Ambient Sulfate Concentrations Research Articles

A Mechanistic Model of Annual Sulfate Concentrations in the United States

Understanding how individual pollution sources contribute to ambient sulfate pollution is critical for assessing past and future air quality regulations. Since attribution to specific sources is typically not encoded in spatial air pollution data, we develop a mechanistic model which we use to estimate, with uncertainty, the contribution of ambient sulfate concentrations attributable specifically to sulfur dioxide (SO2) emissions from individual coal-fired power plants in the central United States. We propose a multivariate Ornstein–Uhlenbeck (OU) process approximation to the dynamics of the underlying space-time chemical transport process, and its distributional properties are leveraged to specify novel probability models for spatial data that are viewed as either a snapshot or time-averaged observation of the OU process. Using US EPA SO2 emissions data from 193 power plants and state-of-the-art estimates of ground-level annual mean sulfate concentrations, we estimate that in 2011—a time of active power plant regulatory action—existing flue-gas desulfurization (FGD) technologies at 66 power plants reduced population-weighted exposure to ambient sulfate by 1.97 μg/m3 (95% CI: 1.80–2.15). Furthermore, we anticipate future regulatory benefits by estimating that installing FGD technologies at the five largest SO2-emitting facilities would reduce human exposure to ambient sulfate by an additional 0.45 μg/m3 (95% CI: 0.33–0.54). Supplementary materials for this article are available online.

High-resolution estimation of ambient sulfate concentration over Taiwan Island using a novel ensemble machine-learning model.

Heavy loadings of sulfate aerosol trigger haze formation and pose great damage to human health in Taiwan Island. Nevertheless, high-resolution spatiotemporal variation of ambient sulfate across Taiwan Island still remained unknown because of the scarce monitoring sites. Thus, we developed a novel ensemble model named extreme gradient boosting coupled with geographically and temporally weighted regression (XGBoost-GTWR) to predict the high-resolution sulfate concentration (0.05°) based on satellite data, assimilated meteorology, and the output of chemical transport models (CTMs). The result suggested that XGBoost-GTWR model outperformed other five models in predicting the sulfate concentration with the highest R2 value (R2 = 0.58) and the lowest relative mean square error (RMSE = 1.96 μg/m3). Besides, the transferability of the XGBoost-GTWR model was also validated based on the ground-level sulfate data in 2019. The result suggested that the R2 value of the extrapolation equation (0.53) did not show notable decrease compared with the 10-fold cross-validation result (0.58), indicating that the model was robust to predict the sulfate concentration. The ambient sulfate concentration in Taiwan Island displayed featured spatial variation with the highest one in Southwest Taiwan and the lowest one in Northeast Taiwan, respectively. It was assumed that the higher anthropogenic emission combined with the adverse meteorological condition led to the higher sulfate level in the southwestern coastal region. The ambient sulfate concentration exhibited significantly seasonal variation with the highest value in spring (5.65 ± 0.84 μg/m3), followed by those in winter (5.45 ± 1.25 μg/m3) and autumn (4.60 ± 0.80 μg/m3), and the lowest one in summer (3.80 ± 0.65 μg/m3). The higher sulfate concentration in spring was mainly contributed by the dense biomass burning and scarce rainfall amount. The present study develops a novel model to capture the high-resolution sulfate map and provides basic data for effective regulations of air pollution and epidemiological studies.

Oxygen isotope effects during microbial sulfate reduction: applications to sediment cell abundances

The majority of anaerobic biogeochemical cycling occurs within marine sediments. To understand these processes, quantifying the distribution of active cells and gross metabolic activity is essential. We present an isotope model rooted in thermodynamics to draw quantitative links between cell-specific sulfate reduction rates and active sedimentary cell abundances. This model is calibrated using data from a series of continuous culture experiments with two strains of sulfate reducing bacteria (freshwater bacterium Desulfovibrio vulgaris strain Hildenborough, and marine bacterium Desulfovibrio alaskensis strain G-20) grown on lactate across a range of metabolic rates and ambient sulfate concentrations. We use a combination of experimental sulfate oxygen isotope data and nonlinear regression fitting tools to solve for unknown kinetic, step-specific oxygen isotope effects. This approach enables identification of key isotopic reactions within the metabolic pathway, and defines a new, calibrated framework for understanding oxygen isotope variability in sulfate. This approach is then combined with porewater sulfate/sulfide concentration data and diagenetic modeling to reproduce measured 18O/16O in porewater sulfate. From here, we infer cell-specific sulfate reduction rates and predict abundance of active cells of sulfate reducing bacteria, the result of which is consistent with direct biological measurements.

