Importance of sulfate emission to sulfur deposition at urban and rural sites in China

Throughfall (TF) and wet only (WO) deposition along with SO₂ and sulfate (SO₄²⁻) concentration in air at 4 urban and rural sites in southwestern China were monitored in order to understand the role of different forms of sulfur (S) emission to the S deposition and its effect in China. The sites were located in Chongqing, Hunan, and Guizhou provinces. S deposition at the most polluted site reached 15 g S m^− 2 yr^− 1. At three of the sites, located in the vicinity of several emission sources, dry S deposition is 2.1–4.2 times that of wet deposition, which is significantly higher than what is found in most other parts of the world.Main components in airborne particles (PM₁₀) are (NH₄)₂SO₄ and CaSO₄ at the highly polluted Tie Shan Ping (TSP) site. Dust particles of gypsum (CaSO₄) in the air are partly due to direct emission and partly from the reaction of calcium oxides and carbonates with sulfuric acid in the air. To illustrate the importance of sulfate emission to total S deposition we analyzed the source of S deposition based on both measurements and models. Results indicated that direct emission of SO₄²⁻ particles could account for high proportion in total S deposition at the three most polluted sites.     

Wet deposition of atmospheric inorganic reactive nitrogen (Nr) across an urban-industrial-rural transect of Nr emission hotspot (India)

The present study comprehensively reports the simultaneous measurement of wet deposition of total inorganic nitrogen (TIN; which is the sum of the NH₄⁺-N and NO₃^?-N) at three different sites in Nr emission hotspot of Indo-Gangetic plain (IGP) over a year-long temporal scale from October 2017 to September 2018. At rural Meetli (MTL) site, urban Baraut (BRT) site and industrial Loni (LNI) site, the annual wet deposition of NH₄⁺-N was estimated as 21.87, 19.48 and 7.43 kg N ha^?1 yr^?1, respectively; the annual wet deposition NO₃^?-N was estimated as 12.96, 12.17 and 4.44 kg N ha^?1 yr^?1, respectively; and the annual wet deposition of TIN was estimated as 34.83, 31.64 and 11.87 kg N ha^?1 yr^?1, respectively. NH₄⁺-N was dominantly contributing species in annual, monsoon and non-monsoon-time wet deposition of TIN at all sites. The spatial gradient (variability) in percent contribution of NH₄⁺ to total annual volume-weighted mean (VWM) concentration of all analyte ions was observed as MTL (43.23%)?>?BRT (37.90%)?>?LNI (30%). On the other hand, the spatial gradient in percent contribution of NO₃^? to total annual VWM concentration of all analyte ions was observed as MTL (7.45%)?>?BRT (6.89%)?>?LNI (5.32%). The extremely narrow range of NH₄⁺-N/NO₃^?-N ratios (ranging from 1.60 at BRT site to 1.69 at LNI site) showed the approximately equal relative abundance of oxidized and reduced nitrogen (N) deposition across all sites. Inferences from enrichment factor analysis, principal component analysis and Pearson’s correlation coefficient analysis suggested that across all sites, virtually all NH₄⁺-N and NO₃^?-N depositions were originated anthropogenically. The annual wet deposition of TIN measured in this study showed?≥?6865%,?≥?6228% and?≥?2274% increment than the natural N deposition rate at MTL, BRT and LNI site, respectively. These empirically measured annual wet depositions of TIN also emanated theoretical transgression of critical N load threshold across all sites therefore signifying probable undermining of long-term elastic stability and resilience of ecosystems against stressor in the study domain.

