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.     

Tests of models of cloudwater deposition to forest canopies using artificial and living collectors

Mechanistic cloud deposition models are very useful in the routine quantification of cloudwater deposition to forest canopies. In order to test, in a natural field situation, several assumptions in these models, a passive string cloudwater collector, a small artificial tree, and a living Norway spruce were exposed to cloudwater on a raised platform at the summit (elevation, 1686 m) of Whitetop Mountain, Virginia over a 5 month period. Cloudwater collection rates by these three collectors were used to examine relationships between these rates and measured values for two important meteorological variables in the models, liquid water content and wind speed, the product of which is the horizontal cloudwater flux. Collection rates for all three collectors were predicted moderately well by horizontal cloudwater flux (R² ranged from 0.54 to 0.73; p<0.0001) across all hours of observation, but were least strongly related when liquid water content was low, probably because of various measurement uncertainties under this condition. For all three collectors, simple linear regressions using the horizontal water flux to predict collection rates were not appreciably improved by inclusion of a cloudwater collection efficiency term or by conversion to binomial or curvilinear models. Cloudwater collection efficiency for all three collectors was related to the logarithm of horizontal water flux, as predicted by the models, only when this relationship was analyzed within individual cloud events. Between individual cloud events, collection efficiency varied across a wide range (0.12–0.50 for the spruce tree), with efficiencies much higher during events of short duration. Cloudwater collection efficiency was often lower than predicted by cloud deposition models, possibly because the models use wind speed measurements which do not take into account reductions in wind speed occurring within needle clusters on branches. Collection rates for all three collectors correlated highly with each other (R² ranged from 0.72 to 0.88; p<0.0001), as well as with a mature red spruce canopy. It was concluded that either the string collector or an artificial tree such as the one used in this study would serve as a good surrogate collector for living spruce tree crowns.     

Monsoon cloudwater chemistry on the Arabian Peninsula

The potentially catastrophic environmental consequences of the conflict in Iraq, Kuwait and Saudi Arabia, from mid 1990 to early 1991, have highlighted the need for background atmospheric chemistry measurements for the region. The only known cloudwater chemistry data obtained in the Arabian Peninsula are presented here. The samples were collected near the coast, in the Dhofar region of southern Oman, from 22 to 30 July 1990, immediately prior to the start of the conflict on 2 August. Analysis of the samples for pH, 10 major ion concentrations and 23 trace elements, demonstrates that the cloud water was very clean. Enrichment factor calculations showed the ions have oceanic and crusal origins, whereas trace elements such as B, V, Mn, Ni, Zn, Se, Sr, Mo and Ba have anthropogenic sources. In comparison with three mountain cloudwater sampling sites in eastern North America, the Omani site has higher pH values, higher Na⁺ and Cl⁻ concentrations, and lower SO₄²⁻ and NH₄⁺ concentrations.     

Intensive studies of Sierra Nevada cloudwater chemistry and its relationship to precursor aerosol and gas concentrations

Measurements of inorganic aerosol and gas phase species are presented for three sites in central California during a 4 day period in April 1988. The measurement sites were located along an east-west transect at Visalia, Ash Mountain, and Lower Kaweah, with elevations of 90, 550 and 1900 m, respectively. Aerosol compositions were nearly neutral at all locations, however large concentrations of NH₃ at Visalia contributed significant excess alkalinity to the air mass sampled there. Concentrations of all major species were observed to decrease with elevation during most of the sampling periods. Concentrations at the upper two sites exhibited diurnal fluctuations, with peaks in the late afternoon, consistent with the transport of pollutants from San Joaquin Valley sources by daytime upslope winds. Concentrations of most of these species reached a maximum at the elevated sites on 28 April, as a weak cold front approached, reducing the atmospheric stability over the valley floor. Concentrations at Visalia on this day were somewhat lower than those observed earlier in the week.Clouds intercepting the mountain slopes on 28 April were sampled at two locations. The coudwater pH at both sites was observed to fall throughout the event, dropping as low as 4.34. Precursor concentrations of aerosol NO₃⁻, SO₄^2- and NH₄⁺, and gas phase HNO₃ and NH₃, were sufficient to account for the observed cloudwater loadings of NO₃⁻, SO₄^2- and NH₄⁺. In-cloud measurements made near the cloud base indicated a considerable S(IV) oxidation potential in the form of H₂O₂, but only low S(IV) concentrations. Cloudwater concentrations of formic acid were approximately three times acetic acid concentrations. Carbonyl concentrations were dominated by formaldehyde and glyoxal.     

