National Science Review

As a promising area in machine learning, multi-task learning (MTL) aims to improve the performance of multiple related learning tasks by leveraging useful information among them. In this paper, we give an overview of MTL by first giving a definition of MTL. Then several different settings of MTL are introduced, including multi-task supervised learning, multi-task unsupervised learning, multi-task semi-supervised learning, multi-task active learning, multi-task reinforcement learning, multi-task online learning and multi-task multi-view learning. For each setting, representative MTL models are presented. In order to speed up the learning process, parallel and distributed MTL models are introduced. Many areas, including computer vision, bioinformatics, health informatics, speech, natural language processing, web applications and ubiquitous computing, use MTL to improve the performance of the applications involved and some representative works are reviewed. Finally, recent theoretical analyses for MTL are presented.

Solar steam generation is emerging as promising solar-energy conversion technology for potential applications in desalination, sterilization and chemical purification. Despite the recent use of photon management and thermal insulation, achieving optimum solar steam efficiency requires simultaneous minimization of radiation, convection and conduction losses without compromising light absorption. Inspired by the natural transpiration process in plants, here we report a 3D artificial transpiration device with all three components of heat loss and angular dependence of light absorption minimized, which enables over 85% solar steam efficiency under one sun without external optical or thermal management. It is also demonstrated that this artificial transpiration device can provide a complementary path for waste-water treatment with a minimal carbon footprint, recycling valuable heavy metals and producing purified water directly from waste water contaminated with heavy metal ions.

Mặc dù đã có nhiều sự chú ý đến việc điều tra và kiểm soát ô nhiễm không khí tại Trung Quốc, nhưng xu hướng nồng độ chất ô nhiễm không khí ở quy mô quốc gia vẫn chưa rõ ràng. Ở đây, chúng tôi đã điều tra định lượng sự biến đổi của các chất ô nhiễm không khí tại Trung Quốc bằng cách sử dụng các tập dữ liệu tổng hợp dài hạn từ năm 2013 đến 2017, trong đó chính phủ Trung Quốc đã nỗ lực lớn để giảm phát thải con người ở các vùng ô nhiễm. Kết quả của chúng tôi cho thấy một xu hướng giảm đáng kể nồng độ PM2.5 ở các khu vực ô nhiễm nặng của miền đông Trung Quốc, với mức giảm hàng năm khoảng 7% so với các số đo năm 2013. Các nồng độ SO2, NO2 và CO (một chỉ số cho các hợp chất hữu cơ bay hơi do con người gây ra) đã giảm cũng giải thích một phần lớn cho sự giảm nồng độ PM2.5 ở các vùng khác nhau. Kết quả là, số ngày ô nhiễm nặng giảm đáng kể ở các vùng tương ứng. Nồng độ aerosol hữu cơ, nitrate, sulfate, ammonium và chloride đo được ở thành phố Bắc Kinh cho thấy một sự giảm đáng kể từ năm 2013 đến 2017, kết nối chặt chẽ sự giảm các tiền chất aerosol với các thành phần hóa học tương ứng. Tuy nhiên, nồng độ ozone bề mặt có xu hướng tăng ở hầu hết các trạm đô thị từ năm 2013 đến 2017, điều này cho thấy ô nhiễm quang hóa mạnh hơn. Chiều cao tầng biên ở các thành phố thủ đô của miền đông Trung Quốc không cho thấy xu hướng đáng kể nào trên các vùng Bắc Kinh - Thiên Tân - Hà Bắc, Đồng bằng sông Dương Tử và Đồng bằng sông Châu Giang từ năm 2013 đến 2017, điều này xác nhận sự giảm phát thải con người. Kết quả của chúng tôi đã chứng minh rằng chính phủ Trung Quốc đã thành công trong việc giảm bụi mịn ở các khu vực đô thị từ năm 2013 đến 2017, mặc dù nồng độ ozone đã tăng đáng kể, điều này gợi ý một cơ chế phức tạp hơn trong việc cải thiện chất lượng không khí tại Trung Quốc trong tương lai.

