Factor analysis of the MAP3S/RAINE precipitation chemistry network: 1976–1980

This paper presents a factor analysis of the MAP3S/RAINE precipitation chemistry data for the period 1976–1980. The purpose of the analysis is to determine the underlying source and/or processes accounting for the variance in the data. Three general sources, ‘acid,’ sea, and ‘ammonium/road dust/soil’ account for most of the variance in the data for the entire network for the aggregated three-and-one-half-year period. Both sulfate and nitrate are found to be significant components of the acid factor. Seasonal and spatial (individual site) analyses were able to distinguish other, less encompassing factors. Seasonal results showed a summer maximum in contribution for the acid factor, while individual site results showed that the contribution of the acid factor increases in going from the midwest to the northeast. Seasonal results show an ‘ammonium/soil’ factor appearing in spring and summer, while an ammonium sulfate factor appears in fall and winter. The results of this study should serve as a baseline for comparison with factor analyses of subsequent MAP3S data sets and factor analysis of other northeastern U.S. precipitation chemistry networks.     

A synoptic climatology for surface ozone concentrations in Southern Ontario, 1976–1981

The synoptic climatology of ozone (O₃) for S Ontario has shown that, over the 1976–1981 period, average summer O₃ concentrations follow a relationship similar to that reported for event analysis during periods of high O₃ concentration. Highest average concentrations, 36 parts per billion (ppb), occur with ‘back of the high’ situations while lowest average concentrations (20 ppb) occur with ‘front of the high’ situations.With similar weather events in the winter, the pattern is reversed with highest average O₃ concentrations on the ‘front of the high’ (19 ppb) and lowest average concentrations on the ‘back of the high’ (13 ppb). Concentration of O₃ in the ‘front of the high’ sector is due in part to the intrusion of O₃ in the vicinity of storms from the stratosphere. The seasonal variation of average concentrations in these situations is low, ranging from 14 to 26 ppb.The very low average concentration during the winter and fall for the ‘back of the high’ situation may be the result of scavenging by NO_x from the urban/industrial areas around the Great Lakes. During the spring and summer, solar energy and warm temperatures cause the photochemical production of O₃ from NO_x and HCs precursors. In the fall and winter, photochemical production of O₃ is either very low or absent, and the NO_x consume O₃ rather than produce it. Thus, average O₃ concentrations for winter ‘back of the high’ situations are one-third of those in the summer months.The synoptic climatology of events during the months from May to September with maximum O₃ concentrations in excess of 80 ppb indicates that 78 % of these events occur under synoptic weather classes generally indicative of back or centre of the high situations.     

Wetfall Deposition and Precipitation Chemistry for a Central Appalachian Forest

Abstract

Although extensive research on acidic deposition has been directed toward spruce-fir forests, less research has been done on the impacts of air pollution on eastern montane hardwood forests. The purpose of this study was to describe precipitation chemistry for several Appalachian hardwood forest sites at or near the Fernow Experimental Forest (FEF) to assess the potential for problems associated with acidic deposition. Emphasis was placed on seasonal patterns of ionic concentrations (H+, Ca++, NH4+; NO3-, and SO4=) and spatial variability of ionic concentrations and deposition among sites. Seasonal patterns of most ions showed highest concentrations during the summer months and deposition of H+ was especially pronounced during this time. Deposition of all ions was generally greater (related to greater precipitation) at three montane forested sample sites compared to a nonforested riverbottom site. Precipitation chemistry at FEF was similar to other sites throughout the eastern United States and contrasted sharply with mid-western and western sites. Eastern sites, including means for FEF sites, were uniformly 3-4 times higher in H+ and SO4= concentration than the mid-western and western sites. Precipitation at FEF was chronically acidic, more so during the growing season, and highest at higher elevations where environmental stresses can be most severe. Furthermore, there were occasional large discrepancies between the low-elevation site and the higher-elevation forested sites for precipitation chemistry and acidic deposition. These results suggest that synoptic-scale (network) data may greatly underestimate the pollutant conditions to which highelevation forest trees are exposed, since network data rarely take elevation into account and typically are based on annual ionic concentration and deposition means that may be considerably lower than those of the growing season.     

