Precipitation Chemistry Data Research Articles

Atmospheric nitrogen inputs to the Delaware Inland Bays: the role of ammonia

A previous assessment of nitrogen loading to the Delaware Inland Bays indicates that atmospheric deposition provides 15–25% of the total, annual N input to these estuaries. A large and increasing fraction of the atmospheric wet flux is NH 4 +, which for most aquatic organisms represents the most readily assimilated form of this nutrient. Particularly noteworthy is a 60% increase in the precipitation NH 4 + concentration at Lewes, DE over the past 20 years, which parallels the increase in poultry production on the Delmarva Peninsula over this period (currently standing at nearly 585 million birds annually). To further examine the relationship between local NH 3 emissions and deposition, biweekly-integrated gaseous NH 3 concentrations were determined using Ogawa passive samplers deployed at 13 sampling sites throughout the Inland Bays watershed over a one-year period. Annual mean concentrations at the 13 sites ranged from <0.5 μg NH 3 m −3 to >6 μg NH 3 m −3, with a mean of 1.6 ± 1.0 μg NH 3 m −3. At most sites, highest NH 3 concentrations were evident during spring and summer, when fertilizer application and poultry house ventilation rates are greatest, and seasonally elevated temperatures induce increased rates of microbial activity and volatilization from soils and animal wastes. The observed north-to-south concentration gradient across the watershed is consistent with the spatial distribution of poultry houses, as revealed by a GIS analysis of aerial photographs. Based on the average measured NH 3 concentration and published NH 3 deposition rates to water surfaces (5–8 mm s −1), the direct atmospheric deposition of gaseous NH 3 to the Inland Bays is 3.0–4.8 kg ha −1 yr −1. This input, not accounted for in previous assessments of atmospheric loading to the Inland Bays, would effectively double the estimated direct dry deposition rate, and is on par with the NO 3 − and NH 4 + wet fluxes. A second component of this study examined spatial differences in NO 3 − and NH 4 + wet deposition within the Inland Bays watershed. In a pilot study, precipitation composition at the Lewes NADP–AIRMoN site (DE 02) was compared with that at a satellite site established at Riverdale on the Indian River Estuary, approximately 21 km southwest. While the volume-weighted mean precipitation NO 3 − concentrations did not differ significantly between sites, the NH 4 + concentration observed at Riverdale (26.3 μmoles L −1) was 73% greater than at Lewes (15.2 μmoles L −1). More recently, a NADP site was established at Trap Pond, DE (DE 99), which was intentionally located within the region of intense poultry production. A comparison of the initial two years (6/2001–5/2003) of precipitation chemistry data from Trap Pond with other nearby NADP–AIRMoN sites (Lewes and Smith Island) reveals fairly homogeneous NO 3 − wet deposition, but significant spatial differences (∼60%) in the NH 4 + wet flux. Overall, these results suggest that local emissions and below-cloud scavenging provide a significant contribution to regional atmospheric N deposition.

Enhanced Wet Deposition Estimates using Modeled Precipitation Inputs

Effective assessments of linkages between atmospheric deposition and sensitive aquatic and terrestrial ecosystems require a knowledge of spatial patterns in deposition of greater resolution than currently available using point estimates and two-dimensional surface algorithms. A high resolution wet deposition model was developed for the Eastern U.S.A. by incorporating daily precipitation measurements from National Oceanic and Atmospheric Administration (NOAA) monitoring sites located within the Eastern U.S.A.; topographic variables, including elevation, slope, and aspect that affect the amount and distribution of precipitation across the region; and precipitation chemistry data from National Atmospheric Deposition Program (NADP) sites within and adjacent to the region. Model performance, which was tested against independent measurements from three physiographic regions of the eastern U.S.A., revealed a statistically significant reduction in interpolation error compared to traditional two-dimensional surface algorithms.

