Long-term Concentration Trends Research Articles

Long-term PM2.5 concentration prediction based on improved empirical mode decomposition and deep neural network combined with noise reduction auto-encoder- A case study in Beijing

Effective prediction of PM2.5 long-term concentration can help reduce exposure risks, but few current studies based on machine learning have been able to credibly predict concentration changing trends after 12 h. To address this issue, this study employed the complementary ensemble empirical mode decomposition (CEEMD) method combined with sample entropy (SE) index, as well as bidirectional long short-term memory (BiLSTM) model stacked denoising auto-encoder (AE) method for better decomposition and prediction of long-term trends in PM2.5 time series data. The new CEEMD-AE-BiLSTM model has the best long-term prediction results at different sites, and the R2 value is 0.871, and the RMSE is 9.28 μg/m3 at T+12 moment in the 1002A site. The new model effectively captures a significant portion of the long-term concentration trends characteristically and has been proven to be an effective tool for long-term predictions. This is attributed to the robust noise resistance of the AE method and the appropriate parameter setting of the CEEMD method.

Using Ice Cores to Evaluate CMIP6 Aerosol Concentrations Over the Historical Era

AbstractThe radiative effect of anthropogenic aerosols is one of the largest uncertainties in Earth's energy budget over the industrial period. This uncertainty is in part due to sparse observations of aerosol concentrations in the pre‐satellite era. To address this lack of measurements, ice cores can be used, which contain the aerosol concentration record. To date, these observations have been under‐utilized for comparison to aerosol concentrations found in state‐of‐the‐art Earth system models (ESMs). Here we compare long term trends in concentrations of sulfate and black carbon (BC) between 15 ice cores and 11 ESMs over nine regions around the world during the period 1850–2000. We find that for sulfate concentration trends model results generally agree with ice core records, whereas for BC concentration the model trends differ from the records. Absolute concentrations of both investigated species are overestimated by the models, probably in part due to representation errors. However, we assume that biases in relative trends are not altered by these errors. Ice cores in the European Alps and Greenland record a maximum BC concentration before 1950, while most ESMs used in this study agree on a post‐1950 maximum. We source this bias to an error in BC emission inventories in Europe. Emission perturbation experiments using NorESM2‐LM support the observed finding that BC concentrations in Northern Greenland ice cores are recording European emissions. Errors in BC emission inventories have implications for all future and past studies where Coupled Model Intercomparison Project Phase 6 historical simulations are compared to observations relevant to aerosol forcing.

Evaluating uncertainty in sensor networks for urban air pollution insights

Abstract. Ambient air pollution poses a major global public health risk. Lower-cost air quality sensors (LCSs) are increasingly being explored as a tool to understand local air pollution problems and develop effective solutions. A barrier to LCS adoption is potentially larger measurement uncertainty compared to reference measurement technology. The technical performance of various LCSs has been tested in laboratory and field environments, and a growing body of literature on uses of LCSs primarily focuses on proof-of-concept deployments. However, few studies have demonstrated the implications of LCS measurement uncertainties on a sensor network's ability to assess spatiotemporal patterns of local air pollution. Here, we present results from a 2-year deployment of 100 stationary electrochemical nitrogen dioxide (NO2) LCSs across Greater London as part of the Breathe London pilot project (BL). We evaluated sensor performance using collocations with reference instruments, estimating ∼ 35 % average uncertainty (root mean square error) in the calibrated LCSs, and identified infrequent, multi-week periods of poorer performance and high bias during summer months. We analyzed BL data to generate insights about London's air pollution, including long-term concentration trends, diurnal and day-of-week patterns, and profiles of elevated concentrations during regional pollution episodes. These findings were validated against measurements from an extensive reference network, demonstrating the BL network's ability to generate robust information about London's air pollution. In cases where the BL network did not effectively capture features that the reference network measured, ongoing collocations of representative sensors often provided evidence of irregularities in sensor performance, demonstrating how, in the absence of an extensive reference network, project-long collocations could enable characterization and mitigation of network-wide sensor uncertainties. The conclusions are restricted to the specific sensors used for this study, but the results give direction to LCS users by demonstrating the kinds of air pollution insights possible from LCS networks and provide a blueprint for future LCS projects to manage and evaluate uncertainties when collecting, analyzing, and interpreting data.