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.     

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.     

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.     

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.     

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.     

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 characteristics of cloudwater over Maoer Mountain district

Samples of cloudwater, rainwater and ambient aerosol were collected over Maoer Mountain in the northeast of Guangxi Province in March 1988. The pH value of cloudwater ranged from 3.37 to 6.20 with a mean value of 4.34. SO42- , NO3- , NH4+, Ca2+ and H+ were the principal ionic species of cloudwater. The advance of cold fronts into Maoer Mountain appeared to lead to higher major ionic concentration and acidity than that of warm fronts. The relative acidity and concentration of NO3- of cloudwater were much greater than that of aerosol samples. With the exception of. NO3- and H+, most of the concentration of SO42- and NH4+ in cloudwater came from the nucleation scavenging of aerosol. Gaseous nitric acid and organic acid from local may be one of the important source of cloudwater acidity. Compared with other sites, Maoer Mountain can be as a clean contrast station for studying acidic precipitation in Guangdong and Guangxi provinces.     

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.     

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.     

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.     

Multiphase chemistry in a microphysical radiation fog model—A numerical study

A microphysical radiation fog model is coupled with a detailed chemistry module to simulate chemical reactions in the gas phase and in fog water during a radiation fog event. In the chemical part of the model the microphysical particle spectrum is subdivided into three size classes corresponding to non-activated aerosol particles, small and large fog droplets. Chemical reactions in the liquid phase are separately calculated in the small and in the large droplet size class. The impact of the chemical constitution of activated aerosols on fogwater chemistry is considered in the model simulations. The mass transfer of chemical species between the gas phase and the two liquid phases is treated in detail by solving the corresponding coupled differential equation system. The model also accounts for concentration changes of gas-phase and aqueous-phase chemical species which are induced by turbulence, gravitational settling and by evaporation/condensation processes.Numerical results demonstrate that fogwater chemistry is strongly controlled by dynamic processes, i.e. the vertical growth of the fog, turbulent mixing processes and the gravitational settling of the particles. The concentrations of aqueous-phase chemical species are different in the two droplet size classes. Reactands with lower water solubility are mainly found in the large droplet size class because the characteristic time for their mass transfer from the gas phase into the liquid phase is essentially longer than the characteristic time for the formation of large fog droplets. Species with high water solubility are rapidly transferred into the small fog droplets and are then washed out by wet deposition before these particles grow further to form large droplets. Thus, the concentrations of the major ions (NO₃⁻, NH₄⁺) are much higher in small than in large droplets, yielding distinctly lower pH values of the small particles. In the present study the reaction of sulfur with H₂O₂ and the Fe(III)-catalysed autoxidation of S(IV) are the major S(VI) producing mechanisms in fog water. Most of the time the sulfur oxidation rates are higher in the large than in the small droplets. Fogwater deposition by gravitational settling occurs mainly in the large droplet size class. However, since in the small droplets the concentrations of chemical species with very good water solubility are relatively high, in both droplet size classes the total wet deposition of these reactands is of the same order of magnitude.     

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 explicity 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.     

