Spatial and temporal patterns in summertime sulfate aerosol acidity and neutralization within a metropolitan area

A study of sulfate aerosol acidity in Metropolitan Toronto was conducted during the summer of 1986. Fine-fraction aerosol (<2.5-μm) were collected using Teflon membrane filters and analyzed for major ionic species (H⁺, NH⁺₄, NO⁻₃, SO²⁻₄). Samples were collected for 6 weeks at three study sites: one in the Center City and the others 13 km (WNW) and 20 km (NE) away. There were very strong correlations among the three sites with respect to measured aerosol species (r² > 0.9 for 24-h data). However, spatial variations in the magnitude of aerosol acidity were observed during sulfate episodes. For example, the peak concentrations for all sites occurred on 25–26 July 1986. While the 24-h data for sulfate were quite uniform at the three sites (34, 34 and 35 μg m⁻³), H⁺ concentrations were 9.4, 8.3 and 6.0 μg m⁻³ (as H₂SO₄) for the NE, WNW and Center City sites, respectively. For most of the summertime episodes, the downtown area also had lower aerosol acidity compared to the two sites in suburban areas.     

Processes affecting concentrations of aerosol strong acidity at sites in eastern England

Concentrations of aerosol strong acidity and related species have been measured at sites in eastern England using a sampler in which ammonia is pre-separated by a denuder. High concentrations occurred at a coastal site and were associated with air advected over the North Sea. At inland sites, ammonia concentrations were higher and the aerosol was more substantially neutralized. Daytime concentrations of aerosol H⁺ exceeded those measured at night, despite higher daytime levels of ammonia, presumably due to more effective production of H₂SO₄ during daytime hours. Concentrations of acidic aerosols were within the range 0–178 neq m⁻³, well below those observed at many eastern North American sites with lower concentrations of ammonia.     

北京中关村地区气溶胶的酸性测量

为了研究大气气溶胶的酸性及其粒径分布,笔者利用自制环状扩散管和三级撞击式组合采样器采样,ｐＨ测量和傅利叶红外光谱技术测量相结合,于１９９４处冬季至１９９５年冬季在北京中关村地区进行了采样分析。结果表明,北京中关村地区气溶胶存在酸性组成,且酸性主要分布于粒径１．５μｍ以下的细粒子中,测得的细粒子最大酸度为５６．６ｎｍｏｌ．ｍ＾－３,各冬季１９９４年冬季和１９９５年夏季气溶胶酸性较强,日平均一般在５ｍ     

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.     

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.     

上海市夏季颗粒物污染过程数值模拟研究

利用观测资料和嵌套网格空气质量模式(Nested Air Quality Prediction Model System,NAQPMS)模拟研究了2010年7月26日—8月26日上海市及周边城市PM10、PM2.5及其无机盐组分的浓度变化趋势及时空分布特点.结果表明,NAQPMS模式较为合理的重现了上海各方位站点及其周边城市PM10、PM2.5及其硫酸盐、硝酸盐等无机化学组分的浓度水平与变化趋势,相关系数在0.7以上.研究期间造成上海颗粒物污染的主要原因是:弱气旋低压系统控制下,西南或西北气流将内陆污染物输送至上海市,当低压中心移至上海附近时带来的辐合气流使得污染进一步累积上升.长三角地区PM2.5主要无机盐组分分布特征表明,上海市及周边城市的硫酸盐、硝酸盐和铵盐的总和占PM2.5浓度的40%~60%,二次气-粒转化过程贡献明显,且以SO2向SO2-4的氧化转换为主;污染上升过程中NO-3/SO2-4比率增大,说明流动源的贡献有所增加.     