Trends in PM2.5 Concentration in Nagoya, Japan, from 2003 to 2018 and Impacts of PM2.5 Countermeasures

In Japan, various countermeasures have been undertaken to reduce the atmospheric concentration of fine particulate matter (PM2.5). We evaluated the extent to which these countermeasures were effective in reducing PM2.5 concentrations by analyzing the long-term concentration trends of the major components of PM2.5 and their emissions in Nagoya City. PM2.5 concentrations decreased by 53% over the 16-year period from fiscal years 2003 to 2018 in Nagoya City. Elemental carbon (EC) was the component of PM2.5 with the greatest decrease in concentration over the 16 years, decreasing by 4.3 μg/m3, followed by SO42− (3.0 μg/m3), organic carbon (OC) (2.0 μg/m3), NH4+ (1.6 μg/m3), and NO3− (1.3 μg/m3). The decrease in EC concentration was found to be caused largely by the effect of diesel emission control. OC concentrations decreased because of the effects of volatile organic compound (VOC) emission regulations for stationary sources and reductions in VOCs emitted by vehicles and construction machinery. NO3− concentrations decreased alongside decreased contributions from vehicles, construction machinery, and stationary sources, in descending order of the magnitude of decrease. Although these findings identify some source control measures that have been effective in reducing PM2.5, they also reveal the ineffectiveness of some recent countermeasures for various components, such as those targeting OC concentrations.

Long-term trends in concentrations and sources of PM2.5–bound metals and elements in central Los Angeles

In this study, we investigated long-term trends in the ambient concentrations and sources of redox-active metals and trace elements in central Los Angeles over the period of 2005–2018. Mass concentrations of PM2.5-bound metals and crustal elements were obtained from the Chemical Speciation Network (CSN) database provided by the US Environmental Protection Agency (US EPA). The recorded metal concentrations showed considerable variations throughout the study period, but they generally followed a descending trend from 2005 to 2018. The decline in annual average concentration of metals was more pronounced for V and Ni. In order to further investigate and interpret the observed decreasing trends, this dataset of 2005–2018 was employed in the positive matrix factorization (PMF) model to determine the contribution of different sources to the total metals’ concentrations and their trends over time. Four major sources were identified by the PMF model, including mineral dust, re-suspended road dust, combustion, and tire wear. Mineral dust (50 ± 8%) and re-suspended road dust (38 ± 13%) were the dominant contributors to total metal concentrations, followed by combustion emissions (9 ± 8%) and tire wear (2 ± 1%). While the PMF results showed generally consistent contributions of mineral dust to total metals concentration throughout the investigation period, the contribution of re-suspended road dust to total metals increased from 2013 to 2018 probably due to the increased road traffic (expressed in the form of vehicle miles traveled, VMT) as well as the growing use of electric vehicles (EVs) (which increases resuspension of road dust particles due to their heavy weight) in the area during the same period. In contrast, the contribution of combustion emissions decreased by almost 88% from 2005 to 2018. Using the PMF-resolved factor profiles, we investigated the contribution of the four identified sources to selected individual metals (i.e., Ba, Br, Fe, Ni, Pb, Ti, V, and Zn). The observed decrease in the mass concentration of V and Ni over time was attributed to the reduction in combustion sources and particularly industrial emissions, further corroborating the effectiveness of aftertreatment and air quality policies in reducing the levels of redox-active metals across Los Angeles.

Probabilistic Prediction of Corrosion Damage of Steel Structures in the Vicinity of Roads

