A laboratory study on the scavenging of SO2 by snow crystals

A laboratory experiment is described where the uptake of SO₂ by dendritic snow crystals was studied. In a first experimental series the uptake of SO₂ was investigated during the growth of the snow crystals from water vapor with and without the presence of H₂O₂ in the air. In a second series of experiments we studied the uptake of SO₂ by snow crystals which had completed their growth. The results of our experiments showed that under both conditions SO₂ became scavenged by snow crystals. The uptake of SO₂ was particularly pronounced during the growth of the snow crystals, and at temperatures close to 0°C where a quasi-liquid layer exists at the surface of ice. As expected, the SO₂ uptake becomes enhanced in the presence of H₂O₂. The present results are in qualitative agreement with previous studies involving bulk ice.     

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.     

The effect of freezing on the composition of supercooled droplets—I. Retention of HCl,HNO3, NH3 and H2O2

Experiments were done to check on the possibility that cloud droplets might, during freezing, lose acidity by evolution of HCl or HNO₃, lose NH₃ or lose dissolved H₂O₂. A spray of droplets with average diameter 39μm was produced by an ultrasonic transducer. The droplets acquired a temperature between −8 and −12°C and fell onto an ice surface, where they froze. Appropriate analytical techniques were applied to compare the composition of the frozen droplets with that of the sprayed liquid. It was found that the four chemical species studied were totally retained in the ice after freezing.     

The effect of freezing on the composition of supercooled droplets—II. Retention of S(IV)

S(IV) dissolved in droplets is partially evolved as SO₂ during freezing. A spray of droplets with average diameter 39μm, produced by an ultrasonic transducer, was let to fall through a controlled atmosphere with known SO₂ concentration, at varying temperatures between −8 and −23°C, attaining thermal and chemical equilibrium. In a first arrangement, the droplets fell by gravitation on an ice surface. Two other arrangements simulated the riming ventilation conditions: in one series of experiments, the droplets were projected by a gas jet at several m s⁻¹ against a target; in another, the droplets were caught by rotating rods. The fraction of S(IV) retained in the ice Γ was determined by analysis of the samples and comparison with the equilibrium concentration in the liquid droplets. Samples collected by gravitation showed a retention coefficient Γ = 0.25 + 0.012 T_s (T_s = supercooling); rime samples showed large dispersion in the results, the retention coefficient being best represented by an average value Γ = 0.62, independent of temperature.     

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.     

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.     

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.     

Wet deposition of volcanic gases and ash in the vicinity of Mount Sakurajima

Wet deposition of soluble materials was monitored for a 7-month period in the vicinity of Mount Sakurajima, which has been very active since 1955. Data obtained for the ionic composition of samples were used as a basis for considering the scavenging of volcanic emissions by precipitation. Precipitation samples with pH < 4 were often collected in the vicinity of Mount Sakurajima during the course of the study. HCl, H₂SO₄ and HF were the primary contributors to the acidity of precipitation. The exCl⁻/exSO₄²⁻ mole ratio of precipitation was several times that of volcanic ash in the vicinity of the volcano and declined with distance from the volcano. Although most of the S in volcanic emissions is in the form of SO₂, relatively little of this is washed out by precipitation in the immediate vicinity of the volcano. The deposition of S in the vicinity of the volcano can be adequately accounted for by assuming the scavenging of SO₄²⁻ particles despite the relatively small share of total atmospheric S loading of particulate SO₄²⁻ in the vicinity of Mount Sakurajima.     

Proportionality between SO2 emissions and wet SO42− concentrations: The effect of area of averaging

