Black Carbon Aggregates Research Articles

Performance of the discrete dipole approximation for optical properties of black carbon aggregates

The optical properties of black carbon (BC) are fundamental for radiative transfer and remote sensing. BC geometry is successfully represented by an idealized model named “fractal aggregate”, and numerous methods are available and widely used to simulate the corresponding optical properties. This study systematically evaluates the performance of the discrete dipole approximation (DDA) for optical simulations of BC aggregates. The Multiple Sphere T-Matrix (MSTM) results are used as references for accuracy evaluation. The differences between the DDA and MSTM can be controlled to be less than 3% by using dipole size much smaller than the monomer size, and the DDA efficiency is sensitive to aggregate structures, e.g. lacy or compact. We find that shape representation for small-sized monomers during DDA discretization leads significant errors, i.e., up to 10%, and relatively large refractive index of BC also affects the DDA accuracy. However, the MSTM treats the BC monomers as perfect spheres without overlapping, and the imperfect structure that is implicitly introduced in the DDA simulations due to the spatial discretization may be a better representation of realistic BC particles. Moreover, the accuracy and efficiency of the DDA can be improved by defining dipoles on the particle boundary to have refractive indices given by the effective medium approximation (EMA). This leads to the adequate shape representation even using larger dipole sizes, and results in the DDA accuracy comparable to that of the reference MSTM solution.

Characterization of Cr (VI) – Containing solid phase particles in dry dust deposition in Daejeon, South Korea

Solid phase speciation of chromium in dry dust deposition and road paint was determined using transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS). Spherical black carbon aggregates in dry dust deposition contained discrete nano-sized lead chromate (PbCrO4) and zinc chromate (ZnCrO4), which likely originated from yellow traffic paint and zinc chromate primer (corrosion resistant pigment), respectively based on their main applications. Road marking paint samples from auto roads included lead chromate particles whose size, shape, composition and crystal structure were similar to those in dry dust deposition. A slight difference was found at the particle boundary. Namely, lead chromate in traffic paint was encapsulated by silica-bearing matrix, while discrete lead chromate in the black carbon of dry deposition was rarely enclosed within silica-bearing matrix. The Cr K edge X-ray absorption near edge structure (XANES) spectrum for dry deposition showed the characteristic pre-edge resonance peak similar to the spectra for the Cr(VI) standard and road paint, but the lower intensity. The amount of Cr(VI) accounted for approximately 45% of the total Cr in the dust sample. The solid phase speciation of Cr observed using TEM and their valence states determined using XANES were consistent with the chemical speciation determined using the sequential extraction. The contribution of this study is that XANES was applied to identify Cr valance states in urban dust deposition and zinc chromate was found as a Cr(VI) phase in dry dust deposition using TEM. These study results provide novel data on Cr speciation and Cr(VI)-containing mineral phases in dry dust deposition and their potential sources. Based on the wide use of lead and zinc chromate, atmospheric contamination with these solid phase speciation of Cr(VI) is expected in other cities in the world.

Applying machine learning to estimate the optical properties of black carbon fractal aggregates

Calculation of the optical properties of black carbon fractal aggregates is important in a variety of applications, whereas the complex morphology of black carbon aggregates makes it particularly computationally expensive for broadband applications. Previous studies have tried to parameterize the optical properties with empirical fitting. In this work, a machine learning method, support vector machine, was applied to estimate the optical properties. The integral optical properties obtained by numerically exact multiple-sphere T-matrix method and trained support vector machine model respectively, were presented and discussed in this study. The comparative results show excellent agreement between the two methods. Even though machine learning may provide inaccurate results when morphological parameters are beyond the range of training dataset, after adding small number of data on the basis of pre-trained support vector machine model to cover the full range, relative errors of extinction efficiency (Qext), absorption efficiency (Qabs), scattering efficiency (Qsca) and asymmetric factor (ASY) are within 2.3%, 1.4%, 5% and 4.8% respectively. The errors are acceptable because the MSTM results may fluctuate from mean values over a small range. Therefore, machine learning can reconstruct the full range of optical properties by small number of training data. This work provides a new method to estimate optical properties of black carbon aggregates and is helpful for simplifying the calculation of optical properties of black carbon fractal aggregates. It may be helpful for atmospheric models calculations, aerosol optical inversions and other fields involved in calculation of BC aggregates with broadband morphological parameters. Moreover, it may provide a new insight for parameterizations of optical properties of BC with more complex morphologies.

