Comparison of annular denuder and filter pack collection of HNO3 (g), HNO2((g), SO2 (g), and particulate-phase nitrate,nitrite and sulfate in the south-west desert

During the 15 January–4 February 1986 SCENES Special Study, a comparison study was conducted to determine atmospheric HNO₃ (g), HNO₂ (g), SO₂ (g), and particle-phase nitrate, nitrite and sulfate sampled with annular diffusion denuder and filter pack sampling systems for 12-h periods. The results of the ion chromatographic analyses of the denuder and filter extracts from the annular denuder system showed that an average of 88% of the total nitrate measured was HNO₃ (g), 97% of the total nitrite was HNO₂ (g), and 91% of the total sulfur was present as SO₂ (g). Analyses of the various gas-phase species collected by replicate annular denuder systems indicated that a precision of ± 3% to ± 18% was achieved using these denuders. The good agreement in HNO₃ (g) concentrations observed between the filter pack and the denuder (r²=0.873, slope=1.06±0.03, intercept=0±3.5 nmol m⁻³) results from the fact that the majority of the atmospheric nitrate consisted of HNO₃ (g), which minimized any positive artifact in HNO₃(g) due to loss of HNO₃(g) from particles collected in the filter pack. The particulate-phase nitrate correlation between the two sampling systems was not as good (r²=0.709, slope=0.519±0.045, intercept =0±1.2 nmol m⁻³) because the lower percentage of nitrate present as the particulate species was more affected by the loss of particulate nitrate during sampling with the filter pack.     

A comparison of measurements of atmospheric ammonia by filter packs,transition-flow reactors,simple and annular denuders and fourier transform infrared spectroscopy

Using data obtained during the 1985 Nitrogen Species Methods Comparison Study (1988,Atmospheric Environment22, 1517), several measurement methods for sampling ambient NH₃ are compared. Eight days of continuous measurements at Pomona College, a smog receptor site in Los Angeles, provided an extensive data base for comparing the following methods: Fourier transform i.r. spectroscopy (FTIR), three filter pack configurations, a simple and an annular denuder, and the transition flow reactor. FTIR was defined as the reference method and it reported hourly NH₃ concentrations ranging from > 60 to 2280 nmol m⁻³ (1.5−57ppb) during the course of the study, the highest values coming from the influence of nearby livestock operations.Although only limited quality assurance procedures were carried out, the following conclusions can, nevertheless, be drawn: most of the methods correlated highly with the FTIR method (correlation coefficientr > 0.96); generally, the linear regression slopes were close to unity and the intercepts were insignificantly different from zero at the 95% confidence level); relative to the FTIR average values, (1) for 4–6 h sampling periods, the averages of the three filter packs from three research groups were 83–130% and the annular denuder average was 87%, and (2) for 10–12 h sampling periods, the simple denuder averaged 90% and the two transition flow reactors were 77–98%. Possible reasons for the reported systematic biases are presented, but these are not able to fully explain the large range of differences reported by the various methods.     

Kinetics of evaporation of ammonium chloride and ammonium nitrate aerosols

The evaporation rates of ammonium chloride and ammonium nitrate were measured by continuously and rapidly removing gaseous NH₃ and HNO₃ or HCl from aerosols in an annular denuder. The experiments gave the evaporation rates in terms of mass loss of chloride or nitrate which can be expressed conveniently as the rates of reduction of aerosol radius with time. Both dry aerosols (humidity 30–60% r.h.) and aqueous aerosols (humidity ca 97% r.h.) were studied. Dry aerosols evaporate at rates of −1.05 Å s⁻¹ for NH₄Cl and −0.45Å s⁻¹ for NH₄NO₃, while the evaporation rates of aqueous aerosols expressed as for equivalent dry particles are −4.52 Å s⁻¹ for NH₄Cl and −0.49 Å s⁻¹ for NH₄NO₃. The experimentally measured rates are independent of particle radius and remarkably low compared with those predicted from existing theories of aerosol evaporation, thus implying that there is an unknown kinetic constraint to the achievement of equilibrium at atmospheric temperature and pressures.     

