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The 22^nd April 2020 marks the 50^th anniversary of Earth Day which is all about climate action this year. Professor Junji Cao, Research Professor in the Division of Aerosol and Environment (DAE) at the Chinese Academy of Sciences (CAS), has 20+ years of experience establishing emission inventories, conducting air quality studies, and performing statistical data analysis.

Cao was the founder of the Key Laboratory of Aerosol Chemistry & Physics at CAS, where he leads a group of 100+ staff and graduate students in conducting ambient and source characterization studies, laboratory filter processing, chemical speciation, and pollution control. Prof. Cao has conducted over 20 aerosol and PM_2.5 studies in China, and has made major contributions with advice to local and central government agencies for air pollution control.

Below you can find an interview with Prof. Cao about the Clean Air Action Plan, his research as well as latest trends.

What is your overall evaluation of the Clean Air Action Plan implemented between 2013 and 2017 and what do you foresee in the upcoming 2018-2020 Three-year Action Plan for Winning the Blue Sky War?            

My overall evaluation of the Clean Air Action Plan is successful. The concentrations of air pollutants decreased largely in the three key areas (Jingjinji areas, Yangtze River Delta regions, Pearl River Delta regions). Especially, the PM_2.5 in Beijing decreased quickly and the days of heavy pollution are obviously reduced. The concentrations of PM_2.5 in Beijing is ~ 60 ug m^-3, which reached the target of Clean Air Action Plan in 2017. 

The PM_2.5 concentration of the Pearl River Delta drops below 35 ug m^-3, reaching the new PM_2.5 standard of the country. In this process, various cities and regions have accumulated a lot of experience in air pollution management and air pollution works have been got unprecedented attentions.

The 2018-2020 Blue Sky Defense War is the second phase of Clean Air Action Plan and instead of the Pearl River Delta region, Fen Wei Plain has been appointed as one of the three key regions to improve air pollution. Air quality improvement became the core work of the National Pollution Battle and has become the top priority of the government at all levels. I think the majority of cities will win the Blue Sky Defense War.

What are some of the main scientific advances in the areas related to the air pollution analysis and control?

• Deep understanding of the causes of air pollution and heavy haze with more scientific and more refined evidences as well as the resolution from daily to hourly
• More intelligent and automated means of pollution monitoring.
• Pollution control methods that are more diversified and specified. Different control measures are adopted for different target sources

What are the current hottest topics in these research areas?

The current hottest topics are: source apportionment and cause of heave haze.

For example, a study led by AN Zhisheng from our institute, published online in  on April15, reviews and synthesizes recent advances in the causes and formation mechanisms of severe haze pollution in northern China. The severe haze events in northern China can be regarded as synergetic effects from the interactions between anthropogenic emissions and atmospheric processes. Researchers found that the seasonally enhanced emissions of pollutants from residential heating and efficient secondary aerosol formation and transformation could cause severe haze. 

Unfavorable meteorological conditions, for example, enhanced air static stability and shallow planetary boundary layer due to aerosol-radiation and aerosol-cloud interactions could also exacerbate the formation of severe haze. In addition, the regional East Asian winter monsoon and westerly circulation, which are influenced by various factors including variations of Arctic sea ice and the Siberian High, the topography of the Tibetan Plateau, and El Nino - Southern Oscillation, may also have significant influence on the formation of severe haze in northern China.

Could you discuss briefly about your view on the increasing ozone pollution in China? 

In recent years, China's ozone pollution is increasing, which is a bad signal. The main reason is related to the proportion of two major precursor pollutants: NOx and VOCs. VOCs has an increasing trend, but NOx decline trend is not obvious. The second reason is related to the atmospheric chemical reaction. When the concentration of PM_2.5 begin to decrease, it leads to an increase in solar radiation, which promotes a more active chemical reaction and forms O₃ pollution.

What are the aims of the journal, Aerosol Science and Engineering? Would you suggest some articles that are most relevant to this interview topic?

 (ASE) publishes high-quality papers that advance aerosol science and engineering. It broadly encompasses various traditional aerosol-related topics. The invention of instrumentation and neo-methods on aerosol sampling and analysis are also included. It emphasizes the application of aerosol technology to environmental and earth issues. It provides a platform not only for basic research but also for industrial interests.

