## Appendix B Expert Interview Process  
In this study, semi-structured interviews were conducted. Questions were posed flexibly based on the interview outline, allowing an in-depth grasp of the experts' views. The research centered on PBSC carbon emission reduction, examining the experts' consensus on both the basis of parameter setting and the parameter setting itself. The interviews aimed to verify the basis' reliability and the parameter setting's scientific nature. We interviewed experts from both universities and enterprises. University experts provided theoretical insights, while enterprise experts shared practical views on PBSC emission reduction. Their suggestions confirmed the parameter basis's reliability and the parameter setting's reasonableness in theory and practice. Over 80% of the experts are aged 35-55, with over ten years of relevant experience, and their valuable suggestions are highly reliable. The remaining younger experts from enterprises have hands-on expertise in PB projects and can offer realistic advice. The experts' details are as follows: (1) Five university experts from Shenyang, researching PBs, PBSCs, and carbon emissions, with recent publications in authoritative journals. (2) Nine enterprise experts, six from Beijing and three from Baoding, all with practical experience in PB construction and some understanding of PBSC emission reduction. Semi-structured interviews were conducted via telephone or face-to-face, with questions comprising two parts: basic and core questions, which are detailed in Tables A1 and A2, respectively. \begin{table}
\begin{tabular}{c c} \hline
**Question Number** & **Question** \\ \hline
1 & What is your age? \\ \hline
2 & What is your gender? \\ \hline
3 & What is your occupation? \\ \hline
4 & What is your current working area? \\ \hline
5 & How many years have you been engaged in work or research related to PBSC carbon emission reduction? \\ \end{tabular}
\end{table} 
Table A1: Basic questions. Following the expert interviews, we categorized their responses and meticulously refined and analyzed the interview opinions. We extracted and organized the numerical data from some experts' answers. We then employed mathematical statistical methods to integrate the results by calculating the mean value and standard deviation and comparing the common intervals. This process was conducted to verify whether the relevant policies, reports, and bases can support the empirical analysis in this paper. Through expert interviews, we derived the following basic conclusions, demonstrating the scientific validity of the parameter assumption basis and the settings we adopted. 1. Experts generally agree that in PBSCs, X and Y have significant inputs in abatement cost, whereas Z and U have relatively minor inputs. 2. In PBSCs, the value-added benefits of joint abatement are approximately 30-70% of those from individual abatement and rarely exceed 100%. Free-riding in emission reduction does exist but can be mitigated through enhanced corporate prevention measures. Free riders are predominantly downstream firms, though some homogeneous firms also engage in free-riding, with the free-riding benefits typically accounting for 0-50% of individual emission reduction benefits. 3. Experts generally concur that it is challenging for enterprises to gather all market-related emission reduction information, with collection rates typically ranging from 40% to 90%. University-based experts primarily rely on online platforms for information collection. A few corporate experts noted that their companies have proprietary collection channels, information networks, and public channels. The acquisition of most information is either free or low-cost. However, information processing and benefit realization are complex, demanding time, resources, and labor costs. In addition to the costs of information collation, analysis, and verification, additional costs are incurred to achieve benefits.