sources both outside the study area and within it.     

Aerosol characteristics of Arctic haze sampled during AGASP-II

As part of the second Arctic Gas and Aerosol Sampling Program (AGASP-II), Arctic aerosol samples were collected by the NOAA WP-3D aircraft in spring 1986. The samples were analyzed in bulk and individual-particle form, using ion chromatography (IC) and electron microscopy (EM), respectively. Information on the chemical composition of the aerosol as determined by various techniques is presented, as well as morphology, concentration, and size distribution data obtained from individual particle analyses. For most flights, a stratospheric sample and a haze profile samople were collected. Haze samples exhibited greater particle concentrations than stratospheric samples, the highest concentrations in haze reaching ∼10³ cm⁻³ (non-volatile particles > 0.05 μm diam). Sulfur was consistently observed to be a major element in both large and small particles in haze samples. Crustal elements such as Si, Al, K, Ca and Fe were often present in significant concentrations together with S. Particles that did not emit X-rays, possibly organic or sooty C, were observed in significant concentrations in both tropospheric and stratospheric samples. Chemical spot tests confirmed that SO₄²⁻ was the major S-containing species and that NO₃⁻ was not nearly as prevalent as SO₄²⁻ in the Arctic aerosol particles. The mass concentrations of major anions (Cl⁻, SO₄²⁻ and NO₃⁻) and cations (Na⁺, K⁺, NH₄⁺, Ca²⁺ and Mg²⁺) in the bulk aerosols were determined using IC. The ratios between ion concentrations, e.g. Ca²⁺/Na⁺, SO₄²⁻/Na⁺ and Cl⁻/Na⁺, may serve as indicators of aerosol origins and mixing status of various air masses. Aerosols collected on six flights demonstrated variability of particle characteristics in relation to sources and transport of Arctic haze.     

Chemical composition of rain in Thessaloniki,Greece, in relation to meteorological conditions

Wet precipitation was collected in Thessaloniki, Greece, during the period March 1989–December 1990 by using an automatic wet-only precipitation sampler.Rainwater samples were analysed for major cations (H⁺, NH₄⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺) and anions (Cl⁻, NO₃⁻, SO₄²⁻), in addition to acidity and conductivity measurements. The majority of rain had a neutral or alkaline character as a result of neutralization, primarily caused by calcareous soil dust and secondarily by atmospheric ammonia. In all rain, SO₄²⁻ concentration exceeded NO₃⁻ concentration. The contribution of maritime sources to the total SO₄²⁻ concentration was very low (<2%).The chemical composition of precipitation was analysed in conjunction with meteorological variables (season of the year, precipitation type, airflow patterns) to evaluate temporal variations and chemical source influence. Rain caused by weak, localized flows showed the highest acidity and the minimum influence of neutralization processes.     

Measurements of atmospheric HNO3, HCl and associated species on a small network in eastern England

Measurements of gaseous HNO₃, HCl and NH₃ and particulate NO₃⁻, SO₄²⁻, Cl⁻ and NH₄⁺ have been made at a small network of sites in eastern England using sampling intervals from 3 h to 7 days. Both HCl and HNO₃ are spatially rather uniform, with some variation apparently due to spatial variations in NH₃, which stoichiometrically exeeded the sum of both gaseous acids. Mean concentrations of NH₃, HCl and HNO₃ between February 1987 and January 1988 were 1.90, 0.67 and 1.01 μg m⁻³, respectively. Pollution roses revealed low NH₃ concentrations, and high associated HCl and HNO₃ with winds from the North Sea. HCl, but not HNO₃ showed an appreciable elevation in concentration on the sector NW from our site, which we speculate may be due to the large capacity of coal-fired power stations in this upwind sector. Three-hourly data have been examined for diurnal effects and its is concluded that nocturnal formation of NO₃⁻ is occurring.     

