Trends In Precipitation Chemistry Research Articles

The evolving role of weather types on rainfall chemistry under large reductions in pollutant emissions

Long-term change and shorter-term variability in the atmospheric deposition of pollutants and marine salts can have major effects on the biogeochemistry and ecology of soils and surface water ecosystems. In the 1980s, at the time of peak acid deposition in the UK, deposition loads were highly dependent on prevailing weather types, and it was postulated that future pollution recovery trajectories would be partly dependent on any climate change-driven shifts in weather systems. Following three decades of substantial acidic emission reductions, we used monitoring data collected between 1992 and 2015 from four UK Environmental Change Network (ECN) sites in contrasting parts of Great Britain to examine the trends in precipitation chemistry in relation to prevailing weather conditions. Weather systems were classified on the basis of Lamb weather type (LWT) groupings, while emissions inventories and clustering of air mass trajectories were used to interpret the observed patterns. Concentrations of ions showed clear differences between cyclonic-westerly-dominated periods and others, reflecting higher marine and lower anthropogenic contributions in Atlantic air masses. Westerlies were associated with higher rainfall, higher sea salt concentrations, and lower pollutant concentrations at all sites, while air mass paths exerted additional controls. Westerlies therefore have continued to favour higher sea salt fluxes, whereas emission reductions are increasingly leading to positive correlations between westerlies and pollutant fluxes. Our results also suggest a shift from the influence of anthropogenic emissions to natural emissions (e.g., sea salt) and climate forcing as they are transported under relatively cleaner conditions to the UK. Westerlies have been relatively frequent over the ECN monitoring period, but longer-term cyclicity in these weather types suggests that current contributions to precipitation may not be sustained over coming years.

Long-term trend of wintertime precipitation chemistry at a remote islet site influenced by anthropogenic emissions from continental East Asia

In this study, the trends and factors influencing wintertime precipitation chemistry at a remote site downwind of continental East Asia were investigated. A total of 336 precipitation samples were collected from 2005 to 2019 at Pengjia Islet, a site ~56 km north of the main island of Taiwan. Overall, the samples had a volume-weighted mean pH of 4.81 (ranging from 3.03 to 7.00), with 74% of samples considered as acid rain (pH < 5). PMF analysis of the samples identified sea-salt, anthropogenic emissions, and dust as sources of major ions in precipitation. Anthropogenic emissions were mostly associated with polluted air from East Asia. Four major air mass transport pathways to Pengjia Islet were identified, in which 87% originated from northern China (i.e. polluted air masses) with a relatively low pH and high ionic concentrations compared to the remaining samples. Wintertime pH values associated with the most common trajectory from China significantly increased at a rate of +1.7% yr−1 (p < 0.01) from 2005 to 2019, similar to those observed at other sites in East Asia in the previous two decades. The increasing pH trend in this study was attributed to the decrease in nss-SO42- concentrations and an increase in Ca2+ concentrations, of which the former aligns with the decrease in nss-SO42- at sites in China caused by the decline of SO2 emissions. The increase of Ca2+ concentrations is supported by the increase of coarse mode aerosol mass concentrations transported to northern Taiwan in winter. By splitting the dataset into two periods (i.e. 2005-2011 and 2012–2019), we found a close relationship between the changes in air-quality control in China and trends in precipitation chemistry at Pengjia Islet, including a consistently decreasing nss-SO42-/NO3− ratio in rainwater. Such a finding has important implications for the effectiveness of air pollution control policies on the regional scale. Additionally, changes in regional transport patterns of air masses might also contribute to the changes in precipitation chemistry at Pengjia Islet.

Trends in precipitation chemistry across the U.S. 1985–2017: Quantifying the benefits from 30 years of Clean Air Act amendment regulation

