The study conducted to deter-
mine the emissions of a particular pollutant is referred to as an emission inventory.
The inventory may include only selected sources, all anthropogenic sources, or all
sources, including natural sources, dependent on the specific objectives of tile
inventory. Preparation of an emission inventory, which is a key element in developing
air quality management strategies, requires detailed knowledge of the emissions
from the relevant sources. This is generally done through the use of emission
factors. Emission factors are defined as the typical emissions (usually in
mass/time or mass per unit of process input or output) that are emitted by a particular
source type based on the specific pollutant, type of process, age, size, type
of control technology in existence, and other pertinent factors affecting the emissions
from the source. Historically, emission factors have been determined
through evaluation of the various types of sources by numerous researchers. As a
result of the critical need for emission factors as a fundamental building block in
emission inventories, the U.S. EPA has compiled these factors into a single multi-
chapter document Compilation of Air Pollutant Emission Factors (AP—42) that
has been maintained on an ongoing basis since the early 1970s. (See the following
section for more details.)