Regional trends in wet deposition of sulfate in the United States and SO2 emissions from 1980 through 1995

Regional trends of seasonal and annual wet deposition and precipitation-weighted concentrations (PWCs) of sulfate in the United States over the period 1980–1995 were developed from monitoring data and scaled to a mean of unity. To reduce some effects of year to year climatological variability, the unitless regional deposition and PWC trends were averaged (hereafter termed CONCDEP). The SO₂ emissions data over the same period from the United States, Canada, and northern Mexico, aggregated by state and province, were weighted appropriately for each deposition region in turn to produce scaled trends of the emissions affecting each region. The emission-weighting factors, which were held constant year to year, were estimated by exercise of a regional transport model. The sulfate CONCDEP regional trends are generally similar to those of regionally weighted SO₂ emissions, although the latter trends are less steep and the former trends have more year to year variability. In eastern regions, sulfate CONCDEPs and SO₂ emissions patterns both generally show an initial decrease, an essentially trendless middle period, and a final decrease as reductions mandated by the Acid Rain Provisions of the 1990 Clean Air Act Amendments began. Linear regressions of regional sulfate CONCDEPs on corresponding regionally weighted SO₂ emissions produced statistically significant relationships in all regions. The analysis indicated that although regional sulfate CONCDEPs decreased relatively faster than did SO₂ emissions during the period in all regions except the Great Plains, in general the slopes were not significantly different from unity.     

Temporal variability of selected air toxics in the United States

Ambient measurements of hazardous air pollutants (HAPs, air toxics) collected in the United States from 1990 to 2005 were analyzed for diurnal, seasonal, and/or annual variability and trends. Visual and statistical analyses were used to identify and quantify temporal variations in air toxics at national and regional levels. Sufficient data were available to analyze diurnal variability for 14 air toxics, seasonal variability for 24 air toxics, and annual trends for 26 air toxics. Four diurnal variation patterns were identified and labeled invariant, nighttime peak, morning peak, and daytime peak. Three distinct seasonal patterns were identified and labeled invariant, cool, and warm. Multiple air toxics showed consistent decreasing trends over three trend periods, 1990–2005, 1995–2005, and 2000–2005. Trends appeared to be relatively consistent within chemically similar pollutant groups. Hydrocarbons such as benzene, 1,3-butadiene, styrene, xylene, and toluene decreased by approximately 5% or more per year at more than half of all monitoring sites. Concentrations of carbonyl compounds such as formaldehyde, acetaldehyde, and propionaldehyde were equally likely to have increased or decreased at monitoring sites. Chlorinated volatile organic compounds (VOCs) such as tetrachloroethylene, dichloromethane, and methyl chloroform decreased at more than half of all monitoring sites, but decreases among these species were much more variable than among the hydrocarbons. Lead particles decreased in concentration at most monitoring sites, but trends in other metals were not consistent over time.     

Reproducibility of regional apportionments of pollution aerosol in the Northeastern United States

Regional apportionments of sulfate and Se in pollution aerosol have been reproduced to within 10% for winter and summer at Narragansett, RI, and for summer at Underhill, VT, over a 3-yr period. Agreement of observed variability of apportionments with earlier estimates of their uncertainties served to empirically confirm both error-propagation methods and original uncertainties assigned to regional signatures. Winter apportionments at Underhill were more variable because of systematic differences in meteorology.     

Spatial distribution of source locations for particulate nitrate and sulfate in the upper-midwestern United States

Two back-trajectory analysis methods designed to be used with multiple site data, simplified quantitative transport bias analysis (SQTBA) and residence time weighted concentration (RTWC), were applied to nitrate and sulfate concentration data from two rural sites (the Mammoth Cave National Park and the Great Smoky Mountain National Park) and five urban sites (Chicago, Cleveland, Detroit, Indianapolis, and St. Louis) for an intensive investigation on the spatial patterns of origins for these two species in the upper-midwestern area. The study was made by dividing the data into five categories: all sites and all seasons, rural sites in summer, rural sites in winter, urban sites in summer, and urban sites in winter. A general conclusion was that the origins of the nitrate in these seven sites were mainly in the upper-midwestern areas, while the sulfate in these seven sites were mainly from the Ohio and Tennessee River Valley areas. The upper-midwestern areas are regions of high ammonia emissions rather than high NO_x emissions. In the winter, metropolitan areas showed the highest nitrate emission potential suggesting the importance of local NO_x emissions. In the summer, ammonia emissions from fertilizer application in the lower midwestern area made a significant contribution to nitrate in the rural sites of this study. The impact of the wind direction prevalence on the source spatial patterns was observed by comparing the urban and rural patterns of the summer. The differences between the results of two methods are discussed and suggestions for applying these methods are also provided.     

