Tests of the use of net throughfall sulfate to estimate dry and occult sulfur deposition

Throughfall and stemflow measurements taken in a mature high elevation red spruce stand, and precipitation measurements made in a nearby clearing, were used to calculate weekly net throughfall (=throughfall + stemflow - precipitation) sulfate deposition and net throughfall volume in the stand over a 20-week period. The study fortuitously was divisible into a low cloudwater deposition period, during which precipitation volumes generally exceeded throughfall volumes, and a high cloud period, during which the reverse was true. Weekly cloudwater deposition volume was estimated independently from continuously recorded cloudwater collections by an artificial tree located on an elevated platform in the clearing. Weekly net throughfall volume correlated well with cloudwater deposition volume (r = +0.86). Precipitation accounted for only 25% of the throughfall sulfate collected throughout the study and only 15% of that collected during the high cloud period, as net throughfall sulfate was 2.4 times greater during the high cloud period than during the low cloud period. Weekly estimates of cloudwater sulfate deposition correlated well (r = +0.74) with measures of net throughfall sulfate during the high cloud period. Dry deposition models were used to estimate weekly dry S deposition; weekly estimates of the wash-off of this dry deposition also correlated well (r = +0.76) with net throughfall sulfate during the low cloud period. During the high cloud period, estimates of cloudwater S plus dry S deposition accounted for 67% of the sulfate collected in net throughfall; however, during the low cloud period only 55% of net throughfall sulfate was accounted for. The low percentage of sulfate accounted for during the low cloud period suggests that the dry models were underestimating S deposition. Possible reasons for underestimation include failure to consider fully topographic complexity and edge effects, underestimates of surface wetness, and the possibility of canopy sources of sulfate (foliar leaching). These results support the use of throughfall sulfate measurements as gross estimates of (1) total S deposition, (2) total dry S deposition (using net throughfall in environments where cloudwater deposition accounts for less than 5% of total sulfate deposition) and/or (3) total cloud S deposition (subtracting precipitation and dry inputs from total throughfall sulfate in high cloud environments).     

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.     

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 comparison of two cloudwater/fogwater collectors: The rotating arm collector and the caltech active strand cloudwater collector

A side-by-side comparison of the Rotating Arm Collector (RAC) and the Caltech Active Strand Cloudwater Collector (CASCC) was conducted at an elevated coastal site near the eastern end of the Santa Barbara Channel in southern California. The CASCC was observed to collect cloudwater at rates of up to 8.5 ml min⁻¹. The ratio of cloudwater collection rates was found to be close to the theoretical prediction of 4.2:1 (CASCC:RAC) over a wide range of liquid water contents (LWC). At low LWC, however, this ratio climbed rapidly, possibly reflecting a predominance of small droplets under these conditions, coupled with a greater collection efficiency of small droplets by the CASCC. Cloudwater samples collected by the RAC had significantly higher concentrations of Na⁺, Ca²⁺, Mg²⁺ and Cl⁻ than those collected by the CASCC. These higher concentrations may be due to differences in the chemical composition of large vs small droplets. No significant differences were observed in concentrations of NO₃⁻, SO₄²⁻ or NH₄⁺ in samples collected by the two instruments.     

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

The wet, dry and cloud water deposition of acidic substances on the forest canopy are considered as major mechanisms for pollutant induced forest decline at high elevations. Direct cloud capture plays a predominant role of intercepting acidic substances in above cloud-base forests. We conducted a field study at Mt. Mitchell, North Carolina (35°44′05″N, 82°17′15″W; 2038 m MSL)—the highest peak in the eastern U.S.—during May–September 1986 and 1987 in order to analyze the chemistry of clouds in which the red spruce and Fraser fir stands stay immersed. It was found that Mt. Mitchell was exposed to cloud episodes 71% of summer days, the cloud immersion time being 28% for 1986 (a record drought summer in southeastern U.S.) and 41% for 1987. Sulfate, NO₃⁻, NH₄⁺ and H⁺ ions were found to be the major constituents of the cloud water, which was collected atop a 16.5 m tall meteorological tower situated among 6–7 m tall Fraser fir trees. The initiation of precipitation in clouds invariably diluted the cloud water acidity. The cloud water pH during short episodes (8 h duration or less), which resulted from the orographic lifting mechanisms, was substantially lower than that during long episodes, which were associated with meso-scale and synoptic-scale disturbances. Sulfate accounted for 65% acidity in cloud water, on the average, and contributed 2–3 times more than the NO₃⁻. Inferential micrometeorological models were used to determine deposition of SO₄²⁻ and NO₃⁻ on the forest canopy and the hydrological input due to direct cloud capture mechanism. The cloud water deposition ranged between 32 and 55 cm a⁻¹ in contrast to the bulk precipitation which was about 130 cm a⁻¹ as measured by an on-site NADP (National Atmospheric Deposition Program) collector. For S compounds, wet, dry and cloud water deposition accounted for 19%, 11% and 70%, respectively for 1986, and 16%, 8% and 76%, respectively for 1987. For N compounds, dry deposition contributed 35% and 23% for 1986 and 1987, respectively, whereas, cloud water deposition contributed 50% and 65% for 1986 and 1987, respectively. Our estimates are compared with the reported literature values for the other sites.     