Trend in Fine Sulfate Concentrations and the Associated Secondary Formation Processes at an Urban Site in North China

There has been a significant decline in SO2, the main precursor of sulfate, in North China over the past decade due to strict sulfur removal measures, whereas the amount of photochemical oxidants such as ozone has continued to increase in this region. In this study, we examined temporal variation in the concentrations of fine sulfate in urban Ji’nan in North China from 2008 till 2015. Over this period, the sulfate concentration decreased by –3.86 ± 2.50 µg m–3 yr–1 (–10.0% yr–1), which is slower that the rate of decrease for SO2 during the same period (–11.6% yr–1). Nevertheless, the sulfur oxidation ratio and the concentrations of ozone and calcium (an indicator of dust particles) increased over this period. An analysis of the seasonal and diurnal variations in sulfate and the related parameters in 2015 indicated that the ambient sulfate concentration was largely influenced by the amount of SO2, atmospheric oxidants, aerosol loading, and meteorological conditions. A detailed investigation of the production of sulfate in eight case studies found that the observed sulfate production rate was in the range of 1.1–10.8 µg m–3 h–1. Numerical calculations revealed that SO2 oxidation by OH and H2O2 was a major contributor to sulfate production during the daytime in warm seasons. At all times of the day in cold seasons and at nighttime in warm seasons, the heterogeneous SO2 reaction on aerosol surfaces contributed 30.1%–65.7% of the sulfate production. The increasing amount of ozone and dust particles in this region, which are associated with photochemical pollution and urban dust emissions, are responsible for the slower decrease in sulfate concentration. Therefore, photochemical smog and urban dust should receive adequate attention in order to mitigate the sulfate pollution.

Microscale reservoir effects on microbial sulfur isotope fractionation

Microbial sulfate reduction can impart strong sulfur isotope fractionation by preferentially using the lighter 32SO42- over the heavier 34SO42-. The magnitude of fractionation depends on a number of factors, including ambient concentrations of sulfate and electron donors. Sulfur isotope compositions in sedimentary rocks thus facilitate reconstruction of past environmental conditions, such as seawater sulfate concentrations, primary productivity, organic carbon burial, and sulfur fluxes into or out of the ocean. Knowing the processes that regulate the magnitude of sulfur isotope fractionation is necessary for the correct interpretation of the geological record, but so far theoretical work has focused mostly on internal cellular processes. In sulfate-limited environments, like low sulfate lakes and the Archean ocean, microbial sulfate reduction can lead to sulfate depletion in the water column and an enrichment in isotopically heavy sulfate. This reservoir effect in turn mutes the fractionation expressed in the water column and ultimately preserved in sediments relative to the biologically induced fractionation. Here we use mathematical modeling to show that similar reservoir effects can also appear at the microscale in close proximity to sulfate-reducing cells. These microscale reservoir effects have the potential to modulate sulfur isotope fractionation to a considerable degree, especially at low (micromolar) sulfate concentrations. As a result, background sulfate concentrations, sulfate reduction rates, and extracellular ion diffusion rates can influence the fractionation expressed even if the physiologically induced fractionation is constant. This has implications for the interpretation of biogenic sulfur isotope fractionations expressed in the geological record, because the correct estimation of the environmental conditions that would promote these fractionations requires consideration of microscale reservoir effects. We discuss these implications, and demonstrate the integration of microscale reservoir effects into geobiological models for low sulfate marine water columns, as perceived for the Archean ocean.

Characterization of ambient-generated exposure to fine particles using sulfate as a tracer in the Chinese megacity of Guangzhou.

