HCP Research Brief: Effect of Climate on Beijing Air Quality

April 7, 2020
Beijing - Winter

Summary of New Project Publications for Non-Specialists

Publication: Gao et al. 2020. “China's emission control strategies have suppressed unfavorable influences of climate on wintertime PM2.5 concentrations in Beijing since 2002.” Atmospheric Chemistry and Physics.

The lead author of this study is Meng Gao, Assistant Professor in the Department of Geography of Hong Kong Baptist University. He began the research as a Postdoctoral Fellow of the Harvard-China Project, with which he now continues to collaborate as an Associate. Co-authors include three more members of the Harvard-China Project: Peter Sherman (Ph.D. student, Department of Earth and Planetary Sciences), Shaojie Song (postdoctoral Research Associate), and Michael B. McElroy (Gilbert Butler Professor of Environmental Studies and Faculty Chair of the Harvard-China Project). The full paper reference is at the bottom.

This HCP Research Brief for non-specialists was written on behalf of the team by Shaojie Song.

Summary of the Research

Haze days with high levels of very small particles less than 2.5 micrometers in diameter (PM2.5) often happen in the North China Plain, including in China’s capital city of Beijing. The haze reduces visibility, makes travel difficult, and more importantly, causes health problems such as respiratory and cardiovascular diseases. The haze problem is particularly severe in winter, when the atmosphere is more stable than other seasons due to weaker sunlight.

January 2013 was reported as the haziest month over the past 60 years in Beijing, with the highest daily mean PM2.5 concentrations exceeding 500 micrograms per cubic meter. Following this episode—sometimes called the “Airpocalypse” in media accounts—the State Council of China issued the “Air Pollution Prevention and Control Action Plan” in September 2013, which explicitly promulgated new pollution control measures and proposed specific goals for concentrations by 2017. These measures included eliminating small coal-fired boilers, phasing out small factories with high emissions, installing control facilities for emissions of volatile organic compounds, and replacing residential coal burning with electricity and natural gas, among others.

Between 2013 and 2017, the annual mean PM2.5 concentrations declined in the Jing-Jin-Ji (Beijing-Tianjin-Hebei) metropolitan region by 40%, as measured by the Ministry of Ecology and Environment. But using only the measured PM2.5 concentrations, it has been difficult to attribute the decline of PM2.5 levels to the effectiveness of these control measures or simply changes in meteorology, or, more simply, weather patterns. What would the climate have done to the variations of PM2.5 if emissions had not changed? This is the question that we wanted to answer in this study, for which we used an air quality model to evaluate the influence of these two factors. The merit of modeling experiments is that we can fix the emissions of primary haze particles and gaseous precursors to a specific year, and use the results to answer this question.

Fixing emissions, we found that meteorological conditions over the study period would have led to an increase in haze in Beijing, but the strict emission control measures more than compensated for the unfavorable influences of the recent climate. The air quality model also allowed us to explore the driving factors of the unfavorable meteorological conditions. Here they are attributed to the weakening of the East Asia winter monsoon associated particularly with an increase in pressure associated with the Aleutian Low, a low-pressure system located near the Aleutian Islands in the Bering Sea during winter in the Northern Hemisphere.

Full reference: Meng Gao, Zirui Liu, Bo Zheng, Dongsheng Ji, Peter Sherman, Shaojie Song, Jinyuan Xin, Cheng Liu, Yuesi Wang, Qiang Zhang, Jia Xing, Jingkun Jiang, Zifa Wang, Gregory R. Carmichael, and Michael B. McElroy. 2020. “China's emission control strategies have suppressed unfavorable influences of climate on wintertime PM2.5 concentrations in Beijing since 2002.” Atmospheric Chemistry and Physics, 20, 3, pp. 1497-1505, doi.org/10.5194/acp-20-1497-2020. The journal is open-access and the paper is freely downloadable at the website of the Harvard-China Project.