The design, construction, and maintenance of steel structures must be carried out in a way that ensures they will be able to reliably operate for the whole duration of their planned service life. To ensure sufficient durability, it is necessary to determine and evaluate the characteristics of the appropriate environment in which the structure will be placed. This submission focuses on the specific environment surrounding roads that are treated with de-icing salts during winter maintenance. It investigates the dependency between corrosive damage to the structure and the relevant parameters of the environment. Basic corrosive factors include temperature, relative humidity, deposition of chlorides and sulfur dioxide, precipitation, the pH of precipitation as well as many other parameters. An accurate estimate of corrosive damage requires an analysis of the long-term trends in concentrations of individual corrosive factors, while respecting their randomly varying attributes. The article, hence, introduces and evaluates stochastic prediction models that are based on long-term programs focusing on the evaluation of the corrosive aggressiveness of the environment, while taking into account random variations of the nature of the input parameters. The use of stochastic prediction models allows us to perform sensitivity analysis that can determine the impact of specific corrosive factors on the corrosive damage caused to the structure. The article is supplemented by sensitivity analysis focusing on an evaluation from the effects of the deposition of chlorides on the corrosive damage to steel bridge structures. The analysis was carried out using data obtained from experimental measurements of the deposition rates of chlorides in the vicinity of roads in the Czech Republic.

Fifteen-year trends in carbon species and PM2.5 in Seoul, South Korea (2003–2017)

This study focused on particulate matter (PM2.5) and carbon species in Seoul, South Korea, to quantitatively evaluate their long-term trends and assess the main correlating factors. Ambient PM2.5 samples were collected over a 24 h period every third or sixth day from March 2003 to December 2017.The mean concentrations of PM2.5, organic carbon (OC), elemental carbon (EC), primary and secondary OC (POC and SOC) in Seoul over 15 years were 32.2 μg/m3 and 7.28 μg/m3, 1.85 μg/m3, 4.29 μg/m3 and 3.54 μg/m3 respectively. The long-term concentration trends in PM2.5, OC, EC, POC, and SOC decreased significantly at rates of −2.09, −3.13, −6.31, −2.86, and −3.88 per year, respectively from 2003 to 2017 (p < 0.001), whereas the long-term trends in OC/EC significantly increased at a rate of 12.9/year (p < 0.001). These long-term decreases in PM2.5 and carbon species concentrations were most pronounced in 2008 but almost disappeared from 2013 onwards. Considering the decrease in wind speed and variations in the concentration of gaseous air pollutants (carbon monoxide, sulfur dioxide, nitrogen dioxide, and volatile organic compounds) without a tendency to increase or decrease since 2013, secondary aerosol formation by atmospheric stagnation alleviated long-term decreases in PM2.5 and carbon species concentrations. The long-term decreases in EC concentration were the most consistent and rapid, strongly suggesting that atmospheric policies related to mobile in South Korea were effective in reducing EC concentration. Future air quality management should focus on the secondary formation of air pollutants based on regional trends in air pollutant concentrations.

METHANE IN THE SURFACE LAYER OF THE ATMOSPHERE: CURRENT CONTENT, LONG-TERM TRENDS, AND INTRA-ANNUAL VARIABILITY

The article deals with the empirical analysis of series of monthly mean concentrations of methane in the near-surface layer of the atmosphere from the global network of monitoring stations. They operate within the Global Atmosphere Watch (GAW) under the auspices of the World Meteorological Organization (WMO). The data is freely available at the World Data Center for Greenhouse Gases GAW/WMO (WDCGG) on its website https://gaw.kishou.go.jp/. The temporal coverage is from the 1980s. Data series from 69 stations are considered, of which 22 stations represent global background conditions. The rest of the stations are regional. Long-term trends in concentrations and the intra-annual (inter-monthly) deviations of monthly mean concentrations from long-term trends were studied. The multi-year trend was estimated using a series of 12-month running averages. To exclude systematic differences in methane concentrations, these series were adjusted to the series for the high-latitude Arctic station Alert (82° 30' N, 62° 21' W). The analysis showed that long-term trends are non-linear (in particular, a known pause in the growth of methane levels in 1999-2006 is observed), but are similar at most stations under consideration. Exceptions are six regional stations classified as “abnormal” in terms of methane. Possibly, this abnormality is due to the influence of certain sources of methane (anthropogenic or natural). Long-term trends at the rest of the stations just slightly differ from the average trend for the global stations. The series of intra-annual (inter-monthly) deviations of monthly mean concentrations from long-term trends for many stations (even those located at very significant distances from each other) show high correlative similarity. However, this similarity manifests itself at an optimal time shift from 5 months towards earlier dates up to 6 months towards later dates. The results of the analysis are consistent with the assumption that the intra-annual variability in methane concentration is largely driven by seasonal factors that are significantly related to latitude, such as vertical mixing in the atmosphere and destruction in the troposphere in reactions with hydroxyl. The root-mean-square values of intra-annual (inter-monthly) fluctuations in methane concentration depend significantly on latitude. In general, the higher the latitude is, the greater is the value. The maximum values are reached in the latitudinal belt within 45-50° N, and further to the North the values decrease. This feature of intra-annual fluctuations in the level of methane content may be explained, among other things, by significant inter-seasonal fluctuations in anthropogenic methane emissions occurring at the indicated latitudes in the countries with developed economies located in North America and Western Europe. The correlations of the series of intra-annual (inter-monthly) fluctuations of the monthly mean concentrations of CH4 and CO2 were estimated as rather high, about 0.8, at optimal time shifts, which is observed both at some polar stations and at tropical ones. This confirms the assumption that natural seasonal biogeochemical and geophysical processes play a significant role in the formation of intra-annual (inter-monthly) deviations of methane and carbon dioxide content in the near-surface layer from long-term trends. These processes include vertical mixing of air, CO2 absorption on the Earth’s surface, and destruction of methane in the troposphere in reactions with hydroxyl.

