Xueshun Chen

High temperature creep behavior of in-situ TiC particulate reinforced Fe-Cr-Ni matrix composite

The high temperature creep properties of the composites containing 5, 10, 16 vol% TiC particulate were investigated in the temperature range 973-1223 K and the stress range 40-160 MPa. The composites exhibit pronounced decrease of the creep strain rate and increase of the apparent activation energy and the threshold stress with increasing the volume fraction of TIC particulate. The creep life of the composites containing 5 and 10 vol% TiC particulate is longer than that of matrix alloy. By considering the threshold stress, all creep strain rates of the composites can be rationalized by a power-law with a stress exponent of 5. Transmission electron microscopy examination of the deformation microstructure reveals the local climb mechanism of dislocations acts as the predominant deformation mode in the composites. The dislocations are generally pinned by TiC particles and are bent to a high curvature on climbing over the particles. Thus a significant amount of new dislocation lines has to be created, leading to an increase of the threshold stress

Radiative and heterogeneous chemical effects of aerosols on ozone and inorganic aerosols over East Asia

Currently, many challenges are faced in simulating ozone(O3), sulfate(SO42-), and nitrate(NO3-) concentrations over East Asia, particularly the overestimation of surface O3 and NO3- concentrations and underestimation of the SO42- concentration during haze episodes. In this study, we examined the radiative and heterogeneous chemical effects of aerosols by incorporating recently reported mechanisms, including self-amplifying SO42- formation, dinitrogen pentoxide (N2O5) hydrolysis, and a heterogeneous reaction converting gaseous nitric acid (HNO3) to nitric oxide (NOx), into a Nested Air Quality Prediction Modeling System. Uptakes by aerosols can be computed through a simple parameterization that is dependent on the aerosol core and shell species, shell thickness, and amount of aerosol liquid water. In this study, a 1-year simulation was conducted for 2013. The updated model successfully reproduced the seasonal and daily observations of O3, fine particulate matter, SO42-, and NO3- concentrations in East Asia. Our results revealed that heterogeneous reactions reduced more surface O3 concentrations (10-20 ppbv) in the polluted regions of East China than did perturbations in photolysis frequencies from aerosols, effectively again improving the comparison between simulations and observations. Oxidation of SO2 by NO2 on wet aerosols significantly enhanced SO42- formation, with sulfate covering approximately ~30-60% of total sulfate concentrations in North China Plain during haze days in winter. The uptake of reactive nitrogen species on aerosols effectively reduced NO3- concentrations and successfully balanced the NOx/HNO3 chemistry in the models. We recommended that larger reductions of gaseous precursors should be considered in China to achieve the national air quality objective. The results show that surface O3 concentrations over East China will increase if the emission of aerosols is reduced without corresponding reductions in O3 precursors.

Simulation on different response characteristics of aerosol particle number concentration and mass concentration to emission changes over mainland China

In this study, Nested Air Quality Prediction Modeling System with Advance Particle Microphysics module (NAQPMS+APM) is applied to simulate the response characteristics of aerosol particle number concentration and mass concentration to emission changes over mainland China. It is the first attempt to investigate the response of both aerosol mass concentration and number concentration to emission changes using a chemical transport model with detailed aerosol microphysics over mainland China. Results indicate that the response characteristics are obviously different between aerosol particle number concentration and mass concentration. Generally, the response of number concentration shows a more heterogeneous spatial distribution than that of mass concentration. Furthermore, number concentration has a higher sensitivity not only to primary particles emission but also to precursor gases than that of mass concentration. Aerosol particle mass concentration exhibits a consistent trend with the emission change and yet aerosol number concentration does not. Due to the nonlinearity of aerosol microphysical processes, reduction of primary particles emission does not necessarily lead to an obvious decrease of aerosol number concentration and it even increases the aerosol number concentration. Over Central-Eastern China (CEC), the most polluted regions in China, reducing primary particles emission rather than precursor gas emissions is more effective in reducing particles number concentration. By contrast, the opposite is true over the northwestern China. The features of fine particles pollution revealed in this study are associated with the spatial differences in Chinas population, geography, climate and economy. Considering the more adverse effects of ultrafine particles on human health and the spatial distribution of population, making different measures in controlling particles number concentration from that controlling mass concentration in different regions over mainland China is indicated. MAIN FINDINGS FPN concentration responds more heterogeneously to emission than FPM. Spatial difference of response of FPN to emission is distinguished by a boundary line.

East Asian dust storm in May 2017: observations, modelling and its influence on Asia-Pacific region

A severe dust storm event originated from the Gobi Desert in Central and East Asia during 2–7 May 2017. Based on Moderate Resolution Imaging Spectroradiometer (MODIS) satellite products, hourly environmental monitoring measurements from Chinese cities and East Asian meteorological observation stations, and numerical simulations, we analysed the spatial and temporal characteristics of this dust event as well as its associated impact on the AsiaPacific region. The maximum observed hourly PM10 (particulate matter with an aerodynamic diameter ≤ 10 μm) concentration was above 1000 μg m−3 in Beijing, Tianjin, Shijiazhuang, Baoding, and Langfang and above 2000 μg m−3 in Erdos, Hohhot, Baotou, and Alxa in northern China. This dust event affected over 8.35 million km2, or 87 % of the Chinese mainland, and significantly deteriorated air quality in 316 cities of the 367 cities examined across China. The maximum surface wind speed during the dust storm was 23– 24 m s−1 in the Mongolian Gobi Desert and 20–22 m s−1 in central Inner Mongolia, indicating the potential source regions of this dust event. Lidar-derived vertical dust profiles in Beijing, Seoul, and Tokyo indicated dust aerosols were uplifted to an altitude of 1.5–3.5 km, whereas simulations by the Weather Research and Forecasting with Chemistry (WRFChem) model indicated 20.4 and 5.3 Tg of aeolian dust being deposited respectively across continental Asia and the North Pacific Ocean. According to forward trajectory analysis by the FLEXible PARTicle dispersion (FLEXPART) model, the East Asian dust plume moved across the North Pacific within a week. Dust concentrations decreased from the East Asian continent across the Pacific Ocean from a magnitude of 103 to 10−5 μg m−3, while dust deposition intensity ranged from 104 to 10−1 mg m−2. This dust event was unusual due to its impact on continental China, the Korean Peninsula, Japan, and the North Pacific Ocean. Asian dust storms such as those observed in early May 2017 may lead to wider climate forcing on a global scale.