Total personal exposures can differ from the concentrations measured at stationary ambient monitoring sites. To provide further insight into factors affecting exposure to particles, chemical tracers were used to separate total personal exposure into its ambient and non-ambient components. Simultaneous measurements of ambient and personal exposure to fine particles (PM2.5) were conducted in eight districts of Guangzhou, a megacity in South China, during the winter of 2011. Considerable significant correlations (Spearman's Rho, rs) between personal exposures and ambient concentrations of sulfate (SO42-; rs>0.68) were found in contrast to elemental carbon (EC; rs>0.37). The average fraction of personal SO42- to ambient SO42- resulting in an adjusted ambient exposure factor of α=0.72 and a slope of 0.73 was determined from linear regression analysis when there were minimal indoor sources of SO42-. From all data pooled across the districts, the estimated average ambient-generated and non-ambient-generated exposure to PM2.5 were 55.3μg/m3 (SD=23.4μg/m3) and 18.1μg/m3 (SD=29.1μg/m3), respectively. A significant association was found between ambient-generated exposure and ambient PM2.5 concentrations (Pearson's r=0.51, p<0.001). As expected, the non-ambient generated exposure was not related to the ambient concentrations. This study highlights the importance of both ambient and non-ambient components of total personal exposure in the megacity of Guangzhou. Our results support the use of SO42- as a tracer of personal exposure to PM2.5 of ambient origin in environmental and epidemiological studies.

Patterns of sulfur isotope fractionation during microbial sulfate reduction.

Studies of microbial sulfate reduction have suggested that the magnitude of sulfur isotope fractionation varies with sulfate concentration. Small apparent sulfur isotope fractionations preserved in Archean rocks have been interpreted as suggesting Archean sulfate concentrations of <200 μm, while larger fractionations thereafter have been interpreted to require higher concentrations. In this work, we demonstrate that fractionation imposed by sulfate reduction can be a function of concentration over a millimolar range, but that nature of this relationship depends on the organism studied. Two sulfate-reducing bacteria grown in continuous culture with sulfate concentrations ranging from 0.1 to 6 mm showed markedly different relationships between sulfate concentration and isotope fractionation. Desulfovibrio vulgaris str. Hildenborough showed a large and relatively constant isotope fractionation ((34) εSO 4-H2S ≅ 25‰), while fractionation by Desulfovibrio alaskensis G20 strongly correlated with sulfate concentration over the same range. Both data sets can be modeled as Michaelis-Menten (MM)-type relationships but with very different MM constants, suggesting that the fractionations imposed by these organisms are highly dependent on strain-specific factors. These data reveal complexity in the sulfate concentration-fractionation relationship. Fractionation during MSR relates to sulfate concentration but also to strain-specific physiological parameters such as the affinity for sulfate and electron donors. Previous studies have suggested that the sulfate concentration-fractionation relationship is best described with a MM fit. We present a simple model in which the MM fit with sulfate concentration and hyperbolic fit with growth rate emerge from simple physiological assumptions. As both environmental and biological factors influence the fractionation recorded in geological samples, understanding their relationship is critical to interpreting the sulfur isotope record. As the uptake machinery for both sulfate and electrons has been subject to selective pressure over Earth history, its evolution may complicate efforts to uniquely reconstruct ambient sulfate concentrations from a single sulfur isotopic composition.

Brachial Artery Responses to Ambient Pollution, Temperature, and Humidity in People with Type 2 Diabetes: A Repeated-Measures Study

Background: Extreme weather and air pollution are associated with increased cardiovascular risk in people with diabetes.Objectives: In a population with diabetes, we conducted a novel assessment of vascular brachial artery responses both to ambient pollution and to weather (temperature and water vapor pressure, a measure of humidity).Methods: Sixty-four 49- to 85-year-old Boston residents with type 2 diabetes completed up to five study visits (279 repeated measures). Brachial artery diameter (BAD) was measured by ultrasound before and after brachial artery occlusion [i.e., flow-mediated dilation (FMD)] and before and after nitroglycerin-mediated dilation (NMD). Ambient concentrations of fine particulate mass (PM2.5), black carbon (BC), organic carbon (OC), elemental carbon, particle number, and sulfate were measured at our monitoring site; ambient concentrations of carbon monoxide, nitrogen dioxide, and ozone were obtained from state monitors. Particle exposure in the home and during each trip to the clinic (home/trip exposure) was measured continuously and as a 5-day integrated sample. We used linear models with fixed effects for participants, adjusting for date, season, temperature, and water vapor pressure on the day of each visit, to estimate associations between our outcomes and interquartile range increases in exposure.Results: Baseline BAD was negatively associated with particle pollution, including home/trip–integrated BC (–0.02 mm; 95% CI: –0.04, –0.003, for a 0.28 μg/m3 increase in BC), OC (–0.08 mm; 95% CI: –0.14, –0.03, for a 1.61 μg/m3 increase) as well as PM2.5, 5-day average ambient PM2.5, and BC. BAD was positively associated with ambient temperature and water vapor pressure. However, exposures were not consistently associated with FMD or NMD.Conclusion: Brachial artery diameter, a predictor of cardiovascular risk, decreased in association with particle pollution and increased in association with ambient temperature in our study population of adults with type 2 diabetes.Citation: Zanobetti A, Luttmann-Gibson H, Horton ES, Cohen A, Coull BA, Hoffmann B, Schwartz JD, Mittleman MA, Li Y, Stone PH, de Souza C, Lamparello B, Koutrakis P, Gold DR. 2014. Brachial artery responses to ambient pollution, temperature, and humidity in people with type 2 diabetes: a repeated-measures study. Environ Health Perspect 122:242–248; http://dx.doi.org/10.1289/ehp.1206136

