Wet Sulfate Deposition Research Articles

A preliminary assessment of the Montreal process indicators of air pollution for the United States.

Air pollutants pose a risk to forest health and vitality in the United States. Here we present the major findings from a national scale air pollution assessment that is part of the United States' 2003 Report on Sustainable Forests. We examine trends and the percent forest subjected to specific levels of ozone and wet deposition of sulfate, nitrate, and ammonium. Results are reported by Resource Planning Act (RPA) reporting region and integrated by forest type using multivariate clustering. Estimates of sulfate deposition for forested areas had decreasing trends (1994-2000) across RPA regions that were statistically significant for North and South RPA regions. Nitrate deposition rates were relatively constant for the 1994 to 2000 period, but the South RPA region had a statistically decreasing trend. The North and South RPA regions experienced the highest ammonium deposition rates and showed slightly decreasing trends. Ozone concentrations were highest in portions of the Pacific Coast RPA region and relatively high across much of the South RPA region. Both the South and Rocky Mountain RPA regions had an increasing trend in ozone exposure. Ozone-induced foliar injury to sensitive species was recorded in all regions except for the Rocky Mountain region. The multivariate analysis showed that the oak-hickory and loblolly-shortleaf pine forest types were generally exposed to more air pollution than other forest types, and the redwood, western white pine, and larch forest types were generally exposed to less. These findings offer a new approach to national air pollution assessments and are intended to help focus research and planning initiatives related to air pollution and forest health.

A Bayesian space‐time analysis of acid deposition data combined from two monitoring networks

In some studies of atmospheric pollution, deposition data may be available from several distinct monitoring networks. Systematic differences may exist between networks, and an effective statistical analysis of the data combined from all networks should account for this possibility. This article develops a Bayesian modeling approach for spatiotemporal data from two monitoring networks that accounts for possible network differences in measurement error biases and variances. Wet sulfate deposition data from two networks in the eastern United States, the National Acid Deposition Program/National Trends Network and the Clean Air Status and Trends Network, are combined and analyzed via our modeling approach. We examine whether the temporal trend in deposition from 1989–1997 varies regionally and whether the measurement error biases and variances differ between networks. We also demonstrate how prediction uncertainty is reduced using the combined data from levels obtained when basing prediction on data from only one of the networks.

A three-dimensional numerical simulation of sulfate transport and redistribution

We have utilized a relatively sophisticated dynamic cloud model combined with standard bulk-parameterized microphysics and simple sulfur chemistry to explore the impact of deep convection on modification and transport of a suite of pollutants. Two base run simulation parameters are used to initialize the cloud-chemistry model. The simulation of the 6 July 1995 case, with continental polluted field initialization, has revealed that a convective storm generates strong vertical transport of gases and particulate compounds from the planetary boundary layer (PBL) to the upper troposphere (UT), perturbation of aerosol physical and chemical properties, modification of pollutant concentration, and change of the spatial distributions of chemical species. The early formation of precipitation and enhanced scavenging contributed to a registration of approximately 2.5 times the concentration of sulfate in the precipitation near the surface than in the air found at this level. The Spring case numerical experiment on 3 April 2000 with a chemical background taken from Macedonia, provided insight into the potential influence of the long-range transport of atmospheric pollutants and ascertained quantitative–qualitative information about processes by which acidic species are incorporated into precipitation. The model-computed parameters are in good agreement with observation. The average equivalent cloud water pH and rainwater pH when the higher acid precipitation occurs are about 5.0 and 4.5, respectively. The results from a number of sensitivity tests of cloud chemistry of the physical processes for the continental nonpolluted and continental polluted environments, indicate that nucleation and impact scavenging of aerosols account for between 20%–24% of the total sulfur mass removed by wet deposition. Liquid-phase oxidation contributes about 20%–28% of the sulfur content in precipitation. It means that neglecting liquid-phase oxidation when considering the chemistry in these clouds may lead to underestimates of about 20%–28% in sulfate wet deposition. Neglect of the ice phase when considering the chemistry in continental nonpolluted and continental polluted clouds may lead to overestimates of about 112%–130% of the total sulfur mass removed by wet deposition. The assumption of Henry's law equilibrium for those types of clouds gives an overestimation of about 100%–120%, respectively. PACS Nos.: 51.10.+y, 92.60.Sz