Meteorological indicators of summer precipitation chemistry in central Pennsylvania

Daily precipitation chemistry data from nine consecutive summers (1993–2001) at a central Pennsylvania site were analyzed by objective and subjective methods and classified into key synoptic-scale circulation types. Significant differences in acidity and analyte concentrations were found among the seven storm types. The Convective type had the highest mean concentrations for all major ions compared to the Warm-Front (WF) type, which featured the lowest concentrations. The highest sulfate-to-nitrate ratio (2.36), assigned to the Warm-Sector (WS) type, indicates that acidity within air-masses transported to central Pennsylvania from the southwest is primarily in the form of H 2SO 4. About 20% of the overall acidity in precipitation during summer is neutralized by base cations. Both the Low-to-South and WF types have the lowest magnitudes of neutralization due to a lack of base cations of crustal origin. Both circulation systems are featured by an onshore flow from the Atlantic Ocean. Meteorological indicators for each weather type were developed based on values and geographical locations of anomalies of synoptic-scale variables, expressed as significant departures from their long-term means (as extracted from the Reanalysis NCEP/NCAR Model). The results suggest distinctive patterns for the regional position of the anomalies for the differing circulation systems. The two warm-weather types (High-to-South and Convective and WS) are featured by negative anomalies of sea-level pressure (SLP) and 850 hPa geopotential height (GPH) located northwest of the site and positive anomalies of the 850 hPa temperature to the east. Both are also associated with deep convection. The Cold-Front and Low-to-East types manifest an opposite symmetrical position of the anomalies, i.e., negative anomalies of SLP and 850 hPa GPH to the northeast and negative 850 hPa temperature anomalies northwest of the site. Both types are characterized by lower lifted indices, indicating weak atmospheric instability.

Monthly and annual bias in weekly (NADP/NTN) versus daily (AIRMoN) precipitation chemistry data in the Eastern USA

Previous comparisons of the data from the National Atmospheric Deposition Program, National Trends Network (NTN) against collocated event sampled data and daily sampled data suggest a substantial bias in the concentration of ammonium [NH 4 +] and concentrations of several base cations, while the comparability of other ion concentrations ranges among the studies. Eight years of collocated data from five NTN and Atmospheric Integrated Research and Monitoring Network (AIRMoN) sites are compared here. Unlike previous analyses, the data from these two data sets were analyzed in the same laboratory using the same analytical methods; therefore, factors that influence concentration differences can be isolated to sampling frequency and sample preservation techniques. For comparison, the relative biases for these data have been calculated using both median value and volume-weighted mean concentrations, following two different approaches in the literature. The results suggest a relative bias of about 10% in [NH 4 +] (NTN less than AIRMoN), which is smaller than previous estimates that included the influence of inter-laboratory comparisons. The annual relative bias of [H +] increases over the analysis period, which results in a larger total relative bias for [H +] than found in a previous analysis of AIRMoN and NTN data. When comparing NTN and AIRMoN data on monthly time scales, strong seasonal variations are evident in the relative bias for [H +], [NH 4 +], and [SO 4 2−]. Large biases in [SO 4 2−] (NTN greater than AIRMoN) on monthly times scales have not been detected in previous analyses where data for all seasons were considered together.

Potential effects of changes in precipitation and temperature on wet deposition in central Pennsylvania

Changes in regional climate can alter conditions that control the transport, chemical transformation, and eventual deposition of atmospheric pollutants. In Pennsylvania, climate change is projected to increase mean annual temperatures by 4°C and annual precipitation amounts by about 5% through 2100. The objective of this study was to determine how increases in temperature and precipitation would affect concentrations and wet deposition of SO 4 2−, NO 3 −, NH 4 +, and H + ions at a National Atmospheric Deposition Program site (NADP, PA15) in central Pennsylvania. Event-based wet deposition data were obtained from the Multi-State Atmospheric Power Production Pollution Study (MAP3S) monitoring program for the period 1976–1989. Forward stepwise regression was used to predict log-normal concentrations per event using mean temperature, precipitation, Julian Date, and interactions of temperature and precipitation with Julian Date as predictor variables. Julian Date was included to adjust for time trends in precipitation chemistry and climate data. Results were categorized by annual periods, growing season/dormant season, and synoptic climate types. Significant positive effects of temperature on concentration and deposition were found for SO 4 2−, NH 4 +, and H + but not for NO 3 −. Precipitation increases reduced the concentration of each ion due to dilution, but the lower concentration only minimally offset the increase in wet deposition due to the increased precipitation. The effects of climate change during the growing season (April–September) were projected to cause greater increases in the magnitude of SO 4 2−, NO 3 −, and H + concentrations and wet deposition than in the dormant season (October–March). Ammonium (NH 4 +) increases were greatest during the dormant season. Climate change effects on wet deposition were generally similar among synoptic climate types with the greatest effects occurring for cold fronts and warm fronts. The impacts of climate change for this central PA site could increase overall annual wet deposition of SO 4 2− by up to 29%, NO 3 − by up to 6%, NH 4 + by up to 23%, and H + by up to 17% through the year 2100, assuming that the statistical relationships hold true and that no changes in the emission of these ions occur.

