Pollutant concentrations and below-cloud scavenging of selected N (-III) species along a mountain slope

Gas and aerosol measurements were performed at 3 ground based measuring sites at Mt. Rigi in central Switzerland during 2 winter seasons. Both NH₃ and NH₄ ⁺ show a strong vertical concentration gradient between the top station (1620 masl) and the bottom station (430 masl). High concentrations of NH₃ with values up to 29 ppbv, were found at the bottom station. HNO₃ concentrations were usually below 1 ppbv, with lower values at the bottom station than at the top station that presumably reflect particulate NH₄NO₃ formation due to high NH₃ concentrations at the lower site. No vertical concentration gradient was found for SO₂. Simple models have been used to estimate below-cloud scavenging of gaseous NH₃ and particulate NH₄ ⁺ by rain between two sites with a vertical separation of 600 m. The calculations used measurements from three case studies. Below-cloud scavenging of NH₃ by rain was found to be more important than below-cloud NH₄ ⁺ scavenging. From 58 to 88 % of the increase of [NH₄ ⁺] in precipitation between the two sampling sites was calculated to result from gas scavenging. Both observations and scavenging calculations were in relatively good agreement for three events. Observations from the present study and tests using different aerosol and raindrop diameters in the calculations point to the importance of using real data in below-cloud scavenging studies considering the relative importance of aerosol and gas scavenging.     

Atmospheric deposition in Fenno-Scandia: Characteristics and trends

Chemical analyses of daily precipitation samples from “background” stations in Europe are discussed together with measurements of airborne SO₂ and sulphate aerosol, and trends in energy usage and SO₂ emissions. Emission sources contributing to the major part of the concentrations of sulphate and nitrate in precipitation are mostly 500 to 1000 km from the receptor area. Although there are no general statistically significant trends in the precipitation chemistry data, minor changes point to an effect of reduced S02 emissions in some areas. The daily data can be used to infer general conclusions with respect to precipitation scavenging efficiency.     

Measurement of acid precipitation in Norway

Since January 1972, chemical analysis of daily precipitation samples from about 20 background stations in Norway has been carried out on a routine basis. Air monitoring is carried out at six stations. The chemical analysis program is: sulphate, pH and acidity in precipitation, sulphates and SO₂ in air. In addition, more detailed chemical analysis of aerosol and precipitation has been carried out at selected stations. Some results for the measurement period 1972 to 1974 are presented. Comparison of air and precipitation concentrations of S compounds show that the precipitation scavenging efficiency is very high under the conditions in southern Norway.     

Bridging the gap between air quality and precipitation chemistry

Data sets recently have become available providing simultaneous, regional scale observations of ambient air quality and precipitation chemistry. The data cover parts of the greater northeastern United States. With certain key assumptions, the observations of ambient SO_x and NO_x concentrations can be linked with precipitation concentrations through Junge's concept of rainout efficiency, assumed to be qualitatively equivalent to the washout ratio. A preliminary comparison between data taken from the Sulfate Regional Experiment (SURE) and the parallel precipitation studies using Junge's approach reveals certain useful consequences. Apparent dramatic differences in SO_x and NO_x scavenging are found. Ratios between sulfate in the aerosol and in precipitation compared with trace elements suggest the importance of particulate scavenging processes. Such results show promise for simplified analysis of these data as approaches to differentiating mechanisms involved in cloud-precipitation chemistry.     

Global emissions and natural processes for removal of gaseous pollutants

This review briefly illustrates the state of the art in the recognition of the various sources and natural sinks of gaseous pollutants. The removal mechanisms include absorption by vegetation, soil, stone, and water bodies, precipitation scavenging, and chemical reactions within the atmosphere. The nature and magnitude of anthropogenic and natural emissions of the gases (H₂S, SO₂, N₂O, NO, NO₂, NH₃, CO, O₃, and hydrocarbons), along with their ambient background concentrations and information on their major sinks identified to date, are discussed.     