Numerical Investigation on Absorption Enhancement of Black Carbon Aerosols Partially Coated With Nonabsorbing Organics

AbstractThis study numerically evaluates the effects of aerosol microphysics, including coated volume fraction of black carbon (BC), shell/core ratio, and size distribution, on the absorption enhancement (Eab) of polydisperse BC aggregates partially coated by organics, which is calculated by the exact multiple‐sphere T‐matrix method. The coated volume fraction of BC plays a substantial role in determining the absorption enhancement of partially coated BC aggregates, which typically have an Eab in the range of ~1.0–2.0 with a larger value for larger coated volume fraction of BC as the shell/core ratio, BC geometry, and size distribution are fixed. The shell/core ratio, BC geometry, and size distribution have little impact on the Eab of coated BC with small coated volume fraction of BC, while they become significant for large coated volume fraction of BC. The Eab of partially coated BC particles can be slightly less than 1.0 for the large BC in the accumulation mode exhibiting large shell/core ratio and small coated volume fraction of BC, indicating that the absorption shows even slight decrease relative to uncoated BC particles. For partially coated BC aggregates in the accumulation and coarse modes, the refractive index uncertainties of BC result in the Eab differences of less than 9% and 2%, respectively, while those of organics can induce larger variations with the maximum differences up to 22% and 18%, respectively. Our study indicates that accounting for particle coating microphysics, particularly the coated volume fraction of BC, can potentially help to understand the differences in observations of largely variable absorption enhancements over various regions.

Absorption enhancement of aged black carbon aerosols affected by their microphysics: A numerical investigation

An idealized spherical model with black carbon (BC) aggregates fully coated by sulfate is developed to study the absorption enhancement (Eab) of polydisperse BC aerosols, which is numerically calculated by the multiple-sphere T-matrix method (MSTM). The aim of this study is to evaluate the effects of aerosol microphysics on the absorption enhancement of fully coated BC particles. The Eab values of accumulation concentric coated BC aggregates with different BC fractal dimensions vary within 2%, while those of coarse coated BC aggregates can alter up to 20% depending on shell-core ratio Dp/Dc (spherical equivalent particle diameter divided by BC core diameter). The BC position inside coating can result in an Eab decrease of fully coated BC aggregates up to approximately 15% and 20% in the accumulation and coarse modes, respectively. Compared with the concentric spherical structure, the off-center coated BC aggregates shows similar Eab with a difference less than 9% in the accumulation mode, whereas it can lead to up to 31% reductions in Eab in the coarse mode. The absorption enhancement of aged BC is sensitive to particle size distribution, and it decreases as particles becomes larger in accumulation mode, whereas the reverse is true in coarse mode. For BC aggregates fully coated with a very thin layer sulfate at different size distributions, Eab values are generally in ranges of 1.5–1.8 and 1.3–1.4 in accumulation and coarse modes, respectively, while with coating reaching Dp/Dc = 2.7, their values range from 1.7–2.4 and 2.0–2.1 in accumulation and coarse modes, respectively. Our study indicates that, larger Dp/Dc with BC aggregates not close to the coating boundary, or BC position closer to particle geometric center enhance BC lensing effect, whereas BC aggregates near the boundary of heavy coating may not further enhance lensing effect significantly with increased coating fraction.