     

Wet deposition of radionuclides derived from the Chernobyl accident

Chernobyl radioactivity in precipitation was measured at Tsukuba, Japan, as were both surface-air concentrations and particle-size distributions of Chernobyl-released radionuclides. To understand the wet removal processes of the Chernobyl radionuclides, i.e.¹³⁷Cs,¹⁰³Ru, and⁹⁰Sr, wet deposition velocities were calculated. The wet deposition velocities of the Chernobyl radioactivity for individual rainfall events varied largely. The wet deposition velocity is given as the product of washout ratio and rainfall rate. Typically, it was found that the washout ratios of⁹⁰Sr are systematically larger than those of¹³⁷Cs. In order to explain this fact, we examined the relationship between the washout ratios and particle sizes of radionuclide-bearing aerosols. A positive correlation between corrected washout ratios and particle size was found with a particle diameter range from 0.35 to 1.2 µm. The result strongly suggests that the factors controlling the wet removal of the Chernobyl radioactivity for individual rainfall events are surface air concentration, particle size, and rainfall rate, rather than precipitation amount, which is in agreement with previous understandings. This suggests that high contamination areas of radioactivity are formed during heavy rainfall events with high rainfall rates in the case of tropospheric injection such as the Chernobyl accident.     

Investigation of wet acidic deposition episodes capable of damaging red spruce in the Mt. Mitchell State Park

Cloud events whose wet deposition is comparable to that shown to trigger negative health symptoms in trees constitute environmentally significant episodes of wet deposition. They accounted for approximately one fourth of all cloud events that traversed the Mt. Mitchell State Park, NC, during the summer field seasons of 1986–1988 and 1993. The environmentally significant episodes of wet deposition most frequently occurred during the month of June. The June occurrence is equivalent to the frequency of acid mist treatments shown to initiate health related decline in Red Spruce. The slightly more than three seasons of data were analyzed for a possible relationship between cloudwater acidity, liquid water content, wind speed and wind direction. The data analysis shows: (i) A scatter plot of the liquid water flux versus the H⁺ ion flux differentiates between environmentally significant and non-environmentally significant cloud events of wet deposition. (ii) The synoptic and resulting local wind speed and direction are important in determining whether a cloud event will be environmentally significant or not, since they are related to the origin and the approach of the airflow into the Mt. Mitchell site. (iii) A characteristic 850 mb height field associated with the most frequently occurring environmentally significant cloud only event of wet deposition. Further investigation of wet acidic deposition episodes capable of damaging Red Spruce in the Mt. Mitchell State Park     

Wet deposition of ammonium in Europe

Ammonium concentration data in precipitation have been compiled to derive a concentration and deposition field for ammonium in Europe. Measurements referring to a total number of 218 measuring sites have been considered. Because of changes in the ammonium concentrations due to the use of improper sampling procedures, a correction procedure is proposed. This makes allowance for the type of sampler used, the length of the sampling period, and whether or not light-protected sample bottles are used. Dependent on the specific sampling procedure used correction factors range from 0.75 to 1.20. According to our calculations, the total wet deposition flux of ammonium in Europe in the early 1980s amounts to 2.4 Mt NH₄ ⁺y^-1. However, for some parts of Europe the flux cannot be estimated very reliably because of the low number or even the absence of measuring sites. Compared to earlier estimates for around 1960, the ammonium wet deposition flux has increased by approximately 25% during the period 1960–1980.     

Rainwater Chemistry and Wet Deposition over the Equatorial Forested Ecosystem of Zoétélé (Cameroon)