Wintertime measurements of aerosol acidity and trace elements in Wuhan,a city in central China

A 2-week intensive ambient aerosol study was conducted in December 1988 in Wuhan (Hubei Province), a city of nearly 2 million located on the Yangtze River in central China (P.R.C.). This is an industrial region where soft coal burning is widespread, and emission controls for vehicles and industrial facilities are minimal. The sampling site was located in one of the civic centers where residential and commercial density is highest. An Andersen dichotomous sampler was operated with Teflon membrane filters to collect fine (d_p < 2.5 μmad) and coarse (2.5 ⩽ d_p < 10 μmad) particles for total mass and element determinations. An annular denuder system (ADS) was used to collect fine fraction aerosols for analyses of ionic species including strong acidity (H⁺).The study was conducted between 18 and 30 December, which was rainless, consistently cool (3–10°C) and overcast, but without fog or acute stagnation. Fine particulate mass (PM, as μ m⁻³) averaged 139 (range 54–207); coarse PM averaged 86 (range 29–179). Trace element concentrations were also high. Crustal elements (Si, Al, Ca and Fe) were found primarily in the coarse fraction, while elements associated with combustion (S, K, Cl, Zn and Se) were enriched in the fine fraction. The concentrations of arsenic and selenium were evidence of a large source of coal burning, while vanadium levels (associated with fuel oil use) were not especially enriched.Despite the seemingly high PM loadings, ionic concentrations were not especially high. The average composition of soluble fine aerosol species (in neq m⁻³) were SO₄²⁻: 520 (range 180–980), NO₃⁻: 225 (range 50–470), Cl⁻: 215 (range 20–640), and NH₄⁺: 760 (range 280–1660). A deficit in accountable FP components (total mass compared to the total of ionic plus element masses) as well as the black appearance of collected materials indicate an abundance of carbonaceous aerosol, as high as 100 μ m⁻³. (total mass compared to the total of ionic plus element masses) as well as the black appearance of collected materials indicate an abundance of carbonaceous aerosol, as high as 100 μ m⁻³Aerosol acidity was negligible during most monitoring periods, H⁺: 14 (range 0–50 neq m⁻³, equivalent to 0–2.5 μm m⁻³ as H₂SO₄). Sulfur dioxide, measured by the West-Gaeke method for part of the study, concentrations were low. Although not directly measured, the aerosol measurments suggested that gaseous HCl (from refuse incineration) and NH₃ (animal wastes) concentrations might have been high. Higher aerosol acidity might be expected if HCl sources were more prominent and not neutralized by local ammonia or other base components.     

天津市2017年重污染过程二次无机化学污染特征分析

基于2017年天津市超级观测站数据,筛选出7次典型重污染过程,从污染物浓度、二次转化方面分析重污染过程二次无机化学污染特征.结果表明,重污染期间NO₃^-和SO₄^2-浓度较清洁天气增长幅度最高,显著高于PM_2.5的增长程度,说明二次无机转化是导致重污染期间PM_2.5污染加重的重要原因;下半年PM_2.5和SO₂污染程度较上半年减轻,与秋冬季采取燃煤治理等活动有关;重污染期间NO₂/SO₂比值为1.5~19.6,其中下半年NO₂/SO₂比值显著高于上半年,说明在各项污染源管控下移动源的影响比例相对增加;大部分重污染期间NO₃^-浓度大于SO₄^2-浓度,SOR值高于NOR值,说明重污染期间硫酸盐和硝酸盐转化均较重要;在SO₂浓度显著降低的情况下,重污染期间SO₄^2-浓度并未明显降低,说明除二次无机转化外,硫酸盐生成还受其他因素影响.     

天津地区大气污染状况和气溶胶硫酸盐的研究

我们于1980—1981年对天津市大气污染物浓度做了同步监测,对天津市大气污染状况,二氧化硫转化为硫酸盐的过程及其对大气能见度的影响做了研究。以超C_i几率和的方法处理数据,研究结果表明,大气污染以早晨最严重,傍晚次之,中午较轻:冬季比夏季污染严重。各种污染物中以硫酸盐对大气能见度的影响最大,总颗粒物次之。当大气相对湿度大于70%时,对大气能见度的影响比较突出。颗粒物中的苯溶物对大气能见度有一定的影响。城市中相对湿度高时硫酸盐浓度大,可能是产生冬季烟雾事件的原因。下风方向的硫酸根浓度与二氧化硫浓度的比值和硫酸根含硫浓度与大气总硫浓度的比值均比上风方向相应的数值高,说明城市燃煤烟气是硫酸盐的来源。城市中硫酸根浓度与二氧化硫浓度的相关系数为0.77,硫酸根与颗粒物浓度的相关系数为0.95。在形成硫酸根的过程中二氧化硫浓度和颗粒物可能有贡献。以一维有源简化模式估算二氧化硫转化为硫酸根的速率常数,约为2.3±1.1%/hr,冬季约1.5%/hr。二氧化硫的迁移距离约为17—340km。     