An earlier analysis of empirical associations between SO₂ emissions and wet SO₄²⁻ concentrations in central North America (Hilst and Chapman, 1990, Atmospheric Environment 24A, 1889–1901) showed that local wet SO₄²⁻ concentrations were not proportional to SO₂ emissions averaged over areas up to ∼10⁶ km². Because it is axiomatic that at a global level of averaging a proportionality between total S emissions and S deposition should exist, we have extended these analyses in an attempt to determine whether there is proportionality between S emissions and wet SO₄²⁻ deposition. We have found that for the eastern half of central North America (an area of about 4.3 × 10⁶ km²), the annual average wet SO₄²⁻ concentration exhibits a linear-proportional dependence on anthropogenic SO₂ emissions. However, the internal structure of this association for subareas of the eastern half of central North America suggests the “global” proportionality is achieved by a combination of imported SO₂ from major source areas and an oxidant-limited conversion of SO₂ to SO₄²⁻ within the major source areas. If this inference is even approximately correct, a rollback SO₂ control strategy for the eastern United States and southeast Canada should result in an immediate proportional decrease in wet SO₄²⁻ concentrations in minor SO₂ source areas, but no appreciable reduction of wet SO₄²⁻ concentrations in major SO₂ source areas until the oxidant limitation has been overcome.     

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.     

In-cloud scavenging and deposition of sulfates and nitrates: Case studies and parameterization

Scavenging of sulfates and nitrates—two most common ions leading the cloudwater acidity—was investigated during field studies atop a site in Mt. Mitchell (35°44′05″N, 82°17′15″W) State Park where the highest peak (2038 m MSL) of the eastern U.S. is located. Experiments were conducted during the growing seasons (15 May–30 September) of 1986 and 1987 using an instrumented meteorological tower (16.5 m tall) and a passive cloudwater collector. A cloud episode that occurred on 12 October 1987, was also comprehensively investigated. Clouds were frequently observed in which the Fraser fir and red spruce stands stayed immersed 28% and 41% of the time during the 1986 and 1987 seasons, respectively. Rate of cloudwater deposition on the forest canopy was determined using an inferential cloud deposition model. It was found by analysing nine short duration (lasting 8 h or less) and 16 long duration cloud events that the ionic concentration (SO₄²⁻ and NO₃⁻) is inversely proportional to the rate (I_c) of cloudwater deposition (in mm h⁻¹) and can be expressed by the following relationship: [SO₄²⁻] = aI_c^−b or [NO₃⁻] = aI_c^−b. Theoretical arguments leading to these relationships are presented. The b values for predicting NO₃²⁻ concentration are found in the range of 0.14–1.24 (mean = 0.48) for short duration and 0.062–0.63 (mean = 0.27) for long duration cloud events, respectively. The corresponding b values for predicting NO₃⁻ concentrations are 0.19–1.16 (mean = 0.49) and 0.072–0.59 (mean = 0.27), respectively. When the b parameter was between 0.2 and 0.6, the correlation coefficients between measured and predicted ionic concentrations were found to exceed 0.7. The parameter a is shown to represent the maximum ionic flux for a given cloud event. The ratio of the a parameter for SO₄²⁻ to NO₃⁻ varied between 1.75 and 6.95, indicating that the SO₄²⁻ contributes to the total ionic concentration substantially more than the NO₃⁻ leading to the conclusion that the cloudwater acidity is primarily due to the presence of sulfuric acid which has been demonstrated to cause foliar injury and growth retardation in red spruce trees. The above parameterization is similar to the one that is frequently used to relate ionic concentration in precipitation to the rainfall rate. In order to understand physico-chemical processes leading to the proposed parameterization schemes, meteorological and chemical variables are comprehensively analysed for one short duration and two long duration cloud events. The concentrations of principal ions (SO₄²⁻, NO₃⁻, H⁺ and NH₄⁺) during the short duration cloud events were found to be much higher than those during the long duration ones, especially at colder temperatures. Such short cloud events have a potential of causing foliar narcosis in red spruce stands because of unusually acidic cloudwater to which these stands stay exposed intermittently during each growing season.     

A long-term (1975–1988) study of atmospheric SO42−: Regional contributions and concentration trends