Trends in eastern U.S. sulfur air quality from the Atmospheric Integrated Research Monitoring Network

[1] Surface air quality data obtained at long-term stations operated by the Atmospheric Integrated Research Monitoring Network of the National Oceanic and Atmospheric Administration are combined with precipitation chemistry data from the National Atmospheric Deposition Program to construct indicators of air chemistry in the lower atmosphere starting in 1985/1986 and extending through 1997. The analysis is based on data from seven stations east of the Mississippi River. On the whole, levels of airborne sulfur reached a maximum in the late 1980s; since then, levels have been declining steadily at a rate of about 2.8% per annum, but with an increase to about 5% per annum for a period of the 1990s. There is no clear-cut step function that could be attributed to the initiation of emissions trading and resulting SO2 emissions reductions in about 1995. Instead, the data appear to indicate that the new regulatory trading philosophies have continued the long-term improvement in overall air quality that was already under way. The data from individual sites indicate that the benefits of emissions decreases are not necessarily evident at all locations. Improvements appear to be moderated to a considerable extent by local factors that may dominate in some situations. Overall, the air quality data correlate well with changes in emissions of sulfur as reported by the Environmental Protection Agency. The analysis shows that the percentage improvement in sulfur concentrations in the planetary boundary layer east of the Mississippi corresponds one to one to the percentage change in emissions from electric utilities (within the statistical uncertainties involved). Overall, the results support the hypothesis that regional air quality is governed by the largest point-source emitters, rather than by the integral of all upwind sources.

Trends in precipitation chemistry in Alaska, 1980–1998

Nineteen years (1980–1998) of precipitation chemistry data from a site in Alaska are examined for trends using a least squares general linear model. The annual concentrations of SO2– 4 show a significant decreasing trend at 0.001 level and the annual change in concentration is —0.012mg 1–1 yr–1. The annual concentrations of NO – 3 show an increasing tendency non‐significant. The annual base cation concentrations show a clear significant decreasing trend at 0.001 level and the decrease is —0.009mg 1–1 yr–1. Ca2+ concentrations exhibit a significant decreasing trend at 0.001 level and the annual change of concentration is —0.003 mg 1–1 yr–1. Mg2+ and Na+ concentrations show a significant decreasing trend at 0.01 level and the annual change is —0.001 mg1–1 yr–1 for Mg2+ and —0.004 mg1–1 yr–1 for Na+. K+ concentrations are characterised by a decreasing trend, significant at 0.05 level. K+ concentrations have decreased —0.002mg1–1 yr–1. The strongest rates of concentration decline for base cations, Ca2+, Mg2+, K+ and Na+ occurred in fall and winter season. The annual values of pH show a decreasing trend non‐significant. The values of pH oscillate between 5.1 and 5.6 during the period considered.