I have been Editor-in-chief of ASE since 2016. The journal began to publish papers in 2017. It has an great impact on aerosol circles in China and across the world.

Recommended articles in ASE:

G. M. Hidy: 

Judith C. Chow: 

Prashant Rajput: 

Yong Zhang: 

Junji Cao: Qiuyue Zhao: 

Zhenxing Shen: 

Daizhou Zhang: 

Why did you become an environmental scientist and what excites you most in your career?

During my PhD work, I mainly did research on dust storm pollution in northern China. More than 20 years ago, air pollution in Xi' an was greatly influenced by dust especially in spring. So my research interest moved from dust pollution to urban atmospheric particulate matter and I began to study the origin and genesis of atmospheric particulate matter in Xi' an. From the study of TSP (total suspended particulate matter) to PM₁₀, then from 2001 I began to specialize in PM_2.5 pollution. The PM_2.5 research lasted nearly 20 years and I gradually became an environmental scientist.

In my career, the early stage was mainly basic research of air pollution and I published a lot of influential papers. From 2013, I began to think about pollution control work and how to improve air quality. So in recent years my works have mainly focused on environmental air purification that is aimed at the ambient air pollutants rather than the source pollutants from diverse emissions. Our environmental air purification mainly based on the use of advanced nanotechnology to enhance the efficiency of air purification. In another word, this is to transfer a scientific paper into tangible results, i.e., follow the instructions of President Jinping Xi: Write the paper on the ground.

We know you have been leading the first anti-pollution key project called HSALSCS (Hybrid Solar Assisted Large Scale Cleaning System) in China, could you introduce more about it?

Since 2013, I began to think about the control of air pollution. In 2014, I worked with Professor David Pui of the University of Minnesota (member of the National Academy of Engineering, USA) and Professor Tao Wenquan (a member of the Chinese Academy of Sciences) is of Xi ' an Jiaotong University to propose a large air cleaning system i.e., HSALSCS (Hybrid The concept of Solar Assisted Large Scale Cleaning System). We developed a demonstration unit in Xi'an with 60-meter-high. The project is based on the thermal energy application, advanced filtration technology, nano-photocatalytic materials. It can remove the particulate matter and important precursors (NOx, VOCs, SO2, etc.) in ambient air. Currently, the experimental results are good, which can reduce the PM_2.5 concentration within 10 square kilometers by about 15%. This is an innovation idea and it will open up a new way of environmental purification of air pollution.

At present, the project has been widely reported at home and abroad. Nature reported it in New Focus section with "China Tests giant air cleaner to combat urban smog" in March 2018. The HSALSCS was selected by the US media at the end of 2018 as one of 28 "Incredible" Make in China "Innovations that can change the world", alongside innovative projects such as quantum satellites, FAST sky eyes, and Hong Kong-Zhuhai-Macao Bridge. We recently proposed to build new HSALSCS in the relevant cities in China as well as in Thailand and South Korea.

Biosketch of Prof. Cao

Dr. Junji Cao, Research Professor in the Division of Aerosol and Environment (DAE), has 20+ years of experience establishing emission inventories, conducting air quality studies, and performing statistical data analysis. He was the founder of the Key Laboratory of Aerosol Chemistry & Physics at the Chinese Academy of Sciences, where he leads a group of 100+ professional staff, technicians, and graduate students. Prof. Cao has conducted over 20 aerosol and PM2.5 studies in China, and has made major contributions with advice to local and central government agencies for air pollution control.

He is the principal author or co-author of ~500 papers on ISI with total citations of ~18000 and h-index 62. His google citations are 23000+ and google-index is 76. Prof. Cao is Editor-in-Chief of Aerosol Science and Engineering, General Secretary of International Aerosol Research Assembly (IARA), President of Asian Aerosol Research Assembly (AARA) (2011-2014) and Vice President of Chinese Society of Particuology. He is also the recipient of the 2018 TWAS Earth, Space and Astronomy Prize and 2013 A&WMA Frank A. Chambers Excellence in Air Pollution Control Award and several outstanding Young Scientist Awards in China.

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