Basis for Determining 1996 Toxics Emissions, CTI

The California Toxics Inventory (CTI) has been developed as a tool in the Air Resources Board's (ARB) efforts to assess the healthfulness of the air Californians breathe. The CTI contains emission estimates for stationary sources, area sources, and mobile sources (both on-road and off-road). The stationary source data are drawn primarily from the point source data associated with the Air Toxics "Hot Spots" Program and collected by the local air districts. Because these estimates are based on information provided by the air districts throughout the State, you may want to contact them directly for more current data. The area and mobile source components of the CTI have been estimated by the ARB. These estimates are for the 1996 inventory year.

The CTI on the web page is presented on a county-by-county basis. There are a number of counties in the State that are part of more than one district or air basin. If a person is interested in district-specific (or air basin-specific or source type-specific) emissions estimates will need to contact ARB staff.

These data are preliminary and subject to change. As new estimation methods are developed or refined, these changes are incorporated into emission inventory database. That database is the basis for the CTI. The ARB will recalculate the emission estimates when significant changes are made. A revised on-road emission estimation model is in the works and the CTI will be recalculated when revisions to that model are finalized. The ARB staff also solicits input from district staff on the validity of the estimates and those estimates may be revised based on district comments.

There may be those that wish to compare the CTI with the U.S. EPA's National Toxics Inventory (NTI). ARB staff is recommending that that be done with caution. The CTI is significantly different from the NTI in the way the emission estimates are segregated and calculated.

The primary difference between the two databases in the estimation methodology. The CTI is based on California specific information collected and developed by the ARB and the local air districts over the years. The NTI is calculated on a national basis and apportioned to counties in California based on population or employment within certain industries. Another difference is in the way the ARB and EPA define the point sources included in the stationary sources category (EPA calls these facilities "major sources"). The ARB definition is more inclusive and the ARB point source data contains facilities considered area sources by the EPA. Which leads to a third difference in the two databases, the differences between the area source categories. The ARB's area source categories are California-specific and therefore do not match up well with the EPA's nationwide area source categories. Finally, the EPA and ARB use different models to estimate mobile emissions (both on-road and off-road). Since mobile sources are the major contributors of a number of toxic substances, those differences tend to be significant for substances like benzene, formaldehyde and

1,3-butadiene.

The CTI is another step in the ARB's efforts to provide the public with information that will help them assess their quality of life in California.