Spatial and temporal distributions of airborne sulfate in parts of the United States

The current knowledge of the spatial and temporal distribution of airborne, water soluble sulfate is surveyed for two “scales” of atmospheric activity. The “urban” scale with episodes extending over a day or two over distances of about 100 km is illustrated for two comparable American cities, Los Angeles and New York. The regional scale with episodes extending up to several days over distances of 1000 km is exemplified by case studies in the greater eastern United States. Examination of available data reveals several features of the spatial and temporal variation in sulfate occurrence, including seasonal changes, and correlations with aerometric parameters. The importance of water vapor and air mass character on sulfate concentrations is assessed in both the urban and regional conditions. The results of initial attempts to simulate the impact of sulfur oxide emissions on ambient sulfate distributions are compared with an episode case extending over several days in July 1974.     

Detection and quantification of 2-methyltetrols in ambient aerosol in the southeastern United States

Filters collected from the Southeastern Aerosol Research and Characterization (SEARCH) air monitoring network were analyzed for the presence of 2-methyltetrols, namely 2-methylthreitol and 2-methylerythritol, two compounds that are products of the photooxidation of isoprene and have been detected in aerosol at a variety of sites around the globe. The 2-methytetrols were detected in ambient filter samples collected at the four SEARCH sites, Birmingham, AL, Centreville, AL, Pensacola, FL, and at Jefferson Street in Atlanta, GA, in late June 2004. Average atmospheric concentrations of 11.9 and 4.8 ng m⁻³ were measured for 2-methylerythritol and 2-methylthreitol, respectively, at the inland sampling sites, whereas average concentrations of 4.9 and 1.6 ng m⁻³ were measured at the coastal sampling location (Pensacola). On average, the aerosol loading from these two compounds accounts for approximately 0.42% and 0.21% of the organic mass collected on a given sampling day at the inland and coastal sites, respectively. The present data on these compounds, which are particulate-phase fingerprints of isoprene photooxidation, add to the growing body of ambient data on secondary organic aerosol from isoprene.     

A survey of polycyclic musks in selected household commodities from the United States

Occurrence of the polycyclic musks, 1,3,4,6,7,8-hexahydro-4,6,6,7,8,8-hexamethylcyclopenta[g]-2-benzopyran (HHCB) and 7-acetyl-1,1,3,4,4,6-hexamethyl-1,2,3,4-tetrahydronapthalene (AHTN), in wastewater influent and effluent, as well as in surface waters, has been reported. HHCB and AHTN were also reported to occur in human and wildlife tissues. The major sources for HHCB and AHTN to wastewater are thought to be consumer products such as shampoos, deodorants, laundry detergents, and household surface cleaners. However, the levels of HHCB and AHTN in consumer products are not known. For evaluation of the sources of human and environmental exposures, characterization of levels of HHCB and AHTN in consumer products is needed. In this study, we measured concentrations of HHCB (Galaxolide), AHTN (Tonalide), and HHCB-lactone (Galaxolidone) from a variety of consumer products, including perfumes, body lotions, and deodorants. Concentrations of HHCB, AHTN, and HHCB-lactone in consumer products ranged from <5 ng/g to over 4000 microg/g, <5 ng/g to 451 microg/g, and <5 ng/g to 217 microg/g, respectively. The highest concentrations were found in perfumes, body creams and lotions, and deodorants. The results suggest that a wide variety of source materials exist for HHCB and AHTN, and that these materials are used on a daily basis.     

