科研成果

Electron sources driven by femtosecond laser have important applications in many aspects, and the research about the intrinsic emittance is becoming more and more crucial. The intrinsic emittance of polycrystalline copper cathode, which was illuminated by femtosecond pulses (FWHM of the pulse duration was about 100 fs) with photon energies above and below the work function, was measured with an extremely low bunch charge (single-electron pulses) based on free expansion method. A minimum emittance was obtained at the photon energy very close to the effective work function of the cathode. When the photon energy decreased below the effective work function, emittance increased rather than decreased or flattened out to a constant. By investigating the dependence of photocurrent density on the incident laser intensity, we found the emission excited by pulsed photons with sub-work-function energies contained two-photon photoemission. In addition, the portion of two-photon photoemission current increased with the reduction of photon energy. We attributed the increase of emittance to the effect of two-photon photoemission. This work shows that conventional method of reducing the photon energy of excited light source to approach the room temperature limit of the intrinsic emittance may be infeasible for femtosecond laser. There would be an optimized photon energy value near the work function to obtain the lowest emittance for pulsed laser pumped photocathode. (C) 2018 Author(s).

Abstract For a long time, there has been an excessive use of synthetic fertilizers applied to the decreasing area of arable land to help meet the increasing food demand, which causes NH3 volatilization and land degradation. In this study, we conducted a nationwide inventory of NH3 emissions from synthetic nitrogen fertilizers in China from 1991 to 2013. We estimated that NH3 emissions increased from 3.20 to 5.21 Tg NH3 yr−1. Because of the different agricultural practices, fertilizer use schedules, and ambient temperature, monthly NH3 emissions have varied greatly. NH3 emissions during the spring and summer accounted for approximately 83% of the national total in 1991, 1998, 2005, and 2013. Similarly, the spatial distribution of NH3 emissions exhibited large heterogeneity in 1991, 1998, 2005, and 2013. High emissions occurred in the eastern and central provinces and eastern Sichuan. Based on NH3 emissions in Chinese counties for 1991–1998, 1999–2005, and 2006–2013, the Pearson correlation coefficient was applied to compute the changing trends in NH3 emissions and fertilization rates. The results showed that the NH3 emissions from the major grain-producing regions increased, whereas those from the eastern provinces, which experienced rapid economic development, decreased. In addition, fertilizer amount, arable land area, grain yield, and primary industry have been shown to be largely correlated to NH3 emissions based on principal component analysis. Therefore, the results of this study have significant implications for improving the efficient use of fertilizers and preventing soil and/or land degradation.

Epidemiological and toxicological studies suggest that exposure to ambient fine particles (PM2.5) can reduce human reproductive capacity. We previously reported, based on spatial epidemiology, that higher levels of PM2.5 exposure were associated with a lower fertility rate (FR) in China. However, that study was limited by a lack of temporal variation. Using first-difference regression, we linked temporal changes in FR and PM2.5 with adjustment for ecological covariates across 2806 counties in China during 2000-2010. Next, we performed a sensitivity analysis of the variation in the PM2.5 -FR association according to (1) geographic region, (2) indicators of the level of development, and (3) PM2.5 concentrations. Also, we quantified the reduction in the FR attributable to ambient PM2.5 in China for the first time. The FR decreased by 3.3% (1.2%, 5.3%) for each 10 mu g/m(3) increment in PM2.5 . The association varied significantly among the geographic regions, but not with the level of development. Nonlinearity analysis suggested a linear exposure-response function with an effect threshold of -8 mu g/m(3). We also found that comparing to the 2000 scenario, increment of PM2.5 in 2010 might result in a reduction of 2.50 (2.44, 2.60) infants per 1000 women aged 15-44 years per year in China. Our results confirm the statistical association between ambient particles and FR and suggest that poor air quality may contribute to childlessness in China. (C) 2018 Elsevier B.V. All rights reserved.

© The Author(s) 2018. Lacking of floor plans is a fundamental obstacle to ubiquitous indoor location-based services. Recent work have made significant progress to accuracy, but they largely rely on slow crowdsensing that may take weeks or even months to collect enough data. In this chapter, we propose Knitter that can generate accurate floor maps by a single random user's one-hour data collection efforts, and demonstrate how such maps can be used for indoor navigation. Knitter extracts high-quality floor layout information from single images, calibrates user trajectories, and filters outliers. It uses a multi-hypothesis map fusion framework that updates landmark positions/orientations and accessible areas incrementally according to evidences from each measurement. Our experiments on three different large buildings and 30+ users show that Knitter produces correct map topology, and 90-percentile landmark location and orientation errors of 3∼5m and 4 ∼ 6 ∘ , comparable to the state of the art at more than 20 × speed up: data collection can finish in about one hour even by a novice user trained just a few minutes.