On the potential high acid deposition in northeastern China

AbstractThere is an acid deposition conundrum in China: contrary to conventional wisdom, extremely high ambient sulfate concentrations in northeastern China are not always accompanied by correspondingly high acidities. To investigate this discrepancy, data from two independent sets of in situ field measurements were analyzed along with Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) satellite observations and Model for Ozone and Related chemical Tracers (MOZART) chemical transport model calculations. The field measurements included soluble aerosol ion concentrations and pH and particulate data from 11 cities, as well as pH measurement data from 74 sites in China. This study explores the basis for and the impacts of the large discrepancy in northeastern China between the major acidity precursors (SO2 and NOx) and measured acidity levels as indicated by pH values. There are extremely high SO2 emissions and ambient concentrations in northeastern China, while the corresponding acidity is unusually low (high pH) in this region. This is inconsistent with the usual situation where high‐acidity precursor pollutants result in low pH (high acidity) values and acid rain conditions. In other regions, such as southern China and the United States, high SO2 concentrations are typically well correlated with high acidities. Using measured soluble particle measurements (including both positively and negatively charged ions), it is seen that there are high values of alkaline ions in northeastern China that play an important role in neutralizing acidity in this region. This result strongly suggests that the high alkaline concentrations, especially Ca2+, increase warm season pH values by about 0.5 in northern China, partially explaining the inconsistency between sulfate concentrations and acidity. This has a very important implication for acid rain mitigation—especially in northeastern China. However, there are additional issues pertaining to the precursor‐acidity relationship that need further investigation. Why is it that the reduction in acidity due to the alkaline ions is only significant in summer? During winter, the measured alkaline ions play a much smaller role in explaining the discrepancy. The measured alkaline ions in this study were mostly obtained from particles in the PM2.5 range. However, the size of calcium particles is typically much larger—extending well beyond 2.5 µm—and so a significant amount of calcium may be underestimated by PM2.5 measurements alone. The under‐sampling of calcium particles is further exacerbated in that the sampling protocol excluded particle (and soluble ion and pH) measurements during dust storms. This all leads to the need for an improved understanding of pollutant‐ion‐particulate interactions in China, and their role in explaining the counter‐intuitive conclusion that dust mitigation strategies in China could have the unintended consequence of exacerbating acid rain conditions.

Size distributions of aerosol sulfates and nitrates in Beijing during the 2008 Olympic Games: Impacts of pollution control measures and regional transport

For the 2008 Olympic Games, drastic control measures were implemented on industrial and urban emissions of sulfur dioxide (SO2), nitrogen oxides (NOx) and other pollutants to address the issues of poor air quality in Beijing. To investigate the effects of SO2 and NOx reductions on the particulate sulfate and nitrate concentrations as well as their size distributions, size-segregated aerosol samples were collected using micro-orifice uniform deposit impactors (MOUDIs) at urban and downwind rural sites in Beijing before and after full-scale controls. During the sampling period, the mass concentrations of fine particles (PM1.8) at the urban and rural sites were 94.0 and 85.9 μg m−3, respectively. More than 90% of the sulfates and ∼60% of nitrates formed as fine particles. Benefiting from the advantageous meteorological conditions and the source controls, sulfates were observed in rather low concentrations and primarily in condensation mode during the Olympics. The effects of the control measures were separately analyzed for the northerly and the southerly air-mass-dominated days to account for any bias. After the control measures were implemented, PM, sulfates, and nitrates were significantly reduced when the northerly air masses prevailed, with a higher percentage of reduction in larger particles. The droplet mode particles, which dominated the sulfates and nitrates before the controls were implemented, were remarkably reduced in mass concentration after the control measures were implemented. Nevertheless, when the polluted southerly air masses prevailed, the local source control measures in Beijing did not effectively reduce the ambient sulfate concentration due to the enormous regional contribution from the North China Plain.