Optical properties of black carbon aggregates with non-absorptive coating

This study develops an idealized model to account for the effects of non-absorptive coating on the optical properties of black carbon (BC) aggregates. The classic fractal aggregate is applied to represent realistic BC particles, and the coating is assumed to be spherical. To accelerate the single-scattering simulation, BC monomers that were overlapped with coating sphere (not those completely inside the coating) are slightly moved to avoid overlapping. The multiple-sphere T-matrix method (MSTM) becomes applicable to calculate the optical properties of inhomogeneous particles with any coating amount, and is generally two orders of magnitude faster than the discrete-dipole approximation for particles we considered. Furthermore, the simple spherical coating is found to have similar effects on the optical properties to those based on more complicated coating structure. With the simple particle model and the efficient MSTM, it becomes possible to consider the influence of coating with much more details. The non-absorptive coating of BC aggregates can significantly enhance BC extinction and absorption, which is consistent with previous studies. The absorption of coated aggregates can be over two times stronger than that of BC particles without coating. Besides the coating volume, the relative position between the mass centers of BC aggregate and coating also plays an important role on the optical properties, and should obviously be considered in further studies.

Lead chromate detected as a source of atmospheric Pb and Cr (VI) pollution.

Spherical black carbon aggregates were frequently observed in dust dry deposition in Daejeon, Korea. They were tens of micrometers in diameter and presented a mixture of black carbon and several mineral phases. Transmission electron microscopy (TEM) observations with energy-dispersive X-ray spectroscopy (EDS) and selected area diffraction pattern (SADP) analyses confirmed that the aggregates were compact and included significant amounts of lead chromate (PbCrO4). The compositions and morphologies of the nanosized lead chromate particles suggest that they probably originated from traffic paint used in roads and were combined as discrete minerals with black carbon. Based on Pb isotope analysis and air-mass backward trajectories, the dust in Daejeon received a considerable input of anthropogenic pollutants from heavily industrialized Chinese cities, which implies that long-range transported aerosols containing PbCrO4 were a possible source of the lead and hexavalent chromium levels in East Asia. Lead chromate should be considered to be a source of global atmospheric Pb and Cr(VI) pollution, especially given its toxicity.

Optical properties of black carbon in cookstove emissions coated with secondary organic aerosols: Measurements and modeling

ABSTRACTCookstoves are a major source of black carbon (BC) particles and associated organic compounds, which influence the atmospheric radiative balance. We present results from experiments that characterize BC emissions from a rocket stove coated with secondary organic aerosol. Optical properties, namely, BC mass absorption cross-section (MACBC) and mass scattering cross-section (MSC), as a function of the organic-to-black carbon ratio (OA:BC) of fresh and aged cookstove emissions were compared with Mie and Rayleigh–Debye–Gans (RDG) calculations. Mie theory reproduced the measured MACBC across the entire OA:BC range. However, Mie theory failed to capture the MSC at low OA:BC, where the data agreed better with RDG, consistent with a fractal morphology of fresh BC aggregates. As the OA:BC increased, the MSC approached Mie predictions indicating that BC-containing particles approach a core–shell structure as BC cores become heavily coated. To gain insight into the implications of our findings, we calculated...

Intercomparison of the GOS approach, superposition T-matrix method, and laboratory measurements for black carbon optical properties during aging

We perform a comprehensive intercomparison of the geometric-optics surface-wave (GOS) approach, the superposition T-matrix method, and laboratory measurements for optical properties of fresh and coated/aged black carbon (BC) particles with complex structures. GOS and T-matrix calculations capture the measured optical (i.e., extinction, absorption, and scattering) cross sections of fresh BC aggregates, with 5–20% differences depending on particle size. We find that the T-matrix results tend to be lower than the measurements, due to uncertainty in theoretical approximations of realistic BC structures, particle property measurements, and numerical computations in the method. On the contrary, the GOS results are higher than the measurements (hence the T-matrix results) for BC radii <100nm, because of computational uncertainty for small particles, while the discrepancy substantially reduces to 10% for radii >100nm. We find good agreement (differences <5%) between the two methods in asymmetry factors for various BC sizes and aggregating structures. For aged BC particles coated with sulfuric acid, GOS and T-matrix results closely match laboratory measurements of optical cross sections. Sensitivity calculations show that differences between the two methods in optical cross sections vary with coating structures for radii <100nm, while differences decrease to ~10% for radii >100nm. We find small deviations (≤10%) in asymmetry factors computed from the two methods for most BC coating structures and sizes, but several complex structures have 10–30% differences. This study provides the foundation for downstream application of the GOS approach in radiative transfer and climate studies.