МЕТАН В ПРИПОВЕРХНОСТНОМ СЛОЕ АТМОСФЕРЫ: СОВРЕМЕННОЕ СОДЕРЖАНИЕ, МНОГОЛЕТНИЕ ТРЕНДЫ И ВНУТРИГОДОВАЯ ИЗМЕНЧИВОСТЬ

The article is devoted to the empirical analysis of series of monthly mean concentrations of methane in the near-surface layer of the atmosphere from the global network of monitoring stations. They operate within the Global Atmosphere Watch (GAW) under the auspices of the World Meteorological Organization (WMO). The data is freely available at the World Data Center for Greenhouse Gases GAW/WMO (WDCGG) on its website https://gaw.kishou.go.jp/ . The temporal coverage is from the 1980s. Data series from 69 stations are considered, of which 22 stations represent the global background conditions. The rest of the stations are regional. Long-term trends in concentrations and intraannual (inter-monthly) deviations of mean monthly concentrations from long-term trends were studied. The multi-year trend was estimated using a series of 12-month running averages. To exclude systematic differences in methane concentrations, these series were adjusted to the series for the high-latitude Arctic station Alert (82° 30' N, 62° 21' W). The analysis showed that long-term trends are non-linear (including with a known pause in the growth of methane levels in 1999-2006), but are similar at most of the stations under consideration. Exceptions are 6 regional stations classified as ‘abnormal’ for methane. A possible cause of the abnormality is being under the influence of certain sources of methane (anthropogenic or natural). Long-term trends at the rest of the stations are just slightly differ from the average trend for global stations. The series of intra-annual (inter-monthly) deviations of mean monthly concentrations from long-term trends for many stations, even those located at very significant distances from each other, show high correlative similarity. However, it manifests itself at an optimal time shift from 5 months. towards earlier dates up to 6 months. towards later dates. The results of the analysis are consistent with the assumption that the intra-annual variability in methane concentration is largely driven by seasonal factors that are significantly related to latitude, including vertical mixing in the atmosphere and destruction in the troposphere in reactions with hydroxyl. The root-mean-square values of the intra-annual (inter-monthly) fluctuations in methane concentration significantly depend on latitude. The higher the latitude, the greater the overall value. Maximum values are reached in the latitudinal belt 45-50° N, and further, to the north, the Кузовкин В.В., Семенов С.М. 20 values decrease. This character of intra-annual fluctuations in the level of methane content may be explained, among other things, by significant inter-seasonal fluctuations in anthropogenic methane emissions at the indicated latitudes in countries with developed economies located in North America and Western Europe. The estimates of correlations of the series of intra-annual (inter-monthly) fluctuations of the average monthly concentrations of CH4 and CO2 showed that at optimal time shifts, they are rather high, about 0.8. Moreover, this is observed both at some polar stations and at tropical ones. This confirms the assumption that natural seasonal biogeochemical and geophysical processes play a significant role in the formation of intra-annual (inter-monthly) deviations of the methane and carbon dioxide content in the near-surface layer from long-term trends, namely, vertical mixing of air, CO2 absorption on the Earth's surface, destruction of methane in the troposphere in reactions with hydroxyl.