Within the framework of IDAF (IGAC DEBITS AFRICA: International GlobalAtmospheric Chemistry/DEposition of Biogeochemically Important TraceSpecies/Africa) network, data analysis is realised on precipitation chemical composition collected in Zoétélé, in Southern Cameroon. This station, located atabout 200 km from the Atlantic Ocean, is representative of a so-called `Evergreen Equatorial Forest' ecosystem. An automatic wet-only precipitation collector was operated at the station from 1996 to 2000. The rainfall regime, associated with eastward advection of moist and cool monsoon air masses, amounts to an average of 1700 mm/year. Inorganic and organic content of the precipitation were determined by IC in 234 rainfall events, representing a total 4,583 mm of rainfall from an overall of 7,100 mm.The mean annual precipitation chemistry and wet deposition fluxes characteristic of an African equatorial forest are quantified. Typical atmospheric gases and particles sources influence the precipitation chemical content and the associated deposition of chemical species. Indeed, hydrogen concentration is the highest (12.0 eq.L^–1) of the IDAF measurements, leading to acid rains with a low mean pH 4.92. The mineral species are dominated by nitrogenous compounds (NH₄ ⁺:10.5 and NO₃ ^–: 6.9 eq.L^–1), Ca²⁺ (8.9 eq.L^–1) and SO₄ ^{2 –} 5.1 eq.L^–1. Relationship between Ca^{2 +} and SO₄ ^{2 –} indicated aterrigeneous particulate source and an additional SO₄ ^{2 –} contributionprobably due to swamps and volcano emissions. Na⁺ and Cl^–concentrations, around 4.0 eq.L^–1, seem very low for this site,accounting for the marine source. Besides, strong correlations between NH₄ ⁺/K⁺/Cl^– indicate the biomass burning originof these species. Accordingly, precipitation chemistry in Zoétéléis influenced by three major sources: biogenic emissions from soil and forest ecosystems, biomass burning from savannah, and terrigenous signature from particles emissions of arid zones; and three minor sources: marine, volcano and anthropogenic. In spite of the relatively low concentration of all these elements, the wet deposition is quite significant due to the high precipitation levels, with for example a nitrogenous compounds deposition of 34 mmol.m^–2.yr^–1.     

Wet deposition of precipitation chemistry during 2005–2009 at a remote site (Nam Co Station) in central Tibetan Plateau

Chemical compositions of precipitation samples collected from a remote and high elevation site (Nam Co Station, 30°46.44??N, 90°59.31??E, 4730?m?a.s.l.) in central Tibetan Plateau (TP, hereafter) from August 2005 to August 2009 are investigated. During the study period, Ca²⁺ and HCO ₃ ^- have the highest concentrations among ions and are the dominant cation and anion in precipitation, taking 27.46?% and 30.84?% to the total ions respectively. Empirical Orthogonal Functions (EOFs) analyses reveal that crustal aerosol inputs significantly contributed to the loading of Ca²⁺, Mg²⁺, SO ₄ ^2- and HCO ₃ ^- in precipitation, while lake salt plays a major source of K⁺ and Cl^-. Seasonal variations of ionic wet deposition fluxes show high values during monsoon seasons due to large precipitation amount. Among the cations, annual Ca²⁺ flux is the largest (86.26?eq hm^?2), Na⁺ and NH ₄ ⁺ fluxes are following. Among anions, HCO ₃ ^- has the highest flux (98.66?eq hm^?2) while that of NO ₃ ^- is the lowest. Annual wet deposition of nitrogen has varied considerably with the average value of 0.70?kg?ha^?1 a^?1 at Nam Co Station. About 80?% of total nitrogen flux occurs during the monsoon seasons when precipitation is concentrated, in which NH ₄ ⁺ and NO ₃ ^- contributed to 61?% and 39?% of the total nitrogen deposition. Thus, our ionic concentrations and wet deposition fluxes in precipitation can provide a useful dataset to assess atmospheric environment and its impacts on ecosystem in the inland TP.     

Asian dust depositions over the Asian region during March 2010 estimated by ADAM2