Measurement and modelling of cloudwater deposition to a snow-covered forest canopy

Measurements are presented of the flux of cloud droplets, as a function of particle size to a forest canopy (Sitka spruce) before and after a light snowfall. The results are compared to the predivtions of the model of Slinn. Good agreement is found provided that appropriate values for the effective target diameter are chosen, both before and after the snowfall. It is shown that the snowfall results in a considerable reduction in the flux of cloud water to the forest canopy (by a factor of about 2). The snow reduces the surface roughness of the canopy but the most important effect is that it increases the effective target diameter for the droplets impacting on the tree. This effect may considerably reduce the deposition of phytotoxic chemical species to forests at high altitude where snow cover and low cloud are common in winter.     

Measurements of ammonia flux to a spruce stand in Denmark

This work demonstrates the existence of a linear relation between the deposition velocity of ammonia and the friction velocity measured above a spruce stand in the western part of Denmark. In order to estimate the ammonia deposition velocity and flux to a Norway spruce forest, concentration gradients of ammonia and several meteorological parameters were measured in a meteorology tower during two periods, 1 week in spring and 1 week in late summer 1991. The estimated deposition velocities lie in the range −0.125 to 0.201 m s⁻¹, with a mean of 0.026 m s⁻¹. The deposition velocity and the flux were generally largest in the afternoon. On the basis of 24-h measurements of ammonia and routine meteorological measurements the relation between deposition velocity and friction velocity is extrapolated to an estimate of the average flux for the growing season May to September 1991. The estimate gave an average flux of 87 μg NH₃N m⁻² h⁻¹ (=0.02 μg NH₃N m⁻² s⁻¹). The average deposition velocity for the period was 0.045 m s⁻¹.     

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.     

Comparison of three dry deposition models applied to field measurements in the Southern Bight of the North Sea

Dry deposition velocities have been calculated using three different approaches. Turbulent wind profile theory has been used to predict the drag coefficient, wind speed and friction velocity at 10 m height when the wind speed is measured at a higher altitude. The resulting parameters were introduced in a two-layer deposition model. The second approach was the well-known model of Slinn and Slinn (1980, Atmospheric Environment14, 1013–1016), whereas the third corresponded to the model published by Williams (1982, Atmospheric Environment16, 1933–1938). Results point to clear differences. However, in a field experiment carried out at the Southern Bight of the North Sea, all three approaches show relatively comparable results. The role played by the size distribution of atmospheric particulate matter is essential. In our case any of the three models could have given satisfactory outcomes taking into account the wide spread of the experimental results cited in the literature for the same airshed.     

采用巯基捕收剂稳定化处理垃圾焚烧飞灰中的重金属

研究了3种巯基捕收剂,二乙基二硫代氨基甲酸钠（乙硫氮）、乙基黄原酸钾（乙基黄药）和二丁基二硫代磷酰铵（丁铵黑药）,对垃圾焚烧飞灰中重金属的稳定化效果。3种巯基捕收剂的用量均为62.5μmol?g-1飞灰。扫描电镜观察发现,飞灰经稳定化处理后,巯基捕收剂均匀地覆盖于飞灰晶体表面,使矿物晶体棱角变得模糊。利用酸浸提程序（TCLP法）和水浸提程序（水平振荡法）评价飞灰中重金属Cu、Pb、Cd、Cr和Zn的浸出毒性。在0.1mol?L-1醋酸浸提条件下,与Na2S相比,巯基捕收剂对Cu和Pb的稳定化效果较好,其中乙硫氮对Cu的稳定化比率接近100%,丁铵黑药对Pb的稳定化比率达到69.2%;在水浸提条件下,乙硫氮、乙基黄药和丁铵黑药对5种重金属的稳定化比率分别为72.6%、73.5%和76.8%,显著高于Na2S处理（52.4%）。三种巯基捕收剂对5种重金属亲和力的强弱顺序大致为Cu＞Pb＞Cr＞Cd＞Zn,并且超过60%的巯基捕收剂与酸可浸提重金属离子发生了螯合沉淀反应。重金属-巯基捕收剂絮凝物在中性和碱性条件下（pH>6）比较稳定,在酸性条件下（pH<6）可发生部分溶解。为获得较好的重金属稳定化效果,维持稳定化飞灰的高酸缓冲容量十分重要。     