Studies of acid deposition and its effects in two small catchments in Hunan, China

Abstract. Acid deposition and its effects were studied by analysing the chemistry in precipitation, stream water, soil water and soils in two catchments in Hunan. One site, Linkesuo (denoted LKS), is on the outskirts of Changsha, the provincial capital of Hunan, the other (Bailutang, denoted BLT) on the outskirts of Chenzhou in southern Hunan. Volume-weighted average pH values and sulphate concentrations in wet deposition were 4.58 (BLT) and 4.90 (LKS) and 174 μmolc L-1 and 152 μmolc L-1, respectively. Wet deposition of sulphate has been estimated as 4.3 gS m-2yr-1 and 3.4 gS m-2yr-1 at BLT and LKS, respectively. Estimates of the corresponding total depositions (dry + wet) are 6.1 gS m-2yr-1 and 5.3 gS m-2yr-1. In precipitation and throughfall, sulphate was the major anion and calcium the major cation. In stream and soil water, nitrate was slightly higher than sulphate on an equivalent basis and magnesium (Mg) not much lower than calcium (Ca). Important soil properties, such as soil pH, soil organic matter (SOM) content, exchangeable acidic cations, exchangeable base cations, effective cation exchange capacity (CECe), base saturation (BS), and aluminium (Al) and iron (Fe) pools, were determined for five forest soil profiles (consisting of four horizons) in each of the two catchments. The soils in BLT are generally more acid, have lower BS and higher Al and Fe pools than the LKS soils. The Al- and Fe-pools were generally higher in the topsoils (i.e. the O and A horizons) than in deeper soils (i.e. E and B horizons) especially at the most acidic site (BLT). There are significant correlations between Fe-pools and the corresponding Al-pools in both catchments except between the amorphous Feox and Alox. Considering the long-term high deposition of sulphate, there is a risk of future ecological damage due to acidification, especially in the BLT catchment, although vegetation damage has yet to be observed in the catchments. This condition appears to be representative of a large part of Hunan. Keyword: acid deposition, soil acidification, catchment, Al-pools, Fe-pools, Hunan

Moment‐based simulation of microphysical properties of sulfate aerosols in the eastern United States: Model description, evaluation, and regional analysis

A six‐moment microphysics module for sulfate aerosols based on the quadrature method of moments has been incorporated in a host 3‐D regional model, the Multiscale Air Quality Simulation Platform. Model performance was examined and evaluated by comparison with in situ observations over the eastern United States for a 40‐day period from 19 July to 28 August 1995. The model generally reproduces the spatial patterns (sulfate mixing ratios and wet deposition) over the eastern United States and time series variations of sulfate mass concentrations. The model successfully captured the observed size distribution in the accumulation mode (radius 0.1–0.5 μm), in which the sulfate is predominately located, while underestimating the nucleation and coarse modes on the basis of the size distributions retrieved from the modeled six moments at the Great Smoky Mountains (GSM). This is consistent with better model performance on the effective radius (ratio of third to second moment, important for light scattering) than on number‐mean and mass‐mean radii. However, the model did not predict some of the moments well, especially the higher moments and during the dust events. Aerosol components other than sulfate such as dust and organics appear to have contributed substantially to the observed aerosol loading at GSM. The model underpredicted sulfate mixing ratios by 13% with about 50% of observations simulated to within a factor of 2. One of the reasons for this underestimation may be overprediction of sulfate wet deposition. Sulfate mass concentrations and number concentrations were high in the source‐rich Ohio River valley, but number concentrations were also high over the mid‐Atlantic coast (New Jersey area). Most (77%) sulfate amount was below 2.6 km, whereas most sulfate number (>52%) was above 2.6 km except over Ohio River valley (41%). These results demonstrate the accuracy, utility, practicality, and efficiency of moment‐based methods for representing aerosol microphysical processes in large‐scale chemical transport models.

