双碳”目标下城市大气CO2来源解析研究进展及应用

Research progress and application of urban atmospheric CO2 source apportionment under the targets of carbon peaking and neutrality

关键词:  碳排放  源解析  城市  碳同位素

Key words:  carbon emissions  source apportionment  city  carbon isotope

摘要:

我国明确提出2030年“碳达峰”和2060年“碳中和”目标，城市是碳排放集中区域和基本减排单元，准确掌握城市大气CO₂不同来源的排放贡献及其变化规律是制定减排措施和评估减排目标的关键。基于前期研究基础以及国内外研究现状，分别从同位素示踪原理和方法、端元值测定、来源定量计算以及误差分析等方面综述利用稳定碳同位素¹³C和放射性碳同位素¹⁴C示踪城市大气CO₂来源的技术流程及最新成果，分析当前研究不足和未来研究重点。结合当前在排放清单数据可靠性的独立评估中存在的问题，针对性地提出利用来源解析方法对排放清单数据进行校验的新思路，旨在为今后开展城市大气CO₂来源解析及应用研究提供方法参考。

Abstract:

Background, aim, and scope China will achieve peak CO₂ emissions before 2030 and achieve carbon neutrality by 2060, and cities are hotspots of carbon emissions and basic units of emission mitigation. Therefore, understanding the emission sources in urban areas is key for formulating measures and evaluating targets. Carbon isotopic tracing is a new method for analyzing CO₂ sources in the atmosphere. In this paper, methods and applications of CO₂ source apportionment are reviewed, aiming to provide a methodological reference for the systematic analysis of urban CO₂ sources and a basis for the formulation of targeted carbon emission reduction policies. Materials and methods On the basis of a summary of previous studies and our recent research progress, the principles and methods of obtaining urban atmospheric CO₂ sources by means of carbon isotopes (¹⁴C and ¹³C), especially the relative contribution rates of different fossil fuels, are explored. Then, these methods are applied to analyze atmospheric CO₂ sources in winter in two Chinese megacities. Results The fossil source components were separated from the total CO₂ on the basis of ¹⁴C measurements, and the relative contributions of different fossil fuels were further quantified on the basis of ¹³C mass conservation and multivariate mixed models. However, it is necessary to strengthen the following aspects of this research to obtain more reliable results: first, the sampling time and frequency should be moderately increased to ensure that the sample represents emissions within the study area, and second, the spatial differences in fossil fuel isotopes should be considered, as it is better to measure end-member values than to use literature data. Overall, there should be significant differences in the carbon isotopes of different fossil fuels; otherwise, additional tracers should be explored to provide additional constraints for assessing the results. Discussion “Bottom-up” emission inventories are the main tools for estimating carbon emissions; however, the uncertainty increases with increasing spatial resolution, and the imperfection of city-level statistical data in China leads to substantial uncertainty in city inventories, which makes scientific support for reducing urban carbon emissions difficult. Estimating the uncertainty of emission inventories is a challenge since different inventories often have common data sources, and fully revealing the uncertainties through comparisons between different inventories is difficult. Therefore, it is necessary to verify the emission inventory independently to improve its accuracy. Generally, the uncertainty of a prior inventory is evaluated on the basis of observations and inversion models. However, due to factors such as the quality of the simulation and the layout of the observation network, new errors can occur in the inversion process. In addition, observations of CO₂ in the urban atmosphere in China are still in their infancy, and existing urban observation networks have difficulty meeting the needs of inversion research. Considering that an emission inventory not only represents the total emission of fossil energy but also reflects the emission contributions of different fossil fuels, on the basis of this common ground with source tracing information, it is possible to verify the emission inventory data without inversion, which is highly important for further improving the accuracy of carbon budgets for Chinese cities. Future work should focus on reducing the uncertainty of source apportionment results by developing additional tracers. Conclusions With Beijing and Xi’an as research cases, the emission contributions of coal, oil and natural gas obtained via the double carbon isotope tracing method are compared with the emission inventory estimates, revealing that the emission contribution of fuel oil may be underestimated in the city-level inventory and confirming that an emission inventory can be verified by using source apportionment results. Recommendations and perspectives To achieve the goals of carbon peaking and carbon neutrality, the consumption structure of fossil energy in China must change greatly in the future, and high-carbon energy, such as coal and oil, must be gradually replaced by green energy. This transformation process could not only reduce carbon emissions but also provide opportunities for the study of atmospheric CO₂ source apportionment. Importantly, more isotopes, such as oxygen isotopes (¹⁸O), should be explored in the future to improve the reliability of source apportionment, and emission trends should be evaluated in a timely manner through ensuring the scientific layout of urban observation networks and long-term monitoring.