Spatio-temporal controls of dissolved organic carbon stream water concentrations

Elevated concentration and high variance of dissolved organic carbon (DOC) in surface waters are a challenge for the production of drinking water. Past studies have indicated a dominant role of wetlands in DOC mobilization, but have mainly been focused on boreal and oceanic catchments. Here we analyze the observational DOC time series from 89 temperate humid catchments which drain into German drinking water reservoirs. We characterize the DOC concentration median and variability and utilize partial least squares regression in order to quantify the relation to catchment characteristics such as land use, climate, and topography. We found that the long-term median DOC concentration in the catchment is well predicted by the 90th percentile of the distribution of the topographic wetness index (0.9P TWI) over the entire catchment area. The 0.9P TWI can be directly connected to the abundance of riparian wetlands in the catchments. DOC concentration variability (represented as the ratio of the interquartile range and the median concentrations) was also found to be well predictable. Concentration variability was highest in cold and wet catchments with a high 0.9P TWI. Here we also found stronger correlations between DOC concentrations and discharge, with positive concentration-discharge patterns. Catchments with elevated DOC-concentration variance also exhibited the most severe long-term increases in concentrations. Our results thus indicate that, in temperate climates, riparian wetlands can be the dominant source zones of DOC and control the hydrological mobilization and potentially also the spatial difference in long-term concentration trends observed in surface waters. We conclude that the dominance of topography and climatic conditions in controlling spatio-temporal patterns in DOC concentrations leads to very limited management options.

Long-term trends of surface-water mercury and methylmercury concentrations downstream of historic mining within the Carson River watershed

The Carson River is a vital water resource for local municipalities and migratory birds travelling the Pacific Flyway. Historic mining practices that used mercury (Hg) to extract gold from Comstock Lode ore has left much of the river system heavily contaminated with Hg, a practice that continues in many parts of the world today. Between 1998 and 2013, the United States Geological Survey (USGS) collected and analyzed Carson River water for Hg and methylmercury (MeHg) concentrations resulting in a sixteen year record of unfiltered total mercury (uf.THg), filtered (dissolved) Hg (f.THg), total methylmercury (uf.MeHg), filtered MeHg (f.MeHg), and particulate-bound THg (p.THg) and MeHg (p.MeHg) concentrations. This represents one of the longest continuous records of Hg speciation data for any riverine system, thereby providing a unique opportunity to evaluate long-term trends in concentrations and annual loads. During the period of analysis, uf.THg concentration and load trended downward at rates of −0.85% and −1.8% per year, respectively. Conversely, the f.THg concentration increased at a rate of 1.7% per year between 1998 and 2005, and 4.9% per year between 2005 and 2013. Trends in flow-normalized partition coefficients for both Hg and MeHg suggest a statistically significant shift from the particulate to the filtered phase. The upwardly accelerating f.THg concentration and observed shift from the solid phase to the aqueous phase among the pools of Hg and MeHg within the river water column signals an increased risk of deteriorating ecological conditions in the lower basin with respect to Hg contamination. More broadly, the 16-year trend analysis, completed 140 years after the commencement of major Hg releases to the Carson River, provides a poignant example of the ongoing legacy left behind by gold and silver mining techniques that relied on Hg amalgamation, and a cautionary tale for regions still pursuing the practice in other countries.

Nebraska's groundwater legacy: Nitrate contamination beneath irrigated cropland.

