Christopher M. Long

Characterization of indoor particle sources using continuous mass and size monitors

ABSTRACT A comprehensive indoor particle characterization study was conducted in nine Boston-area homes in 1998 in order to characterize sources of PM in indoor environments. State-of-the-art sampling methodologies were used to obtain continuous PM2.5 concentration and size distribution particulate data for both indoor and outdoor air. Study homes, five of which were sampled during two seasons, were monitored over week-long periods. Among other data collected during the extensive monitoring efforts were 24hr elemental/organic carbon (EC/OC) particulate data as well as semi-continuous air exchange rates and time-activity information. This rich data set shows that indoor particle events tend to be brief, intermittent, and highly variable, thus requiring the use of continuous instrumentation for their characterization. In addition to dramatically increasing indoor PM25 concentrations, these data demonstrate that indoor particle events can significantly alter the size distribution and composition of indoor particles. Source event data demonstrate that the impacts of indoor activities are especially pronounced in the ultrafine (da < 0.1 um) and coarse (2.5 < da < 10 |um) modes. Among the sources of ultrafine particles characterized in this study are indoor ozone/terpene reactions. Furthermore, EC/OC data suggest that organic carbon is a major constituent of particles emitted during indoor source events. Whether exposures to indoor-generated particles, particularly from large short-term peak events, may be associated with adverse health effects will become clearer when biological mechanisms are better known.

Generation of very flat optical frequency combs from continuous-wave lasers using cascaded intensity and phase modulators driven by tailored radio frequency waveforms

We demonstrate a scheme based on a cascade of lithium niobate intensity and phase modulators driven by specially tailored RF waveforms to generate an optical frequency comb with very high spectral flatness. In this Letter, we demonstrate a 10 GHz comb with 38 comb lines within a spectral power variation below 1 dB. The number of comb lines that can be generated is limited by the power handling capability of the phase modulator, and this can be scaled without compromising the spectral flatness. Furthermore, the spectral phase of the generated combs in our scheme is almost purely quadratic, which, as we will demonstrate, allows for high-quality pulse compression using only single-mode fiber.

Comb-based radiofrequency photonic filters with rapid tunability and high selectivity

Using electro-optically generated frequency combs, scientists demonstrate radiofrequency photonic filters that can potentially provide simultaneous high stopband attenuation, fast tunability and bandwidth reconfiguration.

Critical review of the human data on short-term nitrogen dioxide (NO2) exposures: Evidence for NO2 no-effect levels