Acid rain was first recognized in the 1970s in North America and Europe as an atmospheric pollutant that was causing harm to ecosystems. In response, the U.S. Congress enacted Title IV of the Clean Air Act Amendments (CAA) in 1990 to reduce sulfur and nitrogen emissions from fossil fuel burning power plants. This study reports trends in wet-precipitation chemistry in response to emissions reductions implemented as part of the CAA. Trends were calculated for sulfate (SO4), nitrate (NO3) and ammonium (NH4) from 1985 to 2017 at 168 stations operated by the National Atmospheric Deposition Program (NADP); stations were divided into 9 regions across the United States. Trend analyses were conducted for three time periods: Period 1 (1985–1999), Period 2 (2000–2017), and the entire study period (1985–2017). Seasonal and regional Kendall trend analyses reveal significant decreasing trends in mean wet-precipitation SO4 concentrations in all 9 regions during the entire study period. The largest decreasing trends in monthly mean SO4 precipitation-weighted concentrations were measured in the Mid-Atlantic (−1.29 μeq/l/yr), Midwest (−1.15 μeq/l/yr), and Northeast regions (−1.10 μeq/l/yr). The trends in monthly mean NO3 concentrations were not as strong as those for SO4, but all of the regions had significant decreasing trends in NO3 and again the Mid-Atlantic (−0.53 μeq/l/yr), Midwest (−0.44 μeq/l/yr), and Northeast regions (−0.50 μeq/l/yr) had the strongest trends. Trends were steepest during Period 2 for SO4 and NO3, in fact for NO3 86% of the stations had significant decreasing trends during Period 2 while only 8% of the stations had significant decreasing trends during Period 1. The stations with the highest concentrations of SO4 and NO3 at the beginning of the study had the strongest decreasing trends and the relations were stronger during Period 2 than Period 1. For NH4, 22% of the stations had statistically significant increasing trends in concentration during Period 1. The largest increasing trends in wet-precipitation NH4 concentration occurred in the North-Central region during Period 1, Period 2 and throughout the entire study. By comparison, NH4 trends in the Rocky-North and Rocky-South regions were about half as steep and trends in the South-Central and Midwest regions were about one-third as steep.We compared trends in SO4 and NO3 concentrations from NADP stations to emissions of sulfur dioxide and nitrogen oxides, respectively to determine whether there was a relation between emissions and wet-precipitation concentration trends within proximity to NADP stations. There was a statistically significant relation (r2 = 0.62–0.69, p < 0.01) between the trend in SO4 concentrations at individual NADP stations and total and mean sulfur dioxide (SO2) emissions from power plants within a range of 750 km and 1000 km from each station. There were also significant relations between NO3 concentration trends at NADP stations and power plant emissions of nitrogen oxides, but they were not nearly as strong (r2 = 0.18–0.36, p < 0.01) as those for SO4 and were strongest for emissions within a range of 1000 km and 1500 km from each NADP station. Decreases in wet-precipitation SO4 concentrations were more consistent across regions and through time than decreases in NO3 and SO4 trends were more closely linked to stationary emissions sources than NO3 trends. There were statistically significant increases in NH4 wet-precipitation concentrations, as have been reported in previous studies, but this study found that those increases were strongest during Period 1 and were not consistent across the United States. During the first 3 years of the study period, wet-precipitation acidity was dominated by SO4 in 8 of the 9 regions; by 2017 NO3 dominated the acidity of wet-precipitation in 7 of the 9 regions. There has also been a downward shift in the NO3:NH4 ratio of wet-precipitation as the emissions of nitrogen oxides have declined while ammonia emissions have remained essentially constant. This shift has resulted in an increase in wet-precipitation total nitrogen concentrations in 7 of the 9 regions and indicate that efforts to control NH3 emissions will become increasingly important as emissions of nitrogen oxides continue to decline.

Contrasted spatial and long-term trends in precipitation chemistry and deposition fluxes at rural stations in France

The long-distance effect of atmospheric pollution on ecosystems has led to the conclusion of international agreements to regulate atmospheric emissions and monitor their impact. This study investigated variations in atmospheric deposition chemistry in France using data gathered from three different monitoring networks (37 stations) over the period from 1995 to 2007. Despite some methodological differences (e.g. type of collector, frequency of sampling and analysis), converging results were found in spatial variations, seasonal patterns and temporal trends. With regard to spatial variations, the mean annual pH in particular ranged from 4.9 in the north-east to 5.8 in the south-east. This gradient was related to the concentration of NO3− and non-sea-salt SO42− (maximum volume-weighted mean of 38 and 31 μeq l−1 respectively) and of acid-neutralising compounds such as non-sea-salt Ca2+ and NH4+. In terms of seasonal variations, winter and autumn pH were linked to lower acidity neutralisation than during the warm season. The temporal trends in atmospheric deposition varied depending on the chemical species and site location. The most significant and widespread trend was the decrease in non-sea-salt SO42− concentrations (significant at 65% of the stations). At the same time, many stations showed an increasing trend in annual pH (+0.3 on average for 16 stations). These two trends are probably due to the reduction in SO2 emissions that has been imposed in Europe since the 1980s. Temporal trends in inorganic N concentrations were rather moderate and not consistent with the trends reported in emission estimates. Despite the reduction in NOx emissions, NO3− concentrations in atmospheric deposition remained mostly unchanged or even increased at three stations (+0.43 μeq l−1 yr−1 on average). In contrast NH4+ concentrations in atmospheric deposition decreased at several stations located in western and northern areas, while the estimates of NH3 emissions remained fairly stable. The decrease in non-sea-salt SO42− and NH4+ concentrations was mainly due to a decrease in summer values and can in part be related to a dilution process since the precipitation amount showed an increasing trend during the summer. Furthermore, increasing trends in NO3− concentrations in the spring and, to a lesser extent, in NH4+ concentrations suggested that other atmospheric physicochemical processes should also be taken into account.