A characterization of the detroit wintertime aerosol

The physical characterization of winter-time aerosol in the Detroit area studied over a 7-week period (January–March, 1983) is presented. Total suspended particle levels (<15 μm) were 56 ± 29 μg m^?3, with 66% of the material in the fine (<2.5 μm) particle fraction. Coarse and fine particle masses were reasonably correlated, indicating that local sources, not long-range transport, influenced winter-time aerosol at the site. Mobile sources were responsible for a marked diurnal variation in the nuclei mode particle count and accounted for about 10% of the submicron aerosol mass. Decreases in submicron aerosol concentrations during precipitation appear to be associated with advection of clean air into the area during frontal passage rather than to precipitation scavenging.     

Atmospheric aerosols and gases in rural Nova Scotia and their dependence on air flow

Various species including nitrate, HN0₃ and Pb were sampled in Antigonish, Nova Scotia and their fluctuations were compared to those of simultaneously sampled acid and sulphate (reported previously). The 24 hr average aerosol nitrate, ～0.3 μg m^?3, was similar in the summer and winter. Nitric acid made up ～42 % of the total (aerosol and gaseous) nitrate in the summer and winter, but nitrate and HN0₃ were correlated only in the summer. Windroses are used to show the dependence of sulphate, nitrate and aerosol acid on wind direction. Lead had a 24 hr average concentration of 71 ng m^?3, was of mainly local origin and was not able to be correlated with other parameters.     

Trace inorganic species in precipitation and their potential use in source apportionment studies

Trace species in atmospheric particles have frequently been used to apportion pollutants in acid rain to their sources by comparing the relative concentrations of trace species at the source and receptor sites. Another approach is to use the trace species in precipitation directly in a source apportionment. This has rarely been done due to the paucity of data on trace species in precipitation and the unknown fractionation of the species during the scavenging process. A study was undertaken to analyze precipitation for a large number of trace species and to compare the concentrations with those in atmospheric particles. Precipitation was collected in Lenox, MA during the summer of 1984. Extreme measures were taken to avoid contamination during collection and analysis. Using five analytical methods, including the novel method of freeze-drying the precipitation followed by neutron-activation analysis, 31 species were analyzed in the rain. The trace concentrations measured in this study were lower than those found in earlier studies, where contamination was believed to be a problem. Concentrations of trace species in the precipitation were compared with those in atmospheric particles. A few species were enriched in precipitation relative to particles, including some large-particle species (Ca, Ti, Fe) and species with gaseous precursors (SO₄, NO₃, Br). Due to this enrichment, these species should be carefully considered before using them in source apportionment calculations. However, most species showed similar enrichments in precipitation and in atmospheric particles, suggesting that negligible fractionation had taken place during the scavenging process. These species could be used effectively in the future to apportion the pollutants in precipitation to their various sources.     

Acid deposition at the summit of Mt. Fuji: Observations of gases,aerosols and precipitation in summer,1993 and 1994

Intensive observations of chemical species in aerosols, gases and other samples at the summit of Mt. Fuji and at Tarobo (at 1300m on the mountain'ts southern slope) was performed from July 28 to Aug. 3, 1993 and from July 25 to 30,1994. The most interesting observation was the abrupt increase in the sulfate concentration in aerosol collected in July, 1993 just after the typhoon (number 9306) passed the Japanese archipelago and the wind direction shifted from south to west. Chemical analysis indicated this aerosol was acidic. In contrast, the summit aerosol observed in 1994 was not acidic following a less dramatic rise in sulfate content. Back trajectory analyses were used to extrapolate from these measurement to an inventory of polluted air over the Asian Continent. The concentrations of gaseous SO₂ and HCl remained low during both observation periods, with some higher concentrations of NH₃.     

