type, and correcting fo r the reduction of pollutants by any air pollution control equipment in  operation.  In addition, the SO$_{2 }$emission factors are reflective of the fuel sulfur content of the fuel, which  may be obtained from the fuel supplier and/or the airport operator . PM emission factors are  reflective of the fuel ash content of the fuel. This may be obtained from the fuel supplier, airport  operator, or may be taken from calculation of default ash content of fuels as described in Chapter  1.3,  Fuel Oil Combustion  of EPA’s AP-42 21 .  The methodology for obtaining the appropriate emission factors is expressed by  Equation A5-2  ( Emission Factors for Boilers and Heaters ).  Generators at airports typically are fixed in place and located throughout the  airport to provide  supplementary or emergency power. These generators are likely powered by gasoline or diesel - fueled reciprocating internal combustion engines.  There are two methodologies to calculate emissions from generators: (i) EPA’s AP-42 method,  which calculates emissions based on the capacity rating  (in horsepower) of generator engines,  and (ii) the U.S. Air Force’s method, which calculates emissions based on the amount of fuel  consumed.  Data inputs required for the emissions calculation for both met hodologies (e.g., capacity of  generator engine, time of gene rator usage, type and quantity of fuel burned, and  air pollution  control factor) for each generator should be obtained from the airport operator, the equipment  manufacturer, and/or the fuel supplier.  24$_{ }$ 21  EPA, AP-42, Chapter 1.3,  Fuel Oil Combustion ,  http://www.epa.gov/ttnchie1/ap42/ch01/final/c01s03.pdf.  A5.2.2.1  Methodology  A5.2.2  Generators  Equation A5-2. Emission Factors for Boilers and Heaters  Where:  EF = emission factor of pollutant (i.e., CO, SO$_{2}$, PM$_{10}$, PM$_{2.5}$, NO$_{x}$, or VOC), expressed in  pounds per thousand gallons (for liquid fuels), million cubic feet (for natural gas), or ton of  fuel (for coal).  UEF = uncontrolled emission factor of pollutant, expressed in pounds per thousand gallons  (for liquid fuels), million cubic feet (for natural gas), or ton of fuel (for coal).  CF = air pollution control factor, expressed as a percentage.  FM = fuel modifier (fuel weight percent sulfur for SO$_{2 }$ emission factor; fuel weight percent  ash for PM emission factor; ignore for other pollutants).  EF = UEF x (1- CF/100) x FM  APPENDIX A5$_{ }$