Atmospheric concentrations of particulate sulfate and nitrate in Hong Kong during 1995–2008: Impact of local emission and super-regional transport

The release of large amounts of sulfur dioxide (SO2) and nitrogen oxides (NOx) from the burning of fossil fuel leads to regional air pollution phenomena such as haze and acidic deposition. Despite longstanding recognition of the severity of these problems and the numerous studies conducted in China, little is known of long-term trends in particulate sulfate and nitrate and their association with changes in precursor emissions. In this study, we analyze records covering a 14-year period (1995–2008) of PM10 composition in the subtropical city of Hong Kong, situated in the rapidly developing Pearl River Delta region of southern China. A linear regression method and a Regional Kendall test are employed for trend calculations. In contrast to the decreased levels of SO2 and NOx emissions in Hong Kong, there are increasing overall trends in ambient concentrations of PM10 sulfate and PM10 nitrate, with the most obvious rise seen during 2001–2005. The percentages of sulfate and nitrate in the PM10 mass and rainwater acidity also increased. Backward trajectories are computed to help identify the origin of large-scale air masses arriving in Hong Kong. In air masses dominated by Hong Kong urban sources and ship emissions, there was no statistically significant trend for PM10 sulfate and a small increase for PM10 nitrate; however, the evident increases in PM10 sulfate and PM10 nitrate concentrations were observed in air masses originating from eastern China and are generally consistent with changes in emissions of their precursors in eastern China. Examination of PM10 mass data recorded at a pair of upwind-urban sites also indicates that long-range transport makes a large contribution (>80%) to PM10 loadings in Hong Kong. Together with our previous study on the ozone trend, these results demonstrate the important impact exerted by long-distance sources and suggest a need to consider the impact of super-regional transport when formulating air-quality management strategy in Hong Kong in future.

Characterization of the Eyjafjallajökull volcanic plume over the Iberian Peninsula by lidar remote sensing and ground-level data collection

Abstract In April and May 2010 the eruption of the Eyjafjallajokull volcano disrupted air traffic across Europe. The vast economic impact of this event has stirred interest on accurate plume dispersion estimation and detailed ash characterization, in order to establish a more precise threshold for safe aircraft operation. In this work we study the physical and chemical properties of volcanogenic aerosol detected at ground level at several locations over the Iberian Peninsula, nearly 3000 km away from the Icelandic volcano. Between 4 and 14 May, the volcanogenic plume was detected at ground level, identified by an increase in sulfur dioxide, particle mass concentrations, and particulate sulfate concentration, at most EMEP stations as well as at the CIEMAT site (for the sulfate concentration in PM). At the CIEMAT site, the synergic use of Raman lidar and on-site instruments provided relevant information on the evolution and properties of the plume over the central part of the Iberian Peninsula. Aerosol extinction coefficient profiles provided by the lidar station show the presence of remarkable aerosol layers between 6 May and 15 May. Provenance studies using FLEXTRA backtrajectories confirmed that most of the aerosol layers originated in the Eyjafjallajokull eruption. The large suite of semi-continuous instruments present in the latter site allowed a better characterization of the aerosol properties. Size distribution and chemical composition were continuously monitored during the event, revealing a large increase in the aerosol fine mode, in coincidence with increases in ambient sulfate concentration, while the coarse mode remained almost unaltered. These results show that the plume carried mainly fine particles, with sizes between 0.1 and 0.7 μm in diameter, in contrast with studies of the plume that affected Central Europe in April, where particles with diameters larger than 20 μm were present in the ash layers. A possible explanation for this can be related to the long distance transport suffered by the plume and by the secondary formation of particulate sulfate from the gaseous sulfur dioxide. The information on volcanic aerosol characteristics after long-range transport provided by this study might contribute to better assess the type of aerosol that reach distant locations.

