# Port-Related Shipping Gas Emissions--A Systematic Review of Research
## 2 Review Methodology  
At the very beginning of the review process of the literature in which shipping emissions in ports were explored, it was possible to notice that various methodologies and data were applied, but with frequent similarities and mutual reference connections. Furthermore, it was recognised that the approaches and datasets used were mainly obtained from other studies. Therefore, in order to find a valid port-related ship emissions estimation methodology, a systematic review of the relevant literature needed to be carried out. That is why PRISMA guidelines were followed in this paper according to the proposed bottom-up multi-layered analysis approach presented in Figure 1.  
The review process began with keyword thread search of literature in the Web of Science Core Collection, Scopus and Google Scholar databases, using combinations of terms that included: port, ship, emissions, inventory, gas, pollution, quantification, method. After  
Figure 1: Bottom-up multi-layered analysis approach. record screening, selected reports went through the analysis process, wherein references to the methods and datasets obtained from different sources were collected. These reference strings were used in the second review, expanding the search to websites of relevant organizations whose studies were cited. By applying both keyword and reference literature identification approaches, metrologies and data used in selected papers could finally be connected with the original sources, so that after completing the second screening it was finally possible to produce a full overview of the selected literature with the original sources of methods and datasets applied in them, thus finishing the first layer of the overall analysis. The complete review and analysis process is displayed in Figure 2 and a detailed explanation is provided below. The overview of the selected papers and their references were examined in the second layer, with the aim of determining the most influential methods and datasets through the quantification of their original sources. To ensure the relevance of reference quantification, the mutual citing connections between all sources used in the selected papers were first analysed with the aim of duplicate exclusion. This resulted in defining multiple sources that used the same methods and/or key data as an individual source. By performing mutual-referencing analysis, the exact number of different methodologies and key datasets cited were determined. Thus, the most influential sources could be defined and thoroughly examined. Figure 2: PRISMA 2020 analysis flow diagram for new systematic reviews which included searches of databases, registers and other sources [19] * According to the PRISMA glossary of terms, a study is defined as a larger scientific document that might have multiple reports, while a report is a document that supplies information about a particular study, such as a journal article, a conference abstract, a preprint, etc. [18]. That is why, in this research, the term “study” stands for large emission inventories that were mostly used as reference sources for methodologies and data. Accordingly, “report” is defined as a scientific paper of the sort reviewed in this research. ** Records that were identified were from the Web of Science Core Collection, Scopus and Google Scholar databases. *** Since no automation tool was used, all exclusions of literature were carried out by the authors of this paper. A review of the most prominent studies was performed in the third layer of the process through an analysis based on the methodologies and datasets used in the selected literature. The analysis aimed to define the advantages and similarities of the most frequently applied approaches and datasets from the examined studies. This validation process enabled the determination of all vital components necessary for quantifying gas emissions from ships in ports.