Acid Rain Reduced in Eastern United States

Concentrations of sulfate (SO42-) and free hydrogen ions (H+) in precipitation decreased from 10% to 25% over a large area of the Eastern United States from 1995 through 1997 as compared to the previous 12-year (1983−1994) reference period. These decreases were unprecedented in magnitude and spatial extent. In contrast, nitrate (NO3-) concentrations generally did not change over this period. The largest decreases in both H+ and SO42- concentrations, which nearly mimicked one another, occurred in and downwind of the Ohio River Valley, the same area where Title IV of the 1990 Clean Air Act Amendments (CAAA) set limitations on sulfur dioxide (SO2) emissions from a large number of utility-owned coal-fired sources. Phase I of the CAAA required that these limitations be met by January 1, 1995. On the basis of our analysis of precipitation chemistry and emissions data, we conclude that significant declines in acid rain occurred in many parts of the Eastern United States from 1995 through 1997 because of large re...

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.

Temporal and spatial variations of airborne Mg, Cl, Na, Ca and K in rural areas of Norway

The Norwegian air chemistry monitoring network includes measurements of gaseous and particulate compounds in air at 12 rural sites. The sampling method is designed to determine concentration levels of sulphur and nitrogen compounds and uses a three-stage filter pack sampler. Concentration levels of base-cations and sea salts may also be determined from the chemical analysis of the filter extracts. In this study, concentration levels of water soluble Mg, Cl, Na, Ca and K in air have been investigated in terms of temporal, seasonal and spatial trends during the period 1986–1996. The results have been combined with precipitation chemistry data to estimate total deposition, and to evaluate the relative importance of dry deposition. While Na, Cl and Mg are of marine origin, which is clearly reflected in concentration ratios and spatial gradients, Ca and K are of a mixed origin. Air trajectory analyses indicate that air masses originating in Eastern Europe result in significantly higher airborne concentrations of non-sea salt Ca and K, than air from other sectors. Major sources of airborne Ca and K in Norway are expected to be of anthropogenic origin from combustion plants and industrial processes in Eastern Europe, while aeolian dust from agricultural areas within Europe or from Sahara is of limited importance. Concentration levels of non-sea salt base-cations in air were 20–30% lower during the 1990s, compared to 1986–1989. Precipitation chemistry data available since the early 1980s indicate even larger reductions, and in the order of 50%. Total deposition of non-sea salt base-cations during 1993–1996 was generally below 10 mmol(c) m −2 year −1, whereas in coastal areas with large precipitation amounts, inputs exceeding 15 mmol(c) m −2 year −1 occurred. Deposition of base-cations may thus counteract up to 25% of the strong acid anion input, but more typically in the order of 5–10%. Dry deposition of non-sea salt base-cations of southern Norway is comparatively small, typically contributing 10–30% to the total deposition in the inland areas, and relatively less in other regions.

An integrated biogeochemical model system for the Baltic Sea

The Swedish Meteorological and Hydrological Institute (SMHI) is developing an integrated biogeochemical model system for the Baltic Sea. It consists of three coupled models; a marine biogeochemical-hydrodynamical model (SCOBI), a continuous riverine nitrogen transport model (HBV-N) and an atmospheric transport and chemical model (MATCH). It is supplied with a tool for presentation and analysis. The SCOBI model is a coupled one-dimensional model with high vertical resolution. Horizontal variations are taken into account by dividing the area into smaller boxes. The model includes primary phytoplankton production, nitrogen fixation and secondary zooplankton production. Nitrogen load from land is obtained through the HBV-N model, which simulates organic and inorganic nitrogen separately in catchments. The model is based on the hydrological model HBV, and the subbasin runoff is simulated on a daily basis. The HBV-N model is calibrated and validated against measured concentrations and water flow in rivers. The atmospheric input of oxidised and reduced nitrogen is taken from the annual assessments carried out with the MATCH-Sweden modelling system. MATCH-Sweden combines model calculations, using an atmospheric transport and chemical model, with observations of air- and precipitation chemistry data to give a detailed mapping of concentrations and deposition of nitrogen compounds over Sweden. Supplemented with monitoring data for the specific area of interest, the integrated biogeochemical model system makes a useful tool for environmental protection analyses; e.g. for interpretation of monitoring data as well as creating scenarios for studies of effects in changes of the nutrient loads.