Radon and Lung Cancer in the American Cancer Society Cohort

Case-control studies conducted in North America, Europe, and Asia provided evidence of increased lung cancer risk due to radon in homes. Here, the association between residential radon and lung cancer mortality was examined in a large-scale cohort study. Nearly 1.2 million Cancer Prevention Study-II participants were recruited in 1982. Mean county-level residential radon concentrations were linked to study participants according to ZIP code information at enrollment [mean (SD)=53.5 Bq/m3 (38.0)]. Cox proportional hazards regression models were used to obtain adjusted HR and 95% CI for lung cancer mortality associated with radon. Potential effect modification by cigarette smoking, ambient sulfate concentrations, and other risk factors was assessed on both the additive and multiplicative scales. Through 1988, 3,493 lung cancer deaths were observed among 811,961 participants included in the analysis. A significant positive linear trend was observed between categories of radon concentrations and lung cancer mortality (P=0.02). A 15% (95% CI, 1-31) increase in the risk of lung cancer mortality was observed per 100 Bq/m3 increase in radon. Participants with mean radon concentrations above the EPA guideline value (148 Bq/m3) experienced a 34% (95% CI, 7-68) increase in risk for lung cancer mortality relative to those below the guideline value. This large prospective study showed positive associations between ecological indicators of residential radon and lung cancer. These results further support efforts to reduce radon concentrations in homes to the lowest possible level.

Continuous Lidar Monitoring of Polar Stratospheric Clouds at the South Pole

Polar stratospheric clouds (PSC) play a primary role in the formation of annual ozone holes over Antarctica during the austral sunrise. Meridional temperature gradients in the lower stratosphere and upper troposphere, caused by strong radiative cooling, induce a broad dynamic vortex centered near the South Pole that decouples and insulates the winter polar airmass. PSC nucleate and grow as vortex temperatures gradually fall below equilibrium saturation and frost points for ambient sulfate, nitrate, and water vapor concentrations (generally below 197 K). Cloud surfaces promote heterogeneous reactions that convert stable chlorine and bromine-based molecules into photochemically active ones. As spring nears, and the sun reappears and rises, photolysis decomposes these partitioned compounds into individual halogen atoms that react with and catalytically destroy thousands of ozone molecules before they are stochastically neutralized. Despite a generic understanding of the ozone hole paradigm, many key components of the system, such as cloud occurrence, phase, and composition; particle growth mechanisms; and denitrification of the lower stratosphere have yet to be fully resolved. Satellite-based observations have dramatically improved the ability to detect PSC and quantify seasonal polar chemical partitioning. However, coverage directly over the Antarctic plateau is limited by polar-orbiting tracks that rarely exceed 80 degrees S. In December 1999, a NASA Micropulse Lidar Network instrument (MPLNET) was first deployed to the NOAA Earth Systems Research Laboratory (ESRL) Atmospheric Research Observatory at the Amundsen-Scott South Pole Station for continuous cloud and aerosol profiling. MPLNET instruments are eye-safe, capable of full-time autonomous operation, and suitably rugged and compact to withstand long-term remote deployment. With only brief interruptions during the winters of 2001 and 2002, a nearly continuous data archive exists to the present.

Ambient site, home outdoor and home indoor particulate concentrations as proxies of personal exposures

Despite strong longitudinal associations between particle personal exposures and ambient concentrations, previous studies have found considerable inter-personal variability in these associations. Factors contributing to this inter-personal variability are important to identify in order to improve our ability to assess particulate exposures for individuals. This paper examines whether ambient, home outdoor and home indoor particle concentrations can be used as proxies of corresponding personal exposures. We explore the strength of the associations between personal, home indoor, home outdoor and central outdoor monitoring site ("ambient site") concentrations of sulfate, fine particle mass (PM(2.5)) and elemental carbon (EC) by season and subject for 25 individuals living in the Boston, MA, USA area. Ambient sulfate concentrations accounted for approximately 70 to 80% of the variability in personal and indoor sulfate levels. Correlations between ambient and personal sulfate, however, varied by subject (0.1-1.0), with associations between personal and outdoor sulfate concentrations generally mirroring personal-ambient associations (median subject-specific correlations of 0.8 to 0.9). Ambient sulfate concentrations are good indicators of personal exposures for individuals living in the Boston area, even though their levels may differ from actual personal exposures. The strong associations for sulfate indicate that ambient concentrations and housing characteristics are the driving factors determining personal sulfate exposures. Ambient PM(2.5) and EC concentrations were more weakly associated with corresponding personal and indoor levels, as compared to sulfate. For EC and PM(2.5), local traffic, indoor sources and/or personal activities can significantly weaken associations with ambient concentrations. Infiltration was shown to impact the ability of ambient concentrations to reflect exposures with higher exposures to particles from ambient sources during summer. In contrast in the winter, lower infiltration can result in a greater contribution of indoor sources to PM(2.5) and EC exposures. Placing EC monitors closer to participants' homes may reduce exposure error in epidemiological studies of traffic-related particles, but this reduction in exposure error may be greater in winter than summer. It should be noted that approximately 20% of the EC data were below the field limit of detection, making it difficult to determine if the weaker associations with the central site for EC were merely a result of methodological limitations.