Measurement and evaluation of acid air pollutants in Chicago using an annular denuder system

The cyclone/annular denuder/filter pack sampling system (ADS) was used to collect and evaluate ambient air pollutants in Chicago. Eighty-one 12-h samples, equally divided into day/night intervals, were collected from April 1990 to March 1991. The chemical species measured were HNO₃, HNO₂, SO₂ and NH₃ in the gas phase, and SO₄²⁻, NO₃⁻, NH₄⁺, and H⁺ in the particulate phase.The ADS data were collected simultaneously with PM10 samples. The particulate matter was analysed for elemental composition. These compositions were combined with the ADS observations and subjected to evaluation using a chemical mass balance receptor model (CMB). From the CMB analysis, the sum of the contributions from soil (15%), mobile (14%), incinerator (2%), coal (0.6%), steel (0.3%) and refinery (0.2%) was 32% of the PM10. NO₃⁻, which was not included in the fingerprints, represented an additional 9% of the PM10. Residual SO₄²⁻ and residual organic carbon, possibly formed in the atmosphere, represented an additional 22 and 20% of the PM10, respectively, leaving only 17% from other unidentified sources. From the standpoint of source contributions of sulfur and nitrogen compounds, coal combustion (23%) and refinery emissions (23%) are the major contributors of ambient sulfur (with 49% from unidentified sources). Mobile sources (87%) contributed most of the ambient nitrogen (with only 2% from unidentified sources).     

Scavenging ratios and deposition of sulphur,nitrogen and chlorine species in eastern England

Measurements of wet deposited NH₄⁺, SO₄²⁻, NO₃⁻ and Cl⁻, as well as airborne concentrations of these species and gaseous HNO₃, HCl and NH₃, have been made at a site in eastern England. Scavenging ratios based solely upon aerosol-associated species and upon aerosol plus gaseous airborne species are presented and compared with literature values. It appears that HCl and HNO₃ have only a rather minor influence upon wet deposition at our site. Gaseous NH₃ influences ground-level air chemistry appreciably, but scavenging ratios for NH₄⁺ are low, even when based upon aerosol NH₄⁺ concentrations alone, presumably due to altitudinal gradients in this species. The problems inherent in interpretation of scavenging ratios are discussed. Deposition of nitrogen in various chemical forms is estimated from rainwater and air composition. If a transport-limited deposition velocity is assumed for ammonia gas, dry deposition of this species accounts for around 40% of total nitrogen deposition to the ground.     

European scale application of atmospheric reactive nitrogen measurements in a low-cost approach to infer dry deposition fluxes

A European scale network was established in 2006 as part of the NitroEurope Integrated Project to infer reactive nitrogen (Nr) dry deposition fluxes, based on low-cost sampling of gaseous and aerosol species and inferential modelling. The network provides monthly measurements of NH₃, NH₄⁺, HNO₃ and NO₃⁻, as well as SO₂, SO₄²⁻, HCl, Cl⁻ and base cations at 58 sites. Measurements are made with an established low-cost denuder methodology (DELTA) as a basis to: (1) examine temporal trends and spatial patterns across Europe, (2) improve and calibrate inferential modelling techniques to estimate exchange of Nr species, (3) provide best estimates of atmospheric dry N deposition, and (4) permit an analysis of net GHG exchange in relation to atmospheric and agricultural N inputs at the European scale. Responsibility for measurements is shared among seven European laboratories. An inter-comparison of the DELTA implementation by 6 laboratories at 4 test sites (Montelibretti, Italy; Braunschweig, Germany; Paterna, Spain and Auchencorth, UK) from July to October 2006 provided training for the laboratories and showed that good agreement was achieved in different climatic conditions (87% of laboratory site-means within 20% of the inter-laboratory median). Results obtained from the first year of measurements show substantial spatial variability in atmospheric Nr concentrations, illustrating the major local (NH₃) and regional (HNO₃, NO₃⁻, and NH₄⁺) differences in Nr concentrations. These results provide the basis to develop future estimates of site-based Nr dry deposition fluxes across Europe, and highlight the role of NH₃, largely of agricultural origin, which was the largest single constituent and will dominate dry Nr fluxes at most sites.     

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.     