Influence of continental outflow events on the aerosol composition at Cheju Island, South Korea

The chemical composition of aerosols measured at Cheju Island, Korea, over the 3‐year period March 1992 to February 1995 are presented and discussed, with a particular emphasis on the Pacific Exploratory Mission in the Western Pacific (PEM‐West B) time period. Cheju Island is under the influence of continental outflow conditions nearly 70% of the year, and as a result the aerosol loading of sea salt as well as continental aerosol components is high. The 3‐year mean values derived from the daily tape filters are non‐sea‐salt (nss) SO22− = 6.8 μg/m3, NO3− = 1.2 μg/m3, Cl− = 1.9 μg/m3, Na+ = 1.7 μg/m3, Ca2+ = 0.5 μg/m3, NH4+ = 1.3 μg/m3, Mg2+ = 0.3 μg/m3, and K+ = 0.4 μg/m3. Sea‐salt components show peak values in winter, while calcium, nitrate, potassium, and to a lesser extent, sulfate and ammonium, show higher values in the spring, and all species exhibit a pronounced minimum in summer. Trajectory and principal component analysis show that elevated levels of primary aerosols (both sea salt and soil‐derived) occur with strong wind conditions associated with winter and spring, and high concentrations of non‐sea‐salt components are most strongly associated with springtime continental outflow events. During the PEM‐West B period, nitrate and calcium are found to be ∼60% higher than the annual mean, sea‐salt components ∼30% higher, and sulfate ∼10% higher. The aerosol data are combined with gaseous SO2 concentrations, precipitation chemistry data and companion aerosol measurements taken at the same site, to provide further insights into the aerosol composition at Cheju. At Cheju, sulfate and ammonium are found mostly in the fine fraction (∼80 to 90%), while calcium and nitrate reside in the coarse size fraction. Sulfate and nitrate are not associated with the primary aerosols, but rather become associated with the aerosol during the long‐range transport process.

High elevation fog and precipitation chemistry in southern Quebec, Canada

Results of the seasonal analysis of the Chemistry of High Elevation Fog (CHEF) project data for the years 1985–1991 are presented. The mountain sites are located in southern Quebec, Canada, and range in altitude from 250 to 970 m. Fog, precipitation, and mixed fog and precipitation chemistry data are given for major inorganic ions for all seasons of the year. This paper documents the occurrence of highly acidic fog events; it points out the potential importance of high elevation fog as a major pathway for acidic wet deposition in southern Quebec; and it provides results paralleling those from the American Mountain Cloud Chemistry Project (MCCP), thus supporting the relevance of investigations of high elevation fog to at least latitude 47°N. Data are from 8034 analyses of event-length samples, of which 1140 are mountain fog samples collected as duplicate pairs, making this one of the largest fog/cloud water databases available. The 1985–1991 fog sample mean pH at Roundtop was 3.90 and at Mont Tremblant 3.72. The mean fog pH for all sites and seasons was 3.79. The minimum fog pH was 2.80 and the maximum 7.20. The mean precipitation pH on Roundtop was 4.22 and at Mont Tremblant 4.19. Mixed sample pH values are intermediate between fog and precipitation. There is a small tendency (< 0.1 pH units) for both rain and snow to be more acidic at low elevations, with snow having a higher pH range. The more northerly Mont Tremblant site had higher ion concentrations and higher acidity than did the Roundtop Mountain site. The dominant anion was SO 4 2− followed by NO 3 −. The dominant cation was NH 4 +. The NO 3 − SO 4 2− equivalents ratio showed a strong wintertime peak from year to year with values sometimes exceeding 1.0. Over the 6 yr data set, the average concentrations of the major ions in the mixed and precipitation samples showed small decreases with time in all seasons. In contrast, the average and median wintertime H +, SO 4 2− and NO 3 − concentrations, in the high elevation fog samples, increased. The results show that the mountain areas of southern Quebec have a potential for total wet deposition that is distinct from low elevation sites in the region.