Indonesian smoke aerosols from peat fires and the contribution from volcanic sulfur emissions

The island arc volcanoes in and around Indonesia have been permanently degassing for thousands of years, thereby contributing significantly to the total emissions of sulfur species in that region. The hot and wet tropical weather conditions with high solar irradiation and regular daily precipitation during the wet season lead to efficient removal of oxidised sulfate by wet deposition. This is accumulated in the Indonesian peat areas, which serve as natural sponges, soaking up rain during the wet season and slowly releasing moisture into the atmosphere during the dry season. When peat forests are drained for land clearing purposes, the peat quickly dries out and becomes extremely flammable. When ignited, the composition of the burning peat mainly determines the fire aerosol chemical composition and microphysical properties. In this paper we investigate the contribution of volcanic sulfur emissions to wet deposition of sulfur in Indonesian peat swamp areas based on numerical simulations carried out with a global atmospheric circulation model including the tropospheric sulfur cycle. Our study suggests that the observed hygroscopicity and elevated sulfur content of the Indonesian peat fire aerosols is due to accumulated volcanic sulfur.

Assessment of Wet Sulfate Concentration Changes at CAPMoN Sites, Canada

This paper describes a statistical analysis of wet sulfate deposition data sampled by the Canadian Air and Precipitation Monitoring Network (CAPMoN) since 1988 till 1997. The goal of the investigation is to detect presence of prevailing significant changes in the probability distribution of annual samples collected by the network at each site. The considerations are based on a first order autoregression model with second order polynomial trend and methods used for analysis of variance and multiple comparison. Unlike studies suggesting existence of long term trends in the data, methods applied here indicate absence of any systematic changes in the observed annual concentration patterns at most of the sites.

Wet deposition from clouds and precipitation in three high-elevation regions of the Eastern United States

Three regions are identified in the Eastern United States (US) that contain substantial land area at high elevations: the mid-Appalachian region (MAR), Eastern New York state (ENY), and the New England region (NER). Approximately 75% of the land cover in these areas is forested, with 5.6–29% of the total acreage above 600 m and subject to cloud deposition. Measurements of cloud deposition are scarce. A 6-year data record of measurements at two high-elevation locations is considered, and scaling factors are developed to enable the rough estimation of area-wide cloud deposition at various elevations in each region. Estimates of precipitation and associated ion deposition are made at 12 arc-second resolution for the Eastern US and are used to obtain elevation-resolved precipitation-mediated deposition for the three study regions. At high elevations, clouds account for a substantial proportion of wet deposition (i.e., the sum of that from clouds and precipitation). For the total land area above 600 m , clouds may account for 20–60% of the total wet ion deposition, with the exact proportion depending on both location and ion species. At elevations above 600 m , but below the climatic tree line, the ratio of cloud-to-precipitation-mediated deposition is higher in NER and ENY than in MAR. At the highest elevations of each study region, clouds may account for over 80% of the wet ion deposition. Although the wet deposition of ammonium (NH 4 +), sulfate (SO 4 2−), nitrate (NO 3 −), and hydrogen (H +) ions is enhanced at higher elevations by clouds over precipitation, this enhancement is the largest for ammonium. This study illustrates the major and perhaps dominant role that clouds may play by delivering considerable ion loads to montane ecosystems in selected elevation ranges where these ecosystems may be especially vulnerable.

침적 모형에 의한 습성침적 플럭스 수치모의

The purpose of this study is to estimate wet deposition flux and to investigate wet deposition characteristics by using the ADOM model. Wet deposition flux of highly reactive <TEX>$SO_2$</TEX> is estimated by applying observed meteorological parameters and concentrations of chemical species to the ADOM model. Wet deposition is largely dependent on large scale precipitation and cloud thickness. Wet deposition flux of sulfate depends on <TEX>$SO_2$</TEX> oxidation in clouds. When large amount of <TEX>$SO_2$</TEX> is converted to sulfate, deposition flux of sulfate increases, but wet deposition flux of <TEX>$SO_2$</TEX> is small. On the whole, the pattern of sulfate wet deposition flux agrees with the typical pattern of sulfate wet deposition that is high in the summer(July) and low in the winter(January).