Acidic precipitation in southeastern Arizona: Sulfate,nitrate and trace-metal deposition

Precipitation was collected during 1984 and 1985 at two sites in southeastern Arizona within 100 km of two copper smelters. The precipitation-depth-weighted mean pH was 4.63 and wet sulfate deposition was 8.9 kg ha⁻¹ over a 13-month period. High acidity and sulfate concentration occurred when upper-level winds were from the directions of the smelters. A smelter ‘fingerprint’, based on antimony, arsenic, cadmium, copper, lead and zinc, was identified, and used to evaluate the smelter contribution to precipitation sulfate on an annually-averaged basis. The variations in the relative proportions of these trace metals were too large to permit application of the method to individual precipitation events.     

Calculating mean concentrations for steady sources in recirculating wakes by a particle trajectory method

A discrete vortex model of the recirculating flow behind a two-dimensional backward-facing step is used to calculate the trajectories of particles released from a fixed point. By averaging over a large number of such trajectories, an estimate is made of the mean concentration profile associated with a steady source in the wake. These estimates are verified against experimental data for point- and line-sources. The importance of incorporating a ‘random walk’ in calculating the trajectories is demonstrated. The mean flow in the discrete vortex model used appears to be the most critical factor in determining mean concentrations. The poorest predictions appear to be associated with the longest trajectories.Particle ‘recirculation times’ are also briefly examined and it is shown how these are related to the ‘residence times’ of Vincent (1976, Atmospheric Environment11, 765–774) and others. It is suggested that such residence times may not be an appropriate means of quantifying near-wake dispersion if sources are inside the wake. Advantages of a ‘particle trajectory’ method, as against a diffusion equation method, for dealing with dispersion in inhomogeneous flow are finally presented.     

A theoretical framework for the episodic-urban air quality management plan (e-UAQMP)

The present research proposes the local urban air quality management plan which combines two different modelling approaches (hybrid model) and possesses an improved predictive ability including the ‘probabilistic exceedances over norms’ and their ‘frequency of occurrences’ and so termed, herein, as episodic-urban air quality management plan (e-UAQMP). The e-UAQMP deals with the consequences of ‘extreme’ concentrations of pollutant, mainly occurring at urban ‘hotspots’ e.g. traffic junctions, intersections and signalized roadways and are also influenced by complexities of traffic generated ‘wake’ effects. The e-UAQMP (based on probabilistic approach), also acts as an efficient preventive measure to predict the ‘probability of exceedances’ so as to prepare a successful policy responses in relation to the protection of urban environment as well as disseminating information to its sensitive ‘receptors’. The e-UAQMP may be tailored to the requirements of the local area for the policy implementation programmes. The importance of such policy-making framework in the context of current air pollution ‘episodes’ in urban environments is discussed. The hybrid model that is based on both deterministic and stochastic based approaches predicting the ‘average’ as well as ‘extreme’ concentration distribution of air pollutants together in form of probability has been used at two air quality control regions (AQCRs) in the Delhi city, India, in formulating and executing the e-UAQMP—first, the income tax office (ITO), one of the busiest signalized traffic intersection and second, the Sirifort, one of the busiest signalized roadways.     

The early detection of sulphur emissions reductions using Wet Deposition Measurements

This paper is the outgrowth of a workshop on the Detection of Trends in Wet Deposition Data, attended by atmospheric modellers, atmospheric chemists and statisticians in Toronto, November 1983. Methods for detecting changes or trends in network data which are described and evaluated include statistical and meteorological analyses of ‘before’ and ‘after’ time series at single stations, the analysis of changes in frequency distributions, and the analysis of entire network data sets. Relative advantages of using precipitation concentration vs deposition data sets are examined, and the added information on trends available from air concentration measurements (SO₂ and particle-SO₄²⁻) is shown. The paper concludes with recommendations for data requirements and preferred approaches to trend or change detection using available statistical and modelling techniques.     

Deposition of nitrate-n and sulfate-s by precipitations in Schleswig-Holstein

In order to quantify the atmospheric nitrate and sulfate deposition and to investigate factors related to the variability of deposition during 1983 and 1984, precipitation samples from five different meteorological stations in Schleswig-Holstein (Northern Germany) were collected in weekly intervals, using the bulk-sample method. The average element depositions in kg ha⁻¹ a⁻¹ were: 20 for S and 5.5 for N in List (North Sea Island Sylt) and Schleswig, 12 for S and 4.7 for N in Kiel, 16 for S and 4.3 for N in Luebeck and 18 for S and 4.2 for N in Quickborn near Hamburg.N and S concentrations showed a close relationship to the amount of precipitation and the following functions for the estimation of nitrate-N and sulfate-S deposition in Schleswig-Holstein could be derived: (x = precipitation in mm a⁻¹, y = N or S deposition in kg ha⁻¹ a⁻¹) NO₃-N: y = 0.003x + 2.29; SO₄₋S: y = 0.014x + 4.71. According to these relationships most of the element deposition occurred during atmospheric conditions of predominating winds from the west. Especially in the case of S, atmospheric deposition is the only external source of S supply for plants on many agricultural soils. Sometimes the low sulfur input is not sufficient to cover the requirements of agricultural crops in Schleswig-Holstein. Due to the negative S balance in many soils, future increase of S deficiency is expected.     