Variation of the radiative properties during black carbon aging: theoretical and experimental intercomparison

Abstract. A theoretical black carbon (BC) aging model is developed to account for three typical evolution stages, namely, freshly emitted aggregates, BC coated by soluble material, and BC particles undergoing further hygroscopic growth. The geometric-optics surface-wave (GOS) approach is employed to compute the BC single-scattering properties at each aging stage, which are subsequently compared with laboratory measurements. Theoretical calculations are consistent with measurements in extinction and absorption cross sections for fresh BC aggregates with different BC sizes (i.e., mobility diameters of 155, 245, and 320 nm), with differences of ≤ 25 %. The measured optical cross sections for BC coated by sulfuric acid and for that undergoing further hygroscopic growth are generally captured (differences &lt; 30 %) by theoretical calculations using a concentric core-shell structure, with an overestimate in extinction and absorption of the smallest BC size and an underestimate in scattering of the largest BC size. We find that the absorption and scattering cross sections of fresh BC aggregates vary by 20–40 and 50–65 %, respectively, due to the use of upper (1.95–0.79i) and lower (1.75–0.63i) bounds of BC refractive index, while the variations are &lt; 20 % in absorption and &lt; 50 % in scattering in the case of coated BC particles. Sensitivity analyses of the BC morphology show that the optical properties of fresh BC aggregates are more sensitive to fractal dimension than primary spherule size. The absorption and scattering cross sections of coated BC particles vary by more than a factor of 2 due to different coating structures. We find an increase of 20–250 % in absorption and a factor of 3–15 in scattering during aging, significantly depending on coating morphology and aging stages. This study suggests that an accurate estimate of BC radiative effects requires the incorporation of a dynamic BC aging process that accounts for realistic coating structures in climate models.

The Effects of Monomer Size Distribution on the Radiative Properties of Black Carbon Aggregates

Black carbon (BC), normally existing as aggregates, significantly affects the Earth radiative forcing, energy balance, and climate by scattering and absorbing both solar radiation and terrestrial emission. The BC particles are usually treated as fractal aggregates with same-sized monomers. However, experimental studies show that monomer diameters of BC normally obey a lognormal distribution ranging from 10 nm to over 100 nm. This study investigates the effects of monomer size distribution on the radiative properties of BC particles. The fractal aggregates are generated by a cluster–cluster aggregation (CCA) algorithm, and the Multiple Sphere T-Matrix (MSTM) method is used to simulate the radiative properties of randomly oriented aggregates. The integral radiative properties of aggregates with different-sized monomers have normal distributions with large standard deviations, and it requires to average radiative properties of over 60 aggregate realizations to represent their ensemble-averaged properties. Th...

Particle Emission Characteristics of a Gas Turbine with a Double Annular Combustor

The total climate, air quality, and health impact of aircraft black carbon (BC) emissions depend on quantity (mass and number concentration) as well as morphology (fractal dimension and surface area) of emitted BC aggregates. This study examines multiple BC emission metrics from a gas turbine with a double annular combustor, CFM56-5B4-2P. As a part of the SAMPLE III.2 campaign, concurrent measurements of particle mobility, particle mass, particle number concentration, and mass concentration, as well as collection of transmission electron microscopy (TEM) samples, allowed for characterization of the BC emissions. Mass- and number-based emission indices were strongly influenced by thrust setting during pilot combustion and ranged from <1 to 208 mg/kg-fuel and 3 ×× 1012 to 3 ×× 1016 particles/kg-fuel, respectively. Mobility measurements indicated that mean diameters ranged from 7 to 44 nm with a strong dependence on thrust during pilot-only combustion. Using aggregation and sintering theory with empirical effective density relationships, a power-law relationship between primary particle diameter and mobility diameter is presented. Mean primary particle diameter ranged from 6 to 19 nm; however, laser-induced incandescence (LII) and mass-mobility-calculated primary particle diameters demonstrated opposite trends with thrust setting. Similarly, mass-mobility-calculated aggregate mass specific surface area and LII-measured surface area were not in agreement, indicating both methods need further development and validation before use as quantitative indicators of primary particle diameter and mass-specific surface area.Copyright 2015 American Association for Aerosol Research