A long-term study of aerosol SO₄²⁻ concentrations ([SO₄²⁻]) has been conducted at Mayville in the western and Whiteface Mountain in the northeastern New York State. From 1975 to 1988, 2382 daily aerosol samples were collected at Whiteface Mountain using high-volume samplers. Similarly, 1863 samples were collected at Mayville for the 1981–1988 period. Both sites are downwind of large SO₂ sources in the Midwest. Whiteface Mountain is located approximately 600 km to the northeast of Mayville. The [SO₄²⁻] at Mayville were approximately twice that of Whiteface Mountain. The highest concentrations at both locations were observed in summer and the lowest during winter. Photochemical reactions appear to be the primary reason for this behavior. Air trajectories (Hefter model) were used to relate the observed [SO₄²⁻] with the upwind SO₂ source regions. In addition, a method based on V/Se ratios was used to resolve SO₄²⁻ contributions between Midwestern sources and those in the East Coast. Approximately, two-thirds or more of the total SO₄²⁻ at the two sites was derived from the Midwestern emissions. At Whiteface Mountain the [SO₄²⁻] for summer months from 1975 to 1988 suggest a decrease of approximately 3% per year between 1978 and 1988. A similar decrease was also observed in SO₂ emissions.     

The role of fly-ash particulate material and oxide catalysts in stone degradation

Studies of fly-ash composition identified the presence of calcium and sulphur, indicating their potential role as sources of calcium sulphate. Residual acidity (particularly for oil fly ash) suggested the possibility of enhanced chemical reaction, and the presence of transition metals, probably as oxides, might accelerate the oxidation of SO₂ to SO₄²⁻. Exposure tests in a laboratory-based rig simulating dry deposition on Portland and Monks Park limestone, either seeded or unseeded with fly-ash particulate material or transition metal oxide catalysts, were carried out using an SO₂-containing environment at 95% r.h. Enhanced sulphation of these seeded limestones due to the above factors was minimal; at high loadings of fly ash, there was even evidence of masking the limestone surface, reducing sulphation. However, pure CaCO₃ powder in the exposure rig showed increases in sulphation when seeded with metal oxide catalysts. Thus the limestones examined contained sufficient inherent catalysts for the oxidation of SO₂ to SO₄²⁻ to proceed at such a rate that external catalysts were superfluous. This implies that dissolution rate of SO₂ in moisture films controls the availability of species for reaction with these carbonate-based stones and that fly ash deposited from the atmosphere does not enhance the reaction.     

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.     

Mid-latitude northern hemisphere background sulfate concentration in rainwater

The pH is not sufficient to characterize the acidity of precipitation, but rather its acid-base components must be described. The chemistry of natural emission sources as well as the mechanism of precipitation formation determine the chemistry of precipitation at mid-latitude, Northern Hemisphere locations. With the ocean biota as a source of atmospheric aerosol SO₄²⁻, it is expected that this “background” chemistry will be dominated by SO₄²⁻. For the purpose of this study, background was defined as a remote site generally upwind of urban areas, with the additional requirement that samples with evidence of contamination by anthropogenic sources be excluded. Canadian and U.S. data from long term precipitation monitoring sites along the coasts of British Columbia, Oregon, and Washington were evaluated to estimate a background SO₄²⁻ concentration in rainwater. In addition to screening the data for charge balance, collection efficiency, and anthropogenic influence, the data were corrected for SO₄²⁻ associated with sea salt. The results of this analysis suggest that the mid-latitude, Northern Hemisphere background excess SO₄²⁻ concentration in rainfall occurs most frequently in the range of 2–16 μeqℓ⁻¹ with a mean of 5.5 μeqℓ⁻¹ and an average measured pH of 5.3.     

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.     

Characterization of pollen deposition in a forest environment

Measurements of the dry deposition of pollen were made during the months of May and June 1987 in northern Wisconsin, using a smooth surrogate surface. Samples were taken on a raft located on Little Rock Lake and at a nearby field monitoring station. Rain samples were also collected at the field station. The wet SO₄²⁻ flux was 102.7 mg m⁻², compared with a dry SO₄²⁻ flux of 118 mg m⁻² at the field monitoring site and 45 mg m⁻² at the lake site.The SO₄²⁻ content of pollen ranged from 0.2 to 0.8% of the weight of the pollen, and NO₃⁻ concentrations were an order of magnitude lower. Between 9 and 22% of the pollen weight was available as total organic carbon (TOC) upon addition to water.The addition of pollen to distilled water produced an acid reaction, due to organic acids and not inorganic acidity.     