Effects of Sulfur Dioxide and Oxides of Nitrogen Emission Reductions on Fine Particulate Matter Mass Concentrations: Regional Comparisons

Two thermodynamic equilibrium models were applied to estimate changes in mean airborne fine particle (PM2.5)mass concentrations that could result from changes in ambient concentrations of sulfate, nitric acid, or ammonia in the southeastern United States, the midwestern United States, and central California. Pronounced regional differences were found. Southeastern sites exhibited the lowest current mean concentrations of nitrate, and the smallest predicted responses of PM2.5 nitrate and mass concentrations to reductions of nitric acid, which is the principal reaction product of the oxidation of nitrogen dioxide (NO2) and the primary gas-phase precursor of fine particulate nitrate. Weak responses of PM2.5 nitrate and mass concentrations to changes in nitric acid levels occurred even if sulfate concentrations were half of current levels. The midwestern sites showed higher levels of fine particulate nitrate, characterized by cold-season maxima, and were projected to show decreases in overall PM levels following decreases of either sulfate or nitric acid. For some midwestern sites, predicted PM2.5 nitrate concentrations increased as modeled sulfate levels declined, but sulfate reductions always reduced the predicted fine PM mass concentrations; PM2.5 nitrate concentrations became more sensitive to reductions of nitric acid as modeled sulfate concentrations were decreased. The California sites currently have the highest mean concentrations of fine PM nitrate and the lowest mean concentrations of fine PM sulfate. Both the estimated PM2.5 nitrate and fine mass concentrations decreased in response to modeled reductions of nitric acid at all California sites. The results indicate important regional differences in expected PM2.5 mass concentration responses to changes in sulfate and nitrate precursors. Analyses of ambient data, such as described here, can be a key part of weight of evidence (WOE) demonstrations for PM2.5 attainment plans. Acquisition of the data may require special sampling efforts, especially for PM2.5 precursor concentration data.

Light scattering by fine particles during the Pittsburgh Air Quality Study: Measurements and modeling

Light scattering by fine particulate matter was measured during the Pittsburgh Air Quality Study (PAQS) as close to ambient conditions as possible. Several approaches are used for the theoretical calculation of the scattering coefficient and the results are compared to the direct measurements. The first approach uses ambient high time and daily resolved PM2.5 composition concentrations to estimate the scattering coefficient assuming that the aerosol is an external mixture. The second approach uses a thermodynamic model and Mie theory to predict the scattering coefficient of aerosols from daily size composition distributions. The third approach introduces high time and daily resolved ambient aerosol water concentrations and concentrations of sulfate, nitrate, organic material, and soil with fixed scattering efficiencies. During the summer the first two approaches underestimate the measured scattering coefficient by around 20%. Agreement within experimental error is obtained between the measured scattering coefficient and the model, incorporating measured water aerosol concentrations. During the winter the first two approaches tend to overpredict the measured scattering by around 15%. This overprediction is weakly correlated to the organic mass. The modeling approaches suggest that sulfate and the associated water contribute 65–73% to the scattering coefficient during the summer, with organic material contributing 25–30%. During the winter, sulfate accounts for 35–43%, nitrate accounts for 24–32%, and organic material accounts for 30–40% of the scattering coefficient.