OBJECTIVES: We assessed relationships between indoor black carbon (BC) exposure and urinary oxidative stress biomarkers, 8-hydroxy-2'-deoxyguanosine (8-OHdG) and malondialdehyde (MDA), in participants with chronic obstructive pulmonary disease (COPD). METHODS: Eighty-two participants completed in-home air sampling for one week prior to providing urine samples up to four times in a year. Weekly indoor and daily outdoor concentrations were used to estimate indoor daily lags and moving averages. There were no reported in-home BC sources, thus indoor levels closely represented outdoor BC infiltration. Mixed effects regression models with a random intercept for each participant were used to assess relationships between indoor BC and 8-OHdG and MDA, adjusting for age, race, BMI, diabetes, heart disease, season, time of urine collection, urine creatinine, and outdoor humidity and temperature. RESULTS: There were positive effects of BC on 8-OHdG and MDA, with the greatest effect the day before urine collection (6.9% increase; 95% CI 0.9-13.3%, per interquartile range: 0.22mug/m(3)) for 8-OHdG and 1 to 4days before collection (8.3% increase; 95% CI 0.03-17.3% per IQR) for MDA. Results were similar in models adjusting for PM2.5 not associated with BC and NO2 (10.4% increase, 95% CI: 3.5-17.9 for 8-OHdG; 8.1% increase, 95% CI: -1.1-18.1 for MDA). Effects on 8-OHdG were greater in obese participants. CONCLUSIONS: We found positive associations between BC exposure and 8-OHdG and MDA, in which associations with 8-OHdG were stronger in obese participants. These results suggest that exposure to low levels of traffic-related pollution results in lipid peroxidation and oxidative DNA damage in individuals with COPD.

© The Author(s) 2018. Mainstream indoor localization technologies rely on RF signatures that require extensive human efforts to measure and periodically recalibrate signatures. The progress to ubiquitous localization remains slow. In this chapter, we explore Sextant, an alternative approach that leverages environmental reference objects such as store logos. A user uses a smartphone to obtain relative position measurements to such static reference objects for the system to triangulate the user location. Sextant leverages image matching algorithms to automatically identify the chosen reference objects by photo-taking, and we propose two methods to systematically address image matching mistakes that cause large localization errors. We formulate the benchmark image selection problem, prove its NP-completeness, and propose a heuristic algorithm to solve it. We also propose a couple of geographical constraints to further infer unknown reference objects. To enable fast deployment, we propose a lightweight site survey method for service providers to quickly estimate the coordinates of reference objects. Extensive experiments have shown that Sextant prototype achieves 2–5 m accuracy at 80-percentile, comparable to the industry state of the art, while covering a 150 × 75 m mall and 300 × 200 m train station requires a one-time investment of only 2–3 man-hours from service providers.

© The Author(s) 2018. The lack of indoor maps is a critical reason behind the current sporadic availability of indoor localization service. Service providers have to go through effort-intensive and time-consuming business negotiations with building operators, or hire dedicated personnel to gather such data. In this chapter, we propose Jigsaw, a floor plan reconstruction system that leverages crowdsensed data from mobile users. It extracts the position, size, and orientation information of individual landmark objects from images taken by users. It also obtains the spatial relation between adjacent landmark objects from inertial sensor data, and then computes the coordinates and orientations of these objects on an initial floor plan. By combining user mobility traces and locations where images are taken, it produces complete floor plans with hallway connectivity, room sizes, and shapes. It also identifies different types of connection areas (e.g., escalators, stairs) between stories, and employs a refinement algorithm to correct detection errors. Our experiments on three stories of two large shopping malls show that the 90-percentile errors of positions and orientations of landmark objects are about 1 \~ 2m and 5 \~ 9°, while the hallway connectivity and connection areas between stories are 100% correct.

The influence of the moving on the radiation characteristics is an important research direction on the field of the microwave remote sense. This paper has investigated the influence of the aerodynamic heating effect caused by high-speed moving on the radiation property of the typical plane. Firstly, the temperature distributions on the plane skin under different flight speeds have been calculated through Fluent simulation. Secondly, the emissivity of the skin under different observation angles has been calculated by the method of equivalent transmission lines. Finally, the radiation brightness temperatures of the stealth plane and the background have been calculated based on the blackbody radiation law and the atmospheric radiation transfer model. The results show that the high-speed moving has a significant influence on the radiation characteristic of the stealth target.

Plastic particles are widely present in the natural environment and are highly likely to interact with bacteria (the ubiquitous microbes in the natural environment), which might affect the transport and deposition of bacteria in porous media. In this study, the significance of plastic particles from nanoscale to micrometer-scale (0.02-2 μm) on the transport and deposition behaviors of bacteria (Escherichia coli) in quartz sand was examined under environmentally relevant conditions in both NaCl and CaCl2 solutions at pH 6. The results showed that the presence of different-sized plastic particles did not affect bacterial transport behaviors at low ionic strength (10 mM NaCl and 1 mM CaCl2), whereas, at high ionic strength conditions (50 mM NaCl and 5 mM in CaCl2), plastic particles increased bacterial transport in quartz sand. At low ionic strength conditions, the mobility of both plastic particles and bacteria was high, which might drive the negligible effects of plastic particles on bacterial transport behaviors. The mechanisms driving the enhanced cell transport at high ionic strength were different for different-sized plastic particles. Specifically, for 0.02 μm nanoplastic particles, the adsorption of plastic particles onto cell surfaces and the repel effect induced by suspended plastic particles contributed to the increased cell transport. As for 0.2 μm microplastics (MPs), the suspended plastic particles induced repel effect contributed to the increased cell transport, whereas, for 2 μm MPs, the competition deposition sites by the plastic particles were the contributor to the increased cell transport. © 2018 American Chemical Society.

