An emission inventory is an itemized list of emission estimates for sources of air pollution in a given area for a specified time period. Present and future year inventories are critical components of air quality planning and modeling. The ultimate goal of the planning process is to identify and achieve emission patterns that do not result in violations of ambient air quality standards.

Emissions are calculated by BAAQMD staff using various procedures. Emission computation methodology by source category is outlined in the BAAQMD publication, "Source Category Methodologies." The BAAQMD participates in the California Emission Inventory Technical Advisory Committee (EITAC) and maintains the best available inventory methodologies.

The emission inventory is divided into stationary, mobile, and biogenic sources.

Stationary Source Emissions
Stationary sources are further divided into point and area sources.

• Point Sources
  Sources identified on an individual facility basis or as a single source are called point sources. Refineries and industrial plants are examples of point sources. The emission characteristics of individual facilities vary widely and each facility is examined individually. The Permit Services Division of the BAAQMD collects and maintains a computer data bank with detailed information on point sources. Almost all facilities emitting greater than 2.5 tons/year of any air pollutant are included. The District data bank contains information for about 4,000 facilities.  Data on the activity, seasonal variations, and hours of operation are collected from each facility. Parameters which affect the quantity of emissions are updated regularly. Emissions are calculated using detailed data for each of the facilities by various processes. The emissions from combustion and other general processes are computed using generalized or specific emission factors. These factors are periodically reviewed and updated.

• Area Sources
  Stationary sources not identified individually are called area sources. This term is sometimes extended to cover groups of numerous small point sources, such as dry cleaners or gas stations. Area sources also include the diverse, unpermitted small sources which individually do not emit significant amounts of pollutants but which together make an appreciable contribution to the emission inventory. Examples of area sources are residential heating and use of paints, varnishes, and consumer products. Emissions from these sources are grouped into categories and calculated based on surrogate variables. Information on these surrogates is usually available for the state or by county. Selected surrogates are used to apportion the category emissions into diurnal and spatial patterns. Emissions for some source categories are estimated by the California Air Resources Board (ARB) based on statewide data.

  Many area source categories are further classified into subcategories for better emission computation, specification, regulation development, and future-year projections. For example architectural coating categories are subdivided into various types of coatings and varnishes to account for varying solvent content. There are more than 800 different subcategories used in this inventory. Emissions for categories affected by regulations are adjusted to reflect the controls required and the estimated rule effectiveness achieved.

Mobile Source Emissions
Mobile sources consist of on-road motor vehicles and other mobile sources.

• On-Road Motor Vehicles
  On-road motor vehicles consist of passenger cars, trucks, buses and motorcycles. Emissions from on-road motor vehicles are a major portion of the emission inventory and are estimated using computer models developed by ARB. These are referred to as EMFAC and BURDEN, the latest available version, EMFAC2007 v2.3 was used in this inventory.

• Other Mobile Sources
  These sources include boats and ships, trains, aircraft, garden, farm and construction equipment. Various methodologies are used to estimate emissions from these sources. Emission factors and methodologies for these sources are provided by ARB and EPA. Aircraft mix and activity data specific to each airport are used in estimating emissions at airports.

Biogenic Sources
In addition to man-made air pollution, there are significant quantities of pollutants from natural sources such as plants, animals, marshes, and the earth itself. Vegetation for example, emits large amounts of isoprene, terpenes, and other organic compounds, which are precursors of ozone. The emission estimates are developed using a personal computer version of the Biogenic Emissions Inventory System (BEIGIS) developed by the California Air Resources Board (ARB).

Click here to view the latest Emissions Inventory Summary Report published in December 2008.

Greenhouse Gas Emissions

The District has also prepared an emission inventory of pollutants contributing to climate change, or greenhouse gases (GHG). The Greenhouse Gas Source Inventory estimates direct and indirect emissions from sources within the District’s jurisdiction for the following gases: carbon dioxide, methane, nitrous oxides, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride.

This emissions inventory estimates GHG emissions produced by the San Francisco Bay Area in 2007.  This inventory updates the Air District's previous GHG emissions inventory for base year 2002 (published November 2006).  All activity data has been updated to reflect more current industrial activity, motor vehicle travel, and economic and population growth.  Most of the methodologies for calculating emissions remain the same, with certain exceptions: 1) emissions from electricity consumed in the Bay Area but generated outside the region are now included; 2) emissions for high global warming potential gases such as Hydrofluorocarbons and Perfluorocarbons used as refrigerants etc. are now included; 3) more complete oil refinery process emissions are included in this inventory; 4) certain off-road equipment, such as construction and industrial, was previously included in the transportation sector and is now reported separately; 5) ship emissions are now calculated for travel within 100 miles of California's coastline rather than 3 miles to be consistent with the Air District's criteria pollutant inventory and; 6) biogenic CO2 emissions are calculated but not included in the total CO2 equivalent estimates for the region.

Because of these revisions, caution should be used in comparing this 2007 GHG inventory to the previous 2002 inventory.  These revisions have resulted in different estimates of Bay Area GHG emissions.  Consequently, the percentage contributions from individual sectors may be affected.  For example, estimates of transportation emissions have not changed significantly between the two inventories, but the percentage from the transportation sector has changed because the estimated total emissions are greater in the updated inventory (due mainly to increased estimated emissions resulting from the revisions summarized above).  Such ongoing updates are typical of emission inventories.  Examining emission forecasts and backcasts in a single emission inventory is more useful in determining trends than comparing one inventory against another.

Click here to view the updated Source Inventory of Bay Area Greenhouse Gas Emissions (2007 Base Year).

Click here to view the Source Inventory of Bay Area Greenhouse Gas Emissions (2002 Base Year).

Contacts
Amir Fanai afanai@baaqmd.gov