High-temperature transport properties in high-mobility lattice-matched InAlN/GaN heterostructures have been investigated. An interesting hysteresis phenomenon of the two dimensional electron gas (2DEG) density is observed in the temperature-dependent Hall measurements. After high-temperature thermal cycles treatment, the reduction of the 2DEG density is observed, which is more serious in thinner InAlN barrier samples. This reduction can then be recovered by light illumination. We attribute these behaviors to the shallow trap states with energy level above the Fermi level in the GaN buffer layer. The electrons in the 2DEG are thermal-excited when temperature is increased and then trapped by these shallow trap states in the buffer layer, resulting in the reduction and hysteresis phenomenon of their density. Three trap states are observed in the GaN buffer layer and CGa may be one of the candidates responsible for the observed behaviors. Our results provide an alternative approach to assess the quality of InAlN/GaN heterostructures for applications in high-temperature electronic devices. (C) 2015 AIP Publishing LLC.
The black carbon (BC) mass concentration and the particle chemical compositions were continually measured at Changdao Island, which is a regional receptor site off the eastern coast of China. This island is in the transport passage of the continental outflow to the Pacific Ocean when the East Asia monsoon prevails in the winter and spring. The campaign period was for March and April 2011, which corresponded to heating and non-heating periods in northern China. The effect of BC emission source regions on BC measured at Changdao Island between the heating and non-heating periods was determined by integrating the total potential source contribution function (TPSCF) model with the new monthly emission inventory in 2010 and the fire counts retrieved from MODIS during the campaign. BC concentrations were determined to be highest for similar times of day for both the heating and non-heating periods: 4.27 mu g m(-3) at 8:00 AM and 3.06 mu g m(-3) at 9:00 AM, respectively. The probable source regions for BC were primarily located in Shandong and Jiangsu provinces (and in other neighboring provinces) for both periods. However, the source regions for the non-heating period extended more to the north and southwest than those of the heating period. TPSCF values were correlated with the emission rates from residential, industry, transportation, and power plants sources in the anthropogenic emission inventory. This correlation provides an indirect and qualitative process to verify the emission inventory. In the heating period, the predominant source was the residential source in the emission inventory, and this source had a significant effect on the BC concentration. The differing peak concentrations between the two periods may be observed because of the increased residential heating during the heating period, which suggested that the measures employed by the government and environmental managers to reduce the emissions of pollutants should be stricter in the identified source regions during the heating period. (C) 2014 Elsevier Ltd. All rights reserved.
The black carbon (BC) mass concentration and the particle chemical compositions were continually measured at Changdao Island, which is a regional receptor site off the eastern coast of China. This island is in the transport passage of the continental outflow to the Pacific Ocean when the East Asia monsoon prevails in the winter and spring. The campaign period was for March and April 2011, which corresponded to heating and non-heating periods in northern China. The effect of BC emission source regions on BC measured at Changdao Island between the heating and non-heating periods was determined by integrating the total potential source contribution function (TPSCF) model with the new monthly emission inventory in 2010 and the fire counts retrieved from MODIS during the campaign. BC concentrations were determined to be highest for similar times of day for both the heating and non-heating periods: 4.27 mu g m(-3) at 8:00 AM and 3.06 mu g m(-3) at 9:00 AM, respectively. The probable source regions for BC were primarily located in Shandong and Jiangsu provinces (and in other neighboring provinces) for both periods. However, the source regions for the non-heating period extended more to the north and southwest than those of the heating period. TPSCF values were correlated with the emission rates from residential, industry, transportation, and power plants sources in the anthropogenic emission inventory. This correlation provides an indirect and qualitative process to verify the emission inventory. In the heating period, the predominant source was the residential source in the emission inventory, and this source had a significant effect on the BC concentration. The differing peak concentrations between the two periods may be observed because of the increased residential heating during the heating period, which suggested that the measures employed by the government and environmental managers to reduce the emissions of pollutants should be stricter in the identified source regions during the heating period. (C) 2014 Elsevier Ltd. All rights reserved.
