Marine plastic pollution poses a potential threat to the ecosystem, but the sources and their magnitudes remain largely unclear. Existing bottom-up emission inventories vary among studies for two to three orders of magnitudes (OMs). Here, we adopt a top-down approach that uses observed dataset of sea surface plastic concentrations and an ensemble of ocean transport models to reduce the uncertainty of global plastic discharge. The optimal estimation of plastic emissions in this study varies about 1.5 OMs: 0.70 (0.13–3.8 as a 95% confidence interval) million metric tons yr−1 at the present day. We find that the variability of surface plastic abundance caused by different emission inventories is higher than that caused by model parameters. We suggest that more accurate emission inventories, more data for the abundance in the seawater and other compartments, and more accurate model parameters are required to further reduce the uncertainty of our estimate.
Employing density functional theory calculations, mechanical and electronic properties of stable penta-B2N4 and penta-B3N3 monolayers are investigated. The different mechanical parameters obtained along different tensile directions suggest both the penta-B2N4 and penta-B3N3 demonstrate mechanical anisotropy. Moreover, due to the lower space group symmetry of penta-B3N3, its anisotropy is more prominent than that of the penta-B2N4. It was found that both the penta-B2N4 and penta-B3N3 are fast to fracture along the direction R1 due to the small fracture strain, but hard to be stretched because of the large Young's modulus. Generally, penta-B2N4 shows better mechanical properties than those of penta-B3N3 in terms of Young's modulus and intrinsic strength. Besides, under the tensile strain, penta-B2N4 keeps its metallicity, but the band gap of penta-B3N3 can be effectively tailored, even inducing a transition from the direct to indirect band gap or transition from the semiconductor to metal. Further analysis of partial charge densities indicates breaking of B–N bonds is the main cause for the band gap enlargement, and similarly, formation of B–N bonds is the reason for the semiconductor-to-metal transition of penta-B3N3.
<p>Soil is one of the largest reservoir of mercury in the environment. Globally, most of the mercury in the soil is stored in permafrost, such as the Arctic and the Tibetan Plateau. Mercury in the soil is mainly derived from atmospheric deposition and tightly bound to the organic carbon. The mercury level in the permafrost over the Tibetan Plateau and its influencing factors have been less studied. This study analyzes soil total mercury (STHg) concentrations and its vertical distribution in meadow soil samples collected from the Tibetan Plateau. We adopt a nested-grid high-resolution GEOS-Chem model to simulate atmospheric mercury deposition. The relationship between STHg and soil organic carbon(OCD) as well as atmospheric deposition are explored. We also extend our analysis to data in the Tibetan Plateau and other regions of China in the literature. Our results show that the STHg concentrations in the Tibetan Plateau are 19.9&#177;12.4 ng/g. The concentrations are higher in the south/east and lower in the north/west in the Tibetan Plateau, consistent with the previous results. Our model shows that the average deposition flux of Hg is 3.3 ug m<sup>-2</sup> yr<sup>-1</sup> with 57% contributed by dry deposition of Hg<sup>0</sup>, followed by dry deposition of Hg<sup>II</sup> and Hg<sup>P </sup>(19%) and wet deposition (24%). We calculate the Hg to carbon ratio (R<sub>HgC</sub>) of 5.52 &#177; 5.11 &#956;g Hg/g C and the estimated STHg is 67.45 Gg in alpine grasslands in the Tibetan Plateau, contributing about 2.7% globally. We find a positive correlation between OCD and STHg in the Tibetan Plateau(Log(STHg) = 0.35log(OCD) + 0.99, R<sup>2</sup> = 0.24) and a weak relationship between model residual (defined as the difference between model fitting values and observations) and atmospheric total Hg deposition. We conclude that soil organic carbon(SOC) and atmospheric deposition work simultaneously for STHg. Atmospheric deposition determines the potential levels of STHg in large spatial scales, while SOC and its characteristics modulate STHg locally by influencing the fate and transport of Hg.</p>
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