The Tibetan Plateau and its surroundings are known as the Third Pole (TP). This region is noted for its high rates of glacier melt and the associated hydrological shifts that affect water supplies in Asia. Atmospheric pollutants contribute to climatic and cryospheric changes through their effects on solar radiation and the albedos of snow and ice surfaces; moreover, the behavior and fates within the cryosphere and environmental impacts of environmental pollutants are topics of increasing concern. In this review, we introduce a coordinated monitoring and research framework and network to link atmospheric pollution and cryospheric changes (APCC) within the TP region. We then provide an up-to-date summary of progress and achievements related to the APCC research framework, including aspects of atmospheric pollution's composition and concentration, spatial and temporal variations, trans-boundary transport pathways and mechanisms, and effects on the warming of atmosphere and changing in Indian monsoon, as well as melting of glacier and snow cover. We highlight that exogenous air pollutants can enter into the TP’s environments and cause great impacts on regional climatic and environmental changes. At last, we propose future research priorities and map out an extended program at the global scale. The ongoing monitoring activities and research facilitate comprehensive studies of atmosphere–cryosphere interactions, represent one of China's key research expeditions to the TP and the polar regions and contribute to the global perspective of earth system science.

Since 1971, it has been known that the atmospheric free radicals play a pivotal role in maintaining the oxidizing power of the troposphere. The existence of the oxidizing power is an important feature of the troposphere to remove primary air pollutants emitted from human beings as well as those from the biosphere. Nevertheless, serious secondary air-pollution incidents can take place due to fast oxidation of the primary pollutants. Elucidating the atmospheric free-radical chemistry is a demanding task in the field of atmospheric chemistry worldwide, which includes two kinds of work: first, the setup of reliable radical detection systems; second, integrated field studies that enable closure studies on the sources and sinks of targeted radicals such as OH and NO3. In this review, we try to review the Chinese efforts to explore the atmospheric free-radical chemistry in such chemical complex environments and the possible link of this fast gas-phase oxidation with the fast formation of secondary air pollution in the city-cluster areas in China.

ENSO is the strongest interannual signal in the global climate system with worldwide climatic, ecological and societal impacts. Over the past decades, the research about ENSO prediction and predictability has attracted broad attention. With the development of coupled models, the improvement in initialization schemes and the progress in theoretical studies, ENSO has become the most predictable climate mode at the time scales from months to seasons. This paper reviews in detail the progress in ENSO predictions and predictability studies achieved in recent years. An emphasis is placed on two fundamental issues: the improvement in practical prediction skills and progress in the theoretical study of the intrinsic predictability limit. The former includes progress in the couple models, data assimilations, ensemble predictions and so on, and the latter focuses on efforts in the study of the optimal error growth and in the estimate of the intrinsic predictability limit.

Because of their low cost, natural abundance, environmental benignity, plentiful polymorphs, good chemical stability and excellent optical properties, TiO2 materials are of great importance in the areas of physics, chemistry and material science. Much effort has been devoted to the synthesis of TiO2 nanomaterials for various applications. Among them, mesoporous TiO2 materials, especially with hierarchically porous structures, show great potential owing to their extraordinarily high surface areas, large pore volumes, tunable pore structures and morphologies, and nanoscale effects. This review aims to provide an overview of the synthesis and applications of hierarchically mesoporous TiO2 materials. In the first section, the general synthetic strategies for hierarchically mesoporous TiO2 materials are reviewed. After that, we summarize the architectures of hierarchically mesoporous TiO2 materials, including nanofibers, nanosheets, microparticles, films, spheres, core-shell and multi-level structures. At the same time, the corresponding mechanisms and the key factors for the controllable synthesis are highlighted. Following this, the applications of hierarchically mesoporous TiO2 materials in terms of energy storage and environmental protection, including photocatalytic degradation of pollutants, photocatalytic fuel generation, photoelectrochemical water splitting, catalyst support, lithium-ion batteries and sodium-ion batteries, are discussed. Finally, we outline the challenges and future directions of research and development in this area.