## 2 Materials and Methods
### Life Cycle Assessment  
Life cycle assessment follows the trend presented in the ISO 14044 standard [24]. LCA includes four stages: goal and scope definition, life cycle inventory, evaluation of the consequences, and interpretation of the results. The goal and scope definition discusses how (scope) and why (goals) an LCA is carried out. The goal definition determines the benefits of the study; the scope determination copes with the description of the studied functional unit, production system and its boundaries, data collection and processing, and the environmental consequences. The inventory step examines the natural sources and other inputs of the system, as well as pollutant emissions and the other outputs of the process. The consequence evaluation step involves the presentation of the natural sources and environmental emission inputs in terms of their contribution to the intended sector. The results of the previous steps are finally interpreted in the last step [39].  
The functional unit is a key concept in LCA studies; it enables the comparison of various products and services (ISO, 2006). The functional unit of the present research is based on mass and is determined as the production of one ton of lettuce within one agricultural year.  
Selection of the system boundaries is one of the prominent and essential steps in goal and scope determination. Determination of the boundaries of the research is essential for a more precise calculation of the emission due to the consumption of the farm or after harvesting and exiting the farm during the processing steps [40]. LCA is an insight from start to finish, but it is possible to consider the system boundary as a part of the total system for more focus on the processes. In this way, the results are expressed based on their selected boundaries on a smaller scale [38]. In this study, the farm gate was taken as the system boundary. The boundary of the system is shown in Figure 2.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Input and Output** & **Unit** &
\begin{tabular}{c} **Energy Equivalent** \\ **(Megajoules per Unit)** \\ \end{tabular} & **Source** \\ \hline \multicolumn{4}{c}{**Inputs**} \\ \hline Labor & H & 1.96 & [PERSON], et al. [30] \\ Machines & Kg & 142.7 & [PERSON], et al. [31] \\ Diesel & L & 56.31 & [PERSON], et al. [32] \\ Electricity & kWh & 12 & Albright and [PERSON] [33] \\ Nitrogen & Kg & 66.14 & [PERSON], et al. [34] \\ Phosphorus & Kg & 12.44 & [PERSON], et al. [35] \\ Potassium & Kg & 11.15 & [PERSON], et al. [36] \\ Pesticide & Kg & 120 & [PERSON] [37] \\ Seeds & Kg & 16.7 & [PERSON] and [PERSON] [33] \\ \hline \multicolumn{4}{c}{**Output**} \\ \hline Lettuce & Kg & 0.7 & [PERSON], et al. [38] \\ \hline \hline \end{tabular}
\end{table}
Table 1: All the energy value inputs and outputs in lettuce production.  
Table 2 lists the impact categories of this study. The environmental consequences of lettuce production were analyzed based on the CML 2 baseline 2000 (Pere 2013).  
Data used to assess the environmental impact of a production process are divided into two categories: data related to farm operations and data related to the production of inputs used. The data relating to farm operations are the energy assessment data collected by completing the questionnaire. The data associated with producing consumable inputs were also obtained from the databases available in the SimaPro life cycle assessment software.