## 1 Introduction  
E-commerce is experiencing significant growth in various countries and sectors. This increase in online sales presents never before seen unique logistical challenges for businesses, one of the most critical being the efficient management of the last mile--the final stage of the delivery process that ends at the customer's home or at a designated pickup point [1]. Last-mile logistics (LML) plays a critical role in influencing service levels [2] and contributes significantly to overall logistics costs [3].  
In the search for innovative solutions to address these challenges, automated parcel lockers (APLs) have emerged as a compelling option. APLs are boxes with automated locks, clustered in structures. APLs are often found in public areas such as supermarkets or train stations and serve multiple customers [4]. APLs are a promising alternative to traditional direct delivery (DD) for several reasons: their ease of access to customers [2], their ability to handle higher delivery densities [5], and their significant reduction in the likelihood of delivery failures [6]. Consequently, researchers and practitioners are both exploring the potential benefits of APLs in the e-commerce delivery. Managers also have shown great interest in understanding the economic impact of APLs compared to DD [7; 8].  
The current literature primarily emphasizes delivery cost benefits, but there is still a scarcity of papers addressing vehicle kilometers traveled (VKT), emissions, and customer trips for parcel pick-ups.  
This work develops a simulation model that allows for a comparison between DD and APLs as an LML scheme. Various factors are analyzed that distinguish between these contexts, such as CO2 emissions and VKT, to gain a comprehensive understanding of their implications, as well as taking additional transports for customers to pick up their parcels at the APLs into account. In this context, the approach presented in this paper includes a more detailed model to estimate the emissions of customers to reach APLs.  
Moreover, previous work comparing delivery through APLs and DD is based on an estimated demand of the delivery process that does not accurately reflect real-world scenarios. This paper introduces a more realistic configuration. It investigates the case of the city of Dortmund, which is located in the federal state North Rhine-Westphalia, Germany. With a population of about 600,000 people, it is the seventh largest city in Germany and the 34 th largest city in the European Union. By addressing the described weaknesses and taking a more comprehensive and realistic approach, the research aims to provide valuable insights into the reductions of CO2 emission by using DD and APLs for different delivery contexts and offering more-accurate and applicable conclusions. Addressing all the identified issues will lead to a comprehensive and clear understanding of the advantages that APLs offer over traditional DD. This goal is addressed through the following research questions (RQs):  
* RQ1. What is the average CO2 emission through APLs compared to traditional DD?
* RQ2. How does this comparison change under different demand scenarios and different assumptions regarding customer behavior for picking up parcels?  
To address these questions, the study uses a simulation approach that includes two distinct stages. In the first stage, a comprehensive model has been developed to estimate CO2 emissions for both APLs and DD. In the second stage, a simulation model was applied to provide more-accurate numerical insights to effectively answer the RQs. This simulation-based methodology ensures a thorough analysis of the environmental impact of APL and DD and provides valuable information for informed decision-making.  
The structure of this paper is as follows: First, the results of the literature review are presented. The simulation model and its application are illustrated in the following two sections. Finally, we summarize the conclusions of our work.