Fire and biofuel contributions to annual mean aerosol mass concentrations in the United States

We estimate the contributions from biomass burning (summer wildfires, other fires, residential biofuel, and industrial biofuel) to seasonal and annual aerosol concentrations in the United States. Our approach is to use total carbonaceous (TC) and non-soil potassium (ns-K) aerosol mass concentrations for 2001–2004 from the nationwide IMPROVE network of surface sites, together with satellite fire data. We find that summer wildfires largely drive the observed interannual variability of TC aerosol concentrations in the United States. TC/ns-K mass enhancement ratios from fires range from 10 for grassland and shrub fires in the south to 130 for forest fires in the north. The resulting summer wildfire contributions to annual TC aerosol concentrations for 2001–2004 are 0.26 μg C m⁻³ in the west and 0.14 μg C m⁻³ in the east; Canadian fires are a major contributor in the east. Non-summer wildfires and prescribed burns contribute on an annual mean basis 0.27 and 0.31 μg C m⁻³ in the west and the east, highest in the southeast because of prescribed burning. Residential biofuel is a large contributor in the northeast with annual mean concentration of up to 2.2 μg C m⁻³ in Maine. Industrial biofuel (mainly paper and pulp mills) contributes up to 0.3 μg C m⁻³ in the southeast. Total annual mean fine aerosol concentrations from biomass burning average 1.2 and 1.6 μg m⁻³ in the west and east, respectively, contributing about 50% of observed annual mean TC concentrations in both regions and accounting for 30% (west) and 20% (east) of total observed fine aerosol concentrations. Our analysis supports bottom-up source estimates for the contiguous United States of 0.7–0.9 Tg C yr⁻¹ from open fires (climatological) and 0.4 Tg C yr⁻¹ from biofuel use. Biomass burning is thus an important contributor to US air quality degradation, which is likely to grow in the future.     

Characterization of fine primary biogenic organic aerosol in an urban area in the northeastern United States

Scanning electron microscopy coupled to energy-dispersive x-ray spectroscopy (SEM/EDX) was used to quantify individual bioparticles in PM_2.5 samples collected during the Pittsburgh Air Quality Study. Microscopy-based estimates of primary biogenic organic aerosol (PBOA) mass were compared to carbohydrate mass associated with PM_2.5. Carbohydrates show substantial seasonal variations, with higher concentrations in the spring and the fall. During the summer, carbohydrates were about 30% of the estimated PBOA concentrations, but in the winter carbohydrate concentrations often greatly exceeded the PBOA mass estimate. Spores and insect detritus were the most abundant PBOA types in the summer samples, while winter samples were comprised predominantly of a mixture of microorganisms, insect and vegetative detritus. During the summer PBOA contributed on average 6.9 ± 5.4% by mass of the PM_2.5 versus 3.3 ± 1.4% of the PM_2.5 mass during the winter.     

Nitrogen use in the United States from 1961-2000 and potential future trends

Nitrogen inputs to the US from human activity doubled between 1961 and 1997, with most of the increase in the 1960s and 1970s. The largest increase was in use of inorganic N fertilizer, but emissions of NOx from fossil-fuel combustion also increased substantially. In 1961, N fixation in agricultural systems was the largest single source of reactive N in the US. By 1997, even though N fixation had increased, fertilizer use and NOx emissions had increased more rapidly and were both larger inputs. In both 1961 and 1997, two thirds of reactive N inputs were denitrified or stored in soils and biota, while one third was exported. The largest export was in riverine flux to coastal oceans, followed by export in food and feeds, and atmospheric advection to the oceans. The consumption of meat protein is a major driver behind N use in agriculture in the US Without change in diet or agricultural practices, fertilizer use will increase over next 30 years, and fluxes to coastal oceans may increase by another 30%. However, substantial reductions are possible.     