Atmospheric mercury in northern Wisconsin: Sources and species

The atmospheric chemistry, deposition and transport of mercury (Hg) in the Upper Great Lakes region is being investigated at a near-remote sampling location in northern Wisconsin. Intensive sampling over two years and various seasons has been completed. A multi-phase collection strategy (gas-, particle- and precipitation-phases) was employed to gain insight into the processes controlling concentrations and chemical/physical speciation of atmospheric Hg. Additional chemical and physical atmospheric determinations (e.g. ozone, particulate constituents, meteorology) were also made during these periods to aid in the interpretation of the Hg determinations. For example, correlations of Hg with ozone, sulfur dioxide and synopticscale meteorological features suggest a regionally discernible signal in Hg. Comparison to isosigma backward air parcel trajectories confirms this regionality and implicates the areas south, southeast and northwest of the site to be sources for Hg. Particle-phase Hg (Hg_p) was found to be approximately 40% in an oxidized form, or operationally defined as “reactive”. However, this was quite variable from year-to-year. Hg_p and other particle constituents (esp. sulfate) show significant correlation and similarity in behavior (concentration ratios in precipitation and in particles). These observations are part of the growing evidence to support the hypothesis that precipitation-phase Hg arises in large part from the scavenging of atmospheric particulates bearing Hg. Observed concentrations of rain and particle-Hg fit broadly the theoretical expectations for nucleation and below-cloud scavenging. Significant increases in the Hg/aerosol mass ratio appear to take place during transport. Enrichment of aerosols is taken as evidence of gas/particle conversion which could represent the step linking gas-phase Hg with rain. The refined budget indicates ca. 24% of total deposition is from summer particle dry deposition, and that this deposition also contributes ca. 24% of all reactive Hg deposition. Additionally, almost all (86%) deposition (wet and dry) occurs during the summer months.     

Stoichiometry of rain across the U.S.A.: Evidence of independent neutralization of sulfate and nitrate acidities

Annual wet-deposition fluxes of ions in the rains for 1982–1984 at the NADP/NTN sites across the U.S.A. were corrected for contributions from sea salt and anomalous rains (∫cations/∫ anions ratios outside 0.85 to 1.15). The resulting fluxes of normal rains in gram equivalents/ha/yr were used to determine the flux ratios of (NH₄ ⁺ + H⁺)/SO₄ ^2? and (Ca²⁺ + Mgt+ + K+ + Na+)/N03 . The first ratio is 1 ± 0.15 over a large area including most sites west of the Mississippi River, except California. A smaller core of this area has the second ratio equal to 1 ± 0.15. These results are attributed to mineral carbonate aerosol from the Western Plains being distributed by the prevailing westerly winds. This aerosol acts as a sink for NO_x and forms mineral nitrates. When the supply of carbonate is limited, free HN0₃ occurs. The aerosol NH₄HSO₄ is the main sink for NH₃ and SO₂. Thus, a major conclusion is that sulfate and nitrate acidities are neutralized independently.     

The influence of a sulfur dioxide point source on the rain chemistry of a single storm in the Puget sound region

Sampling and chemical analysis of a rainstorm were undertaken to determine the influence of a large source of SO₂ on regional precipitation chemistry. A definite correlation was obtained between hydrogen ion, sulfate ion and As concentration increases downwind of the source. Estimated sulfate deposition in a 1500 km² downwind area was compared to existing models.     

Utilization of atmospheric H2S by plant foliar tissue: Its interaction with sulfate assimilation

Exposure of spinach cotyledons to H₂S or sulfate resulted in a concentration and time dependent accumulation of water-soluble non-protein thiols. The increase in thiol content upon a combined exposure to H₂S and sulfate was comparable to that of the sum of the increases obtained by the individual exposures. However, it never exceeded the maximal level attained upon exposure to H₂S alone. Evidently, plants are able to utilize atmospheric H₂S as a sulfur source, which competes with the assimilatory sulfate reduction for thiol synthesis.     