Nitrogen dioxide (NO2) is a ubiquitous atmospheric pollutant due to the widespread prevalence of both natural and anthropogenic sources, and it can be a respiratory irritant when inhaled at elevated concentrations. Evidence for health effects of ambient NO2 derives from three types of studies: observational epidemiology, human clinical exposures, and animal toxicology. Our review focuses on the human clinical studies of adverse health effects of short-term NO2 exposures, given the substantial uncertainties and limitations in interpretation of the other lines of evidence. We examined more than 50 experimental studies of humans inhaling NO2, finding notably that the reporting of statistically significant changes in lung function and bronchial sensitivity did not show a consistent trend with increasing NO2 concentrations. Functional changes were generally mild and transient, the reported effects were not uniformly adverse, and they were not usually accompanied by NO2-dependent increases in symptoms. The available human clinical results do not establish a mechanistic pathway leading to adverse health impacts for short-term NO2 exposures at levels typical of maximum 1-h concentrations in the present-day ambient environment (i.e., below 0.2 ppm). Our review of these data indicates that a health-protective, short-term NO2 guideline level for susceptible (and healthy) populations would reflect a policy choice between 0.2 and 0.6 ppm. Extended abstract Nitrogen dioxide (NO2) is a ubiquitous atmospheric pollutant due to the widespread prevalence of both natural and anthropogenic sources, and it can be a respiratory irritant when inhaled at elevated concentrations. Natural NO2 sources include volcanic action, forest fires, lightning, and the stratosphere; man-made NO2 emissions derive from fossil fuel combustion and incineration. The current National Ambient Air Quality Standard (NAAQS) for NO2, initially established in 1971, is 0.053 ppm (annual average). Ambient concentrations monitored in urban areas in the United States are ~0.015 ppm, as an annual mean, i.e., below the current NAAQS. Short-term (1-h peak) NO2 concentrations outdoors are not likely to exceed 0.2 ppm, and even 1-h periods exceeding 0.1 ppm are infrequent. Inside homes, 1-h NO2 peaks, typically arising from gas cooking, can range between 0.4 and 1.5 ppm. The health effects evidence of relevance to ambient NO2 derives from three lines of investigation: epidemiology studies, human clinical studies, and animal toxicology studies. The NO2 epidemiology remains inconsistent and uncertain due to the potential for exposure misclassification, residual confounding, and co-pollutant effects, whereas animal toxicology findings using high levels of NO2 exposure require extrapolation to humans exposed at low ambient NO2 levels. Given the limitations and uncertainties in the other lines of health effects evidence, our review thus focused on clinical studies where human volunteers (including asthmatics, children, and elderly) inhaled NO2 at levels from 0.1 to 3.5 ppm during short-term (½–6-h) exposures, often combined with exercise, and occasionally combined with co-pollutants. We examined the reported biological effects and classified them into (a) lung immune responses and inflammation, (b) lung function changes and airway hyperresponsiveness (AHR), and (c) health effects outside the lungs (extrapulmonary). We examined more than 50 experimental studies of humans inhaling NO2, finding that such clinical data on short-term exposure allowed discrimination of NO2 no-effect levels versus lowest-adverse-effects levels. Our conclusions are summarized by these six points: For lung immune responses and inflammation: (1) healthy subjects exposed to NO2 below 1 ppm do not show pulmonary inflammation; (2) at 2 ppm for 4 h, neutrophils and cytokines in lung-lavage fluid can increase, but these changes do not necessarily correlate with significant or sustained changes in lung function; (3) there is no consistent evidence that NO2 concentrations below 2 ppm increase susceptibility to viral infection; (4) for asthmatics and individuals having chronic obstructive pulmonary disease (COPD), NO2-induced lung inflammation is not expected below 0.6 ppm, although one research group reported enhancement of proinflammatory processes at 0.26 ppm. With regard to NO2-induced AHR: (5) studies of responses to specific or nonspecific airway challenges (e.g., ragweed, methacholine) suggest that asthmatic individuals were not affected by NO2 up to about 0.6 ppm, although some sensitive subsets may respond to levels as low as 0.2 ppm. And finally, for extra-pulmonary effects: (6) such effects (e.g., changes in blood chemistry) generally required NO2 concentrations above 1–2 ppm. Overall, our review of data from experiments with humans indicates that a health-protective, short-term-average NO2 guideline level for susceptible populations (and healthy populations) would reflect a policy choice between 0.2 and 0.6 ppm. The available human clinical results do not establish a mechanistic pathway leading to adverse health impacts for short-term NO2 exposures at levels typical of maximum 1-h concentrations in the present-day ambient environment (i.e., below 0.2 ppm).

Carbon black vs. black carbon and other airborne materials containing elemental carbon: Physical and chemical distinctions

Airborne particles containing elemental carbon (EC) are currently at the forefront of scientific and regulatory scrutiny, including black carbon, carbon black, and engineered carbon-based nanomaterials, e.g., carbon nanotubes, fullerenes, and graphene. Scientists and regulators sometimes group these EC-containing particles together, for example, interchangeably using the terms carbon black and black carbon despite one being a manufactured product with well-controlled properties and the other being an undesired, incomplete-combustion byproduct with diverse properties. In this critical review, we synthesize information on the contrasting properties of EC-containing particles in order to highlight significant differences that can affect hazard potential. We demonstrate why carbon black should not be considered a model particle representative of either combustion soots or engineered carbon-based nanomaterials. Overall, scientific studies need to distinguish these highly different EC-containing particles with care and precision so as to forestall unwarranted extrapolation of properties, hazard potential, and study conclusions from one material to another.

