Systematic error and uncertain carbon dioxide emissions from U.S. power plants

Carbon dioxide (CO₂) emissions from U.S. power plants are independently reported by the U.S. Energy Information Administration (EIA) and the Clean Air Markets Division (CAMD) within the U.S. Environmental Protection Agency (EPA). Differences between the CAMD and EIA emission tallies show that the amount of CO₂ produced by an individual power plant is less certain than might be imagined or desired. These differences are attributed to systematic error and random measurement error. Random error cannot be retroactively corrected, whereas systematic error can be corrected where relevant data are available. Accordingly, this study identified and, where possible, corrected systematic error affecting the CAMD and EIA CO₂ emission tallies for 1065 power plants that emitted more than 25,000 tons of CO₂ during 2013. The EIA tallies were corrected by accounting for emission factor error, acid-gas sorbent consumption, and combustion of biogenic fuel. The CAMD tallies were likewise corrected by accounting for unreported unit emissions. It was not possible to objectively correct systematic error affecting about 11% of the power plants, and subjective corrections were not attempted. At these plants, the CAMD and EIA emission tallies sometimes differed by more than 20% due to missing unit error, plant identification error, temporal measurement error, or inferred reporting error. Comparisons of the CAMD and EIA emission tallies before and after correction for systematic error show the effectiveness of these corrections. The comparisons also show the persistence of random measurement error. <i>Implications</i>: Understanding the uncertainty of CO2 emission tallies for USA power plants might inform emission inventories, atmospheric flow models or inversions, and emission reduction policies. Knowing the cause and size of measurement errors that contribute to this uncertainty might also help to identify ways to improve the measurement methods and reporting protocols that these CO2 emission tallies are based on.

美国发电厂的二氧化碳（Carbon dioxide, CO₂）排放数据由美国能源信息署（U.S. Energy Information Administration, EIA）与美国环境保护署（U.S. Environmental Protection Agency, EPA）下属的清洁空气市场部门（Clean Air Markets Division, CAMD）独立上报。CAMD与EIA的排放统计值之间存在差异，这表明单座发电厂的二氧化碳排放量并不如人们预想或期望的那般确定。此类差异可归因于系统误差与随机测量误差。随机误差无法进行回溯修正，而系统误差在具备相关数据的前提下可予以修正。
据此，本研究针对2013年二氧化碳排放量超过25000吨的1065座发电厂，识别并在可行范围内修正了影响CAMD与EIA二氧化碳排放统计的系统误差。其中，EIA的统计数据通过核算排放因子误差、酸性气体吸附剂消耗量以及生物质燃料燃烧情况完成修正；CAMD的统计数据则通过核算未上报的机组排放量完成修正。
约11%的发电厂的系统误差无法通过客观方式修正，且未尝试进行主观修正。在这些发电厂中，由于机组误差缺失、电厂标识错误、时间维度测量误差或推断性报告误差，CAMD与EIA的排放统计数据有时差异超过20%。
对修正系统误差前后的CAMD与EIA排放统计数据进行对比，可验证这些修正措施的有效性，同时也能展现随机测量误差的持续性。
<i>启示</i>：明晰美国发电厂二氧化碳排放统计的不确定性，可为排放清单、大气流场模型或反演模型以及减排政策制定提供参考。厘清导致该不确定性的测量误差的成因与规模，还有助于探索改进这些二氧化碳排放统计所依托的测量方法与报告规程的路径。