[en] The effects of soot aggregate quantities on the optical properties of their semi-external mixture with sulfate host particle were investigated. In this study, the individual soot-containing mixtures were simulated as sulfate host point-contact attached to a specified amount of soot aggregates with the same monomer numbers and fractal parameters. The total numbers and volumes of soot monomers were also constant. Optical properties of this type of aerosol mixture were calculated using the numerically exact superposition T-matrix method (STM). The random-orientation averaging results indicated that the optical properties of the soot-containing mixtures may be influenced by the soot aggregate quantities. In these simulations, the absorption Ångström exponent (AÅE) values ranged from 0.9 to 1.1, which agree with the observations. The relative deviations of scattering Ångström exponent (SÅE) values between different numbers of soot aggregates attached to the surface of a sulfate host were upwards of ~11%. The results showed that the greater number of attached soot aggregates may lead to smaller SÅE values in the soot-containing mixtures. For most cases of simulated mixtures, a more compact morphology of soot aggregates, larger soot monomer radii and smaller soot volume fractions (F_s_o_o_t) may also generate smaller SÅE values. Moreover, in the visible range, the simulated scattering cross sections of soot-containing mixtures with two, three, four, and six soot aggregates (D_f=1.8) were ~5%, ~10%, ~15%, and ~30% larger than those with only one soot aggregate, respectively, on the condition that volumes of soot and sulfate are comparable. However, these relative deviations between different numbers of attached soot aggregates on the scattering cross sections of mixtures may be weakened for larger volume of non-absorbing sulfate particle (<5% for the cases of F_s_o_o_t=1/10, sulfate volume is 9 times of soot). - Highlights: • Optical properties of sulfate mixing varying numbers of soot aggregates are simulated. • Absorption Ångström exponents ranged from 0.9 to 1.1, which agree with observations. • Effect of soot number on scattering is raised (5–30%) for more soot volume fractions. • More attached soot aggregates may bring stronger effect on scattering of the mixture.

[en] Light absorption enhancement of aged soot aerosols is highly sensitive to the morphologies and mixing states of soot aggregates and their non-absorbing coatings, such as organic materials. The quantification of these effects on the optical properties of thinly coated soot aerosols is simulated using an effective model with fixed volume fractions. Fractal aggregated soot was simulated using the diffusion limited aggregation (DLA) algorithm and discretized into soot dipoles. The dipoles of non-absorbing aerosols, whose number was fixed by the volume fraction, were further generated from the neighboring random edge dipoles. Their optical properties were calculated using the discrete dipole approximation (DDA) method and were compared with other commonly used models. The optical properties of thinly coated soot calculated using the fixed volume fraction model are close to (less than ~10% difference) the results of the fixed coating thickness model, except their asymmetry parameters (up to ~25% difference). In the optical simulations of thinly coated soot aerosols, this relative difference of asymmetry parameters and phase functions between these realistic models may be notable. The realizations of the fixed volume fraction model may introduce smaller variation of optical results than those of the fixed coating thickness model. Moreover, the core-shell monomers model and homogeneous aggregated spheres model with the Maxwell-Garnett (MG) theory may underestimate (up to ~20%) the cross sections of thinly coated soot aggregates. The single core-shell sphere model may largely overestimate (up to ~150%) the cross sections and single scattering albedo of thinly coated soot aggregates, and it underestimated (up to ~60%) their asymmetry parameters. It is suggested that the widely used single core-shell sphere approximation may not be suitable for the single scattering calculations of thinly coated soot aerosols. - Highlights: • Fixed volume fraction model is applied for optical simulations of thinly coated soot. • Optical diversity between different models may be caused by morphological differences. • This model simulated similar (<10%) cross sections from fixed coating thickness model. • Differences of asymmetry parameters between these models may be obvious (up to ~25%). • Three other widely used models were investigated with large diversities (up to ~150%).

[en] Anthropogenic soot aerosols are shown as complex, fractal-like aggregated structures with high light absorption efficiency. In atmospheric environment, soot monomers may tend to acquire a weakly absorbing coating, such as an organic coating, which introduces further complexity to the optical properties of the aggregates. The single scattering properties of soot aggregates can be significantly influenced by the coated status of these kinds of aerosols. In this article, the monomers of fractal soot aggregates are modelled as semi-external mixtures (physical contact) with constant radius of soot core and variable sizes of the coating for specific soot volume fractions. The single scattering properties of these coated soot particles, such as phase function, the cross sections of extinction and absorption, single scattering albedo (SSA) and asymmetry parameter (ASY), are calculated using the numerically exact superposition T-matrix method. The random-orientation averaging results have shown that the single scattering properties of these coated soot aggregates are significantly different from the single volume-equivalent core–shell sphere approximation using the Mie theory and the homogeneous aggregates with uncoated monomers using the effective medium theory, such as Maxwell-Garnett and Bruggemann approximations, which overestimate backscattering of coated soot. It is found that the SSA and cross sections of extinction and absorption are increased for soot aggregates with thicker weakly absorbing coating on the monomers. Especially, the SSA values of these simulated aggregates with less soot core volume fractions are remarkably (∼50% for core volume fraction of soot aggregates of 0.5, ∼100% for a core volume fraction of 0.2, at 0.67 μm) larger than for uncoated soot particles without consideration of coating. Moreover, the cross sections of extinction and absorption are underestimated by the computation of equivalent homogeneous fractal aggregate approximation (within 5% for the T-matrix method and 10–25% for the Rayleigh–Debye–Gans approximation due to different soot volume fractions). Further understanding of the optical properties of these coated soot aggregates would be helpful for both environment monitoring and climate studies. -- Highlights: • Optical properties of soot aggregates with coated monomers are calculated. • Homogeneous aggregated approximations overestimate backscattering of coated soot. • A relationship between absorption cross section and core volume fraction is built