南京2013年冬季三级分粒径雾水化学特征

为研究南京冬季不同粒径雾滴的化学成分的特征,利用three-stage CASCC主动式分档雾水采集器,于2013年12月7日~12月9日南京郊区发生浓雾期间,分时段采集三级分档雾水样本,分档粒径为4~16μm(三级)、16~22μm(二级)、>22μm(一级),共计23个分档雾水样本;用瑞士万通850professional IC型色谱仪器测定水溶性阴、阳离子浓度,分析探讨了三级分粒径雾水中阴、阳离子组分的分布特征,不同粒径雾滴中阴、阳离子浓度的相关性,雾水离子浓度与污染气体以及微物理之间的关系.结果表明,南京雾水的pH值多呈酸性,雾水中的各离子成分分布都与雾滴的大小存在着尺度依赖性关系,小雾滴与大雾滴相比,小雾滴中主要离子成分浓度(NH4+,NO3-,SO42-)高、pH值小且电导率(EC)值高.同时南京雾水中的各离子浓度呈现出夜间高白天低.统计分析显示,南京雾过程中雾水组分的变化,主要源于污染源的贡献差异.结合雾滴谱和污染气体资料分析得出,雾水化学组成的变化与微物理特征以及空气中污染气体有关.     

Origin of soluble chemical species in bulk precipitation collected in Tokyo,Japan: Statistical evaluation of source materials

An iterative least-squares method with a receptor model was applied to the analytical data of the precipitation samples collected at 23 points in the suburban area of Tokyo, and the number and composition of the source materials were determined. Thirty-nine monthly bulk precipitation samples were collected in the spring and summer of 1987 from the hilly and mountainous area of Tokyo and analyzed for Na⁺, K⁺, NH₄⁺, Mg²⁺, Ca²⁺, F⁻, Cl⁻, Br⁻, NO₃⁻ and SO₄²⁻ by atomic absorption spectrometry and ion chromatography. The pH of the samples was also measured. A multivariate ion balance approach (Tsurumi, 1982, Anal. Chim. Acta138, 177–182) showed that the solutes in the precipitation were derived from just three major sources; sea salt, acid substance (a mixture of 53% HNO₃, 39% H₂SO₄ and 8% HCl in equivalent) and CaSO₄. The contributions of each source to the precipitation were calculated for every sampling site. Variations of the contributions with the distance from the coast were also discussed.     

A comparison of urban and suburban precipitation chemistry

Precipitation samples at an urban Chicago site and a nearby suburban site were compared in order to examine the influence of emissions within a large urban area on local precipitation chemistry. Precipitation samples were collected from June 1981 to May 1982, initially for events and subsequently weekly, and precipitation-weighted concentrations (PWCs) of the major chemical constituents were calculated from concurrent urban-suburban pairs of samples, stratified according to the estimated mixed-layer wind quadrant. Overall, PWCs at the urban site were higher than those at the suburban site for Ca²⁺, Mg²⁺, NH₄⁺, NO₃⁻ and Cl⁻; approximately equal for Na⁺ and SO₄²⁻; and lower for H⁺. For precipitation in southwesterly flow, in which the suburban site was upwind of the urban site and most urban emissions, PWCs of all species except Na⁺ were higher at the urban site. For the few precipitation cases in northeasterly flow, however, differences between sites did not have a pattern consistent with a reversal in the upwind-downwind relationship.     

Urban precipitation chemistry: A review and synthesis

Urban sampling sites have been specifically excluded from recent large networks for measuring chemical composition of precipitation. Because information on precipitation composition in urban areas is needed for a variety of current applications, it is useful to summarize present knowledge. Most of the available information is based on samples of bulk precipitation, collected by continuously open collectors. This method is now widely acknowledged to be of limited value because of its poor sampling characteristics for dry deposition. For many ions, particularly those residing on large airborne particles, urban bulk sampling yielded considerably higher concentrations than found in samples collected in ‘wet-only’ samplers. Spatial variability of ionic concentrations in urban areas is expressed in terms of the sample standard deviation of site precipitation-weighted means, as a percentage of the overall urban mean. Median values of the most major ions were near 30%; half of the available measurements were between 20 and 47%. Differences between urban and nearby rural concentrations of ions in precipitation were often reported. Those ions with some tendency to occur in higher concentrations in cities included Na⁺, Mg²⁺, Ca²⁺, NO₃⁻, SO₄²⁻ and Cl⁻. These differences should be viewed with caution, however, because they were usually based on one or two, possibly unrepresentative, rural sampling sites. Seasonal variations of urban [H⁺] were different in Europe and the U.S. Highest concentrations occurred in winter in Europe, but in summer in the U.S. There is a pressing need for additional urban precipitation chemistry monitoring and research.