Estimates of ion sources in deciduous and coniferous throughfall

Estimates of external and internal sources of ions in net througfall deposition were derived for a deciduous and coniferous canopy by use of multiple regression. The external source component appears to be dominated by dry deposition of Ca²⁺, SO₂ and NO₃⁻ during dormant and growing seasons for the two canopy types. Increases in the leaching rates of K⁺ and Mg²⁺ during the growing season reflect the presence of leaves in the deciduous canopy and increased physiological activity in both canopies. Internal leaching rates for SO₄²⁻ doubled during the growing season presumably caused by increased physiological activity and uptake of SO₂ through stomates. Net deposition of SO₄²⁻ in throughfall during the growing season appears highly dependent on stomatal uptake of SO₂. Estimates of SO₂ deposition velocities were 0.06 cm s⁻¹ and 0.13 cm s⁻¹ for the deciduous and coniferous canopies, respectively, during the dormant seasons, and 0.30 cm s⁻¹ and 0.43 cm s⁻¹ for the deciduous and coniferous canopies, respectively, during the growing season. For the ions of major interest with respect to ecosystem effects, namely H⁺, NO₃⁻ and SO₄²⁻, precipitation inputs generally outweighed estimates of dry deposition input. However, net throughfall deposition of NO₃⁻ and SO₄²⁻ accounted for 20–47 and 34–50 per cent, respectively, of total deposition of those ions. Error estimates of ion sources were at least 50–100 per cent and the method is subject to several assumptions and limitations.     

The spatial distribution and particle size of some inorganic nitrogen,sulphur and chlorine species over the North Sea

The use of filter packs and a cascade impactor during a series of research cruises in the southern area of the North Sea has yielded detailed spatial distribution patterns of aerosol concentrations, Cl⁻, NO₃⁻, SO₄²⁻¹ and NH₄⁺ and gaseous concentrations, HCl, HNO₃ and NH₃. The overall distribution of the atmospheric concentrations closely parallels published modelled results for metallic species. The chemical transformations of these aerosols and gases are investigated together with their interactions with the seasalt aerosol. Aerosol chloride loss is greatest in the more polluted areas, whilst concentrations products of NH₃ with HNO₃ and HCl appear insufficient to sustain the existence of NH₄NO₃ and NH₄Cl. Nitrate is associated predominantly with larger particles and appears to be present substantially as a surface coating on marine aerosol. The total dry deposition input for nitrogen species is calculated for the southern sector with extrapolation to the whole of the North Sea, using particle size weighted deposition velocities of 0.63 and 0.21 cm s⁻¹ for NO₃⁻¹ and NH₄⁺, respectively, and literature-derived values for the gaseous constituents. Finally the use of air-mass back trajectories illustrates the role of source regions in influencing the chemical composition of the North Sea atmosphere.     

The vertical distribution of aerosols and acid related compounds in air and cloudwater

Vertical profiles (surface to 5 km) of aerosol particle number concentration, NO_y′ mixing ratio, and cloudwater SO₄²⁻ and NO₃⁻ equivalent concentration were obtained in three field studies: North Bay, Ontario, during the summer of 1982 and the winter of 1983–1984, and Syracuse, New York, during the fall of 1984. The measurements from these locations and different seasons are compared. Generally, airborne concentrations are highest with air-mass back trajectories from the south and lowest with back trajectories from the north. For the southerly trajectories, median particle number concentrations (0.2–2 μm) near ground level (950 mb) vary from 1700 cm⁻³ during the summer project to 800 cm⁻³ during the winter project. At 700 mb, the south trajectory particle number concentration ranged between 60 and 170 cm⁻³. Median NO_y′ mixing ratios for southerly back trajectories were approximately 6 and 9 ppb at 950 mb and 0.4 and 0.8 ppb at 700 mb for the fall and winter projects, respectively. Comparison of particle number concentration profiles outside of cloud with cloud droplet plus interstitial aerosol particle number concentrations inside cloud indicate that cumulus clouds can transport aerosols vertically from below cloud base. In contrast, stratiform clouds have similar concentrations inside the clouds as outside at the same altitude. The vertical variations of cloudwater sulphate and nitrate concentrations and the NO₃⁻/SO₄²⁻ equivalent concentration ratio are discussed for each of the three field studies.     

The chemical composition of intercepted cloudwater in the Sierra Nevada

The chemical composition of cloudwater in the Sierra Nevada is dominated by NO₃⁻, SO₄²⁻, and NH₄⁺. 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 NO₃⁻, SO₄²⁻, and NH₄⁺. In the absence of inputs of NH₃, 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 NO₃⁻, SO₄²⁻, and NH₄⁺ 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.