Trends in precipitation chemistry in the United States: A national perspective, 1980–1992

Thirteen years (1980–1992) of precipitation chemistry data from 58 National Atmospheric Deposition Program/National Trends Network (NADP/NTN) sites are examined for trends using a general linear least squares model. SO 4 2- concentrations are decreasing throughout the United States; significant ( p < 0.05) trends are evident at 42 of the 58 sites included in this analysis and the average decrease is 12.026 μeq ℓ −1. The largest concentration of sites with significant decreasing SO 4 2− trends is located in the north central and western regions of the country. Eleven sites exhibit significant NO 3 − trends, nine of which are decreasing. Ca 2+ and Mg 2+ concentrations show the most widespread decline (44 and 52 sites, respectively) of all the major cations and anions in precipitation. Ca 2+ concentrations have decreased 4.587 μeq ℓ −1 since 1980; Mg 2+ concentrations have decreased 2.034 μeq ℓ −1. The most consistent and statistically most significant ( p < 0.001) Ca 2+ and Mg 2+ trends occur in the northeast. Decreasing trends are also evident at 28 sites for Na + and at 35 sites for K +. NH 4 + concentrations exhibit very little change over this 13-year period. The widespread decline in base cations, particularly Ca 2+ and Mg 2+, appears to have offset the effects that decreasing SO 4 2− concentrations should have on free acidity in precipitation. Only 17 sites exhibit both significant decreasing H + and SO 4 2− concentration trends, most of which are located in the north central portion of the United States. The average decrease in H + at the 17 sites is 9.991 μeq ℓ −1. Hawaii is the only site in the NADP/NTN network to exhibit a significant increasing H + trend.

Long-term trends in NADP/NTN precipitation chemistry data: Results of different statistical analyses

This paper summarizes the results of four statistical approaches for the estimation of long-term trends (1983–92) in sulfate concentration data from 90 monitoring sites across the United States. Least squares regression models and nonparametric techniques were applied to these data. Sulfate concentrations were found to be generally decreasing on the order of 0–4% at most sites. There was general agreement that trends were significant in the Great Lakes, Pacific northwest, and southwest regions. Although strengths and weaknesses are described for each approach, all of these approaches are useful for long-term trend estimation. Visualization techniques are recommended for displaying trend patterns and associated levels of statistical significance.

A Meteorology-Based Approach to Detecting the Relationship between Changes in SO2Emission Rates and Precipitation Concentrations of Sulfate

In this paper, the authors present an analysis of correlations between SO2 emissions and wet SO4(2-) concentrations over eastern North America that includes adjustments for the impact of meteorological variability. The approach uses multiple-regression models and readily available meteorological information to analyze precipitation chemistry data collected from 1979 to 1986 at six Utility Acid Precipitation Study Program sites. On an event-to-event basis, from 25% to 50% of the variation in concentrations, depending on site, was found to be related to meteorology. Precipitation amount, temperature, upwind emissions, and upwind mean lower-tropospheric relative humidity (indicator for upwind precipitation) were related to the natural log of SO4(2-) concentrations. Inclusion of this information resulted in a decrease in the uncertainty associated with the emission change to concentration relationship at all sites, but the results were inconsistent. Year-to-year and season-to-season changes in SO2 emissions were found to be significantly related (p less than 0.033) to variations in event and seasonal concentrations at three of the sites, but not at the other three sites. Possible explanations for this discrepancy are discussed in this paper. When all sites were examined simultaneously, strong statistical correlations (p less than 0.007) were found between emissions and concentrations, indicating that SO4(2-)more » concentrations decreased in response to seasonal and annual emission changes.« less

Comparison of the source locations and seasonal patterns for acidic species in precipitation and ambient particles in Southern Ontario, Canada

The Potential Source Contribution Function (PSCF) is a probability function based on the air parcel trajectory data coupled with information regarding the nature of the contaminants measured in that air parcel. The PSCF value is the ratio of the probability of a contaminated air parcel having traversed a 1° latitude by 1° longitude area to the probability that any air parcel traversed that area. Regions with high PSCF values thus have a higher probability of contributing pollutants to the measured concentrations at the receptor site. Previously, PSCF analysis has been used on precipitation chemistry data obtained from the Acidic Precipitation in Ontario Study (APIOS) to locate the sources of the acidic precipitation. In this present study, the methodology has been applied to both precipitation samples and particulate samples collected by the APIOS network. Analyses have been performed separately for the winter and summer seasons so that the comparison between particles and precipitation and/or winter and summer can be made. From these results it is possible to identify the possible source locations for the observed constituents and at the same time, derive information on atmospheric process by this study.