Non-cancer health effects of diesel exhaust: a critical assessment of recent human and animal toxicological literature.

We reviewed laboratory and clinical studies bearing on the non-cancer health effects of diesel exhaust (DE) published since the 2002 release of the US EPA Health Assessment Document for Diesel Engine Exhaust. We critically evaluated over 100 published articles on experimental research, focusing on their value for predicting the risk of non-cancer health effects in humans exposed to DE. Human controlled-exposure studies provide new evidence of lung inflammatory effects and thrombogenic and ischemic effects of inhaled DE, albeit for older-model diesel engines and concentrations that are much higher (~300 μg/m3) than typical ambient or even occupational levels. Recent animal studies provide insight into the potential mechanisms underlying observed respiratory and cardiovascular health responses; however, because of unrealistically high DE concentrations, the mechanisms elucidated in these studies may not be relevant at lower DE exposure levels. Although larger in number, and suggestive of possible mechanisms for non-cancer health effects at elevated DE levels, interpretation of this recent group of clinical-study findings and laboratory-animal results remains hindered by inconsistencies and variability in outcomes, potentially irrelevant DE-exposure compositions, limitations in exposure protocols and pathways, and uncertainties in extrapolation and generalization. A mechanism of action that allows reliable prediction of adverse health effects at DE-exposure levels typical of the present-day ambient and occupational environment has not emerged. Because of changing diesel-engine technology, inhalation studies using realistic environmental and occupational exposures of new-technology diesel exhaust are of critical importance.

Reconfigurable and Tunable Flat-Top Microwave Photonic Filters Utilizing Optical Frequency Combs

We demonstrate reconfigurable and tunable flat-top microwave photonic filters based on an optical comb source and a dispersive medium. Complex taps allowing flexible and tunable filter characteristics are implemented by programming the amplitude and phase of individual comb lines using an optical line-by-line pulse shaper. First, we implement a flat top filter by applying positive and negative weights across the comb lines, then tune the filter center frequency by adding a phase ramp onto the tap weights.

Health effects research and regulation of diesel exhaust: an historical overview focused on lung cancer risk.

The mutagenicity of organic solvent extracts from diesel exhaust particulate (DEP), first noted more than 55 years ago, initiated an avalanche of diesel exhaust (DE) health effects research that now totals more than 6000 published studies. Despite an extensive body of results, scientific debate continues regarding the nature of the lung cancer risk posed by inhalation of occupational and environmental DE, with much of the debate focused on DEP. Decades of scientific scrutiny and increasingly stringent regulation have resulted in major advances in diesel engine technologies. The changed particulate matter (PM) emissions in “New Technology Diesel Exhaust (NTDE)” from today’s modern low-emission, advanced-technology on-road heavy-duty diesel engines now resemble the PM emissions in contemporary gasoline engine exhaust (GEE) and compressed natural gas engine exhaust more than those in the “traditional diesel exhaust” (TDE) characteristic of older diesel engines. Even with the continued publication of epidemiologic analyses of TDE-exposed populations, this database remains characterized by findings of small increased lung cancer risks and inconsistent evidence of exposure–response trends, both within occupational cohorts and across occupational groups considered to have markedly different exposures (e.g. truckers versus railroad shopworkers versus underground miners). The recently published National Institute for Occupational Safety and Health (NIOSH)-National Cancer Institute (NCI) epidemiologic studies of miners provide some of the strongest findings to date regarding a DE-lung cancer association, but some inconsistent exposure–response findings and possible effects of bias and exposure misclassification raise questions regarding their interpretation. Laboratory animal studies are negative for lung tumors in all species, except for rats under lifetime TDE-exposure conditions with durations and concentrations that lead to “lung overload.” The species specificity of the rat lung response to overload, and its occurrence with other particle types, is now well-understood. It is thus generally accepted that the rat bioassay for inhaled particles under conditions of lung overload is not predictive of human lung cancer hazard. Overall, despite an abundance of epidemiologic and experimental data, there remain questions as to whether TDE exposure causes increased lung cancers in humans. An abundance of emissions characterization data, as well as preliminary toxicological data, support NTDE as being toxicologically distinct from TDE. Currently, neither epidemiologic data nor animal bioassay data yet exist that directly bear on NTDE carcinogenic potential. A chronic bioassay of NTDE currently in progress will provide data on whether NTDE poses a carcinogenic hazard, but based on the significant reductions in PM mass emissions and the major changes in PM composition, it has been hypothesized that NTDE has a low carcinogenic potential. When the International Agency for Research on Cancer (IARC) reevaluates DE (along with GEE and nitroarenes) in June 2012, it will be the first authoritative body to assess DE carcinogenic health hazards since the emergence of NTDE and the accumulation of data differentiating NTDE from TDE.