Black carbon radiative forcing over the Tibetan Plateau

AbstractWe estimate the snow albedo forcing and direct radiative forcing (DRF) of black carbon (BC) in the Tibetan Plateau using a global chemical transport model in conjunction with a stochastic snow model and a radiative transfer model. The annual mean BC snow albedo forcing is 2.9 W m−2 averaged over snow‐covered plateau regions, which is a factor of 3 larger than the value over global land snowpack. BC‐snow internal mixing increases the albedo forcing by 40–60% compared with external mixing, and coated BC increases the forcing by 30–50% compared with uncoated BC aggregates, whereas Koch snowflakes reduce the forcing by 20–40% relative to spherical snow grains. The annual BC DRF at the top of the atmosphere is 2.3 W m−2 with uncertainties of −70–85% in the plateau after scaling the modeled BC absorption optical depth to Aerosol Robotic Network observations. The BC forcings are attributed to emissions from different regions.

Effects of internal mixing and aggregate morphology on optical properties of black carbon using a discrete dipole approximation model

Abstract. According to recent studies, internal mixing of black carbon (BC) with other aerosol materials in the atmosphere alters its aggregate shape, absorption of solar radiation, and radiative forcing. These mixing state effects are not yet fully understood. In this study, we characterize the morphology and mixing state of bare BC and BC internally mixed with sodium chloride (NaCl) using electron microscopy and examine the sensitivity of optical properties to BC mixing state and aggregate morphology using a discrete dipole approximation model (DDSCAT). DDSCAT is flexible in simulating the geometry and refractive index of particle aggregates. DDSCAT predicts a higher mass absorption coefficient (MAC), lower single scattering albedo (SSA), and higher absorption Angstrom exponent (AAE) for bare BC aggregates that are lacy rather than compact. Predicted values of SSA at 550 nm range between 0.16 and 0.27 for lacy and compact aggregates, respectively, in agreement with reported experimental values of 0.25 ± 0.05. The variation in absorption with wavelength does not adhere precisely to a power law relationship over the 200 to 1000 nm range. Consequently, AAE values depend on the wavelength region over which they are computed. The MAC of BC (averaged over the 200–1000 nm range) is amplified when internally mixed with NaCl (100–300 nm in radius) by factors ranging from 1.0 for lacy BC aggregates partially immersed in NaCl to 2.2 for compact BC aggregates fully immersed in NaCl. The SSA of BC internally mixed with NaCl is higher than for bare BC and increases with the embedding in the NaCl. Internally mixed BC SSA values decrease in the 200–400 nm wavelength range, a feature also common to the optical properties of dust and organics. Linear polarization features are also predicted in DDSCAT and are dependent on particle size and morphology. This study shows that DDSCAT predicts complex morphology and mixing state dependent aerosol optical properties that have been reported previously and are relevant to radiative transfer, climate modeling, and interpretation of remote sensing measurements.

The single-scattering properties of black carbon aggregates determined from the geometric-optics surface-wave approach and the T-matrix method

The single-scattering properties of eight black carbon (BC, soot) fractal aggregates, composed of primary spheres from 7 to 600, computed by the geometric-optics surface-wave (GOS) approach coupled with the Rayleigh–Gans–Debye (RGD) adjustment for size parameters smaller than approximately 2, are compared with those determined from the superposition T-matrix method. We show that under the condition of random orientation, the results from GOS/RGD are in general agreement with those from T-matrix in terms of the extinction and absorption cross-sections, the single-scattering co-albedo, and the asymmetry factor. When compared with the specific absorption (m2/g) measured in the laboratory, we illustrate that using the observed radii of primary spheres ranging from 3.3 to 25nm, the theoretical values determined from GOS/RGD for primary sphere numbers of 100–600 are within the range of measured values. The GOS approach can be effectively applied to aggregates composed of a large number of primary spheres (e.g., >6000) and large size parameters (≫2) in terms of computational efforts.