Characteristics of atmospheric speciated mercury concentrations (TGM,Hg(II) and Hg(p)) in Seoul,Korea

Ambient speciated mercury concentrations including total gaseous mercury (TGM), gaseous divalent mercury (Hg(II)), and particulate mercury (Hg(p)) were measured on the roof of the Graduate School of Public Health building in Seoul, Korea from February 2005 to February 2006. The average concentrations were 3.22 ± 2.10 ng m^?3, 27.2 ± 19.3 pg m^?3, and 23.9 ± 19.6 pg m^?3 for TGM, Hg(II), and Hg(p), respectively. Hg(II) and Hg(p) concentrations were higher during the daytime than during the nighttime, probably because of high photochemical activity. Hg⁰ concentrations were not significantly correlated with ozone however a positive correlation between ozone and Hg(II) was found during periods of high humidity. Eighteen days were characterized as pollution events with 24 h average PM_2.5 concentrations >65 μg m^?3. The average concentrations of TGM and Hg(p) during these events were 1.4–2 times higher than those during non-pollution events. In order to identify the contribution of long-range transported mercury to the enhanced mercury concentrations in Korea, an episode was defined as a period with hourly average TGM and CO concentrations higher than the monthly average TGM and CO concentrations and with significant enhancement of both TGM and CO concentrations for at least 10 h. A total of 70 episodes were identified during the sampling period: 36 local episodes and 34 long-range transport episodes. The mean ΔTGM/ΔCO slope for all episodes was 0.0063 ng m^?3 ppbv^?1 which agreed well with the slope (0.0036–0.0074 ng m^?3 ppbv^?1) found in previous studies that identified long-range transport of TGM from China. The mean slope during non-events was 0.0011 ng m^?3 ppbv^?1. Back-trajectory analysis showed that during episodes, air parcels arrived mostly from the major industrial areas in China (n = 25, 73%), followed by Japan (n = 4, 12%), Yellow Sea (n = 3, 9%), and Russia (n = 2, 6%).     

An assessment of source contributions to the ozone concentrations in southern Ontario, 1979–1985

Source contributions to the surface O₃ concentrations in southern Ontario were assessed for the 1979–1985 period. Ozone episode analyses indicate a frequency of about nine episodes per year (15 episode-days). These occur primarily in the summer months and are generally manifestations of the northern extent of the O₃ problem in eastern North America. Widespread elevated O₃ levels tend to occur under weather classes indicative of back or centre of the high pressure situations and associated flow/trajectory from areas south/southwest of the lower Great Lakes. These episodes vary considerably from year-to-year. Local impacts on O₃ levels are generally small.A study of O₃ levels during cloud-free summer days for the period 1981–1985 gave local ‘background’ O₃ levels of about 20–30 ppb daily and 30–50 ppb hourly maxima. The O₃ contributions from the U.S. to southern Canada (assuming local ‘background’ O₃ levels to be independent of wind direction) were estimated to be 30–35 ppb daily and 30–50 ppb hourly maxima. These results indicate an overall O₃ contribution of about 50–60% from the U.S. to southern Ontario. For episode-days, the U.S. contribution is even more significant.     