Trajectory statistical analysis of atmospheric transport patterns and trends in precipitation chemistry of a rural site in NE Spain in 1984–2009

The aim of this study is to interpret the variation of precipitation chemistry during the last 25 years in Montseny (NE Spain) by taking into account the main air mass transport routes and emission sources. To this end, trajectory cluster analysis and source–receptor models were applied to an early monitoring period (1984–1993) and compared to a more recent one (1998–2009). A decrease of Atlantic advections and increase of African and European air flows was found. Cluster analysis and source–receptor approaches showed that this region is under the influence of natural and anthropogenic sources from the local and long-range scale. Sulphate and H+ source areas extended over a vast stretch of central Europe in the early period, but were drastically reduced in the recent period, showing the effectiveness of pollution abatement measures for S. On the other hand, NO3− sources areas from central Europe strikingly increased. Ship emissions and industrialisation in Eastern Europe and North Africa seem to be acquiring a greater role in the recent period.

Temporal and spatial trends in precipitation chemistry in the Georgia Basin, British Columbia

During the 1960s and 1970s, scientific evidence on the impacts of atmospheric acid deposition led to international negotiations to control emissions of compounds (primarily sulphur (S) and nitrogen (N) oxides) that undergo long-range transport. More recently, there has been growing interest in acid rain research in western Canada, e.g., the Georgia Basin, British Columbia, where S and N emissions are expected to increase during the next two decades. In the current study, long-term trends (1990–2007) in precipitation chemistry were evaluated at four stations (Saturna, British Columbia; Egbert, Ontario; Chapais Quebec; and Kejimkujik, Nova Scotia) using the Mann-Kendall statistical test. Precipitation chemistry at Saturna suggests that the Georgia Basin is less influenced by anthropogenic (S and N) emissions compared with monitoring stations in eastern Canada; nonetheless, long-term (1990–2007) trends in precipitation chemistry at Saturna [pH (significant increase) sulphate (significant decrease) and nitrate (decrease)] mirrored those at sites in eastern Canada suggesting all stations (across Canada) have responded similarly to large-scale emissions reductions under the Canada-United States Air Quality Agreement. The chemistry of precipitation and snow in the current study was well characterised by location and elevation, providing a way to estimate long-term mean annual base cation and chloride precipitation chemistry (ions showing no trend) at unmonitored sites for contiguous regions in the Georgia Basin.

Response of lake chemistry to changes in atmospheric deposition and climate in three high-elevation wilderness areas of Colorado

Trends in precipitation chemistry and hydrologic and climatic data were examined as drivers of long-term changes in the chemical composition of high-elevation lakes in three wilderness areas in Colorado during 1985–2008. Sulfate concentrations in precipitation decreased at a rate of −0.15 to −0.55 μeq/l/year at 10 high-elevation National Atmospheric Deposition Program stations in the state during 1987–2008 reflecting regional reductions in SO2 emissions. In lakes where sulfate is primarily derived from atmospheric inputs, sulfate concentrations also decreased although the rates generally were less, ranging from −0.12 to −0.27 μeq/l/year. The similarity in timing and sulfur isotopic data support the hypothesis that decreases in atmospheric deposition are driving the response of high-elevation lakes in some areas of the state. By contrast, in lakes where sulfate is derived primarily from watershed weathering sources, sulfate concentrations showed sharp increases during 1985–2008. Analysis of long-term climate records indicates that annual air temperatures have increased between 0.45 and 0.93°C per decade throughout most mountainous areas of Colorado, suggesting climate as a factor. Isotopic data reveal that sulfate in these lakes is largely derived from pyrite, which may indicate climate warming is preferentially affecting the rate of pyrite weathering.