Black and brown carbon fractal aggregates from combustion of two fuels widely used in Asian rituals

Incense sticks and mustard oil are the two most popular combustion fuels during rituals and social ceremonies in Asian countries. Given their widespread use in both closed and open burning activities, it is important to quantify the spectral radiative properties of aerosols emitted from the combustion of both fuels. This information is needed by climate models to assess the impact of these aerosols on radiative forcing. In this study, we used a 3-wavelength integrated photoacoustic-nephelometer – operating simultaneously at 405, 532 and 781nm – to measure the optical coefficients of aerosols emitted from the laboratory combustion of mustard oil lamp and two types of incense sticks. From the measured optical coefficients at three wavelengths, time-varying single scattering albedo (SSA), absorption Ångström exponent (AAE), and scattering Ångström exponent (SAE) were calculated. For incense smoke particles, the time-averaged mean AAE values were found to be as high as 8.32 (between 405 and 532nm) and 6.48 (between 532 and 781nm). This spectrally-varying characteristic of AAE indicates that brown carbon – a class of organic carbon which strongly absorbs solar radiation in the blue and near ultraviolet – is the primary component of incense smoke aerosols. For aerosols emitted from the burning of mustard oil lamp, the time-averaged mean AAE values were ∼1.3 (between 405 and 781nm) indicating that black carbon (BC) is the primary constituent. Scanning electron microscopy combined with image processing revealed the morphology of incense smoke aerosols to be non-coalescing and weakly-bound aggregates with a mean two-dimensional (2-d) fractal dimension (Df)=1.9±0.07, while the mustard oil smoke aerosols had typical fractal-like BC aggregate morphology with a mean 2-d Df=1.85±0.09.

T-matrix modeling of linear depolarization by morphologically complex soot and soot-containing aerosols

We use state-of-the-art public-domain Fortran codes based on the T-matrix method to calculate orientation and ensemble averaged scattering matrix elements for a variety of morphologically complex black carbon (BC) and BC-containing aerosol particles, with a special emphasis on the linear depolarization ratio (LDR). We explain theoretically the quasi-Rayleigh LDR peak at side-scattering angles typical of low-density soot fractals and conclude that the measurement of this feature enables one to evaluate the compactness state of BC clusters and trace the evolution of low-density fluffy fractals into densely packed aggregates. We show that small backscattering LDRs measured with ground-based, airborne, and spaceborne lidars for fresh smoke generally agree with the values predicted theoretically for fluffy BC fractals and densely packed near-spheroidal BC aggregates. To reproduce higher lidar LDRs observed for aged smoke, one needs alternative particle models such as shape mixtures of BC spheroids or cylinders.

Black carbon fractal morphology and short-wave radiative impact: a modelling study

Abstract. We investigate the impact of the morphological properties of freshly emitted black carbon aerosols on optical properties and on radiative forcing. To this end, we model the optical properties of fractal black carbon aggregates by use of numerically exact solutions to Maxwell's equations within a spectral range from the UVC to the mid-IR. The results are coupled to radiative transfer computations, in which we consider six realistic case studies representing different atmospheric pollution conditions and surface albedos. The spectrally integrated radiative impacts of black carbon are compared for two different fractal morphologies, which brace the range of recently reported experimental observations of black carbon fractal structures. We also gauge our results by performing corresponding calculations based on the homogeneous sphere approximation, which is commonly employed in climate models. We find that at top of atmosphere the aggregate models yield radiative impacts that can be as much as 2 times higher than those based on the homogeneous sphere approximation. An aggregate model with a low fractal dimension can predict a radiative impact that is higher than that obtained with a high fractal dimension by a factor ranging between 1.1–1.6. Although the lower end of this scale seems like a rather small effect, a closer analysis reveals that the single scattering optical properties of more compact and more lacy aggregates differ considerably. In radiative flux computations there can be a partial cancellation due to the opposing effects of different error sources. However, this cancellation effect can strongly depend on atmospheric conditions and is therefore quite unpredictable. We conclude that the fractal morphology of black carbon aerosols and their fractal parameters can have a profound impact on their radiative forcing effect, and that the use of the homogeneous sphere model introduces unacceptably high biases in radiative impact studies. We emphasise that there are other potentially important morphological features that have not been addressed in the present study, such as sintering and coating of freshly emitted black carbon by films of organic material. Finally, we found that the spectral variation of the absorption cross section of black carbon significantly deviates from a simple 1/λ scaling law. We therefore discourage the use of single-wavelength absorption measurements in conjunction with a 1/λ scaling relation in broadband radiative forcing simulations of black carbon.