Simulation of the tropospheric sulfur cycle in a global model with a physically based cloud scheme

The treatment of the sulfur cycle in the CSIRO global climate model (GCM) is described. It is substantially based on the scheme developed previously for the European Center/Hamburg (ECHAM) model, but the treatment of wet scavenging has been completely rewritten to better reflect the different properties of liquid and frozen precipitation, and the treatment of these in the model's cloud microphysical scheme. The model is able to reproduce the observed finding that wet deposition of sulfur over Europe and North America is larger in summer than in winter, but the seasonal cycle of sulfate over Europe is not well simulated. The latter is improved when the amplitude of the seasonal cycle of European emissions is increased. Below‐cloud scavenging makes an important contribution in our scheme: On omitting it, the global sulfate burden increases from 0.67 to 0.93 Tg S. On reverting to the less efficient scavenging treatment used in ECHAM, the global sulfate burden again increases from 0.67 to 0.93 Tg S, and excessive sulfate concentrations are obtained in Europe and North America. Some deficiencies in the simulation are investigated via further sensitivity tests. In particular, during the Arctic winter, the modeled sulfur dioxide (SO2) concentrations are too large, and the modeled sulfate concentrations are too small (as in most global sulfur‐cycle models). Recent laboratory experiments suggest that SO2 oxidation in ice clouds is nonnegligible. We obtain a much improved Arctic simulation when a simple treatment of SO2 oxidation in ice clouds is included.

Status and trends in atmospheric deposition and emissions near Atlanta, Georgia, 1986–99

Wet and dry atmospheric deposition were investigated from weekly data, 1986–99 (1986–97 for dry deposition) at the Panola Mountain Research Watershed (PMRW), a forested research site 25 km, southeast of Atlanta, Georgia. Furthermore, the wet deposition was compared to that at three adjacent National Atmospheric Deposition Program's National Trends Network (NTN) sites (GA41, 50 km south of PMRW; AL99, 175 km northwest; NC25, 175 km north-northeast) and dry deposition was compared to that at adjacent Clean Air Status and Trends Network (CASTNET) sites, co-located at the NTN sites. The pH of precipitation is acidic and the dominant acid anion is SO 4; the pH (derived from the volume-weighted mean H concentration) averages 4.44 for 1986–99, and varies seasonally with average lowest values in summer (4.19) and highest in winter (4.63). From 1986–99, the annual wet deposition of sulfur (S) and nitrogen (N) averaged 400 and 300 eq ha −1 (6.4 and 4.2 kg ha −1), respectively. Inferential model estimates of annual dry S and N deposition from 1986–97 averaged 130 and 150 eq ha −1 (2.1 and 2.1 kg ha −1), respectively. From 1993–99, net S deposition (dry deposition plus canopy interactions) for coniferous and deciduous throughfall (throughfall minus wet-only deposition) averaged 400 and 150 eq ha −1 (6.4 and 2.1 kg ha −1), respectively. The annual wet deposition of S and N species at PMRW was comparable to that at NTN sites, with the exception of higher N species deposition at AL99 and relatively lower H, SO 4 and NO 3 deposition at GA41. Dry S deposition at PMRW differed markedly from the CASTNET sites despite similarity in S concentrations for all but NC25; the differences are attributed to differences in model parameters associated with the landscape and vegetation characteristics at the sites. At PMRW, atmospheric deposition trends were not detected for the entire sampling period, but were detected for shorter periods (4–5 yr). Annual S and N deposition increased from 1986 to 1991, decreased to 1995 and then increased to 1999. SO 2 emissions from seven major point sources within 120 km of PMRW decreased markedly from the late 1980s to 1995 and have remained relatively constant or increased slightly from 1995 to 1998 Annual wet and dry S deposition at PMRW significantly correlates ( p<0.01) with SO 2 emissions, and the correlation is dominated by the large SO 2 emissions decreases in the early 1990s, consistent with the implementation of Phase I of Title IV of the 1990 Clean Air Act Amendments.