Sampling and physico-chemical analysis of precipitation: a review

Wet deposition is one of two processes governing the transfer of beneficial and toxic chemicals from the atmosphere on to surfaces. Since the early 1970s, numerous investigators have sampled and analyzed precipitation for their chemical constituents, in the context of "acidic rain" and related atmospheric processes. Since then, significant advances have been made in our understanding of how to sample rain, cloud and fog water to preserve their physico-chemical integrity prior to analyses. Since the 1970s large-scale precipitation sampling networks have been in operation to broadly address regional and multi-regional issues. However, in examining the results from such efforts at a site-specific level, concerns have been raised about the accuracy and precision of the information gathered. There is mounting evidence to demonstrate the instability of precipitation samples (e.g. with N species) that have been subjected to prolonged ambient or field conditions. At the present time precipitation sampling procedures allow unrefrigerated or refrigerated collection of wet deposition from individual events, sequential fractions within events, in situ continuous chemical analyses in the field and even sampling of single or individual rain, cloud and fog droplets. Similarly analytical procedures of precipitation composition have advanced from time-consuming methods to rapid and simultaneous analyses of major anions and cations, from bulk samples to single droplets. For example, analytical techniques have evolved from colorimetry to ion chromatography to capillary electrophoresis. Overall, these advances allow a better understanding of heterogeneous reactions and atmospheric pollutant scavenging processes by precipitation. In addition, from an environmental perspective, these advances allow better quantification of semi-labile (e.g. NH4+, frequently its deposition values are underestimated) or labile species [e.g. S (IV)] in precipitation and measurements of toxic chemicals such as Hg and PCBs (polychlorinated biphenyls). Similarly, methods now exist for source-receptor studies, using for example, the characterization of reduced elemental states and/or the use of stable isotopes in precipitation as tracers. Future studies on the relationship between atmospheric deposition and environmental impacts must exploit these advances. This review provides a comprehensive and comparative treatment of the state of the art sampling methods of precipitation and its physico-chemical analysis.     

Mechanisms of episodic acidification in low-order streams in Maine, USA

Five factors contribute to episodic depressions in pH and ANC during hydrologic events in low-order streams in Maine: (1) increases of up to 50 microeq litre(-1) NO3; (2) increases of up to 75 microeq litre(-1) organic acidity; (3) increases of as much as 0.3 in the anion fraction of SO4; (4) as much as 100 microeq litre(-1) acidity generated by the salt-effect in soils; and (5) typically < or = 40% dilution by increased discharge. In conjunction with increased discharge, factors 1, 2 or 4 appear necessary to depress pH to less than 5.0. The chemistry of individual precipitation events is irrelevant to the generation of acidic episodes, except those caused by high loading of neutral salts in coastal regions. Increases in discharge, but not necessarily in dilution of solutes, in combination with the chronically high SO4 from atmospheric deposition, provide the antecedent chemical conditions for episodic acidification. Differences in antecedent moisture conditions determine the processes that control output of either ANC or acidifying agents to aquatic systems.     

The effect of precipitation on the vertical profiles of atmospheric ammonia: A modeling investigation

This paper describes theoretical calculations of atmospheric ammonia profiles during precipitation events. The PLUVIUS reactive storm model (Hales, 1982, Atmospheric Environment16,1775–1783) was used in a simplified form relevant to this specific system. Calculated NH₃ profiles for dry atmospheres were consistent with distributions previously reported in the literature. NH₃ profiles in the gas and cloud phases, following a simulated convective storm, showed large variations with height. These variations are strongly dependent on storm duration. In some cases, NH₃ gas concentrations were depleted by more than 50% by the precipitation scavenging process. Modeled NH₃ gas profiles regenerate slowly with time after storm termination.     

A theoretical assessment of pollutant deposition to individual land types during a regional-scale acid deposition episode

A mesoscale model of pollutant transport, transformation and deposition was used to perform a detailed analysis of acidic deposition to the states of New York and Ohio during a 3-day springtime deposition episode. This model can be used to assess the roles of wet and dry deposition to individual land types in the removal of pollutants from the atmosphere. Over two-thirds (67 %, Ohio; 78 %, New York) of the acidic deposition during this rainy period fell as wet deposition, primarily in the form of H₂SO₄. Dry deposition of SO₂ accounted for 70–75 % of the total dry acidic deposition in both areas, and most of the remaining dry deposition occurred as HNO₃. Over both deposition areas, particulate sulfate deposition accounted for <1 % of the total acid deposition. Due to the highly surface-specific nature of the dry deposition process, individual land types displayed unique patterns of pollutant uptake. Water surfaces absorbed primarily SO₂, while rougher forested areas absorbed a larger proportion of HNO₃ vapor. Urban areas, with their associated material surfaces, were found to absorb significantly less acid in the dry form, and during dry periods most of this deposition may occur as HNO₃ vapor, although considerable uncertainty exists regarding the treatment of rainfall-wetted surfaces. These model results suggest that dry pollutant fluxes to individual surface types will show significant variability from any ‘averaged’ flux estimates over larger areas encompassing numerous land types.     

