Spatio-Temporal Characteristics and Variation Pattern of the Atmospheric Particulate Matter Concentration: A Case Study of the Beijing–Tianjin–Hebei Region, China

Based on measurement data from air quality monitoring stations, the spatio-temporal characteristics of the concentrations of particles with aerodynamic equivalent diameters smaller than 2.5 and 10 μm (PM2.5 and PM10, respectively) in the Beijing–Tianjin–Hebei (BTH) region from 2015 to 2018 were analysed at yearly, seasonal, monthly, daily and hourly scales. The results indicated that (1) from 2015 to 2018, the annual average values of PM2.5 and PM10 concentrations and the PM2.5/PM10 ratio in the study area decreased each year; (2) the particulate matter (PM) concentration in winter was significantly higher than that in summer, and the PM2.5/PM10 ratio was highest in winter and lowest in spring; (3) the PM2.5 and PM10 concentrations exhibited a pattern of double peaks and valleys throughout the day, reaching peak values at night and in the morning and valleys in the morning and afternoon; and (4) with the use of an improved sine function to simulate the change trend of the monthly mean PM concentration, the fitting R2 values for PM2.5 and PM10 in the whole study area were 0.74 and 0.58, respectively. Moreover, the high-value duration was shorter, the low-value duration was longer, and the concentration decrease rate was slower than the increase rate.

Quantum-electrodynamic magnetic reconnection as the origin of the FRB 200428-associated X-ray burst from SGR J1935+2154

Magnetars, often under the name soft gamma-ray repeaters (SGRs) or anomalous X-ray pulsars, are highly magnetized neutron stars that exhibit diverse X-ray activities. Recently, a unique non-thermal X-ray burst with cut-off energy up to 84 keV is detected and thought to be associated with the fast radio burst (FRB) 200428 in the same single explosive event from SGR J1935+2154, as their spectra show similar feature of narrow double peaks that are emitted almost simultaneously. However, the physical origin of this FRB 200428-associated X-ray burst is still unknown yet. Here, with the first cross-scale numerical simulation in which modeling of particle acceleration by magnetic reconnections is self-consistently coupled with that of photon emission by multiple Compton scatterings, we identify that magnetic reconnection at the quantum-electrodynamic field strength inside the magnetar magnetosphere is the much likely driving source of such FRB-associated non-thermal X-ray burst. Both its temporal and spectral features are well reproduced in our simulations by assuming the plasma magnetization parameter σ~102-103 in consistency with the astronomical observations. The results could greatly promote our understandings of various X-ray burst events from magnetars.

New insights in the mechanisms of the reaction 3.65 Å phase = clinoenstatite + water down to nanoscales

The reaction of 3.65 Å phase <=> clinoenstatite + water was investigated in five experiments at 10 GPa, 470–600 ∘C, using a rotating multi-anvil press. Under these P/T conditions, clinoenstatite exists in its high-pressure modification, which, however, is not quenchable to ambient conditions but transforms back to low-pressure clinoenstatite. The quenched run products were characterized by electron microprobe analyses (EMPA), powder X-ray diffraction (XRD), Raman spectroscopy and by high-resolution transmission electron microscopy (HRTEM) on focused ion beam (FIB)-cut foils. We bracketed the reaction in the T range 470 to 510 ∘C (at 10 GPa). The hydration of clinoenstatite to the 3.65 Å phase at 470 ∘C was very sluggish and incomplete even after 96 h. Clinoenstatites range in size from less than 1 to up to 50 µm. Usually clinoenstatite has a very small grain size and shows many cracks. In sub-micron-sized broken clinoenstatite, an amorphous phase (0.91Mg:1.04Si, with about 20 wt % H2O) was observed, which further transformed with increasing reaction time into the 3.65 Å phase (1Mg:1Si, with 34 wt % H2O). Thus, the sub-micron-sized fractured clinoenstatite transformed via an amorphous water-bearing precursor phase to the 3.65 Å phase. The dehydration to clinoenstatite was faster but still incomplete after 72 h at 600 ∘C. From the backscattered electron images of the recovered sample of the dehydration experiment, it is obvious that there is a high porosity due to dehydration of the 3.65 Å phase. Again, the grain size of clinoenstatite ranges from less than 1 up to 50 µm. There are still some clinoenstatite crystals from the starting material present, which can clearly be distinguished from newly formed sub-micron-sized clinoenstatite. Additionally, we observe a water-rich crystalline phase, which does not represent the 3.65 Å phase. Its Raman spectra show the double peaks around 700 and 1000 cm−1 characteristic for enstatite and strong water bands at 3700 and 3680 cm−1. The Mg:Si ratio of 0.90:1.04 was determined by EMPA, totalling to 81 wt %, in accordance with its high water content. Diffraction patterns from high-resolution images (fast Fourier transform – FFT) are in agreement with an orthoenstatite crystal structure (Pbca). The surprising observation of this study is that, in both directions of the investigated simple reaction, additional metastable phases occur which are amorphous in the hydration and crystalline in the dehydration reaction. Both additional phases are water rich and slightly deviate in composition from the stable products 3.65 Å phase and clinoenstatite, respectively. Thus, as a general remark, conventional investigations on reaction progress should be complemented by nanoscale investigations of the experimental products because these might reveal unpredicted findings relevant for the understanding of mantle processes. The extreme reduction in grain size observed in the dehydration experiments due to the formation of nanocrystalline clinoenstatite rather than the slowly released fluids might cause mechanical instabilities in the Earth's mantle and, finally, induce earthquakes.