Aboveground vegetation effects on the deposition and cycling of atmospheric sulfur: Chemical and stable isotope evidence

Up to 60% of the sulfate in upland forest throughfall and stemflow at Plastic Lake in central Ontario is non-precipitation by origin, but is derived from aboveground vegetation. The sources of this aboveground vegetation sulfate include dry deposited aerosols and SO₂, and mineralized plant organic S. σ³⁴S data indicate that atmospheric S dominates the upland forest ecosystems of southern and central Ontario, with little S isotope fractionation. Seasonal σ³⁴S variations in precipitation sulfate may be due to mixing of bacteriogenic and anthropogenic S. σ¹⁸O and concentration data indicate that oxidation of dry-deposited SO₂, and mineralization of organic S on vegetation surfaces may contribute one third or more of throughfall sulfate in summer and autumn, but less in late spring, perhaps due to foliar uptake of S during this season. Oxidized SO₂, or mineralized organic S contributes one third or more of stemflow sulfate during these seasons.     

雨滴击溅对耕作层土壤团聚体粒径分布的影响

为研究不同雨滴直径的降雨对耕作层团聚体的破碎及其粒径分布特征的影响,该文选取4个雨滴直径(2.67~3.79 mm)对耕层土壤(0~20 cm)团聚体进行雨滴击溅试验,每次试验各滴5 000滴,每1 000滴收集1次溅蚀团聚体。结果表明:1)所有收集次序中雨滴直径3.79 mm溅蚀量最大,累积雨滴数为2 000、3 000和4 000时,溅蚀量与雨滴直径均呈显著的指数函数关系。2)各雨滴直径的溅蚀量随粒径减小呈增大-减小-增大趋势,2 mm粒径的溅蚀量几乎为0,0.053 mm粒径的溅蚀量随雨滴直径增大而增大。3)相同雨滴直径不同累积雨滴数之间平均重量直径值差异不显著,相同累积雨滴数不同雨滴直径之间平均重量直径值差异不显著(P0.05)。4)不同雨滴直径溅蚀团聚体富集率随粒径变化一致,1 mm粒径溅蚀量团聚体富集率值接近0,0.053~1 mm粒径团聚体富集,1 mm粒径团聚体主要破碎成0.053~1 mm粒径团聚体,且粒级越小,富集率越高。研究可为黄土高原地区水土保持提供理论依据。     

Occurrence and turnover of atmospheric mercury over the nordic countries

A seasonal variation of both particle and gaseous Hg concentrations in the atmosphere is present in south-western Sweden. An average gaseous Hg level of 3.7 ng m^?3 is found in winter, compared to 2.8 ng m^?3 in summer. A weak decreasing south-north gradient for gaseous Hg in air over the Nordic countries is also present, with yearly average values from 3.2 to 2.8 ng m^?3. A gradient for particulate Hg is less clear. An air parcel trajectory sector classification of gaseous Hg levels in air, and to some extent the particulate associated Hg, clearly demonstrates the increased concentrations in the southern sectors, especially in south-western Sweden where the gaseous Hg increase is about I ng m^?3. These observations are consistent with an influence from the European continent. The average concentrations of Hg in precipitation at the various stations show a pronounced decreasing south-north gradient. A major portion of the total Hg present in precipitation is associated with particles. For the southern stations, a strong correlation between Hg and sulfate, or pH, is present suggesting a connection between Hg in precipitation and anthropogenic activities.     