The relationship between hydrogen and sulphate ions in precipitation-A numerical analysis of rain and snowfall chemistry

Acidic rain has been identified as potentially harmful to the aquatic and terrestrial components of the ecosystem. Sulphate measured in rain and snow has been used as a surrogate indicator of acidic deposition. If sulphur dioxide controls are the means to limit acidic deposition, then the association between sulphate and hydrogen ion concentrations in precipitation is an important factor in establishing such limits. Selected data on rain and snowfall chemistry from the National Atmospheric Deposition Program (NADP), the Electric Power Research Institute's SURE, the utility industries' UAPSP, and the Department of Energy's MAP3S were reviewed. Numerical analyses were performed to assess the relationship between hydrogen and sulphate ion concentrations. The strength of the association between hydrogen and sulphate ions varied from site to site. In the Midwestern and Eastern regions, the Pearson correlation coefficient was over 0.50 while in the Central and Upper Midwestern parts of the United States, the correlation coefficients were less than 0.25. Regardless of the strength of the association between hydrogen and sulphate ions, all but one of the NADP/NTN sites used in our analysis exhibited at least 30% of the anions (sulphate, nitrate, and chloride) associated with cations other than hydrogen. For sites where the strength of the association was weak, between 65% and 98% of the anions appeared to be associated with cations other than hydrogen. Because a large percentage of the anions (i.e. sulphate, nitrate, and chloride) appear to be associated with cations other than hydrogen even at those sites where the association between hydrogen and sulphate ions was strong, the complex chemistry controlling the acidity in precipitation may make it difficult to predict the impact of a reduction in sulphate concentration.     

Non-linear regression model for spatial variation in precipitation chemistry for South India

Chemical composition of rainwater changes from sea to inland under the influence of several major factors – topographic location of area, its distance from sea, annual rainfall. A model is developed here to quantify the variation in precipitation chemistry under the influence of inland distance and rainfall amount. Various sites in India categorized as ‘urban’, ‘suburban’ and ‘rural’ have been considered for model development. pH, HCO₃, NO₃ and Mg do not change much from coast to inland while, SO₄ and Ca change is subjected to local emissions. Cl and Na originate solely from sea salinity and are the chemistry parameters in the model.Non-linear multiple regressions performed for the various categories revealed that both rainfall amount and precipitation chemistry obeyed a power law reduction with distance from sea. Cl and Na decrease rapidly for the first 100 km distance from sea, then decrease marginally for the next 100 km, and later stabilize. Regression parameters estimated for different cases were found to be consistent (R² ～ 0.8). Variation in one of the parameters accounted for urbanization. Model was validated using data points from the southern peninsular region of the country. Estimates are found to be within 99.9% confidence interval.Finally, this relationship between the three parameters – rainfall amount, coastline distance, and concentration (in terms of Cl and Na) was validated with experiments conducted in a small experimental watershed in the south-west India. Chemistry estimated using the model was in good correlation with observed values with a relative error of ～5%. Monthly variation in the chemistry is predicted from a downscaling model and then compared with the observed data. Hence, the model developed for rain chemistry is useful in estimating the concentrations at different spatio-temporal scales and is especially applicable for south-west region of India.     

Source apportionment of PAHs and n-alkanes in respirable particles in Tehran,Iran by wind sector and vertical profile

The vertical concentration profiles and source contributions of polycyclic aromatic hydrocarbons (PAHs) and n-alkanes in respirable particle samples (PM₄) collected at 10, 100, 200 and 300-m altitude from the Milad Tower of Tehran, Iran during fall and winter were investigated. The average concentrations of total PAHs and total n-alkanes were 16.7 and 591 ng/m³, respectively. The positive matrix factorization (PMF) model was applied to the chemical composition and wind data to apportion the contributing sources. The five PAH source factors identified were: ‘diesel’ (56.3 % of total PAHs on average), ‘gasoline’ (15.5 %), ‘wood combustion, and incineration’ (13 %), ‘industry’ (9.2 %), and ‘road soil particle’ (6.0 %). The four n-alkane source factors identified were: ‘petrogenic’ (65 % of total n-alkanes on average), ‘mixture of petrogenic and biomass burning’ (15 %), ‘mixture of biogenic and fossil fuel’ (11.5 %), and ‘biogenic’ (8.5 %). Source contributions by wind sector were also estimated based on the wind sector factor loadings from PMF analysis. Directional dependence of sources was investigated using the conditional probability function (CPF) and directional relative strength (DRS) methods. The calm wind period was found to contribute to 4.4 % of total PAHs and 5.0 % of total n-alkanes on average. Highest average concentrations of PAHs and n-alkanes were found in the 10 and 100 m samples, reflecting the importance of contributions from local sources. Higher average concentrations in the 300 m samples compared to those in the 200 m samples may indicate contributions from long-range transport. The vertical profiles of source factors indicate the gasoline and road soil particle-associated PAHs, and the mixture from biogenic and fossil fuel source-associated n-alkanes were mostly from local emissions. The smaller average contribution of diesel-associated PAHs in the lower altitude samples also indicates that the restriction of diesel-fueled vehicle use in the central area of Tehran has been effective in reducing the PAHs concentration.     

