Urban areas account for 2% of the world's land but are responsible for 40%–70% of global anthropogenic, or human-made CO₂ emissions. As a case study, Beijing represents a region experiencing a fast-growing economic cycle. Likewise, the city is heavily populated and developed. Due to rapid urbanization in Beijing, quantitative fossil-fuel and greenhouse gas (GHG) variation tracking in the city’s most densely populated areas is greatly important.

Prof. LIU Yi and his team—a group of researchers from the Carbon Neutrality Research Center (CNRC), of the Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences (CAS) wrote a call to action for humans to focus on quantifying urban CO₂ emissions using top-down methods. They recently published the results of their study in Advances in Atmospheric Sciences.

Currently, carbon emissions are assessed by "bottom-up" methods, used for emission inventories obtained by aggregating statistical data from relevant economic sectors at a given terrestrial scale relevant for mitigation policy. Bottom-up emission inventories are typically constructed from annual statistics.

Complementary to the bottom-up emission inventories and reported to the United Nations Framework Convention on Climate Change (UNFCCC), Greenhouse Gases (GHG) emissions can be estimated "top-down" using atmospheric measurements and inverse modelling. This method uses an atmospheric transport model and emission inventories as a priori estimates of the surface fluxes for land, ocean, and other CO₂ emission attributions.  

The current air pollution control policies in Beijing are directly related to the country's overall pollution reduction goals. To determine the combustion efficiency, or how much carbon and oxygen converts to CO₂ rather than CO in a vehicle's combustion reactions, researchers deployed a portable Fourier-Transform Spectrometer at an urban site in Beijing. This apparatus observes column concentrations of CO and CO₂, which are used to correlate analyses between CO and CO₂ enhancements, therefore providing useful information to quantify combustion efficiency.

"When using CO as a tracer, anthropogenic CO₂ signals are higher than other urban stations, such as Karlsruhe, Pasadena, Tsukuba, and Paris, in developed countries, suggesting high pollution in Beijing." said CHE Ke, the paper's lead author and PhD student at IAP/CAS. 

The research team also compared the observed combustion efficiency data with the current "bottom-up" emission inventories. They found that the multi-resolution emission inventory (MEIC) and Peking University (PKU) emission inventories are 42.54% and 101.15% higher than the observed values, respectively, indicating that traditional combustion inventory methods underestimate the real values.

"Combustion efficiency in Beijing is easily affected by transportation." said CAI Zhaonan, corresponding author and associate professor at IAP/CAS. "Air masses passing over the southern part of Beijing increased the proportion of anthropogenic signal."  

This work highlights necessary long-term column measurements in the heavily CO-emitting Beijing region. However, with one station, scientists could only capture limited information on a local scale within a larger urban region. In future research, an intensive CO₂ measuring station combined with model data could open up more potential pathways for regional emissions studies. 

Citation: Che, K., Y. Liu, Z. N. Cai, D. X. Yang, H. B. Wang, D. H. Ji, Y. Yang, P. C. Wang, 2022: Characterization of regional combustion efficiency using ΔXCO:ΔXCO₂ observed by a portable Fourier-Transform Spectrometer at an urban site in Beijing. Adv. Atmos. Sci., https://doi.org/10.1007/s00376-022-1247-7.

Media contact: Ms. LIN Zheng, jennylin@mail.iap.ac.cn