APPENDIX A1 - Aircraft Emissions Inventory for Criteria  Pollutants  A1.1  Overview  • Taxi/idle  includes the time an aircraft taxis between the runway and a terminal, and all  ground-based delay incurred through the aircraft  taxi route. The taxi/idle-delay mode  1$_{ }$ Aircraft engines produce emissions from the combustion of fuel and these emissions depending on aircraft engine type, fuel type, number of engines, power settings,  and amount of  fuel burned. The criteria pollutants (and their precursors)  of primary significance include  volatile organic compounds (VOCs), carbon monoxide (CO), oxides of nitrogen (NO $_{x}$), sulfur  dioxide (SO$_{2}$), and particulates (PM).  When computing aircraft engine emissions, it is customary to simulate aircraft activity patterns  within the traditional landing and takeoff (LTO) cycle. An LTO cycle consists of the following  operational modes:  Source EPA,  Procedures for Emission Inventory Preparation, Volume IV: Mobile Sources , December 1992, [EPA-420-R-92- 009].  Aircraft Use  Description  Commercial  Includes aircraft used for scheduled service transporting passengers, freight, or both.  Air Taxi  Includes aircraft that fly scheduled service carrying passengers and/or freight but usually  are smaller aircraft and operate on a more limited basis.  General Aviation  (GA)  Includes most other non -military aircraft used for recreational flying, transportation, business travel (usually on an unscheduled basis) and activities.  Military  Covers a wide range of aircraft sizes, uses, and operating missions. While they often are  similar to civil aircraft, they are handled separately because they typically exclusively out of military air bases and frequently have distinctive flight profiles.  Table A1-1. Aircraft Uses Categorization  This appendix discusses the recommended emissions inventory methodologies, models, and  data sources for computing criteria pollutant emissions from aircraft  engines.  Section 3  ( Sources and Types of Air Emissions ) of the  Handbook  provides a description of the  types of criteria pollutants most commonly associated with aircraft engines and  Section 6.1  ( Preparing an Emissions Inventory – Criteria Pollutants ) provides a discussion of the available  guidance for preparing and reporting a criteria pollutant emissions inventory for Federal Aviation  Administration (FAA) projects/actions.  For this discussion, three overall approaches, or methods, are addressed and are referred to as the  (i) Environmental Protection Agency ( EPA), (ii) Emissions and Dispersion Modeling System  (EDMS), and (iii) Aviation Environmental Design Tool (AEDT) methods. First, some common  terms and concepts are discussed that apply to all three methods.  As summarized in  Table A1-1  ( Aircraft Uses Categorization ), for emission inventory  applications, aircraft are typically categorized by their general use as commercial (including air  carrier and cargo), air taxi, general aviation (GA), or military.  APPENDIX A1$_{ }$