A COMPARISON OF HEALTH EFFECTS FROM EXPOSURE TO AMBIENT AND NON-AMBIENT PARTICLES

ISEE-89 Introduction: The plausibility of epidemiological associations between adverse health effects and outdoor concentrations of airborne particulate matter (PM) has been questioned by studies demonstrating low correlations between individual exposures to ambient PM and ambient PM concentrations. Since personal exposure to PM is frequently dominated by exposure to non-ambient particles, it is important to evaluate the potential health impacts of both ambient and non-ambient exposures. Methods: In summer 1998, personal exposures to PM2.5 and sulfate, and ambient concentrations of PM10, PM2.5 and sulfate were measured for a repeated measures (7 repeats) panel study of respiratory and cardiovascular effects in chronic obstructive pulmonary disease (COPD) patients (n=16). In a further analysis of this dataset, we used an estimation method, based on time-activity data and the personal/ambient ratio of sulfate as a marker for the infiltration of ambient particles with the same size distribution, to develop separate estimates of exposures to ambient PM2.5, PM10 and PM10-2.5 and to non-ambient PM2.5. Health outcome measures included lung function, blood pressure, heart rate variability, and arrhythmia. The concentrations and association with health effects of these estimated exposures were compared to the originally measured total personal exposures and to measured ambient concentrations. Use of mixed models and elimination of one day of exposure associated with transported Asian dust led to larger and more significant associations than found in an earlier analysis. Results: Personal exposures to PM2.5 were dominated by exposures to non-ambient PM2.5 (56% on average), which were uncorrelated with exposures to ambient PM2.5 and to measured ambient PM2.5. For most outcomes, ambient exposure provided greater and more significant effect estimates in the expected direction than non-ambient or total exposure. For most outcomes, the effect estimates between PM indicators and health outcomes were similar in direction and magnitude for ambient concentrations and their respective ambient exposures. However, for most outcomes, effect estimates for ambient exposures were larger and more significant than those for ambient concentrations. Discussion: This study demonstrates a methodology that permits epidemiological analyses using separate indicators for ambient and non-ambient exposure. The results support the hypotheses that ambient exposures provide more precise and stronger estimates of effect in epidemiological analyses than ambient concentrations and that ambient exposures and non-ambient exposures will demonstrate different associations with health outcomes. These results also support the plausibility of associations between ambient concentrations and adverse health outcomes.

Commentary: Health Review Committee

Epidemiologic work conducted over several decades has suggested that long-term residence in cities with elevated ambient levels of air pollution from combustion sources is associated with increased mortality. Subsequently, two prospective cohort studies, the Six Cities Study (as reported in Dockery et al., 1993) and the American Cancer Society (ACS) Study (as reported in Pope et al., 1995), estimated that annual average all-cause mortality increased in association with an increase in fine particles (all particles less than 2.5 µm in median aerodynamic diameter [PM2.5]). As part of the Six Cities Study, Dockery and colleagues (1993) had prospectively followed a cohort of 8111 adult subjects in the northeastern and midwestern United States for 14 to 16 years beginning in the mid-1970s. The authors found that higher ambient levels of fine particles and sulfate (SO ) were associated with a 26% increase in mortality from all causes when comparing the most polluted to the least polluted city and that an increase in fine particles was also associated with increased mortality from cardiopulmonary disease. The relative risks in all-cause mortality were associated with a difference (or range) in ambient fine particle concentrations of 18.6 µg/m 3 and a difference of ambient sulfate concentrations of 8.0 µg/m 3 , comparing the least polluted city to the most polluted city.

Chemical indicators of sulfate sensitivity to nitrogen oxides and volatile organic compounds

The formation of aerosol sulfate (SO42−) in eastern North America is chemically linked to the emissions of nitrogen oxides (NOx) and volatile organic compounds (VOC) through oxidation of the gaseous precursor, sulfur dioxide (SO2). The response of sulfate production to controls in NOx and VOC emissions depends, in part, on the resulting changes in oxidant levels and the competition that naturally exists between the gas‐ and aqueous‐phase pathways for SO2 oxidation. We propose the use of a combination of concentrations of nitric acid, particulate nitrate, hydrogen peroxide, and ambient sulfate as a nondimensional indicator of the effectiveness of VOC or NOx controls in decreasing SO42− abundance. The concentrations of these indicator species were calculated from a series of photochemical model simulations with varying rates of NOx and VOC emissions using a three‐dimensional Eulerian model (MODELS‐3) that covers the northeastern United States. This study shows that ambient sulfate concentrations are likely to decrease more effectively as VOC emissions are reduced, when the nondimensional indicator is less than a certain threshold. However, a higher value of the indicator identifies a regime in which NOx emissions reductions are more effective for reducing sulfate than are VOC emissions. In addition, a description of the sulfate‐formation pathways, along with a theoretical analysis of the transition between NOx‐ and VOC‐sensitive regimes, provides a strong rationale for the use of the sulfate sensitivity indicator.