Characteristics and Source Identification of Particulate Matter in Wintertime in Beijing

Aerosol samples were collected during the wintertime from Nov. 24, 1998 to Feb. 12, 1999 in Beijing, China. Chemical composition was determined using several analytical techniques, including inductive coupled plasma atomic emission spectroscopy (ICP-AES), graphite furnace atomic absorption spectroscopy (GFAAS) and flame atomic absorption spectroscopy (FAAS) for trace elements, ion chromatography (IC) for water-soluble ions and CHN elemental analyzer for organic carbon (OC) and elemental carbon (EC). The average concentration of aerosol was 375?±?169 μg m^?3, ranging from 136 to 759 μg m^?3. Multilinear regression (MLR) analysis was performed and crustal matter, secondary particles and organics were identified as three major components of aerosol in wintertime in Beijing, accounting for 57.3%?±?9.8%, 13.4%?±?8.0%, and 22.8%?±?5.9% of the total concentration, respectively. Based on performance evaluation, Al, SO₄ ^2? and OC were selected as tracers of the three components, with the regression coefficients of 23.5, 1.78 and 1.26, respectively. A regression constant of 19.6 was obtained, which accounts for other minor components in aerosol. On average 93.5% of the total aerosol concentration, ranging from 82% to 105%, was explained by crustal matter, secondary particle and organics. Meteorological conditions are important factors that can influence the concentration level and chemical composition of aerosols. Wind would be favorable for the pollutant dilution, leading to low aerosol levels, whereas too strong a wind may cause regional soil dust and local road dust to be resuspended resulting in a high contribution of crustal matter. Circuitous air movement, high RH% and low wind speed facilitated the secondary particle formation, not only inorganic salts, such as sulfate and nitrate, but also secondary organic carbon in a similar way.     

Comparison of aerosol sulfur and crustal element concentrations in particle size fractions from continental U.S. locations

A comparison of element concentrations in aerosols is made for S, Si, K, Ca, Ti, Fe, and Zn as a function of particle size during April 1976 in nonurban New Mexico, Colorado, and New Hampshire and urban and nearby rural St Louis, Missouri. In the submicrometer diameter range at all sites, S greatly exceeds Si, K, Ca, Ti, and Fe, which occur predominantly in the coarse particle (>1 μm) fraction. Particle size distributions of each of these elements are similar among the five sites, but prominent concentration anomalies and secondary size distribution differences are measureable and suggest differences in aerosol production processes and relative pollution source strengths. The results are potentially useful in evaluating causes of atmospheric visibility differences between the western and eastern U.S.A.     

A Study on Rain Acidification Processes in Ten Cities of China

A physico-chemical sub-cloud rain acidification model is used to simulate the acidification processes of rainwater in ten cities of China, based on the observation data of the chemical components of cloud- and rainwater and the concentrations of gaseous and aerosol species. The results show that there are in-cloud process and below-cloud process in the formation of acid rain, but the relative importance of these two processes is different in various cities. The acidification of rainwater under cloud is very important in Guiyang, Shanghai, Jilin, Beijing, Changsha and Chongqing, and the in-cloud and below-cloud process contribute equally to the acidification in Guilin, however, the acidification of rainwater in cloud is more important in Nanchang and Guangzhou. Overall, the aerosol has an alkalization effect on the rain, about 11%–25% of H⁺ concentration in the rainwater is neutralized over Southern China and 60%–70% is neutralied over Northern China by aerosol species. The effect of atmospheric SO₂ on acidification of rainwater is nonlinear and is different in various regions. The effects of the variation of atmospheric NH₃, H₂O₂ and aerosol species on pH of rainwater are also discussed in detail.     

Some approximations for the wet and dry removal of particles and gases from the atmosphere

Semi-empirical formulae are presented which can be used to estimate precipitation scavenging and dry deposition of particles and gases. The precipitation scavenging formulae are appropriate both for in- and below-cloud scavenging and comparisons with data indicate the importance of accounting for aerosol particle growth by water vapor condensation and attachment of the pollutant to plume or cloud particles. It is suggested that both wet and dry removal of gases is usually dictated by other than atmospheric processes. Dry deposition of particles to a canopy is shown to depend on canopy height, biomass, vegetative type and mean wind. Two large-scale practical problems are addressed dealing with the relative importance of wet and dry deposition and with the sources which contribute to deposition in a specific location.