California Air Resources Board

Source Categories Used in Risk Maps

Page Reviewed July 28, 2010
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Emission source categories: Introduction: Top of Page

In order to determine where emissions and inhalation cancer risk are coming from, it is necessary to group pollution sources into general categories. The general categories are onroad and offroad mobile, industrial, nonindustrial, and biogenic emissions sources.

Each of these categories has its diesel component split out separately, because it has such a high risk factor associated with it.

Onroad mobile sources:

Automobiles, trucks, motorcycles, as well as other motorized vehicles that travel on the roadways, are in this category. This is a major component of the emissions inventory and the largest contributor to risk in many areas.

Offroad mobile sources:Top of Page

This category includes such sources as offroad mobile equipment, aircraft, trains, ships, and boats. Because of the large amount of diesel emissions attributed to this category, it is very important in risk analysis. 

Industrial sources:

This category contains stationary emission sources that are usually larger than individual nonindustrial sources defined, below. This category is sometimes referred to as "stationary point sources." Industrial sources may have specific geographical coordinates assigned. They may also reflect emissions that have been geographically distributed using demographic and/or business category distributions. The amount of emissions coming from industrial sources varies widely. A great deal of work has been done to correctly assign these sources to geographical coordinates. However there are still a large number of these sources for which accurate geographical coordinates are not available to ARB staff. Work is ongoing to try to improve industrial source geographic assignment.

Nonindustrial sources (paints, solvents and consumer products): Top of Page

Nonindustrial source emissions typically come from the use of paints, solvents, as well as consumer products. This category is sometimes referred to as "area sources." Residential wood combustion and various other types of widely dispersed emission sources are also included in this category. For this category, the region-specific emissions are distributed geographically by using surrogates, such as population, employment, land-use, etc.

Biogenic sources: Top of Page

The biogenic source category consists of reactive organic gas emissions from trees, agricultural crops and other plants. This category is not ready for inclusion into the toxic risk analysis. As knowledge of this category continues to develop, its inclusion in the toxic risk analysis will be reexamined.

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A department of the California Environmental Protection Agency