Diesel exhaust particulate (DEP) and nanoparticle exposures: What do DEP human clinical studies tell us about potential human health hazards of nanoparticles?

Engineered nanoparticles (ENPs) are increasingly tested in cellular and laboratory-animal experiments for hazard potential, but there is a lack of health effects data for humans exposed to ENPs. However, human data for another source of nanoparticle (NP) exposure are available, notably for the NPs contained in diesel exhaust particulate (DEP). Studies of human volunteers exposed to diesel exhaust (DE) in research settings report DEP-NP number concentrations (i.e., >106 particles/cm3) that exceed number concentrations reported for worst-case exposure conditions for workers manufacturing and handling ENPs. Recent human DE exposure studies, using sensitive physiological instrumentation and well-characterized exposure concentrations and durations, suggest that elevated DE exposures from pre-2007 engines may trigger short-term changes in, for example, lung and systemic inflammation, thrombogenesis, vascular function, and brain activity. Considerable uncertainty remains both as to which DE constituents underlie the observed responses (i.e., DEP NPs, DEP mass, DE gases), and as to the implications of the observed short-term changes for the development of disease. Even so, these DE human clinical data do not give evidence of a unique toxicity for NPs as compared to other small particles. Of course, physicochemical properties of toxicological relevance may differ between DEP NPs and other NPs, yet overall, the DE human clinical data do not support the idea that elevated levels of NPs per se (at least in the DEP context) must be acutely toxic by virtue of their nano-sized nature alone.

Indoor-Outdoor Relationships and Infiltration Behavior of Elemental Components of Outdoor PM2.5 for Boston-Area Homes

In order to investigate the relationship between indoor and outdoor elemental concentrations and to characterize the infiltration behavior of elemental PM2.5 constituents, we conducted an analysis of indoor and outdoor PM2.5 elemental data collected during a comprehensive particle characterization study of nine nonsmoking homes in Boston, MA. Using data from nighttime periods when little or no particle-generating activity occurred, analyses focused on six elements that were consistently detected in both indoor and outdoor samples and that spanned a range of particle sizes: sulfur, nickel, zinc, iron, potassium, and silicon. Results showed that outdoor levels of all the elements were highly correlated with their corresponding indoor levels. Correlations remained high for different air exchange rate conditions, building characteristics, and seasons, suggesting that variability in ambient elemental infiltration into residences may not be a large source of variability affecting personal-ambient correlations for these elements. Elemental infiltration factors showed strong relationships with air exchange rate and season and were suggestive of an effect of particle size, which was likely obscured by remaining indoor source impacts. Analyses of this small dataset provided an indication that several elements—in particular nickel—could potentially serve as accurate tracers for infiltration of total PM2.5 mass- and size-resolved particles into residential buildings. Similar to previously reported findings for sulfur, these elemental tracers showed the poorest performance for smaller and larger particle sizes.