Chemical characteristics of fine particles and their impact on visibility impairment in Shanghai based on a 1-year period observation

In this work, a one-year observation focusing on high time resolution characteristics of components in fine particles was conducted at an urban site in Shanghai. Contributions of different components on visibility impairment were also studied. Our research indicates that the major components of PM_2.5 in Shanghai are water-soluble inorganic ions and carbonaceous aerosol, accounting for about 60% and 30% respectively. Higher concentrations of sulfate (SO₄²⁻) and organic carbon (OC) in PM_2.5 occurred in fall and summer, while higher concentrations of nitrate (NO₃⁻) were observed in winter and spring. The mass concentrations of Cl⁻ and K⁺ were higher in winter. Moreover, NO₃⁻ increased significantly during PM_2.5 pollution episodes. The high values observed for the sulfate oxidizing rate (SOR), nitrate oxidizing rate (NOR) and secondary organic carbon (SOC) in OC indicate that photochemical reactions were quite active in Shanghai. The IMPROVE (Interagency Monitoring of Protected Visual Environments) formula was used in this study to investigate the contributions of individual PM_2.5 chemical components to the light extinction efficient in Shanghai. Both NH₄NO₃ and (NH₄)₂SO₄ had close relationships with visibility impairment in Shanghai. Our results show that the reduction of anthropogenic SO₂, NO_x and NH₃ would have a significant effect on the improvement of air quality and visibility in Shanghai.     

The investigation of air quality and acid rain over the Gulf of Mexico

A research cruise was conducted in the summer of 1986 by a group of scientist from the U.S.A. and Mexico to investigate air chemistry over the Gulf of Mexico. Chemical, physical, meteorological and oceanographic measurements were carried out to survey temporal and spatial variations of diverse parameters throughout the Gulf. Emphases were placed on air-sea-land exchange of gases and aerosols, natural air quality, transport of anthropogenic air pollution, and acid rain deposition to the Gulf. Although the prevailing winds were easterly from the sea during the cruise, the air was highly polluted with continental aerosols, probably caused by local shifting winds and the oscillation between sea breeze and land breeze. Aerosol number concentrations were measured from 10⁵ cm⁻³ at ports to 10³ cm⁻³ in the open Gulf. The average aerosol mass concentration was ∼25μg M⁻³, consisting of 60% insoluble crustal particles that contained Si, Al, Fe; 30% seasalt particles that contained Na⁺ and Cl⁻; and 10% anthropogenic sulfate and nitrate particles. Samples of rain water collected near the coast were acidic (pH ∼4). The concentrations of dimethyl sulfide correlated with bio-particle concentrations in surface seawater and could be a significant precursor of atmospheric SO₄²⁻ particles. The life cycles of the aerosols in the Gulf, including sources, transport, transformation, and wet and dry deposition are discussed.     

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.     

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.     

大气PM2.5中水溶性离子在线观测技术的应用研究

为了解北京大气PM2.5污染状况,评估大气细颗粒物快速捕集-化学成分在线分析系统(RCFP-IC)在追踪污染生成-消散过程中的适用性,于2011年3月对北京PM2.5中NO3-、SO24-、NH4+和Cl-这4种污染型水溶性离子浓度变化进行了连续高时间分辨率观测,并结合同期气象要素的变化,探讨了污染过程形成的原因.结果表明,一个月的观测期内捕捉到了5次较为明显的污染过程,4种水溶性离子的浓度变化趋于一致,并呈现出典型的"慢积累、快清除"的锯齿型污染物浓度时间序列变化特征.NO3-和NH4+在典型污染事件中峰值浓度是清洁时期浓度的10倍以上,而SO24-和Cl-污染峰值浓度仅为清洁时期的2～4倍.停暖后4种离子浓度较采暖期下降了15%～60%.RCFP-IC与高分辨率飞行时间气溶胶质谱(HR-TOF-AMS)同期观测结果变化趋势具有高度的一致性,但RCFP-IC定量水溶性离子浓度更为准确.     