The Asian Dust Aerosol Model 2 with the MM5 meteorological model has been employed to estimate the dust emission, dust concentration, and wet and dry deposition of dust in the Asian region for the month of March in 2010. It is found that the model simulates quite reasonably the dust (PM₁₀) concentrations both in the dust source region and the downstream region of Korea. The starting and ending times of most dust events and their peak concentration occurrence times are well simulated. The monthly mean maximum surface dust concentration (PM₁₀) is found to be 267???g?m^?3 in the domain of central northern China (CNC). Monthly total maximum dust emission of more than 32?t km^?2 and that of deposition of more than 25.4?t km^?2 (dry deposition of 24?t km^?2 and wet deposition of 1.4?t km^?2) are found to occur in the domain CNC, whereas the monthly mean minimum surface dust concentration (PM₁₀) is found to be 0.2???g?m^?3 in the domain of the Tibetan Plateau, where the monthly total dust emission (4?kg?km^?2) and the monthly total dust deposition (9?kg?km^?2) are found to be minimum. This monthly total dust deposition of 9?kg?km^?2 (dry deposition of 7?kg?km^?2 and wet deposition of 2?kg?km^?2) is as large as 2.25 times of that of emission (4?kg?km^?2), suggesting net dust influx toward the Tibetan Plateau from the surrounding dust source regions. It is also found that the ratio of the total dust deposition to the total dust emission in the source region increases toward the downstream direction from 0.4 in the upstream source region of Taklimakan to 0.80 in the downstream source region of northeastern China. More than 90% of the total dust deposition is found to be contributed by dry deposition due to the lack of precipitation in the dust source region. The monthly mean dust concentration (PM₁₀) is found to decrease with distance away from the dust source region. The monthly mean dust concentration of 62???g?m^?3 over the Yellow Sea (YES) decreases to 4.3???g?m^?3 over the Northwestern Pacific Ocean (NWP). The monthly total dust deposition in the downstream region is also found to decrease away from the source region from 2.33?t km^?2 (dry deposition of 1.36?t km^?2 and wet deposition of 0.97?t km^?2) over the domain YES to 1.45?t km^?2 (dry deposition of 0.16?t km^?2 and wet deposition of 1.30?t km^?2) over the domain NWP. A large amount of the total dust deposition over the seas is contributed by wet deposition (more than 90%), causing a small decreasing rate of the total dust deposition with distance from the source region. The estimated dust deposition could adversely impact the eco-environmental system significantly in the downstream regions of the Asian dust source region, especially over the seas.     

An investigation of the controls on Irish precipitation δ18O values on monthly and event timescales

This two-year study investigates the relative influence of meteorological variables (precipitation amount and temperature), atmospheric circulation, air mass history, and moisture source region on Irish precipitation oxygen isotopes (δ¹⁸O_p) on event and monthly timescales. Single predictor correlations reveal that on the event scale, 20% of δ¹⁸O_p variability is attributable to the amount effect and 7% to the temperature effect while on the monthly timescale the North Atlantic Oscillation accounts for up to 20% of δ¹⁸O_p variability and the amount and temperature effects are not significant. In comparison, multivariate linear regression reveals that the interaction of temperature and precipitation amount explains up to 40% of δ¹⁸O_p variance at event and monthly timescales. Five-day kinematic back trajectories suggest that the amount-weighted mean δ¹⁸O_p value of southerly- and northerly-derived events are lower by 2‰ relative to events derived from the west. Because air mass history and atmospheric circulation appear to influence δ¹⁸O_p in Ireland, Irish paleo-δ¹⁸O_p proxy records are best interpreted as reflecting a combination of parameters, not just paleotemperature or paleorainfall.     

Rainwater Chemistry and Wet Deposition over the Wet Savanna Ecosystem of Lamto (Côte d'Ivoire)

