An international research team has conducted a study of new particle formation (NPF) events in the atmosphere of Beijing, which provides the first evidence of the importance of transport in NPF events in the urban atmosphere. Such atmospheric NPF events influence air quality, climate, and human health.
The team's findings are published in the journal Particulology on January 9, 2023.
NPF events are an important source of secondary particles in the atmosphere, significantly influencing the cloud albedo and air quality. The mechanisms by which NPF events occur under high aerosol loadings (so called "polluted" NPF events) in the atmosphere previously have not been fully understood, resulting in limited precision of climate models and particle pollution control.
To better understand how these "polluted" NPF events occur, the research team conducted a one-month comprehensive field measurement in Beijing during the summer of 2016. The team discovered that the "clean" NPF events were caused by local nucleation and growth, while the "polluted" NPF events were caused by both local nucleation-growth and regional transport. Regional transport is the process in which the pollutants from upwind sources impact the air quality in a downwind location. The team's findings emphasize the importance of the transport for nanoparticles in relatively polluted atmospheres. This result shows that regional joint air pollution control is an essential policy.
Researchers have widely studied the atmospheric NPF in China because of its negative impact on air quality, climate factors and human health. Since 2004, the PKU aerosol team has conducted continuous measurements of NPF and observed a unique polluted type of NPF process in the atmosphere of Beijing. During this NPF process they observed, the background particle level was high, and the particle burst covered a wide range (3-20 nm). "Although many mechanisms of NPF events in clean atmosphere have been established, the mechanism of how polluted NPF events occurred remained ambiguous," said Min Hu, a professor at Peking University.
To analyze the effects of transport on NPF events, the team conducted their one-month observation during the summer of 2016 at a suburban site in Beijing, about 40 km northwest of the urban center. This site is strongly influenced by regional transport controlled by mountain and valley breeze. They comprehensively investigated both "clean" and "polluted" NPF events using particle, precursor and meteorological data. To discover if the NPF events occurred on a larger scale, they conducted simultaneous measurements in the urban area at the main campus of Peking University.
During the summer of 2016, the team found that the polluted NPF events are caused by both regional transport and local nucleation of the nanoparticles in the atmosphere. The transport brings 3-20 nm particles from upwind areas in the morning, and local nucleation contributes molecule clusters, such as sulfuric acid dimers, from around 12:00 local time. They found that the wind direction is also different on the "polluted" NPF days, compared with the normal "clean" NPF days and the days without NPF events. "Our findings imply that even when the local emission of particles and NPF gaseous precursors is strictly controlled, the transport can still produce large amounts of secondary particles in the local atmosphere and then trigger the haze events. Thus, joint control measures are highly required on the regional scale to achieve further particle pollution mitigation," said Hu.
Looking ahead to future research, the team plans to conduct on-site monitoring campaigns and laboratory simulations with more parameters including organic molecules, to gain a deeper and molecular understanding of the "polluted" NPF events. "The ultimate goal is to improve the mechanisms of NPF events, both clean and polluted type, in the air quality models and climate models, and to reduce uncertainties in policies addressing climate change and controlling air pollutions," said Hu.
The research team includes Dongjie Shang, Min Hu, Lizi Tang, Xin Fang, Ying Liu, Yusheng Wu, Zhuofei Du, Xuhui Cai from Peking University; Min Hu, Zhijun Wu, Song Guo, and Yuanhang Zhang from Peking University and Nanjing University of Information Science & Technology; Shengrong Lou from Shanghai Academy of Environmental Sciences; and Mattias Hallquist from University of Gothenburg.
This research is funded by the National Natural Science Foundation of China (NSFC), the NSFC - Creative Research Group Fund, the NationalKey Research and Development Program of China, and the bilateral Sweden–China framework program "Photochemical smog in China: formation, transformation, impact and abatement strategies."