Transfer of atmospheric constituents into an alpine snow field

Simultaneous aerosol and snow sampling was performed during a field campaign at the Alpine site Weissfluhjoch Davos, Switzerland (2540 m a.s.1.) from 1 January through 30 March 1988. In addition, a snow pit was sampled on 30 March 1988. Very good agreement between the new snow and pit snow samples was found for the measured major ions as well as for the stable isotopes δ¹⁸O and δD. Thus, snow pit samples obtained at this site during the winter months yield representative deposition patterns with a conserved stratigraphy. Generally, concentrations in snow were very low, with 3.5, 8.5, 5.2 and 2.4 μeq ℓ⁻¹ for Cl⁻, NO₃⁻, SO₄²⁻, respectively. The ³⁶Cl and ¹⁰Be concentrations as well as the ¹⁰Be/³⁶Cl ratios were comparable to the ones measured at Arctic sites. With the exception of NO₃⁻, no linear relation was obtained between atmospheric and snow concentrations, showing that the concept of scavenging ratios must be used with caution when looking at single snowfall events. The following precipitation-weighted mean scavenging ratios were found: 300 for NH₄⁺, 350 for SO₄²⁻, 940 for total NO₃⁻(NO₃⁻+HNO₃), 175 for ²¹⁰Pb, and 750 for ¹⁰Be.     

Aspects of wet,acidifying deposition in Arnhem: Source regions,correlations and trends (1984–1991)

A time series of wet deposition in Arnhem, the Netherlands, was analysed for the period 1984–1991. Precipitation was collected with four samplers on a daily basis. A comparative study by the Dutch National Precipitation Network showed significant biases for the observations of the National Network station due to longer exposure to dry deposition. Simultaneous operation of wet-only and bulk collectors demonstrated a concentration bias of about 10% for daily bulk sampling.Using a cluster analysis of backward trajectories, clear distinctions could be made between precipitation from continental and maritime origin. Event-to-event variations in deposition seemed to be determined largely by meteorological influences. As major anthropogenic source regions, the U.K., France, Belgium and the Netherlands itself were identified. The contribution of Dutch sources to wet acid deposition in Arnhem was estimated at 30–40%.Trends and seasonal variations were analysed with an advanced time-series model based on Kalman filtering. Similar seasonal variations were found for SO₄²⁻ and NH₄⁺. Also, seasonal variations in the concentrations of H⁺ and NO₃⁻ corresponded. Significant long-term changes in deposition and concentration were found for SO₄²⁻ (about −3% yr⁻¹) and H⁺ (about −9% yr⁻¹) only. The analysed trends were decreasing, but decreases were larger in the years 1984–1986 than in the following years. The relative decrease in the wet deposition of SO₄²⁻ was substantially smaller than decrease in dry-deposited SO₂ and SO₄²⁻.     

Chemistry of atmospheric precipitation at the Western Arabian Gulf Coast

Rainwater and atmospheric bulk deposition samples were collected at a station on the rooftop of the Research Institute of King Fahd University of Petroleum and Minerals in Dhahran. Continuous sampling was carried out manually throughout the rainy season between December 1987 and February 1988, and for one rainfall event in March 1987. A total number of 13 samples were collected and investigated for pH and dissolved ionic composition using inductivity coupled plasma emission spectrometry (ICP) and ion chromatography (IC). The range and volume-weighted average pH were 5.1–7.2 and 5.48, respectively. Significant negative linear correlations were observed between the precipitation pH and rain depth, and between pH and the summation of dissolved {(Ca²⁺ + Mg²⁺)−(SO₄²⁻ + NO₃⁻ + NO₂⁻)} (in μeqℓ⁻¹). The ionic summation also correlated negatively with rain depth. The ionic abundance in rainwater (in μeqℓ⁻¹) expressed in concentration order showed the general trend SO₄²⁻ > HCO₃⁻¹ = Cl⁻ = NO₃⁻ > NO₂⁻ for anions and Ca²⁺ > Na⁺ > Mg²⁺ > NH₄⁺ > K⁺ > H⁺ > Sr²⁺ for cations. Good mass balance between cations and anions was observed. Total NO₃⁻ contribute equally to precipitation acidity as SO₄²⁻ and Ca²⁺ plus Mg²⁺ in alkaline suspended particulates from natural sources are the major ions which buffer the acidity of precipitation. The NH₄⁺ ion which is also present plays an insignificant role in the acid/base equilibrium of rainwater.