[en] Highlights: • Metal analysis of tea powder samples with laser-induced breakdown spectroscopy. • Using oxide nanoparticles owing to excellent adsorption capability of metal ions. • Applying laser sintering induced adsorption of metal ions on oxide nanoparticles. • Enhancing LIBS signals of Mg, Ca, Na, and K in tea samples 3–7 times. • Facilitating quantitative metal analyses owing to matrix effect reducing. -- Abstract: Oxide nanoparticles have shown excellent absorption capability of metal elements in fluid environment. Here for elemental analysis of solid powder samples (tea as an example) with laser-induced breakdown spectroscopy (LIBS), we successfully applied laser sintering induced adsorption of metal elements on oxide nanoparticles. Doped with Al₂O₃ or SiO₂ nanoparticles followed by using continuous-wave laser to sinter the mixture pellets, the LIBS signals for the tea mixture pellets increased to 3–7 times for most metal elements, such as Mg, Ca, Na, and K, compared with those analyzed directly without laser sintering. Furtherly the enhancement was due to the laser sintered adsorption of metal elements on oxide nanoparticles. Owing to a high percentage addition of Al₂O₃ nanoparticles in the tea pellets, the matrix effect from different kinds of tea was also reduced efficiently, which facilitate quantitative analyses for different tea samples. This method may find promising applications in elemental analysis of various solid samples.

[en] Objective: To explore the aberrance of brain activity in healthy young adults with apolipoprotein E (APOE) ε2, ε3 and ε4 allele by the method of functional connectivity density (FCD). Method: Two hundred and thirteen young healthy adults underwent the 3 T resting-state functional MRI, the neuropsychological tests and genotype testing for ε2, ε3 and ε4 allele. Age-and gender-matched individuals, including 14 subjects with APOE ε2, 31 subjects with APOE ε4 and 31 subjects with APOE ε3 were enrolled for final analysis. FCD mapping was used to compare the brain functional connective networks among the three groups. All results were corrected with a Gaussian random field (GRF) (voxel-level of P < 0.01 and joint cluster-level of P < 0.05). Correlation analysis was performed between abnormal short-and long-range FCD values and neuropsychological scores. Results: Compared with APOE ε3 carriers, the short-and long-range FCD values of APOE ε2 carriers were increased in the frontal lobe, particularly in the medial prefrontal lobe (37 voxels, t = 3.54), anterior cingulate cortex (36 voxels, t = 3.19) and orbital frontal cortex (41 voxels, t = 3.72), while APOE ε4 carriers showed decreased short-range FCD in the bilateral cuneus (38 voxels, t = -3.68). Moreover, the long-range FCD values of APOE ε4 group were decreased in right orbital frontal cortex (46 voxels, t = -4.56) and increased in the right inferior parietal lobe (31 voxels, t = 3.49) compared with that of APOE ε2 group. No significant correlation was detected with Bonferroni correction. Conclusions: The opposite FCD alteration in cuneus and inferior parietal lobe might be the early existence of inhibitory and compensatory mechanism modulated by the ε4 allele in the young age, while the increased FCD in frontal lobe might be the underlying protective mechanism of delaying the onset of Alzheimer's disease. (authors)

[en] This is the first part of a series of white papers that represent Expert Consensus Documents on dual-energy computed tomography developed by the Society of Computed Body Tomography and Magnetic Resonance at the end of 2016. This article mainly introduces the first and the second parts: part 1 describes the fundamentals of the physical basis and the technology of dual-energy computed tomography, and part 2 addresses radiation dose and iodine sensitivity in dual-energy computed tomography. (authors)