Dynamical influences on the distribution and loading of SO2 and sulfate over North America, the North Atlantic, and Europe in April 1987

The loading of tropospheric aerosols is highly variable spatially and temporally as a consequence of large spatial variability in sources, temporal variability in transport winds, and short residence times (days), and additionally for secondary sulfate, intermittent production associated with aqueous‐phase reaction in clouds, and intermittent removal, mainly by precipitation. We have used a chemical transport and transformation model for atmospheric sulfur driven by observationally derived meteorological data to calculate the geographical distribution of sulfate and sulfur dioxide (SO2) over North America, the North Atlantic, and Europe as a function of time for April 1987. We present the results of these calculations, mainly as animations showing the time dependence of the column burden (vertical integral of concentration) and of sulfate wet deposition, and interpret the temporal evolution in terms of the controlling meteorological phenomena as discerned from synoptic analyzes at 925 and 500 hPa. The analyzes show highly localized and episodic buildup and removal of sulfate and SO2 and circulations under the influence of high‐ and low‐pressure systems as well as instances of rapid meridional and zonal transport over distances of thousands of kilometers. Key meteorological phenomena giving rise to large amounts of sulfate include not only slow moving high‐pressure systems, as previously recognized, but also low‐pressure systems. Whether or not a given low‐pressure system results in large sulfate loading depends strongly on its location relative to sources of sulfur species and of the water vapor that is required for formation of clouds and precipitation. We present a detailed analysis of the evolution of sulfate and SO2 over the eastern North Atlantic April 3–8, 1987, under the influence of a cutoff low‐pressure system during which sources in northern Europe continued to feed a system that repeatedly circulated over source regions. Animations such as these facilitate identification of episodes of large aerosol loading and relating these loadings to the controlling meteorological phenomena.

Relationship between wet deposited sulphur and sulphur dioxide emissions

It is well established that wet deposition of sulphate in the UKhas fallen by a much smaller fraction than have emissions of sulphur dioxide. Dry deposition of sulphur has decreased in proportion to the decline in emissions. A number of suggestionshave been made which offer possible explanations for this non-linearity between emissions and wet deposition. In this paper amodel of the processing of sulphur dioxide by aqueous phase cloudchemistry in a cloudy boundary layer is presented. This work doesnot simulate all the details of the mechanisms by which sulphate is incorporated into precipitation. It does, however, explorethe non-linearity of this oxidation process. It is shown that theoxidation of sulphur dioxide, in these conditions, over the UK isdominated by oxidation by ozone. The rate of sulphate productionis then controlled by the availability of the one basic trace gasin the atmosphere ammonia. Using realistic concentrations of thereacting species this is found to simulate well the observed non-linearity. The model predicts that sulphate and sulphur dioxideconcentrations will be uncorrelated in the cloudy boundary layerbut that ammonium and sulphate will be well correlated. Fieldobservations at a cloudy site in Northern England are consistentwith these predictions.

