Air Pollution Sources

There are three types of air pollution sources: point, area, and line sources. A point source is a single facility that has one or more emissions points. An area source is a collection of smaller sources such as emissions from residential heating within a particular geographic area. A line source is a one-dimensional, horizontal configuration such as roadway. Most emissions emanate from a specific stack or vent. Emissions emanating from sources other than stacks, e.g., storage piles or unpaved lots, are classified as fugitive emissions.

EPA requires that each step develop emissions inventories for all primary pollutants and precursors to secondary pollutants that are classified as criteria or hazardous air pollutants. In clean, rural areas, country-wide emissions totals for individual pollutant species may be all that is needed as long as emissions from large point sources are inventoried separately. For urban areas having severe air pollution problems, gridded emissions inventories are required. An area is divided into grids, typically 5 to 10 km to a side, and area-and line- source emissions are calculated for each grid. Large point sources are listed individually. Such inventories are used as inputs to sophisticated air quality models, employed to develop air pollution control strategies.

Emissions rate for a specific source can be measured directly by inserting sampling probes into the stack or vent and this has been done for most large point sources. It would be an impossible task to do for every source in an area inventory, however. Instead, emission factors, based on measurements from similar sources or engineering mass-balance calculatios, are applied to most sources. An emissions factor is a statistical average or quantitative estimate of the amount of a pollutant emitted from a specific source type as a function of the amount of raw material processed, product produced, or fuel consumed. Emission factors for most sources have been compiled. Emission factors for motor vehicle are determined as a function of vehicle model year, speed, temperature, etc. The vehicles are operated using various driving patterns on a chassis dynamometer. Dynamometer-based emissions data are use in EPA's MOBILE 4 model to calculate total fleet emissions for a given roadway system.