## 4 Greenhouse Gas Emissions  
As the construction industry has come under increasing scrutiny for its GHG emissions, interest in evaluating pipeline projects has increased. Several studies in the past decade have documented and quantified GHG emissions, and different models have been developed to estimate the emissions of pipeline construction activities. Some of the most widely recognized models are those developed by the EPA's Nonroad model in 2010 and the California Off-Road model [31]. A study used the Environmental Protection Agency (EPA) Nonroad model for the quantification of emissions from equipment and transportation involved in a utility installation using HDD. An emissions calculator, implemented based on the EPA model, was employed whereby site-specific information and equipment operating hours were used as inputs to calculate overall emissions. This model thus acts as a useful tool by policymakers in weighing different construction methods based on estimated emissions and thus aiding in minimizing airborne pollution from future pipeline installations. [31; 32; 33]. The USEPA also provided a standard formula to estimate the quantity of GHG emitted by construction equipment. It is given by Equation (1) below:  
\[Emissions_{x}=EF_{x}\times HRS\times HP\times LF \tag{1}\]  
where:  
\(Emissions_{x}\) means total emissions for pollutant x in gallons;  
\(EF_{x}\) represents the emission factor for pollutant x in grams per hp-hr;  
\(HRS\) denotes the number of hours the equipment is used;  
\(HP\) denotes the average rated horsepower of the equipment;  
\(LF\) represents the load factor or operating hp/maximum rated HP. This equation provides an accurate quantification of GHGs for the specific pollutants such as CO\({}_{2}\), sulfur dioxide (SO\({}_{2}\)), nitrogen oxides (NO\({}_{x}\)), carbon monoxide (CO), particulate matter (PM), and high hydrocarbon (HC) by the amount described in the equations of the emission factor [33]. This provides a systematic approach to identifying and mitigating environmental impacts in construction activities [31]. The transportation footprint for pipeline construction projects can be quantified using a standardized Equation (2), which estimates emissions from transportation activities. This methodology has been presented as follows:  
\[Emissions_{x}=EF_{x}\times n\times(DO+DR) \tag{2}\]  
where:  
\(Emission_{x}\) is the transportation emissions for pollutant x, grams/mile;  
\(EF_{x}\) represents the emission factor (EF) for pollutant x, g/mi;  
\(n\) is the number of trips needed to move materials and equipment;  
\(DO\) is the one-way distance to the site;  
\(DR\) is the return distance from the site. The USEPA has given complete sets of EF formulas, transportation-based, which are specifically designed for CO\({}_{2}\), SO\({}_{2}\), NOx, PM, and HC. These EF values are the keys to quantifying transportation emissions properly. This formula can be utilized for evaluating and potentially reducing the environmental consequences of material and equipment transportation at a construction site [31; 34; 35]. Pipeline installation (PI) activities contribute to the increasing atmospheric concentration of CO\({}_{2}\) and other GHGs, which are major drivers of global warming [31; 36]. The phenomenon, commonly referred to as the \"greenhouse effect\", makes Earth habitable by trapping heat. However, human activities have altered the chemical composition of theatmosphere, primarily through the accumulation of greenhouse gases. These gases act just like the glass of a greenhouse, allowing sunlight in while prohibiting the heat from escaping back out [31, 36]. In the U.S., CO\({}_{2}\) comprised 82% of all human-caused GHG emissions in 2013 [31, 37]. Most of these are the result of burning fossil fuels--coal, oil, and natural gas--used in producing electricity, in transportation, and in industrial processes. Other important GHGs are CH\({}_{4}\), N\({}_{2}\)O, BC, and a suite of fluorinated gases.