Temporal trends of black carbon concentrations and regional climate forcing in the southeastern United States

The effect of black carbon (BC) on climate forcing is potentially important, but its estimates have large uncertainties due to a lack of sufficient observational data. The BC mass concentration in the southeastern US was measured at a regionally representative site, Mount Gibbes (35.78°N, 82.29°W, 2006 m MSL). The air mass origin was determined using 48-h back trajectories obtained from the hybrid single-particle Lagrangian integrated trajectory model. The highest average concentration is seen in polluted continental air masses and the lowest in marine air masses. During the winter, the overall average BC value was 74.1 ng m⁻³, whereas the overall summer mean BC value is higher by a factor of 3. The main reason for the seasonal difference may be enhanced thermal convection during summer, which increases transport of air pollutants from the planetary boundary layer of the surrounding urban area to this rural site. In the spring of 1998, abnormally high BC concentrations from the continental sector were measured. These concentrations were originating from a biomass burning plume in Mexico. This was confirmed by the observations of the Earth probe total ozone mapping spectrometer. The BC average concentrations of air masses transported from the polluted continental sector during summer are low on Sunday to Tuesday with a minimum value of 256 ng m⁻³ occurring on Monday, and high on Wednesday to Friday with a maximum value of 379 ng m⁻³ occurring on Friday. The net aerosol radiative forcing (scattering effects plus absorption effects) per unit vertical depth at 2006 m MSL is calculated to be −1.38×10⁻³ W m⁻³ for the southeastern US. The magnitude of direct radiative forcing by aerosol scattering is reduced by 15±7% due to the BC absorption.     

Spatial and temporal trends of precipitation chemistry in the United States, 1985-2002

A Seasonal Kendall Trend (SKT) test was applied to precipitation-weighted concentration data from 164 National Atmospheric Deposition Program National Trends Network (NADP/NTN) sites operational from 1985 to 2002. Analyses were performed on concentrations of ammonium, sulfate, nitrate, dissolved inorganic nitrogen (DIN, sum of nitrogen from nitrate and ammonium), and earth crustal cations (ECC, sum of calcium, magnesium, and potassium). Over the 18-year period, statistically significant (p< or =0.10) increases in ammonium concentrations occurred at 93 sites (58%), while just three sites had statistically significant ammonium decreases. Central and northern Midwestern states had the largest ammonium increases. The generally higher ammonium concentrations were accompanied by significant sulfate decreases (139 sites, 85%), and only one significant increase which occurred in Texas. In the west central United States there were significant nitrate increases (45 sites, 27%), while in the northeastern United States there were significant decreases (25 sites, 15%). Significant DIN decreases were observed in the northeastern United States (11 sites, 7%); elsewhere there were significant increases at 75 sites (46%). ECC decreased significantly at 65 sites (40%), predominantly in the central and southern United States, and increased at 11 sites (7%). The concentrations of sulfate, nitrate, and ammonium in precipitation have changed markedly over the time period studied. Such trends indicate changes in the mix of gases and particles scavenged by precipitation, possibly reflecting changes in emissions, atmospheric chemical transformations, and weather patterns.     

Regional visibility modeling for the Eastern United States

Visibility modeling over long transport distances is complicated not only by the chemical and removal processes, but also by the multiplicity of sources from different areas that contribute to the particle and gaseous concentrations—and visibility impairment—at specific locations. To study interregional pollutant exchanges and their effect on visibility, a regional model has been developed and applied to the eastern United States and the visibility reduction budgeted according to area of origin. The new model, called VISMAP-1, produces short-term (three-hour) and long-term (monthly) sulfate concentrations; visibility calculations are made by applying a mass-to-light-scattering function to the aerosol concentrations. This function is weighted according to relative humidity to account for hygroscopic particle growth. One of the most useful features of this model is its capability to budget fine-particulate and gaseous concentrations in various receptor regions according to the contribution of source regions.In this analysis for visibility effects, three-hour SURE (Sulfate Regional Experiment) sulfate measurements for July 1978 are compared to fine-particle calculations to evaluate the model's ability to predict the transport of aerosol sulfur for the shorter averaging period. Visibility is modeled from the sulfate calculations using an empirical mass-to-light-scattering function. This technique is commonly used to determine aerosol light-scattering properties at given relative humidities. National Weather Service visual range observations have been compared with the model's visibility calculations; both regional patterns of visibility degradation and the absolute magnitude of the reduction in visual range are evaluated. Preliminary results are encouraging and the VISMAP modeling approach appears to be a useful step toward identifying long-range source/receptor relationships that affect visibility.     