Comparability and precision of MAP3S and NADP/NTN precipitation chemistry data at an acidic site in eastern North America

Precipitation chemistry data derived from more than six years of concurrent sampling under the MAP3S and NADP/NTN protocols at the Penn State site in rural Pennsylvania are compared. Preprocessing of the data (sample and gage depths, concentrations of H 3O +, SO 4 2−, NO 3 −, NH 4 +) included aggregating the event and daily MAP3S data into weekly averages that coincide with the weekly data of the NADP/NTN program. A simple statistical model was applied to the differences of the paired data to look for systematic biases between the networks. The most notable bias occurred for ammonium concentration (NADP/NTN low), a finding consistent with other studies. A minor, but statistically significant, bias was also found for sulfate coccentration (NADP/NTN high). Both the ammonium and sulfate biases may be related to the relatively long time that the NADP/NTN samples remain in the field, thus strengthening arguments in favor of daily sampling protocols. The hydronium and nitrate concentrations exhibited remarkably high levels of comparability between the daily and weekly protocols. Overall, the data from the MAP3S and NADP/NTN networks are comparable, with the possible exception of ammonium, for some applications. The field results from about two years of duplicate sampling under the MAP3S protocol demonstrate that measurement precisions of only a few percent are possible when the protocol is consistently followed and the equipment is well maintained.

Temporal variation of sulphate and nitrate concentration in precipitation in eastern North America: 1979–1990

Precipitation chemistry data from the U.S. National Atmospheric Deposition Program/National Trends Network and the Canadian Air and Precipitation Monitoring Network were used to investigate the temporal variation of sulphate and nitrate concentrations in precipitation at 24 sites across eastern North America. Two different techniques were applied—a kernel regression smoother to investigate the character of the temporal variations, and a geometric model fitted by least-squares. Of the 24 monitoring sites investigated, all produced statistically significant long-term sulphate trends, while only 13 produced significant nitrate trends. In contrast to previously published studies which assumed monotonically increasing or decreasing temporal variations, only 2 of the 48 long-term trends in this study exhibited monotonic behaviour. Long-term cycles with periods between 2 and 5.5 years were found at 58 and 83% of the sites for SO 2− 4 and NO − 3 , respectively. At more than 58% of the sites more complicated seasonal patterns than a simple sinusoidal cycle were obtained for both ions. These results indicate that more complicated models than the ones that have been used up to now should be employed. It is recommended that a smoothing technique (e.g. kernel smoothing regression) be used to help in identifying the best parametric model to use with least-squares techniques and to test for statistical significance in the temporal model.

Associations between pollutant emissions and precipitation chemistry: An empirical analysis

Empirical analyses were conducted of the associations between anthropogenic SO2 emissions and precipitation SO42− concentrations as well as between anthropogenic NOx emissions and precipitation NO3− concentrations across central North America. Both the 1982 Electric Power Research Institute (EPRI) National Air Pollutant Emissions Inventory and the 1985 National Acid Precipitation Assessment Program (NAPAP) Emissions Inventory, along with precipitation chemistry data from up to 130 monitoring sites, were used in the analyses. Comparisons of the EPRI and NAPAP emissions inventories in their geographic area of overlap indicate that SO2 emissions differ more than do the NOx emissions, on both absolute and fractional difference scales. Empirical analysis results indicate that the associations between anthropogenic SO2 emissions and precipitation SO42− are stronger than associations between anthropogenic NOx emissions and precipitation NO3− concentrations, with the associations equally well represented by either a power law or linear law function. Distinct regional and seasonal differences exist in the strength of the associations.