Most pollutants in the Earth's atmosphere are removed by oxidation with highly reactive hydroxyl radicals. Field measurements have revealed much higher concentrations of hydroxyl radicals than expected in regions with high loads of the biogenic volatile organic compound isoprene(1-8). Different isoprene degradation mechanisms have been proposed to explain the high levels of hydroxyl radicals observed(5,9-11). Whether one or more of these mechanisms actually operates in the natural environment, and the potential impact on climate and air quality, has remained uncertain(12-14). Here, we present a complete set of measurements of hydroxyl and peroxy radicals collected during isoprene-oxidation experiments carried out in an atmospheric simulation chamber, under controlled atmospheric conditions. We detected significantly higher concentrations of hydroxyl radicals than expected based on model calculations, providing direct evidence for a strong hydroxyl radical enhancement due to the additional recycling of radicals in the presence of isoprene. Specifically, our findings are consistent with the unimolecular reactions of isoprene-derived peroxy radicals postulated by quantum chemical calculations(9-11). Our experiments suggest that more than half of the hydroxyl radicals consumed in isoprene-rich regions, such as forests, are recycled by these unimolecular reactions with isoprene. Although such recycling is not sufficient to explain the high concentrations of hydroxyl radicals observed in the field, we conclude that it contributes significantly to the oxidizing capacity of the atmosphere in isoprene-rich regions.
Human exposure to pollutants from e-waste is an important scientific issue for their health effects. In this study, organohalogen pollutants in human serum sample from an e-waste dismantling site (n = 35) and a control site (n = 21), both located in Tianjin, Northern China, were analyzed using GC-ECNI-MS. Geometric mean concentrations of tetra- through hexa-BDEs, hepta- through nona-BDEs, PCBs, PBB-153, and DP in the exposure group were 2.77, 12.2, 44.1, 0.52, and 7.64 ng g(-1) lipid, respectively, which ranged from 1.5 to 7.4-fold higher than those in the control group through multivariate regression analysis, indicating that working and/or living in the e-waste site was associated with elevated body concentrations of these pollutants. Pollutants with low vapor pressures (i.e., hepta- through nona-BDEs and DP) were at significantly higher levels for e-waste dismantling workers than for local residents living around the e-waste site, suggesting higher exposure to these pollutants might exist for the occupational workers. (C) 2012 Elsevier Ltd. All rights reserved.
A self-terminating gate recess etching technique is first proposed to fabricate normally off AlGaN/GaN MOSFET. The gate recess process includes a thermal oxidation of the AlGaN barrier layer for 40 min at 615 degrees C followed by 45-min etching in potassium hydroxide solution at 70 degrees C, which is found to be self-terminated at the AlGaN/GaN interface with negligible effect on the underlying GaN layer, manifesting itself easy to control, highly repeatable, and promising for industrialization. The fabricated device based on this technique with atomic layer deposition Al2O3 as gate insulator exhibits a threshold voltage as high as 3.2 V with a maximum drain current over 200 mA/mm and a 60% increased breakdown voltage than that of the conventional high electron mobility transistors.
It is in urgent need to develop fast and efficient transcoding methods so as to remarkably save the storage of surveillance videos and synchronously transmit conference videos over different bandwidths. Towards this end, the special characteristics of these videos, e. g., the relatively static background, should be utilized for transcoding. Therefore, we propose a fast and efficient transcoding method (FET) based on background modeling and block classification in this paper. To improve the transcoding efficiency, FET adds the background picture, which is modeled from the originally decoded frames in low complexity, into stream in the form of an intra-coded G-picture. And then, FET utilizes the reconstructed G-picture as the long-term reference frame to transcode the following frames. This is mainly because our theoretical analyses show that G-picture can significantly improve the transcoding performance. To reduce the complexity, FET utilizes an adaptive threshold updating model for block classification and then adopts different transcoding strategies for different categories. This is due to the following statistics: after dividing blocks into categories of foreground, background and hybrid ones, different block categories have different distributions of prediction modes, motion vectors and reference frames. Extensive experiments on transcoding high-bit-rate H. 264/AVC streams to low-bit-rate ones are carried out to evaluate our FET. Over the traditional full-decoding-and-full-encoding methods, FET can save more than 35% of the transcoding bit-rate with a speed-up ratio of larger than 10 on the surveillance videos. On the conference videos which should be transcoded more timely, FET achieves more than 20 times speed- up ratio with 0.2 dB gain.
Present and emerging biotechnological applications for iron (oxyhydr)oxide nanomaterials depend on their interaction with microorganisms, as do their toxicity, transport, and fate in biological and environmental systems. However, mass or electron transfer along key molecular pathways at microbe–nanomaterial interfaces is extremely difficult to quantify because of system complexity. Inspired by Fe(II)-oxidizing microbes widespread in nature, we isolate and characterize one such pathway by examining the oxidation of Fe3–xTixO4 (magnetite-titanomagnetite) nanoparticles by the bacterial electron transfer enzyme MtoA, a decaheme c-type cytochrome. Oxidation by MtoA was studied as a function of the thermodynamic driving force for electron transfer by controlling the Ti(IV) doping content (x), which tunes the solid-state Fe(II)/Fe(III) ratio built into the nanoparticles. A higher Fe(II)/Fe(III) ratio appears to systematically increase the electron transfer kinetics to the cytochrome. In situ X-ray diffraction indicated that, during oxidation, the spinel ferrite lattice remains intact while structural Fe(II) is progressively depleted. Surface and atomic site specific Fe L2,3-edge X-ray magnetic circular dichroism indicated that MtoA directly accesses magnetically ordered B-sublattice Fe(II) at the interface. This study provides the first quantitative insights into an isolated molecular pathway for biotransformation of iron (oxyhydr)oxide nanomaterials, and more generally, it also illustrates new techniques for probing these pathways in detail, featuring use of tailored nanoparticles, purified metalloenzyme, and synchrotron X-ray absorption spectroscopies.