Seasonal variability of aerosol optical properties over Beijing

The knowledge of aerosol properties at local and regional scale is important in understanding of the global climate change. In this study, the aerosol optical properties over Beijing have been presented from the Aerosol Robotic Network (AERONET) measurements during 2002–2007. The aerosol optical depth (AOD) showed a distinct seasonal variation with high values in spring (March–May) and summer (June–August). The magnitude of Ångström exponent (α) was found to be relatively high throughout the year and the highest values (1.27) occurred in summer and the lowest (1.0) in spring. The water vapor retrieved from AERONET was found to be highest (2.60 cm) in summer. The fine modes of aerosol volume size distributions showed the highest peak around radius 0.15 μm in spring, autumn (September–November) and winter (December–February), and radius 0.19 μm in summer. The coarse modes showed the maxima peak at radius 3.0 μm in all seasons. The asymmetry factor (g) has considered as 0.65 at 440, 675, 870 and 1020 nm over Beijing in climate and radiation models. The average values of the single scattering albedo (SSA) at the four wavelengths were taken as 0.89, 0.91, 0.87 and 0.86 in spring, summer, autumn and winter, respectively. Both real and imaginary parts of the refractive index showed low wavelength dependence. The highest averages of real (1.52) and imaginary parts (0.0165) were found in spring and winter respectively in the wavelength range of 440–1020 nm. The aerosol properties over Beijing were found to highly dependent on season, and changes in aerosol properties were mainly attributed to the presence of dust as the main component during the spring season and the dominance of anthropogenic pollutants during the winter season.     

The effect of CO on sulfate aerosol formation

A series of experiments were performed in an outdoor smog chamber to determine the effect of CO on the rate of SO₂ oxidation in a rural air photochemical system. The presence of added CO, at concentrations ranging from 15 to 30 ppm, decreased the aerosol formation rate as measured by a condensation nuclei counter, an electrical aerosol analyzer, and X-ray fluorescence (XRF) analysis of collected aerosol. Enhanced O₃ production due to the action of CO was also detected in one of the experiments. Comparison of the XRF data with results of a photochemical model shows good agreement concerning the effect of CO on the SO₂ oxidation rate. This agreement supports the hypothesis that SO₂ is oxidized principally by the hydroxyl and hydroperoxy radicals in the system studied.     

Geographic distribution of selected elements in the livers of polar bears from Greenland, Canada and the United States

To assess geographic distributions of elements in the Arctic we compared essential and non-essential elements in the livers of polar bears (Ursus maritimus) collected from five regions within Canada in 2002, in Alaska between 1994 and 1999 and from the northwest and east coasts of Greenland between 1988 and 2000. As, Hg, Pb and Se varied with age, and Co and Zn with gender, which limited spatial comparisons across all populations to Cd, which was highest in Greenland bears. Collectively, geographic relationships appeared similar to past studies with little change in concentration over time in Canada and Greenland for most elements; Hg and Se were higher in some Canadian populations in 2002 as compared to 1982 and 1984. Concentrations of most elements in the polar bears did not exceed toxicity thresholds, although Cd and Hg exceeded levels correlated with the formation of hepatic lesions in laboratory animals.     

Characterization of spatially homogeneous regions based on temporal patterns of fine particulate matter in the continental United States

Statistical analyses of time-series or spatial data have been widely used to investigate the behavior of ambient air pollutants. Because air pollution data are generally collected in a wide area of interest over a relatively long period, such analyses should take into account both spatial and temporal characteristics. The objective of this study is 2-fold: (1) to identify an efficient way to characterize the spatial variations of fine particulate matter (PM2.5) concentrations based solely upon their temporal patterns, and (2) to analyze the temporal and seasonal patterns of PM2.5 concentrations in spatially homogenous regions. This study used 24-hr average PM2.5 concentrations measured every third day during a period between 2001 and 2005 at 522 monitoring sites in the continental United States. A k-means clustering algorithm using the correlation distance was used to investigate the similarity in patterns between temporal profiles observed at the monitoring sites. A k-means clustering analysis produced six clusters of sites with distinct temporal patterns that were able to identify and characterize spatially homogeneous regions of the United States. The study also presents a rotated principal component analysis (RPCA) that has been used for characterizing spatial patterns of air pollution and discusses the difference between the clustering algorithm and RPCA.