Wet deposition and related atmospheric chemistry in the São Paulo metropolis, Brazil. Part 3: Trends in precipitation chemistry during 1983–2003

The metropolitan area of São Paulo (MASP), with 18 million inhabitants, is one of the largest urban areas of the world, with serious environmental problems, including atmospheric pollution. Even though there is an official network monitoring the air quality, it does not include data on the chemical composition of precipitation. However, some research groups have sampled and measured common analytes in rainwater on several sites at MASP during restricted periods. Eventually, available data of pH values and ion concentrations in rainwater since 1983 in the city of São Paulo have been compiled and are, now, discussed in this work, which, besides showing possible trends, gives support for future studies planned in MASP. A decreasing trend of rainwater free acidity in São Paulo occurred along the 2 decades considered. Since 1995, no monthly volume weight mean (VWM m) pH values below 4.5 have been observed which, in part, reflects the gradual decrease in the atmospheric concentrations of the SO 2, with annual average of 65 μg m −3 in 1983 and 15 μg m −3 in 2003. Scarce data on sulfate concentration in the rain shows a similar decay from 33±35 μmol L −1 in 1989–1990 to 19±18 μmol L −1 in 2002–2003. Despite the fact that no significant trends can be inferred in the limited nitrate database, for ammonium, high concentrations (47±42 μmol L −1 of NH 4 +) have been observed in recent years, indicating the contribution of the NH 3 present in the São Paulo's atmosphere to the neutralization of the rain acidity.

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.

Assessing decadal changes in rainwater alkalinity at a rural Mediterranean site in the Montseny Mountains (NE Spain)

Annual volume-weighted mean (VWM) concentrations in rainwater collected at La Castanya (LC, Montseny Mountains, NE Spain) were analysed from 1983 to 2000 to study the temporal trends in precipitation chemistry, and the causes behind the changes. A significant positive correlation was found between annual rainwater SO 4 2− concentrations at LC and Spanish SO 2 emissions ( r=0.73, P=0.0008) both decreasing remarkably during this period. Rainwater alkalinity increased during the period, shifting from negative values at the beginning (VWM in the 5 initial years=−2.7 μeq l −1) to alkaline values in recent years (VWM in the 5 final years=18.0 μeq l −1). Stepwise regression analysis indicated that 88% of the variation of alkalinity could be accounted for by the variability of non-marine Ca 2+ and non-marine SO 4 2−, with a more prominent dependence on Ca 2+. Rains of African provenance were highly enriched in alkalinity and Ca 2+, but no significant increases in their occurrence were found for the study period. Because of the reported higher dust updraft in northern Africa during years of high North Atlantic Oscillation (NAO) index, we also explored the relationship between rainwater variables associated with an African provenance and NAO. Annual precipitation was inversely related to NAO ( r=−0.61, P=0.007). The annual wet deposition of African dust-related elements showed no correlation with NAO, probably because wet deposition of these elements depends on two factors (precipitation and dust updraft) which have opposite behaviour with respect to NAO. We hypothesise that dry deposition of African dust during dry spells (not sampled in this study) might be higher during high NAO-index years.

Changes in the Chemistry of Precipitation in the United States, 1981–1998

Regulatory measures in the United States, such as Title IV of the Clean Air Act Amendments of 1990, have primarily restricted sulfur dioxide emissions as a way to control acidic deposition. These restrictions, coupled with increasing concentrations of NH4+ in wet deposition in some regions of the U.S. and continued high emissions of nitrogen oxides have generated a significant shift in the chemistry of precipitation as measured at National Atmospheric Deposition Program/National Trends Network sites. Trends in precipitation chemistry at NADP/NTN sites were evaluated for statistical significance for the period 1981–1998 using a Seasonal Mann-Kendall Test, a robust non-parametric test for detection of monotonic trends. SO42− declines were detected at 100 of the 147 sites examined while no sites exhibited increasing SO42− trends. On average, SO42− declined 35% over the period 1981–1998 with downward SO42− trends being most pronounced in the north-eastern United States. In contrast, no consistent trends in NO3− concentrations were observed in precipitation in any major region of the United States. Although the majority of sites did not exhibit significant trends in NH4+ concentration, 30 sites exhibited upward trends. For Ca2+ concentration in precipitation, 64 sites exhibited a significant decreasing trend and no sites exhibited an upward trend.

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.