Analysis of regional budgets of sulfur species modeled for the COSAM exercise

The COSAM intercomparison exercise (comparison of large-scale sulfur models) was organized to compare and evaluate the performance of global sulfur cycle models. Eleven models participated, and from these models the simulated surface concentrations, vertical profiles and budget terms were submitted. This study focuses on simulated budget terms for the sources and sinks of SO2 and sulfate in three polluted regions in the Northern Hemisphere, i.e., eastern North America, Europe, and Southeast Asia. Qualitatively, features of the sulfur cycle are modeled quite consistently between models, such as the relative importance of dry deposition as a removal mechanism for SO2, the importance of aqueous phase oxidation over gas phase oxidation for SO2, and the importance of wet over dry deposition for removal of sulfate aerosol. Quantitatively, however, models may show large differences, especially for cloud-related processes, i.e., aqueous phase oxidation of SO2 and sulfate wet deposition. In some cases a specific behavior can be related to the treatment of oxidants for aqueous phase SO2 oxidation, or the vertical resolution applied in models. Generally, however, the differences between models appear to be related to simulated cloud (micro-)physics and distributions, whereas differences in vertical transport efficiencies related to convection play an additional rôle. The estimated sulfur column burdens, lifetimes and export budgets vary between models by about a factor of 2 or 3. It can be expected that uncertainties in related effects which are derived from global sulfur model calculations, such as direct and indirect climate forcing estimates by sulfate aerosol, are at least of similar magnitude.

Uncertainty in Estimation of Wet Deposition of Sulphur

A model is described for predicting wet deposition ofsulphur in Britain from rainfall and site measurementsof ion concentration in precipitation. This modelincludes orographic enhancement of both rainfall andion concentration. The model output is comparedagainst available measurement data. Sensitivity anduncertainty analyses are used to predict the outputuncertainty. If the stated assumptions can be shown tobe correct, the wet deposition for Britain at the 5 kmscale is accurate to ±35% across the country. Theanalyses show a larger uncertainty in central Englandand a possible bias towards underestimation of wetdeposition, the latter being of importance incalculating critical load exceedances in remote areas.

Simulation of Future Aerosol Distribution, Radiative Forcing, and Long-Range Transport in East Asia.

Distributions of aerosol concentrations, optical properties, and wet deposition fluxes are simulated for the next fifty years using an aerosol transport model coupled with an atmospheric general circulation model. Treated species are sulfur dioxide, and all the main tropospheric aerosols, i.e., carbonaceous (black and organic carbons), sulfate, soil dust, and sea salt. We especially pay attention to distributions of anthropogenic carbonaceous aerosols, sulfate aerosols, and sulfur dioxide. The simulation uses the Special Report on Emissions Scenarios (SRES) of the Intergovernmental Panel on Climate Change (IPCC) as the future emission scenarios of anthropogenic pollutants. Simulated results suggest that carbonaceous aerosols continue to increase over industrial and densely populated regions for the next five decades, whereas sulfate aerosols decrease around Europe and North America. The aerosol single scattering albedo in the future is, therefore, calculated to become small gradually in the mid- and high-latitudes of the Northern Hemisphere. Sulfate aerosols and sulfur wet deposition fluxes are, on the other hand, simulated to increase only over East Asia. Black carbon and sulfate aerosols around Japan in 2050 are simulated to be two or three times as large as those in 2000 with one of the SRES scenarios. Hence this suggests that pollutants originating from the East Asian continent can seriously affect the atmospheric quality in Japan in the next several decades.

Estimation of wet deposition of sulfate using routinely available meteorological data and air-monitored data in Korea

A method is developed to estimate wet deposition of sulfate using routinely available meteorological data and air-monitored data. This approach takes into account different mechanisms of precipitation formation that determines sulfate concentration in precipitation water. Four different precipitating cloud types, including cold cloud, warm cloud, stratified layered cloud and convective cloud, according to their precipitation formations are incorporated differently to estimate sulfate concentration in precipitation water with the airborne sulfate concentration estimated by routinely available monitored data by regression equations. Two different regression equations for the estimation of airborne sulfate concentration are developed: one is applicable in winter, and the other in other seasons except for winter. The presently developed algorithm has been implemented in Seoul to estimate wet deposition of sulfate in 1997 when the measured wet deposition of sulfate is available. The estimated annual total wet deposition of sulfate in Seoul is found to be 3450 and 3770 kg km −2 yr −1 without and with consideration of the oxidation of SO 2 in clouds, respectively, which is quite comparable with the measured value of 3510 kg km −2 yr −1. This result suggests that the present model can be used more practically in the field of acid rain study, especially in cases of the lack of monitoring sites for wet deposition.