From the IGAC-DEBITS Africa network (IDAF), data sets on precipitation chemistry collected from the ‘wet savanna ecosystem’ site of Lamto (Côte d'Ivoire), are analyzed (1995–2002). Inorganic (Ca^{2 +}, Mg^{2 +}, Na⁺, K⁺, NH₄ ⁺, Cl^?, SO₄ ^{2 ?}, NO₃ ^?) and organic (HCOO^?, CH₃COO^?) ions content were determined using Ion Chromatography. The analyzed 631 rainfall events represent 8420.9 mm of rainfall from a 9631.1 mm total. The precipitation chemistry at Lamto is influenced by four main sources: natural biogenic emissions from savanna soils (NO _x and NH₃), biomass burning (savanna and domestic fires), terrigeneous particles emissions from dry savanna soils, and marine compounds embedded in the summer monsoon. The inter-annual variability of the weighted volume mean concentration of chemical species linked with wet deposition fluctuates by ～ 20% over the period. Ammonium concentration is found to be the highest (17.6 μ eq.l^{? 1}) from all IDAF sites belonging to the West Africa ecosystems. Ammonia sources are from domestic animals, fertilizers and biomass burning. In spite of the high potential acidity of 30.5 μ eq.l^{? 1} from NO₃ ^?, SO₄ ^{2 ?}, HCOO^? and CH₃COO^?, a relatively weak acidity is measured: 6.9 μ eq.l^{? 1}. The 40% acid neutralization is explained by the acid gas – alkaline soil particles interaction. The remaining neutralization is from inclusion of gaseous ammonia. When results from Lamto, are compared with those from Banizoumbou (dry savanna) and Zoetele (equatorial forest), a regional view for wet tropospheric chemistry processes is obtained. The high concentration of the particulate phase in precipitation emphasizes the importance of multiphases processes between gases and particles in the atmospheric chemistry of the West Africa ecosystems. For example, the nss Ca^{2 +} precipitation content, main indicator of terrigeneous particles, goes from 30.8 μ eq.l^{? 1} in dry savanna to 9.2 μ eq.l^{? 1} at Lamto and 8.9 μ eq.l^{? 1} in the Cameroon forest. A similar gradient is obtained for rainfall mineral particles precipitation content with contribution of 80% in dry savanna, 40% in wet savanna, and 20% in the equatorial forest.     

Comparison of collection methods to determine atmospheric deposition in a rural Mediterranean site (NE Spain)

Wet-only, dry-only, bulk deposition and deposition of sedimentary particles and gases deposited after the last rain (DAR) were collected weekly at La Castanya station in the Montseny mountains (NE Spain, 41°46′N, 2°21′E) from February 2009 to July 2010. These samples were analysed for pH, alkalinity, and the concentrations of major ions (Cl^?, NO₃ ^?, SO₄ ^2?, Na⁺, K⁺, Ca²⁺, Mg²⁺, NH₄ ⁺). Significant differences were observed between bulk and wet-only precipitation, with an enrichment of ions associated to coarse particles in bulk deposition. The comparison between wet and dry fluxes revealed that the removal of compounds at Montseny occurred mainly by wet deposition, which accounted for 74 % of total deposition. The dry flux was characterised by the predominance of K⁺, Ca²⁺ and Mg²⁺, which are related to coarse particles. Bulk collection methods at Montseny were considered representative of total atmospheric deposition, since bulk deposition plus DAR accounted for 97 % of total deposition measured with wet and dry-only collection devices. Thus, bulk deposition collectors can be recommended for deposition networks at remote sites (lacking electricity connection) in environments, where coarse particles are a predominant fraction of the aerosol mass.     

Wet deposition due to fog in the Po Valley,Italy

Wet deposition due to radiation fog is examined in this paper. The area where the reported measurements were performed, the Po Valley, northern Italy, is characterized by both a high fog occurrence during the fall-winter months and fog water solutions of high ionic concentration and acidity.Estimated wet deposition for NH ₄ ⁺ , NO _inf3 ^sup- and SO _inf4 ^sup2- ions due to fog droplet settling to the ground accounts for 13.2, 12.1 and 5.3 percent with respect to bulk precipitations over the same period: January–March and October–December (fog season).Fog deposition rates show that this process can be an important pathway of trace gases and particles loss from the air. First indicative results of fog removal efficiency with respect to air particulate matter are presented.Dry deposition parameters should be taken into account in evaluating the potential effect of fog droplet deposition on vegetation.     