Ten years trends (1984–1993) in the precipitation chemistry in central Austria

Since 1984, wet precipitation samples have been collected from five sampling sites in the eastern Alpine region of Austria. The chemical analysis performed included pH, NH 4 +, Na +, K +, Ca 2+, Mg 2+, Cl −, NO 3 − SO 4 2− and precipitation amount. These data were used to determine the 10 years (1984–1993) trends in precipitation chemistry and deposition for all major ions. The statistical data treatment by the use of least-squares linear regression revealed a significant decrease of sulfate and hydrogen ion concentrations and depositions at all sites in consideration (above 65% for hydrogen ion and over 33% for sulfate concentration and 40% for hydrogen and 36% for sulfate deposition). An effort is made to explain the acidity decrease not only by the parallel decrease of sulfate concentration but by the participation of the other constituents of the wet precipitation events. It has been found that when the hydrogen ion concentration decrease is stronger than that of sulfate ion concentrations a compensation mainly by ammonium ion concentration increase is achieved. On the contrary, when the hydrogen ion concentration decrease is weaker than sulfate concentration decrease, a calcium ion concentration decrease is present for the sites in consideration. The results obtained indicate that the precipitation chemistry in Central Austria follows the global trends of controlled reduction of atmospheric acidity.

Deposition of sulphur and nitrogen compounds in Croatia

Since precipitation is an efficient scavenger of pollutants, concentrations of major ions in precipitation reflect changes in chemistry of the atmosphere and in the subsequent exposure of various ecosystems to deposition. The National Atmospheric Precipitation Chemistry programme was initiated in 1978 and operated by Meteorological and Hydrological Service of Croatia to provide needed information on geographical patterns and temporal trends in precipitation chemistry in Croatia. To accomplish this, a network of about 20 stations, settled in different geographical regions, operates on a daily basis for 15 years now. Some monitoring stations are site- and study- specific; others are included in long-term, regional, or European monitoring networks (EMEP, GAW, MEDPOL, GEMS). The purpose of this work was to summarise existing data from the whole network for the period 1981–1992 and to compare data from measurements with EMEP model calculations of acid deposition. Results presented here show that annual average concentration and deposition values at remote sites agree reasonably well compared to modelled ones.

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.

The chemical composition of atmospheric deposition collected from six Louisiana sites from 1983 to 1992

Since 1983, precipitation samples have been collected weekly from six Louisiana sites. Analyses performed included pH, EC, Ca 2+, Na +, Mg 2+, K +, NH 4 +, Cl, NO 3 −, SO 4 2−, PO 4 3−, and precipitation amount. These data were used to determine quarterly and yearly regional trends in precipitation chemistry and deposition. Concentrations of K + (< 0.2 mgℓ −1), Mg 2+ (< 0.02 mgℓ −1) and PO 4 3− (< 0.5 mgℓ −1) were below detection limits more than 90% of the time. Calcium, Na +, NH 4 +, SO 4 2−, and Cl − had the highest deposition during the spring at most sites. The highest EC values were found in the summer, while the lowest were in the fall. Nitrate deposition was highest in the summer and lowest in the fall. Sulfate and NH 4 + had the least deposition in the fall for most sites. The most acidic pH values were in the summer for all six sites. The lowest precipitation amounts were in the summer and fall, while the highest were in the winter. Yearly deposition amounts of nutrients were not sufficient to meet major crop requirements. Sulfur was deposited in quantities to meet one-half to one-third of its requirement in cotton, rice, soybeans, and wheat.

Long-term trends in precipitation chemistry at Hubbard Brook, New Hampshire

A continuous, 19-year record (1963–1982) of weekly, bulk precipitation chemistry at the Hubbard Brook Experimental Forest in West Thornton, New Hampshire shows no statistically significant trend in annual volume-weighted concentrations of hydrogen ion and nitrate, but a 34% decrease in sulfate, a 34% decrease in ammonium, a 63% decrease in chloride, a 79% decrease in magnesium and an 86% decrease in calcium during the period. Nitrate concentrations increased from 1964 to 1971 and H-ion concentrations decreased after 1970. Frequency distributions of the concentrations of the chemical components of precipitation are skewed. The range of H-ion concentrations in weekly samples has narrowed, and the frequency distribution has shifted toward higher concentrations (lower pH) during the last 19 years. Highest concentrations generally occur with lowest amounts of precipitation for most ions, but low concentrations can occur with either low or high amounts of precipitation. Time trends in deposition generally parallel trends in concentration over the 19-year period. Chemical deposition generally increases with increasing amount of precipitation in weekly samples.

Seasonal trends in precipitation chemistry

Using precipitation chemistry data from several North American sites, significant seasonal fluctuations in the concentrations of certain precipitation ions were found. These fluctuations are mathematically described using the sine function.