A 31 year record of ∼44,000 nitrate analyses in ∼11,500 irrigation wells was utilized to depict the decadal expansion of groundwater nitrate contamination (N ≥ 10 mg/L) in the irrigated corn-growing areas of eastern and central Nebraska and analyze long-term nitrate concentration trends in 17 management areas (MAs) subject to N fertilizer and budgeting requirements. The 1.3 M contaminated hectares were characterized by irrigation method, soil drainage, and vadose zone thickness and lithology. The areal extent and growth of contaminated groundwater in two predominately sprinkler-irrigated areas was only ∼20% smaller beneath well-drained silt loams with thick clayey-silt unsaturated layers and unsaturated thicknesses >15 m (400,000 ha and 15,000 ha/yr) than beneath well and excessively well-drained soils with very sandy vadose zones (511,000 ha and 18,600 ha/yr). Much slower expansion (3700 ha/yr) occurred in the 220,000 contaminated hectares in the central Platte valley characterized by predominately gravity irrigation on thick, well-drained silt loams above a thin (∼5.3 m), sandy unsaturated zone. The only reversals in long-term concentration trends occurred in two MAs (120,500 ha) within this contaminated area. Concentrations declined 0.14 and 0.20 mg N/L/yr (p < 0.02) to ∼18.3 and 18.8 mg N/L, respectively, during >20 years of management. Average annual concentrations in 10 MAs are increasing (p < 0.05) and indicate that average nitrate concentrations in leachates below the root zone and groundwater concentrations have not yet reached steady state. While management practices likely have slowed increases in groundwater nitrate concentrations, irrigation and nutrient applications must be more effectively controlled to retain nitrate in the root zone.

Anthropogenic changes of CO2, CH4, N2O, CFCl3, CF2Cl2, CCl2FCClF2, CHCl3, CH3CCl3, CCl4, SF6 and SF5CF3 mixing ratios in the atmosphere over southern Poland

800x600 An overview of long-term, systematic observations of trace gas composition of the atmosphere over southern Poland is given. Three major greenhouse gases (CO 2 , CH 4 , N 2 O) and selected halocarbons (freons F-11, F-12 and F-113; chloroform; 1,1,1-trichloroetane; carbon tetrachloride; sulfur hexafluoride and trifluoromethyl sulfur pentafuoride) were monitored. Measurements were performed at two locations of contrasting characteristics: (1) the high-mountain site Kasprowy Wierch, High Tatras, representing atmospheric conditions relatively free of local influences, and (2) two sites located in the Krakow agglomeration, representing a typical urban atmosphere. The data available for Krakow and Kasprowy Wierch were compared with the Mace Head data, representing a marine regional background. The impact of continental sources for some of the measured gases is clearly seen in the Krakow and Kasprowy Wierch records. The mean offset between CH 4 concentrations recorded at Kasprowy Wierch and at Mace Head for the period 1998-2012 is 20.7 ppb and stems from continental emissions of this gas originating mainly from anthropogenic activities (leaks of natural gas distribution networks, landfills, livestock). For N 2 O, a similar offset of ca. 1 ppb for the period 2009-2012 was observed. Although the long-term concentration trends of selected halogenated compounds measured in Krakow coincide in general with the respective trends in Mace Head data, the Krakow records contain numerous spikes and periods of enhanced concentrations, reflecting the impact of local sources of these compounds. The impact of a legislative framework enforced in Poland in July 2002, regulating the trade, storage and disposal of ozone-depleting substances, is visible in the Krakow record of halogenated compounds. Normal 0 21 false false false PL X-NONE X-NONE MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:Standardowy; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:Times New Roman,serif;} Normal 0 21 false false false MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:Standardowy; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:Times New Roman; mso-ansi-language:#0400; mso-fareast-language:#0400; mso-bidi-language:#0400;}

Controls Over Base Cation Concentrations in Stream and River Waters: A Long-Term Analysis on the Role of Deposition and Climate

Significant concern has emerged over the past decades regarding decreases in available base cations (that is, calcium, magnesium, potassium, and sodium) in forest soils and surface waters. Base cations (BCs) are important for buffering against changes in soil and water acidity, and their concentrations can be indicative of environmental management problems such as those linked to acid deposition and land use. Climate variability is also a potentially large factor influencing the dynamics of BCs in soils and surface waters, but our understanding of these interactions at broad scales remains elusive. We used a hierarchical Bayesian model to evaluate the long-term (1990–2010) patterns and drivers of BC concentrations for 60 stream and river monitoring stations across Sweden. Results indicated that the long-term trends in concentration, and the associated environmental drivers, differed among individual BCs and geographical regions. For example, we found that concentrations of Ca2+, Mg2+, K+, and Na+ have decreased in southern Sweden since 1990 and that this is strongly related to concurrent declines in sulfate (SO4 2−) over the same period of record. In contrast, concentrations of Ca2+, Mg2+, K+, and Na+ in northern Sweden did not exhibit significant directional trends, despite declines in SO4 2−, nitrate (NO3 −), and chloride (Cl−) over the same period. Instead, BC dynamics in the north are characterized by inter-annual variability that is most closely linked to climate variables. Results suggest that the interaction between climatic variability and historical acid deposition determines the regional pattern and long-term trends of BC concentrations across streams and rivers of Sweden. Understanding the strength of the interaction between climate features and historic deposition will greatly improve our ability to predict long-term trends of Ca2+, Mg2+, K+, and Na+ and their inter-annual dynamics in the future.