华东典型地区大气硫沉降通量的观测和模拟研究

本文使用中国科学院常熟和鹰潭生态实验站和气象站的观测资料，应用区域酸沉降模式系统（RegADMS）和大叶阻力相似模型来研究华东地区不同下垫面条件上的大气硫沉降问题，定量估计了农田下垫面上大气硫化物的沉降通量。SO2和硫酸盐的干沉降速率使用大叶阻力相似模型来估计，使用与降水量有关的参数化方案来确定湿沉降系数。结果表明，常熟地区农田下垫面的大气硫沉降通量为19．0gm^-2 a^-1，其中干沉降占42％；而位于江西红壤地区的鹰潭站的大气硫沉降通量为10．4gm^-2a^-1，其中干沉降占83％。比较发现，两地硫干沉降通量绝对值相差不大，但其在总沉降通量中所占的份额有较大差异；常熟站的硫湿沉降通量比鹰潭站要大9．23gm^-2a^-1，该差异是由两地污染状况和气象条件的不同造成。华东地区的年硫沉降总量为1．88Mt（1Mt=10^6t），其中72．8％沉降在农田下垫面上。硫沉降中43％是干沉降，57％是湿沉降。     

7Be deposition in a high-rainfall area of New Zealand

Concentrations of natural ⁷Be in air and rainwater were monitored for one year at Hokitika, New Zealand. The mean airborne concentration was 3.1±1.3 mBq m^–3, the mean Hokitika, New Zealand. The mean airborne concentration was 3.1±1.3 mBq m^–3, the mean concentration in rainwater was 2600±1200 Bq m^–3, and the mean total deposition was estimated to be 130±99 Bq m^–2 wk^–1. Most of the ⁷Be was wet deposited and the washout ratio was independent of precipitation amount. A significant linear relationship exists between the weekly wet deposition flux and weekly precipitation at this high-rainfall site.     

Chemistry of Precipitation Events and Inter-Relationship with Ambient Aerosols over a Semi-Arid Region in Western India

The precipitation events (n = 91), collected for 3 years (2000–2002) during the period of SW-monsoon (Jun–Aug) from an urban site (Ahmedabad, 23.0°N, 72.6°E) of a semi-arid region in western India, are found to exhibit characteristic differences in terms of their solute contents. The low solute (<700 μeq L⁻¹) events are either marked by heavy precipitation amount or successive events collected during an extended rain spell; whereas light precipitation events occurring after antecedent dry period are characterized by high solutes (>700 μeq L⁻¹). The ionic composition of low solute events show large variability due to varying contribution of anthropogenic species (: 1%–74%; : 1%–25%; and : 8%–68%) to the respective ion balance. In high solute events, ionic abundances are dominated by mineral dust (Ca²⁺ and ) and sea-salts (Na⁺ and Cl⁻). These differences are also reflected in the pH of low solute events (range: 5.2–7.4, VWM: 6.4) and high solute events (range: 6.6–8.2, VWM: 7.3). The comparison of Ca²⁺/Na⁺ and nss- ratios (on equivalent basis) in rain and aerosols suggests that the ionic composition of high solute events is influenced by below-cloud scavenging; whereas evidence for in-cloud scavenging is significantly reflected in low solute events. The annual wet-deposition fluxes of and are 330 and 480 mg m⁻² y⁻¹, respectively, in contrast to their corresponding dry-deposition fluxes (14 and 160 mg m⁻² y⁻¹); whereas wet and dry removal of Ca²⁺, Mg²⁺ and are comparable.     