降尘收集方法对降尘效率的影响

大气降尘收集方法较多,但目前还没有通用的野外观测方法,由于降尘收集方法的多样性以及降尘缸收集效率的不同,所观测的结果很难进行对比.目前对各类降尘缸收集降尘效率的研究较少.在腾格里沙漠地区,利用国家标准规定降尘缸,对干法、湿法、过滤网法、过滤网+玻璃球法和减速沿法这5种降尘收集方法进行对比研究,旨在探讨不同降尘收集方法的收集效率,为准确评价区域降尘量提供依据.干法收集的降尘量仅占湿法的5%～62%;减速法仅占湿法的50%～95%;过滤网法仅占湿法的43%～89%;玻璃球+过滤网法仅占湿法的40%～80%.不同的降尘收集方法,风速明显影响降尘效率,平均风速<2.5m·s-1时,降尘效率随平均风速增加而降低;但在平均风速>2.5 m·s-1时,降尘效率与平均风速之间无明显规律.湿法收集的降尘粒度最细,依次为减速法、玻璃球+过滤网法、过滤网法和干法.平均风速,>5 m·s-1的平均风速与湿法,减速法收集的降尘量之间可以用指数函数表示.不同降尘方法观测的数据可以相互转化,干法、过滤网、过滤网+玻璃球法、减速沿法与湿法收集的降尘量之间为线性相关.在干旱及半干旱地区,减速法能够作为一种替代湿法进行收集降尘的有效方法.     

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.     

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.     

A design for an occult precipitation collector

A prototype design for a cloud and mist collector is presented. The collector is a relatively simple and inexpensive design and excludes precipitation, making it suitable for long term collection of chemical data related to occult deposition. Chemical data obtained from weekly samples taken during an 18 month period using the occult collector and an adjacent bulk collector are presented.     

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.     

冲激式湿式除尘器在燃用高挥发份煤锅炉除尘的应用

本文总结了各种类型除尘器在使用当地含挥发份较高的长焰煤种过程中的除尘效率,及其性能特点,详细阐述了冲击式湿式除尘器在燃用高挥发份煤锅炉除尘中的应用效果。     

湿排渣多管除尘器在10t/h锅炉上的除尘应用实践

如何抑制多管除尘器落尘返混是提高多管除尘器除尘效率的关键，通过在10t／h锅炉上采用湿排渣除尘设计的实践应用表明，湿排渣多管除尘器能够有效地提高多管除尘器的除尘效率。     

Interannual variability in acidic deposition on the Mt. Mitchell area forest

Recently the forest decline at high elevation mountains in the eastern U.S. has been suggested to be associated with the deposition of acidic substances on the forest canopy through dry, wet and cloud deposition pathways. To determine the relative importance of these deposition mechanisms, a field study was initiated in May 1986, at Mt. Mitchell, NC. Since Mt. Mitchell is frequently immersed in clouds (immersion time being in the range of 28–41%), our investigations were primarily focused on the collection of cloud water and the monitoring of meteorology and ambient air quality. The precipitation data and related chemistry were obtained from a nearby NADP (National Atmospheric Deposition Program) site (Clingman's Peak). To estimate the dry and cloud deposition, the deposition velocities for gases and the rates of cloud deposition for cloud droplets are calculated with the ATDD (Atmospheric Turbulence and Diffusion Division, National Oceanic and Atmospheric Administration) model and a cloud deposition model (CDM), respectively. The wet deposition is obtained from NADP annual reports. Computations show the deposition velocities for SO₂, NO₂ and O₃ to be in the range of tenths of cm s⁻¹. The mean rate of cloud deposition is about 0.13–0.21 mm h⁻¹. The rainfall ranged from 40 to 60 cm during the growing seasons (from mid-May to the end of September) of 1986–1988. Using these deposition parameters and the 3 year database, the deposition fluxes of sulfur (S) compounds are found primarily contributed through cloud capture mechanism (60%) followed by incident precipitation (25%) and dry deposition (15%). As to the deposition fluxes of nitrogen (N) compounds, cloud, wet and dry deposition contributedf about 50%, 25% and 25%, respectively. A comparison of deposition estimates at Mt. Mitchell with those at other sites shows that the sulfate deposition at sites exceeding 1200 m MSL in elevation in Bavaria, F.R.G., and the eastern U.S. is a almost identical within error limits. Reasons for large uncertainties in deposition estimates are also discussed as the mechanisms for redistribution of the deposited material on the forest canopy.     

An intercomparison of semiempirical diffusion models under low wind speed,stable conditions

The performances of some diffusion models are analysed using concentration data measured at ground level up to 400 m from the emission point, in a series of diffusion tests conducted by U.S. National Oceanic and Atmospheric Administration (NOAA) under inversion conditions with light winds. All tested models are simple semiempirical formulae based on the Gaussian formulation, with different assumptions concerning dispersion parameters; each model utilizes a minimum set of information, i.e. vertical stability category, mean wind speed and standard deviation of the horizontal wind direction over the considered averaging time, σ_θ.Results show that for cases with very low wind speed and large plume spread, explicit consideration of diffusion along the mean wind direction, which is neglected in the standard plume model, significantly improves model results; moreover, when σ_θ is very large (greater than 50–60°), the analysis suggests that standard deviations of the horizontal wind speed may significantly differ from the estimates commonly found in the literature.