应用扩散管测量霾污染期间大气氮硫化合物浓度的方法

活性氮和硫化合物在大气颗粒物形成过程中扮演重要角色,但对它们气相/颗粒相的同步观测结果比较缺乏.本研究尝试基于扩散管的DELTA系统测量氮和硫化合物短时累积浓度,以期捕捉它们在霾污染期间的演变规律.结果表明,DELTA系统收集气态污染物的扩散管中以及颗粒物滤膜上NH_4~+和NO-3空白干扰较小,适用于研究NH_3、HNO_3、NH_4~+和NO-3的日均浓度,可以作为城市环境空气质量监测参数的有效补充;但采样系统中SO_2-4背景含量较高,仅适合监测48 h以上时间尺度的SO_2浓度和周~月尺度SO_2-4浓度,用于大气硫沉降观测.北京2016年5月9日~6月7日观测期间,大气NH_3、HNO_3、NH_4~+和NO-3浓度具有明显的逐日演变规律,呈现出随着风向转变而发生周期性波动的典型特征;这些含氮污染物与PM_(2.5)、CO、SO_2和NO_2浓度的变化规律一致,其来源可能与化石燃料燃烧源有关.污染天NH_3、HNO_3、NH_4~+和NO-3浓度约为清洁天的2倍,但还原性氮和氧化性氮的相态分布在清洁天和污染天无明显差异;整个观测期间,HNO_3/NO-3约为1.2,NH_3/NH_4~+为4.5,春夏之交较高的温度有利于活性氮在气粒平衡过程中偏向于气态形式存在.     

The spatial distribution and particle size of some inorganic nitrogen,sulphur and chlorine species over the North Sea

The use of filter packs and a cascade impactor during a series of research cruises in the southern area of the North Sea has yielded detailed spatial distribution patterns of aerosol concentrations, Cl⁻, NO₃⁻, SO₄²⁻¹ and NH₄⁺ and gaseous concentrations, HCl, HNO₃ and NH₃. The overall distribution of the atmospheric concentrations closely parallels published modelled results for metallic species. The chemical transformations of these aerosols and gases are investigated together with their interactions with the seasalt aerosol. Aerosol chloride loss is greatest in the more polluted areas, whilst concentrations products of NH₃ with HNO₃ and HCl appear insufficient to sustain the existence of NH₄NO₃ and NH₄Cl. Nitrate is associated predominantly with larger particles and appears to be present substantially as a surface coating on marine aerosol. The total dry deposition input for nitrogen species is calculated for the southern sector with extrapolation to the whole of the North Sea, using particle size weighted deposition velocities of 0.63 and 0.21 cm s⁻¹ for NO₃⁻¹ and NH₄⁺, respectively, and literature-derived values for the gaseous constituents. Finally the use of air-mass back trajectories illustrates the role of source regions in influencing the chemical composition of the North Sea atmosphere.     

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.     

Atmospheric concentrations and deposition of nitrogen and major ions in conifer forests in the United States and Federal Republic of Germany

Emission densities of air pollutants are higher in Europe than in the U.S. as a whole, suggesting similar differences in atmospheric deposition. We determined air concentrations and deposition during the warm season at conifer forests in Tennessee and northern Germany. Our results confirmed major differences in both chemistry and fluxes. Atmospheric and precipitation concentrations of all ions except H⁺ were higher at the German site, most significantly for the nitrogen species. The much higher levels of NH₄⁺ at this site reflect higher emissions of NH₃, which was the species largely responsible for the lower levels of H⁺. Total airborne nitrate was dominated by HNO₃ in Tennessee. In Germany we found comparable amounts of HNO₃ and aerosol NO₃⁻, the concentration of which varied seasonally, apparently in response to agricultural emissions of NH₃ that reacted to form NH₄NO₃. Total deposition of all major ions was much higher at the German site, particularly for the nitrogen species, which exhibited a marked edge effect in throughfall. Dry deposition was determined from air concentrations by using a canopy resistance model and from a statistical model of throughfall fluxes, each of which yielded comparable fluxes for several ions. Dry deposition contributed 10–70% of the ion input and was most important at the German site. Both forest canopies absorbed 40–50% of total deposited nitrogen, primarily from dry deposition.     

Interactions of aerosols (ammonium sulfate,ammonium nitrate and ammonium chloride) and of gases (HCl,HNO3) with fogwater

The concentrations of aerosols (NH₄NO₃, (NH₄)₂SO₄ and NH₄Cl) and of gases (HCl_(g), HNO_3(g), NH_3(g) were determined by denuder methods under different conditions (in the absence of fog, before, during and after fog events). At this site situated in an urban region, high concentrations of the gaseous strong acids HCl_(g) and HNO_3(g) are observed. NH₄Cl and NH₄NO₃ aerosols represent a major fraction of the Cl⁻ and NO₃⁻ aerosols (<2.4 μm)collected by denuders. During a fog event, very high concentrations of SO₄²⁻ were found in small aerosols, which are attributed to the aqueous phase oxidation of SO₂ under the influence of high pH due to the presence of NH₃. Differences in SO₄²⁻ concentrations measured in aerosols (<2.4 μm) and in fog droplets were probably due to mass-transport limitations of the SO₂ oxidation. Ammonium sulfate aerosols represent in some cases a significant fraction of the total S present (SO_2(g) + SO₄²⁻. Soluble aerosols and gases contribute to the composition of fogwater and are released again after fog dissipation.     