On the wet removal of gaseous and particulate sulfur and nitrogen species from the atmosphere

The role of trace gases and aerosol particles in the control of sulfur and nitrogen levels in atmospheric precipitation is estimated on the basis of the enrichment factor in the precipitation of these elements relative to particulate matter in the air. By using air and precipitation chemistry data obtained at a Hungarian background air pollution station (K-puszta) it is found that the fraction of ammonium, nitrate and sulfate in precipitation, due to the removal of particulate matter is at least 59, 27 and 31%, respectively. The relationship between wet depositions and air concentrations of different species is determined statistically by applying daily data set. The regression equations obtained make the estimation of the sub-cloud scavenging ratios possible and they give some information on the magnitude of in-cloud scavenging processes. The results show that the in-cloud scavenging is a determining factor for precipitation sulfate, while it is relatively unimportant in the case of ammonium. The sub-cloud scavening of NO₂ and SO₂ is not too significant. However, for HNO₃, and NH₃ it is an effective process. The sub-cloud scavenging ratio of sulfur and nitrogen-containing particles varies around 0.25×10⁶.     

Sulfur dioxide in rain clouds: Gas-liquid scavenging efficiencies and wet deposition rates in the presence of formaldehyde

The interaction of formaldehyde with SO₂ dissolved in the aqueous phase of clouds leads to the formation of hydroxymethane sulfonate. The impact of this process upon the gas-liquid equilibrium distribution of SO₂ in rain clouds and the ensuing wet SO₂ precipitation rate is explored. Model vertical SO₂ distributions are derived from observational data for three atmospheric regions: continental polluted, continental remote, and marine. The wet deposition rate for SO₂ in the polluted atmosphere increases by about a factor of two in the presence of formaldehyde compared with its absence. The effect is much stronger in the remote atmosphere leading to a potentially significant enhancement of wet SO₂ deposition. In the marine atmosphere, wet deposition of SO₂ may contribute as much as 35% to the total removal rate for SO₂ by all processes including dry deposition and chemical conversion to sulfate.     

Increases of wet deposition at remote sites in Japan from 1991 to 2009

Temporal trends in wet deposition of major ions were explored at nationwide remote sites in Japan from April 1991 to March 2009 by using the seasonal Kendall slope estimator and the nonparametric seasonal Kendall test. For the trend analysis, datasets from eight remote sites (Rishiri, Echizenmisaki, Oki, Ogasawara, Shionomisaki, Goto, Yakushima, and Amami) were selected from the Japanese Acid Deposition Survey (JADS) conducted by the Ministry of the Environment. Deposition of H⁺ has been increasing at remote sites in Japan on a national scale. Significant (p????0.05) increases in H⁺ deposition were detected with changes of +3?C+9?%?year^?1 at seven sites, while insignificant increases were observed at one site. Depositions of non-sea salt (nss)-SO ₄ ^2? and NO ₃ ^? significantly increased at four and six sites, respectively, with changes of +1?C+3?%?year^?1. Significant increases in precipitation at four sites would have contributed to the increase in depositions of H⁺, nss-SO ₄ ^2? , and NO ₃ ^? . The emission trends of SO₂ and NO_x did not corresponded to the deposition trends of nss-SO ₄ ^2? and NO ₃ ^? . The different trends indicated that temporal variation of precipitation amount trend dominated the deposition trends.     

Covariations in oceanic dimethyl sulfide,its oxidation products and rain acidity at Amsterdam Island in the Southern Indian Ocean

Simultaneous measurements of rain acidity and dimethyl sulfide (DMS) at the ocean surface and in the atmosphere were performed at Amsterdam Island over a 4 year period. During the last 2 years, measurements of sulfur dioxide (SO₂) in the atmosphere and of methane sulfonic acid (MSA) and non-sea-salt-sulfate (nss-SO₄ ^2-) in rainwater were also performed. Covariations are observed between the oceanic and atmospheric DMS concentrations, atmospheric SO₂ concentrations, wet deposition of MSA, nss-SO₄ ^2-, and rain acidity. A comparable summer to winter ratio of DMS and SO₂ in the atmosphere and MSA in precipitation were also observed. From the chemical composition of precipitation we estimate that DMS oxidation products contribute approximately 40% of the rain acidity. If we consider the acidity in excess, then DMS oxidation products contribute about 55%.