[en] Laser-induced plasma from an aluminum target in one-atmosphere argon background has been investigated with ablation using nanosecond ultraviolet (UV: 355 nm) or infrared (IR: 1064 nm) laser pulses. Time- and space-resolved emission spectroscopy was used as a diagnostics tool to have access to the plasma parameters during its propagation into the background, such as optical emission intensity, electron density, and temperature. The specific feature of nanosecond laser ablation is that the pulse duration is significantly longer than the initiation time of the plasma. Laser-supported absorption wave due to post-ablation absorption of the laser radiation by the vapor plume and the shocked background gas plays a dominant role in the propagation and subsequently the behavior of the plasma. We demonstrate that the difference in absorption rate between UV and IR radiations leads to different propagation behaviors of the plasma produced with these radiations. The consequence is that higher electron density and temperature are observed for UV ablation. While for IR ablation, the plasma is found with lower electron density and temperature in a larger and more homogenous axial profile. The difference is also that for UV ablation, the background gas is principally evacuated by the expansion of the vapor plume as predicted by the standard piston model. While for IR ablation, the background gas is effectively mixed to the ejected vapor at least hundreds of nanoseconds after the initiation of the plasma. Our observations suggest a description by laser-supported combustion wave for the propagation of the plasma produced by UV laser, while that by laser-supported detonation wave for the propagation of the plasma produced by IR laser. Finally, practical consequences of specific expansion behavior for UV or IR ablation are discussed in terms of analytical performance promised by corresponding plasmas for application with laser-induced breakdown spectroscopy.

[en] Highlights: • BC absorption during aging is quantified with diverse primary particle sizes (a). • Uncertain a leads to ~10%−20% relative variation on BC absorption enhancement (E_abs). • Importance of a to BC e_abs is between volume-equivalent size and fractal dimension. • Optical relative variation of BC with 50–70 nm volume-equivalent radii are up to ~50%. Light absorption enhancement (E_abs) of black carbon (BC) aerosol following atmospheric aging is one of the most challenging issues in the assessment of aerosol radiative forcing. BC E_abs is constrained by complex particle morphologies; however, large uncertainties continue to occur due to certain morphological parameters, including primary particle size. The values of E_abs during BC aging is quantified with diverse primary particle sizes using the superposition T-matrix method (STM). The results show that the uncertainty of absorption enhancement due to the primary particle size of fully aged BC particles ranges from ~10% to 20%, while the uncertainties arising from varied BC volume-equivalent size and fractal dimension are ~20–30% and ~8–12%, respectively. The optical properties of BC particles with volume-equivalent radii ranging from 50 to 70 nm were largely influenced (up to ~50%) by inappropriate assumptions regarding primary particle size. The specific assumptions of primary particle size in optical modeling plays an important role in constraining BC E_abs.

[en] Atmospheric black carbon (BC) is the most important aerosol contributor to global warming. However, there is a lack of understanding about the climate impact of BC aerosols because of systematic discrepancies between model and observation estimates of light absorption enhancements (Eabs) in atmospheric processes after emissions, and such discrepancies are transferred directly into large uncertainties of aerosol radiative forcing assessments. In this study, we quantify Eabs of atmospheric BC aerosols with diverse particle morphology distributions using a multi-dimensional aerosol model. We show that current widely used Mie method may overestimate BC Eabs by ∼50% because variations in particle morphology are not considered. Although absorption calculation can be improved by including complex particle morphology and heterogeneity in composition, we find that neglect of the diverse particle morphology distributions in modeling may lead to 15% ∼ 30% relative deviations on Eabs estimations of BC aerosol ensembles. The results thus imply that particle morphology distribution should be included in models to accurately represent the radiative effects of BC aerosols. (letter)

[en] Measurements with laser-induced breakdown spectroscopy (LIBS) usually take place in the atmospheric air. For quantitative analysis of metallic elements, oxidation may represent an important issue which can significantly modify the stoichiometry of the plasma. Molecule formation in plasma should be therefore studied and taken into account in the LIBS practice. In this work, we experimentally investigated the temporal evolution and transformation of the plasma induced on an aluminum target by a nanosecond infrared (1064 nm) laser in the atmospheric air, in terms of its temperatures over a large interval of time from hundreds of nanoseconds to tens of microseconds. Such evolution was then correlated to the temporal evolution of the emission intensity from AlO molecules in the ablation plume. In particular, for a given ablation laser pulse energy, the appearance of the molecular emission while the plume cools down allows determining a minimal delay, τ_m_i_n, which corresponds to a maximal value of the temperature, T_m_a_x, below which the molecular emission begins to be clearly observed and to grow as a function of the delay. Such delay or such temperature indicates the longest delay or the lowest temperature for laser-induced plasma to be suitable for a correct analysis of metallic elements without significant influence of the alternation of the stoichiometry by oxidation. In our experiment, the values of τ_m_i_n and T_m_a_x have been determined for a range of ablation laser pulse energies from 5 mJ to 50 mJ. These values lie respectively in the range of 3 to 15 μs for τ_m_i_n, and 4500 K to 6600 K in terms of the molecule temperature for T_m_a_x. Beyond the practical interest for LIBS, our results provide also insights to the kinetics of the AlO molecule formation in laser-induced plasma. - Highlights: • Determination of the temperatures in laser-induced plasma up to tens of microseconds • Determination of the molecule temperature by fitting the emission spectrum • The delay and the temperature for the emission from AlO molecules in plasma