Time-resolved measurements of black carbon light absorption enhancement in urban and near-urban locations of southern Ontario, Canada

Abstract. In this study a photoacoustic spectrometer (PA), a laser-induced incandescence instrument system (LII) and an Aerosol Mass Spectrometer were operated in parallel for in-situ measurements of black carbon (BC) light absorption enhancement. Results of a thermodenuder experiment using ambient particles in Toronto are presented first to show that LII measurements of BC are not influenced by the presence of non-refractory material thus providing true atmospheric BC mass concentrations. In contrast, the PA response is enhanced when the non-refractory material is internally mixed with the BC particles. Through concurrent measurements using the LII and PA the specific absorption cross-section (SAC) can be quantified with high time resolution (1 min). Comparisons of ambient PA and LII measurements from four different locations (suburban Toronto; a street canyon with diesel bus traffic in Ottawa; adjacent to a commuter highway in Ottawa and; regional background air in and around Windsor, Ontario), show that different impacts from emission sources and/or atmospheric processes result in different particle light absorption enhancements and hence variations in the SAC. The diversity of measurements obtained, including those with the thermodenuder, demonstrated that it is possible to identify measurements where the presence of externally-mixed non-refractory particles obscures direct observation of the effect of coating material on the SAC, thus allowing this effect to be measured with more confidence. Depending upon the time and location of measurement (urban, rural, close to and within a lake breeze frontal zone), 30 min average SAC varies between 9 ± 2 and 43 ± 4 m2 g−1. Causes of this variation, which were determined through the use of meteorological and gaseous measurements (CO, SO2, O3), include the particle emission source, airmass source region, the degree of atmospheric processing. Observations from this study also show that the active surface area of the BC aggregate, which is measured by the LII as the PPS, is an important parameter for inferring the degree of particle collapse of a BC particle. In addition, PPS could be a useful measurement for indicating the importance of recently emitted BC (e.g. from gasoline or diesel engines) relative to the total measured BC in the atmosphere.

Cytotoxic effects of aggregated nanomaterials

This study deals with cytotoxicity assays performed on an array of commercially manufactured inorganic nanoparticulate materials, including Ag, TiO 2, Fe 2O 3, Al 2O 3, ZrO 2, Si 3N 4, naturally occurring mineral chrysotile asbestos and carbonaceous nanoparticulate materials such as multiwall carbon nanotube aggregates and black carbon aggregates. The nanomaterials were characterized by TEM, as the primary particles, aggregates or long fiber dimensions ranged from 2 nm to 20 μm. Cytotoxicological assays of these nanomaterials were performed utilizing a murine alveolar macrophage cell line and human macrophage and epithelial lung cell lines as comparators. The nanoparticulate materials exhibited varying degrees of cytoxicity for all cell lines and the general trends were similar for both the murine and human macrophage cell lines. These findings suggest that representative cytotoxic responses for humans might be obtained by nanoparticulate exposures to simple murine macrophage cell line assays. Moreover, these results illustrate the utility in performing rapid in vitro assays for cytotoxicity assessments of nanoparticulate materials as a general inquiry of potential respiratory health risks in humans.