Investigate the Double Peaks in Main Emission of UVB LEDs

In this study we suppressed the parasitic emission caused by electron overflow found in typical UVB light-emitting diodes (LEDs). Furthermore, modulation of the p-layer structure and doping profile allowed us to decrease the relaxation time of the holes to reach conditions of quasi-charge neutrality in the UVB quantum well. Our UVB LED (sample A) exhibited a clear exciton emission, with its peak near 306 nm and a band-to-band emission at 303 nm. The relative intensity of the exciton emission of sample A decreased as a result of a thermal energy effect. At temperatures of up to 363 K, sample A displayed the exciton emission. Our corresponding UVC LED (sample B) exhibited only a Gaussian peak emission at a wavelength of approximately 272 nm.

Magneto-Optical Transport Properties of Type-II Nodal Line Semimetals

We investigate the magneto-optical transport properties and Landau levels of type-II nodal line semimetals. The tilted liner dispersion in type-II nodal line semimetals makes the conduction band and valence band asymmetric, and Landau levels are coupling in the presence of a magnetic field. We find the background of absorption peaks is curved. The oscillation peaks are tailless with the change of magnetic field. Through tuning tilt term, we find the absorption peaks of optical conductivity change from incomplete degenerate structure to splitting double peaks structure. We also find interband absorption peaks is no longer zero in the imaginary part of Hall conductivity. With the change of the tilt term, the contribution of the absorption peak has two forms, one is that the negative peak only appears at high frequencies, and the other is two adjacent peaks with opposite signs. In addition, the resistivity, circularly polarized light and magnetic oscillation of Hall conductivity are studied.

Double peaks of gravitational wave spectrum induced from inflection point inflation

We investigate the possibility of inducing the gravitational waves (GWs) with double peak energy spectrum from primordial scalar perturbations in inflationary models with three inflection points. Here the inflection points can be generated from a polynomial potential or generated from a Higgs-like $$\phi ^4$$ ϕ 4 potential with the running of quartic coupling. In such models, the inflection point at large scales predicts the scalar spectral index and tensor-to-scalar ratio to be consistent with current CMB constraints, and the other two inflection points generate two large peaks in the scalar power spectrum at small scales, which can induce GWs with a double peak energy spectrum. We find that for some choices of the parameters the double peak spectrum can be detected by future GW detectors, and one of the peaks around $$f\simeq 10^{-9}{-}10^{-8}$$ f ≃ 10 - 9 - 10 - 8 Hz can also explain the recent NANOGrav signal. Moreover, the peaks of the power spectrum allow for the generation of primordial black holes, which accounts for a significant fraction of dark matter.

Non-Symmetrical Collapse of an Empty Cylindrical Cavity Studied with Smoothed Particle Hydrodynamics

The non-symmetrical collapse of an empty cylindrical cavity is modeled using Smoothed Particle Hydrodynamics. The presence of a nearby surface produces an anisotropic pressure field generating a high-velocity jet that hits the surface. The collapse follows a different dynamic based on the initial distance between the center of the cavity and the surface. When the distance is greater than the cavity radius (detached cavity) the surface is hit by traveling shock waves. When the distance is less than the cavity radius (attached cavity) the surface is directly hit by the jet and later by other shock waves generated in the last stages of the of the collapse. The results show that the surface is hit by a stronger shock when distance between the center of the cavity and the surface is zero while showing more complex double peaks behavior for other distances.