Chemical and optical properties of aerosols and their interrelationship in winter in the megacity Shanghai of China

A field campaign on air quality was carried out in Shanghai in winter of 2012. The concentrations of NO, NO₂, NO_x, SO₂, CO, and PM_2.5 increased during haze formation. The average masses of SO₄^2-, NO₃^- and NH₄⁺ were 10.3, 11.7 and 6.7 μg/m³ during the haze episodes, which exceeded the average (9.2, 7.9, and 3.4 μg/m³) of these components in the non-haze days. The mean values for the aerosol scattering coefficient (b_sp), aerosol absorption coefficient (b_ap) and single scattering albedo (SSA) were 288.7, 27.7 and 0.91 Mm^-1, respectively. A bi-peak distribution was observed for the mass concentrations of CO, NO, NO₂, and NO_x. More sulfate was produced during daytime than that in the evening due to photochemical reactions. The mass concentration of NH₄⁺ achieved a small peak at noontime. NO₃^- showed lower concentrations in the afternoon and higher concentrations in the early morning. There were obvious bi-peak diurnal patterns for bsp and bap as well as SSA. bsp and bap showed a positive correlation with PM_2.5 mass concentration. (NH₄)₂SO₄, NH₄NO₃, organic mass, elemental carbon and coarse mass accounted for 21.7%, 19.3%, 31.0%, 9.3% and 12.3% of the total extinction coefficient during non-haze days, and 25.6%, 24.3%, 30.1%, 8.1% and 8.2% during hazy days. Organic matter was the largest contributor to light extinction. The contribution proportions of ammonium sulfate and ammonium nitrate to light extinction were significantly higher during the hazy time than during the non-haze days.     

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.     

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.     

上海市浦东城区二次气溶胶生成的估算

利用2010年5～10月不同日最高O3小时浓度(O3,max)下PM10浓度变化评估不同O3浓度水平下二次气溶胶生成量.CO作为一次颗粒物的标志物,O3作为光化学反应水平的指示物种.结果表明不同光化学水平条件下,PM10中一次与二次气溶胶浓度及比例存在较大差异.随着光化学水平的增加,PM10中一次气溶胶排放浓度增长不大(0.036～0.044 mg.m-3),二次气溶胶的生成量却呈数倍增长(0.018～0.055 mg.m-3);二次与一次气溶胶浓度的比例也从49.8%增加到124.5%.O3,max出现的时间也随着光化学水平的增强由13:00推迟到14:00,二次气溶胶生成的主要时段也从11:00～20:00增加到09:00～20:00;此外,PM2.5中主要组分SO24-、NO3-、OC等比例随着光化学水平(即O3,max浓度)的不同而存在一定差异,当O3,max<0.10 mg.m-3时,PM2.5主要由12.0%有机碳(OC)、18.7%硫酸盐、13.1%硝酸盐和4.5%元素碳(EC)组成,而O3,max>0.20mg.m-3时,PM2.5主要由20.0%有机碳(OC)、22.9%硫酸盐、23.1%硝酸盐和4.7%元素碳(EC)组成.说明SO24-、NO3-、OC受光化学水平影响较大.     

不同采样膜对大气颗粒物酸性测定影响的研究

颗粒物特别是酸性细粒子对人体呼吸系统具有毒性效应,且颗粒物呈酸性还会加速许多气溶胶二次组分的形成,从而影响环境和气候.因此,本文在美国环保署(EPA)制定的颗粒物酸性标准测定方法的基础上,分别采用玻璃膜、石英膜、Teflon膜和复合纤维素酯滤膜(MCE膜)在实验室中进行平行提取测定,探讨各种膜对颗粒物酸性测定的影响,确定不同条件下加入H+量与提取H+量之间的标准曲线,评价不同采样膜对颗粒物酸性测定的准确性与精密度.结果显示,Teflon膜、MCE膜、石英膜、玻璃膜用于颗粒物酸性测定,其测定的精密度和准确性分别为9.4%和7.8%、9.9%和13.0%、15.6%和18.1%、17.8%和16.1%.玻璃膜用于H+的数据测定时,由于其标准曲线的斜率变化范围大(0.745～1.048),基质碱性物质背景值高,导致测定的潜在误差最大.     

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.