Observational studies are prone to bias due to confounding either measured or unmeasured. While measured confounding can be controlled for with a variety of sophisticated methods such as propensity score-based matching, stratification and multivariable regression model, the unmeasured confounding is usually cumbersome, leading to biased estimates. In econometrics, instrumental variable (IV) is widely used to control for unmeasured confounding. However, its use in clinical researches is generally less employed. In some subspecialties of clinical medicine such as pharmacoepidemiological research, IV analysis is increasingly used in recent years. With the development of electronic healthcare records, more and more healthcare data are available to clinical investigators. Such kind of data are observational in nature, thus estimates based on these data are subject to confounding. This article aims to review several methods for implementing IV analysis for binary and continuous outcomes. R code for these analyses are provided and explained in the main text.

This paper presents a high-integration miniaturized dielectric spectroscopy system for sensing the change of permittivity at 240 GHz in the SiGe BiCMOS technology. The sensor features a transducer with a resonator to perform bandpass frequency response, whose complex value of S21 is varied with the permittivity of the sample under test. This variation can be detected and recorded as the change of amplitude and phase of the 240-GHz in-phase and quadrature direct conversion mixer. An external 30-GHz source is employed with cascade frequency multiplier chain to deliver a signal through the system with a wide tuning range of 215–245 GHz. An additional probe is employed to carry the sample and implement chip measurements on the probe station. The sensing function of this system is performed with the leaded wire as a metallic sample to be placed on the top of the transducer. Based on the measured dc output voltage changes, the calculated magnitude and phase of IQ signal in the 215–245-GHz range are used to estimate the complex permittivity change of MUTs. This dielectric spectroscopy system is also suitable for sensing the complexy permittivity change at higher frequencies in the future terahertz Lab-on-Chip measurements.

Background: Bacterial communities are essential to the biogeochemical cycle in riverine ecosystems. However, little is presently known about the integrated biogeography of planktonic and sedimentary bacterial communities in large rivers. Results: This study provides the first spatiotemporal pattern of bacterial communities in the Yangtze River, the largest river in Asia with a catchment area of 1,800,000 km(2). We find that sedimentary bacteria made larger contributions than planktonic bacteria to the bacterial diversity of the Yangzte River ecosystem with the sediment subgroup providing 98. 8% of 38,906 operational taxonomic units (OTUs) observed in 280 samples of synchronous flowing water and sediment at 50 national monitoring stations covering a 4300 km reach. OTUs within the same phylum displayed uniform seasonal variations, and many phyla demonstrated autumn preference throughout the length of the river. Seasonal differences in bacterial communities were statistically significant in water, whereas bacterial communities in both water and sediment were geographically clustered according to five types of landforms: mountain, foothill, basin, foothill-mountain, and plain. Interestingly, the presence of two huge dams resulted in a drastic fall of bacterial taxa in sediment immediately downstream due to severe riverbed scouring. The integrity of the biogeography is satisfactorily interpreted by the combination of neutral and species sorting perspectives in meta-community theory for bacterial communities in flowing water and sediment. Conclusions: Our study fills a gap in understanding of bacterial communities in one of the world's largest river and highlights the importance of both planktonic and sedimentary communities to the integrity of bacterial biogeographic patterns in a river subject to varying natural and anthropogenic impacts.

Dinitrogen pentoxide (N2O5) is one of the basic trace gases which plays a key role in nighttime atmosphere. An intercomparison and validation of different N2O5 measurement methods is important for determining the true accuracy of these methods. Cavity ring down spectroscopy (CRDS) and cavity enhanced absorption spectrometer (CEAS) were used to measure N2O5 at the campus of the University of Chinese Academy of Sciences (UCAS) from February 21, 2016 to March 4, 2016. The detection limits were 1.6 ppt (1 sigma) at 30 s intervals for the CEAS instrument and 3.9 ppt (1 sigma) at 10 s time resolution for the CRDS instrument respectively. In this study, a comparison of the 1 min observations from the two instruments was presented. The two data sets showed a good agreement within their uncertainties, with an absolute shift of 15.6 ppt, slope of 0.94 and a correlation coefficient R-2 = 0.97. In general, the difference between the CRDS and CEAS instruments for N2O5 measurement can be explained by their combined measurement uncertainties. However, high relative humidity (> 60%) and high PM2.5 concentration (> 200 mu g/m(3)) may contribute to the discrepancies. The excellent agreement between the measurement by the CRDS and CEAS instruments demonstrates the capability of the two instruments for accurately measuring N2O5 with high sensitivity. (C) 2017 Elsevier B.V. All rights reserved.