engines, power settings, amount of fuel burned, and other variables (e.g., temperature, altitude, etc.). 

Computational methods and modeling t echniques for assessing aircraft engine emissions are discussed in Section 6 ( Preparing an Emissions Inventory ) and Section 7 ( Conducting Dispersion Modeling ). For detailed instructions and guidance on how to compute aircraft engine emissions refer to Appendix A1 ( Aircraft Emission Inventory for Criteria Pollutant s ) of this Handbook. 

## 3.1.2. Auxiliary Power Units 

Auxiliary Power Units (A PUs) are smaller turbine engines that provide power to an aircraft during approach, while taxiing, or positioned at the terminal gate. APUs power the aircraft 's instruments, lights, heat and air conditioning, and other equipment and are used for starting the main aircraft’s engines. APUs burn jet fuel and is common on both large commercial and military aircraft and some air taxi and GA aircraft. 

APUs typically remain in use while the aircraft is parked and serviced at the gate, unless an alternative source of electricity and preconditioned air is made available. In such cases, APUs are reactivated at least five to ten minutes before the aircraft leaves the gate or parking space. Normally, APUs are turned off after the main engines have been started, prior to takeoff. 

Computational methods and modeling techniques for assessing APU emissions are discussed in Section 6 ( Preparing an Emissions Inventory ) and Section 7 ( Conducting Dispersion Modeling ). For detailed instruction and guidance on how to compute APU emissions refer to Appendix A2 (Auxiliary Power Units Emission Inventory for Criteria Pollutants ). 

## 3.1.3. Ground Support Equipment 

Ground Support Equipment (GSE) is a term used to describe the equipment that service aircraft while loading and unloading passengers and freight at an airport. The types of GSE and their use depend on the aircraft type and the designated category of an aircraft operation (e.g., passenger, cargo, etc.). GSE consist of aircraft tugs, air start units, forklifts, tractors, air-conditioning units, ground power units (GPUs), baggage tugs, belt loaders, fuel or hydrant trucks, catering trucks, cabin trucks, deicer trucks, water trucks, lavatory trucks, and cargo loaders, among others. 

Air emissions resulting from the operation of GSE vary depending on the number and type of equipment used to service each aircraft, along with the duration of equipment operation aircraft landing-takeoff (LTO) cycle and the fuel type (e.g., gasoline, diesel, propane, electric, etc.). 

Computational methods and modeling techniques for assessing GSE emissions are discussed in Section 6 ( Preparing an Emissions Inventory ) and Section 7 ( Conducting Dispersion Modeling ). For detailed instruction and guidance on how to compute GSE emissions refer to Appendix A3 ( Ground Support Equipment Emission Inventory for Criteria Pollutants ). 

## 3.1.4. Ground Access Vehicles 

Ground access vehicles encompass motor vehicles traveling upon on- and off-airport roadways, within airport parking facilities, and idling along terminal curbsides. These include airport passenger vehicles ( e.g., private autos, taxis/limos, shuttles, vans, buses, rental cars, etc.); 