Estimation of sulfur deposition in South Korea

Dry and wet depositions of sulfur have been estimated using the routinely available meteorological data at 70 sites, air-monitored data at 27 sites and upper air soundings made daily twice (0000 and 1200 UTC) at three stations over South Korea for 4 yr from 1994 to 1997. The average annual total dry deposition and wet deposition of sulfur in South Korea are found to be 3 and 1.7 t km −2 yr −1, respectively. The ratio of dry-to-wet deposition of sulfur is found to vary with season with a maximum value of 6.6 in winter to a minimum value of 0.5 in summer in association with both precipitation amount and SO 2 concentration. The maximum of annual total sulfur deposition exceeding 10 t km −2 yr −1 occurs in the southeastern part of Korea where big cities and industrial complexes are located, suggesting the importance of SO 2 emission for deposition of sulfur.

The sensitivity of sulfur wet deposition to atmospheric oxidants

The wet deposition of sulfate (SO 4 2−) in eastern North America is chemically linked to primary emissions of sulfur dioxide (SO 2) via atmospheric oxidants and therefore also to the emissions of nitrogen oxides (NO x ) and hydrocarbons (RH) within the region. Despite the major reductions in SO 2 sources mandated by the Clean Air Act Amendments of 1990, large sulfate deposition events are still observed in eastern North America. A case study of one such event in June 1996 in central Pennsylvania is used to gain an understanding of the linkages between sulfur deposition and oxidant chemistry and transport. These issues are addressed here using a combination of meteorological and chemical models (HY-SPLIT and OZIPR) in order to interpret air and precipitation chemistry data (AIRMoN) from the Penn State research sites. The large deposition of sulfate resulting from this rain event can be accounted for by the scavenging of ambient sulfate formed prior to the storm. The potential impacts that future NO x controls might have on ambient sulfate, visibility, and acidic deposition depend in part on the resulting changes in total oxidant levels and the competition that naturally exists between the clear-air and in-cloud oxidation pathways. The findings here suggest that lowered levels of NO x , even when they result in reduced production of ozone (O 3), may nevertheless lead to enhanced generation of hydrogen peroxide (H 2O 2) and therefore to the continued occurrence of sulfate episodes and high-deposition events in the future.

Changes in sulfate deposition in eastern USA following implementation of Phase I of Title IV of the Clean Air Act Amendments of 1990

Phase I of Title IV of the Clean Air Act Amendments of 1990 (CAAA) was implemented on 1 January 1995. On that date limitations were imposed on sulfur dioxide (SO 2) emissions from 110 coal-fired electric utility plants (263 units) in 21 states of which 17 are east of the Mississippi River. Sulfur dioxide emissions at these plants dropped an average 2.86 million tons (38%) from 1995 through 1997, when compared to mean 1993–1994 emissions from these plants. As a result of these reductions, wet sulfate (SO 4 2−) deposition decreased an average of 2.4–4.0 kg ha −1 (10–25%) over a large area of the eastern USA. Based on more than 15 years of data, the decrease in wet deposition was unprecedented in magnitude and spatial extent. The largest decrease occurred across the Mid-Appalachian and Northeast regions of the country, the regions specifically targeted by Phase I for reductions in acidic deposition. Although the reductions in wet SO 4 2−deposition were generally consistent with observed reductions in SO 4 2− concentrations, deviations in precipitation volumes between the pre- and post-Phase I periods did result in some sites recording significantly ( p<0.05) lower post-Phase I concentrations but higher wet depositions. Deviations in precipitation within and between regions also influenced the magnitude and spatial distribution of the observed changes in wet SO 4 2− deposition. Clearly, deviations in the amount and distribution of precipitation need to be considered along with concentration changes in evaluating the CAAA effectiveness in reducing acidic deposition. This is particularly important in the eastern USA, since many of the most acid-sensitive ecosystems are located in regions, mostly mountainous and forested, that are subject to considerable fluctuations in precipitation volumes and subsequently acidic wet deposition.