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.     

Measurements of atmospheric HNO3, HCl and associated species on a small network in eastern England

Measurements of gaseous HNO₃, HCl and NH₃ and particulate NO₃⁻, SO₄²⁻, Cl⁻ and NH₄⁺ have been made at a small network of sites in eastern England using sampling intervals from 3 h to 7 days. Both HCl and HNO₃ are spatially rather uniform, with some variation apparently due to spatial variations in NH₃, which stoichiometrically exeeded the sum of both gaseous acids. Mean concentrations of NH₃, HCl and HNO₃ between February 1987 and January 1988 were 1.90, 0.67 and 1.01 μg m⁻³, respectively. Pollution roses revealed low NH₃ concentrations, and high associated HCl and HNO₃ with winds from the North Sea. HCl, but not HNO₃ showed an appreciable elevation in concentration on the sector NW from our site, which we speculate may be due to the large capacity of coal-fired power stations in this upwind sector. Three-hourly data have been examined for diurnal effects and its is concluded that nocturnal formation of NO₃⁻ is occurring.     

A multiple-system,multi-channel diffusion denuder sampler for the determination of fine-particulate organic material in the atmosphere

Measurement of the loss of semi-volatile organic compounds from particles collected with a filter is carried out by comparing the amounts collected by comparable filter pack and diffusion denuder samplers. The sorbents used to collect organic compounds in the denuder and sorbent filters must have the same efficiency for collection of all gas-phase organic compounds present. Interpretation of the data requires that the efficiency of collection of gas-phase compounds by the denuder be known. In theory this can be accomplished by determination of the deposition pattern of all organic compounds collected in the denuder, but in practice this is very difficult if the organic material consists of a wide variety of compounds. An alternative approach is to determine the breakthrough of organic compounds in a sampler with a particle-collection filter preceding the denuder and sorbent filter. In such a sampler only gas-phase organic material enters the denuder. We have developed both a multi-channel parallel plate diffusion denuder sampler and a comparable sampler in which the denuder is preceded by a filter. Samples can be collected with the multisystem sampler at a flow rate of 35 sLpm. The denuder surfaces and the sorbent filters are made from sheets of an activated charcoal-impregnated filter paper. Collection of semi-volatile compounds by the samplers has been characterized and the systems have been field tested. The samplers are now being used for the routine collection and determination of semi-volatile organic compounds in particles at Canyonlands National Park in southeastern Utah. Available data from this field program show significant losses of particulate organic compounds on a quartz filter due to volatilization during sampling.     

Cloud chemistry at Mt Rigi,Switzerland: Dependence on drop size and relationship to precipitation chemistry

The chemical composition of winter and spring cloud water sampled at 1620 masl elevation on Mt Rigi in central Switzerland was dominated by NO₃⁻, SO₄²⁻, NH₄⁺ and H⁺. A wide range of concentration levels was observed, with maxima of 3700, 1800 and 4600 micronormal for NO₃⁻, SO₄²⁻ and NH₄⁺, respectively. Concentrations at a lower elevation (1030 masl) site on the mountain were higher due to lower cloud liquid water contents and higher pollutant levels at that site. The lowest pH observed was 2.95; large concentrations of NH₃ in the region prevented pH values from falling even lower. A comparison of simultaneously sampled cloud water and precipitation revealed much higher concentrations for most species in the cloud water, except in one case of extreme precipitation riming when the concentrations in the two phases converged. An exception to the pattern was H⁺; at times the precipitation was more acidic than the cloud water. The chemical composition of the cloud drops varied with drop size. Drops smaller than 10 μm diameter were enriched in NO₃⁻, SO₄²⁻ and NH₄⁺ relative to larger drops. Since the larger drops are the ones most effeciently captured by snow crystals, knowledge of their composition is essential to understanding the chemical implications of accretional growth of precipitation.     