Predictions in ungauged basins (PUBs) are an increasingly important question in the water resource and quality management fields, especially for small watersheds where the phenomenon of data scarcity is prevalent. The transfer of hydrological parameters on the basis of regionalization is a common approach for solving PUB problems, and plenty of research has been undertaken on how to transfer calibrated parameters from gauged to ungauged watersheds. However, hydrological parameters estimation is substantially influenced by calibration objectives, which may affect model performance on the recipient watershed as well. In this paper the influence of calibration objectives on the transfer of parameter sets from one watershed to another is discussed. The HSPF hydrological model and the PEST automatic calibration model with four calibration objectives (squared error of daily flow, squared error of monthly flow, squared error of exceedance flow time, and sum of these squared errors) were applied. One gauged watershed of the Lake Dianchi Basin with daily flow data from 1999 to 2010 was calibrated by the combined HSPF-PEST model to obtain the transferrable parameters. Then the entire parameter sets were transferred to another neighborhood watershed, and model performance was tested by conventional goodness-of-fit statistics. Results show that (1) parameters transferred from all four calibration objectives perform well on the target watershed; (2) selection of calibration objective has a significant impact on model performance for both donor and recipient watersheds; and (3) the differences among objectives are similar in the two watersheds, suggesting that the objectives' features are transferrable. Therefore, the selection of calibration objective should be considered as a significant factor when transferring parameters to ungauged watersheds. (C) 2014 American Society of Civil Engineers.
Oxygenated volatile organic compounds (OVOCs) are important products of the photo-oxidation of hydrocarbons. They influence the oxidizing capacity and the ozone-forming potential of the atmosphere. In the summer of 2008, 2 months of emission restrictions were enforced in Beijing to improve air quality during the Olympic Games. Observational evidence reported in related studies that these control measures were efficient in reducing the concentrations of primary anthropogenic pollutants (CO, NOx and non-methane hydrocarbons, i.e., NMHCs) by 30-40%. In this study, the influence of the emission restrictions on ambient levels of OVOCs was explored using a neural network analysis with consideration of meteorological conditions. Statistically significant reductions in formaldehyde (HCHO), acetaldehyde (CH3CHO), methyl ethyl ketone (MEK) and methanol were found to be 12.9, 15.8, 17.1 and 19.6%, respectively, when the restrictions were in place. The effect of emission controls on acetone was not detected in neural network simulations, probably due to pollution transport from surrounding areas outside Beijing. Although the ambient levels of most NMHCs were reduced by similar to 35% during the full control period, the emission ratios of reactive alkenes and aromatics closely related to automobile sources did not present much difference (< 30%). A zero-dimensional box model based on the Master Chemical Mechanism version 3.2 (MCM3.2) was applied to evaluate how OVOC production responds to the reduced precursors during the emissions control period. On average, secondary HCHO was produced from the oxidation of anthropogenic alkenes (54%), isoprene (30%) and aromatics (15%). The importance of biogenic sources for the total HCHO formation was almost on par with that of anthropogenic alkenes during the daytime. Anthropogenic alkenes and alkanes dominated the photochemical production of other OVOCs such as acetaldehyde, acetone and MEK. The relative changes of modeled HCHO, CH3CHO, methyl vinyl ketone and methacrolein (MVK + MACR) before and during the pollution controlled period were comparable to the estimated reductions in the neural network, reflecting that current mechanisms can largely explain secondary production of those species under urban conditions. However, it is worth noting that the box model overestimated the measured concentrations of aldehydes by a factor of 1.4-1.7 without consideration of loss of aldehydes on aerosols, and simulated MEK was in good agreement with the measurements when primary sources were taken into consideration. These results suggest that the understanding of the OVOCs budget in the box model remains incomplete, and that there is still considerable uncertainty in particular missing sinks (unknown chemical and physical processes) for aldehydes and absence of direct emissions for ketones