Trends in snowpack chemistry and comparison to National Atmospheric Deposition Program results for the Rocky Mountains, US, 1993–2004

Seasonal snowpack chemistry data from the Rocky Mountain region of the US was examined to identify long-term trends in concentration and chemical deposition in snow and in snow-water equivalent. For the period 1993–2004, comparisons of trends were made between 54 Rocky Mountain Snowpack sites and 16 National Atmospheric Deposition Program wetfall sites located nearby in the region. The region was divided into three subregions: Northern, Central, and Southern. A non-parametric correlation method known as the Regional Kendall Test was used. This technique collectively computed the slope, direction, and probability of trend for several sites at once in each of the Northern, Central, and Southern Rockies subregions. Seasonal Kendall tests were used to evaluate trends at individual sites. Significant trends occurred during the period in wetfall and snowpack concentrations and deposition, and in precipitation. For the comparison, trends in concentrations of ammonium, nitrate, and sulfate for the two networks were in fair agreement. In several cases, increases in ammonium and nitrate concentrations, and decreases in sulfate concentrations for both wetfall and snowpack were consistent in the three subregions. However, deposition patterns between wetfall and snowpack more often were opposite, particularly for ammonium and nitrate. Decreases in ammonium and nitrate deposition in wetfall in the central and southern rockies subregions mostly were moderately significant ( p<0.11) in constrast to highly significant increases in snowpack ( p<0.02). These opposite trends likely are explained by different rates of declining precipitation during the recent drought (1999–2004) and increasing concentration. Furthermore, dry deposition was an important factor in total deposition of nitrogen in the region. Sulfate deposition decreased with moderate to high significance in all three subregions in both wetfall and snowpack. Precipitation trends consistently were downward and significant for wetfall, snowpack, and snow-telemetry data for the central and southern rockies subregions ( p<0.03), while no trends were noted for the Northern Rockies subregion.

Long Term Trends in Concentration of Major Pollutants (SO2, CO, NO, NO2, O3 and PM10) in Prague – Czech Republic (Analysis of Data Between 1992 and 2005)

To assess the effect of changes in traffic density and fuels used for heating at the beginning of the 1990s, 1992–2005 monthly averages of PM10, SO2, NO2, NO, CO and O3 from Prague, the Czech capital, were analyzed together with long term trends in emissions of major pollutants, fuel consumption and number of vehicles registered in Prague. The data from all monitoring stations were retrieved from the database of the state automated monitoring system. Correlation coefficients between ambient monthly averaged temperature and all pollutants of concern showed distinct seasonal trends. The results showed that while SO2 and to some extent also CO concentrations dropped namely in the first half of the analyzed period (1992–1997) as a result decreased fossil fuel consumption for local heating, the behaviour of other pollutant concentrations followed a different pattern. PM10 concentrations decreased during the beginning of the 1990s but showed a sign of increase after 2000. Concentrations of ozone and NO2 did not reveal any significant change throughout the whole studied period. It can be concluded that during the studied period traditional urban sources of pollution, such as coal and oil combustion, lost their importance but were simultaneously substituted by pollutants from automotive transport (namely PM and NO2) making the problem of air quality even worse.