被动采样法观测研究京津冀区域大气中气态污染物

为了对京津冀区域的大气污染物进行观测,揭示污染物浓度和组分的时空分布和变化规律,深入了解区域复合型污染,从2007年12月开始使用造价低、操作简捷的被动采样方法对区域10个站点大气中的SO2、NO2、O3和NH3等主要污染物进行了监测,对被动采样方法的区域适用性进行了比较全面的评估并根据监测结果对污染物的浓度水平和区域分布进行了研究.方法适用性评估表明被动采样方法在污染较严重的京津冀区域能进行长时间采样,采样频率设定为每月1次;平行采样结果显示,SO2、NO2、O3和NH3的变异系数分别为6.4%、7.1%、4.2%和3.9%,方法表现出良好的稳定性;每月1次的被动采样浓度结果与主动采样仪器观测结果月平均值相比具有较好的一致性,SO2、NO2和O3这2种方法监测结果的相关系数达到0.91、0.88和0.93,拟合曲线斜率分别为1.25、0.98和0.93,平均相对标准偏差分别为23.3%、14.9%和8.5%,能基本满足大气采样的要求,NH3的短时监测也表明2种方法具有可比性.评估结果说明被动采样方法是一种可靠的大气污染监测方法,可用于区域污染的监测.2008年夏季京津冀区域10个站点SO2、NO2、O3和NH3的被动采样平均浓度分别(12.3±6.3)×10-9、(13.2±7.0)×10-9、(40.5±9.5)×10-9和(24.0±13.7)×10-9.浓度区域分布显示SO2和NO2在城市站点具有较高浓度,而NH3在农业站点的浓度较高,SO2、NO2和NH3的大气浓度水平明显受局地排放影响,浓度分布较直观的反应了站点的局地源排放;而O3除了背景站兴隆,在北京和天津周边的大小城市,平均浓度都在40×10-9左右,表现出区域协同污染特征.     

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.     

Chemical composition of rain in Thessaloniki,Greece, in relation to meteorological conditions

Wet precipitation was collected in Thessaloniki, Greece, during the period March 1989–December 1990 by using an automatic wet-only precipitation sampler.Rainwater samples were analysed for major cations (H⁺, NH₄⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺) and anions (Cl⁻, NO₃⁻, SO₄²⁻), in addition to acidity and conductivity measurements. The majority of rain had a neutral or alkaline character as a result of neutralization, primarily caused by calcareous soil dust and secondarily by atmospheric ammonia. In all rain, SO₄²⁻ concentration exceeded NO₃⁻ concentration. The contribution of maritime sources to the total SO₄²⁻ concentration was very low (<2%).The chemical composition of precipitation was analysed in conjunction with meteorological variables (season of the year, precipitation type, airflow patterns) to evaluate temporal variations and chemical source influence. Rain caused by weak, localized flows showed the highest acidity and the minimum influence of neutralization processes.     

Transported acid aerosols measured in southern Ontario

During the period 29 June 1986–9 August 1986, a field health study assessing the acute health effects of air pollutants on children was conducted at a summer girls' camp on the northern shore of Lake Erie in SW Ontario. Continuous air pollution measurements of SO₂, O₃, NO_x, particulate sulfates, light scattering, and meteorological measurements including temperature, dew point, and wind speed and direction were made. Twelve-hour integrated samples of size fractioned particles were also obtained using dichotomous samplers and Harvard impactors equipped with an ammonia denuder for subsequent hydrogen ion determination. Particulate samples were analyzed for trace elements by X-ray fluorescence and Neutron Activation, and for organic and elemental carbon by a thermal/optical technique. The measured aerosol was periodically very acidic with observed 12-h averaged H⁺ concentrations in the range < 10–560 nmoles m⁻³. The aerosol H⁺ appeared to represent the net strong acidity after H₂SO₄ reaction with NH₃(g). Average daytime concentrations were higher than night-time for aerosol H⁺, sulfate, fine mass and ozone. Prolonged episodes of atmospheric acidity, sulfate, and ozone were associated with air masses arriving at the measurement site from the west and from the southwest over Lake Erie. Sulfate concentrations measured at the lakeshore camp were more than twice those measured at inland sites during extreme pollution episodes. The concentration gradient observed with onshore flow was potentially due to enhanced deposition near the lakeshore caused by discontinuities in the meteorological fields in this region.