Trajectory analysis of wet deposition at Whiteface Mountain: A sensitivity study

Meteorological trajectory analysis, MAP3S precipitation chemistry data, and hourly precipitation data at Whiteface Mountain (WFM) were used to determine the source direction of sulfate deposited in precipitation for 1978. A sensitivity study was done on source direction from different trajectory models, using different transport layers, and different precipitation information to determine the time deposition occurred. Results obtained from the different precipitation information indicate that precipitation data in 6-h increments or less are essential to correctly identify the trajectories associated with the wet deposition. Bias resulted when trajectories were not only associated with wet deposition. The direction of source regions, defined by 30° sectors, was much less dependent on the trajectory model or transport layer. Results suggest that over one-third of the wet sulfate deposition at WFM in 1978 arrived from the west-southwest, passing through the eastern Great Lakes region.     

Estimates of regional contributions to wet acid deposition in western Massachusetts during the summer of 1984

The General Motors mobile Atmospheric Research Laboratory was situated in rural western Massachusetts in the Berkshire Mountains for 62 days during the summer of 1984. One purpose of this study was to determine the source regions of wet acid deposition for this northeastern U.S. location. First, to apportion the precursors (sulfate, sulfur dioxide and nitrate) to source regions, daily ambient air samples were analyzed for the precursors as well as for tracer species that are associated with particular sources. Factor and trajectory analyses were then used to deduce the contributions of the Midwest and the Northeast to these precursors. Finally, the contribution of the precursors to precipitation acidity was estimated by analyzing the chemical constituents in the rain during seven precipitation events. Averaged over the entire duration of the study, the data show that Northeast sources accounted for about 60% of the precipitation sulfate and nitrate, while Midwest sources accounted for about 30%. The balance (~ 10%) was accounted for by background sulfate. A more useful way of examining the data is according to the type of storm that caused the wet deposition. The site was affected by two basic types of storms: coastal low-pressure systems that traveled up the Atlantic Coast, and cold fronts that approached from the west. During the coastal lowpressure events, the Midwestern contribution to precipitation acidity was zero, as easterly flows from the Atlantic Ocean dominated. The cold front events, however, were all associated with southwesterly flows, and the Midwest contributions exceeded the Northeast contributions. During these events, the average Midwest contribution to precipitation acidity was about 50%. For all events, the ratio of sulfate to nitrate was approximately 2:1 on an equivalent basis. During the coastal lows, the relative nitrate contributions were the highest. It was estimated that particulate sulfate scavenging was responsible for about half of the sulfate in the rain, while the other half was due to in-cloud oxidation of gaseous sulfur dioxide.     

Regional-scale impacts of Phase I of the Clean Air Act Amendments in the USA: the relation between emissions and concentrations,both wet and dry

A comparison of data records in the 1990s, both before (1991–1994) and after (1995–1997) implementation of Phase I of the Clean Air Act Amendments (CAAA) of 1990 for the eastern US, shows a significant reduction in SO₂ emissions for most states, except for Texas, North Carolina, Illinois, Florida, and Alabama. However, of the major NO_x emitting states, only two eastern states (New York and Pennsylvania) show significant declines in NO_x. A pattern of large declines in SO₂ emissions (>20%) after CAAA implementation, and large declines in precipitation SO₄²⁻ and H⁺, as well as air concentrations of SO₂ and SO₄²⁻ (components of dry deposition), exists for most regions of the eastern US. In most cases, the emission/concentration relations are close to 1 : 1 when the source region based on 15-h back trajectories is used for the New England region, and source regions based on 9-h back trajectories are used for the six other eastern US regions that were studied. The southern Appalachian Mountain region, an acid-sensitive area receiving high levels of acidic deposition, has not seen an appreciable improvement in precipitation acidity. This area has also shown the least improvement in wet and dry sulfur concentrations, of the areas examined. Precipitation base cations (Ca²⁺ and Mg²⁺) show a pattern of either increasing or level concentrations when comparing 1990–1994 to 1995–1998 data, for six of the seven regions examined. Ammonium concentrations have generally changed <10%, except for the Illinois and southern Appalachian Mtn. regions, which increased >15%.