Temporal Variation in Daily Concentrations of Ozone and Acid-Related Substances at Saturna Island, British Columbia

A multiple linear regression model was used to investigate seasonal and long-term trends in concentrations of ozone (O3) and acid-related substances at the Saturna Island monitoring station in southwestern British Columbia from 1991 to 2000. Statistically significant primary (dominant) cycles with a period of 1 yr were found for O3, sulfur dioxide (SO2), nitric acid (HNO3), and aerosol concentrations of sulfate (SO4 2−), calcium (Ca2+) and chloride (Cl−). Of these, peak median concentrations occurred during the spring for O3 and Ca2+, during the warmer, drier months (April-September) for SO4 2− and HNO3, and during the cooler, wetter months (October-March) for SO2 and Cl−. Statistically significant secondary cycles of 6 months duration were seen for concentrations of O3, SO4 2−, HNO3, Ca2+, and Cl−. Daily maximum O3 concentrations exhibited a statistically significant increase over the period of record of 0.33 ± 0.26 ppb/yr. Statistically significant declines were found for concentrations of SO2, SO4 2−, HNO3, Ca2+, and potassium, ranging from 20 to 36% from levels at the start of the sampling period. Declines in ambient concentrations of SO2, SO4 2−, and HNO3 reflect local declines in anthropogenic emissions of the primary precursors SO2 and NOx over the past decade. Trends in Ca2+ and potassium ion concentrations are in line with a broader North American declining trend in acid-neutralizing cations.

Perfluorooctane sulfonate in oysters, Crassostrea virginica, from the Gulf of Mexico and the Chesapeake Bay, USA.

Concentrations of perfluorooctane sulfonate (PFOS), a metabolite of several sulfonated perfluoroorganic compounds, were measured in oysters collected from 77 locations in the Gulf of Mexico and Chesapeake Bay of the United States. PFOS was detected in oysters collected from 51 of the 77 locations at concentrations ranging from < 42 to 1,225 ng/g on a dry weight basis. This study provides baseline data for future monitoring programs to examine long-term trends in concentrations of PFOS.

Long-term trends in concentrations of halocarbons and radiatively active trace gases in atlantic and european air masses monitored at mace head, Ireland from 1987–1994

Long-term trends in trace gas concentrations over the period 1987–1994 are reported here for air masses advected to the Mace Head monitoring station on the remote west coast of Ireland. The trace gases covered include the principal halocarbons: CFC-11, CFC-12, CFC-113, CCl 4 and methyl chloroform; the radiatively active trace gases: carbon dioxide, methane, nitrous oxide and ozone; together with carbon monoxide, the major photochemical ozone precursor. By careful sorting using two independent techniques, it is possible to distinguish air masses that arrive at Mace Head from over the North Atlantic Ocean and those that have recently travelled over polluted European continental land areas. Concentration trends have been derived for each trace gas in polluted European continental and baseline North Atlantic maritime air and they appear to be distinctly different. Using a simple long-range transport model, estimates have been made of the European source strengths required to sustain the observed concentrations of each trace gas and their recent trends. These are compared with published emission inventories where they are available. The European continent appears to be a significant source of methane, carbon dioxide and nitrous oxide, a net sink for ozone and a declining source of the principal halocarbons and carbon monoxide.

Model of the Long-Term Exchange of PCBs between Soil and the Atmosphere in the Southern U.K.

Measurements of concentrations of four PCB congeners (PCB-28, -52, -138, and -153) in air, herbage, and soil of the southern U.K. over the period 1942-1992 are compiled to demonstrate that emissions of PCB to the atmosphere increased from low levels in 1940 to a peak in the late 1960s and then fell to low current levels. This emission pulse resulted in a corresponding peak in air concentrations. Soil concentrations have responded to this pulse but with some delay, reaching a maximum in the early 1970s. At that time PCB fugacities in soil apparently exceeded atmospheric fugacities by about a factor of 10 because of nondiffusive deposition processes. Since then, fugacities in air and soil have fallen with a half-life of 10-20 yr probably as a result of volatilization or «degassing» of PCB, although it is possible (but not proved) that the degradation processes are also occurring with a half-life of approximately 15 yr. A simple two-compartment (air-soil) mass balance model is developed and applied to these data which gives a reasonable simulation of the long-term trends in concentration. It is concluded that there is a need to develop and improve models of this type to explain and predict the extent to which soils and vegetation act as a buffer of atmospheric concentrations of persistent chemicals