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.

• 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, EMFAC2002 v2.2 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. Reactive organic emissions from Bay Area vegetation total about 170 tons per day. 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).

Planning Inventory
A planning inventory is a seasonal inventory representing emissions when a pollutant's concentrations are at their highest levels. For example, the emission inventory for the ozone season represents emissions occurring during the summer season, when ozone levels are highest. The emission inventory for the carbon monoxide season represents emissions occurring during the winter season, when carbon monoxide levels are highest. The seasonal inventories (summer and winter) are prepared according to ARB guidelines.

Table I and Table II contain general statistics for the Bay Area as well as individual county statistics, including population and total area. Light-duty vehicle population, as well as estimated vehicle miles traveled (VMT) and total gasoline sales are also included. VMT figures were obtained from the EMFAC2002 v2.2 mentioned above.

Table III shows total daily summer 2005 ozone planning emissions.
Table IV shows percent contribution from major groups of source categories.
BAAQMD staff work continually to update the emission inventory. More detailed information and estimates for additional years have been included in Section 2 of the San Francisco Bay Area Ozone Strategy for the 1-Hour National Ozone Standard published in January 2006. More detailed emission data are available to the public upon request. Updates on some individual source categories may also be available. Some public requests may involve a charge; this is usually the case if significant staff time, copying, or computer work is required.

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 emissions from sources within the District’s jurisdiction for the following gases: carbon dioxide, methane, nitrous oxides, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride.

This GHG Source Inventory does not report indirect emissions, for example emissions associated with electricity that is consumed in the Bay Area but produced outside of the region. These indirect emissions are of interest to communities developing GHG reduction strategies, and that seek to identify their total “carbon footprint”. The District is working with ICLEI and other organizations to assist communities in calculating both direct and indirect emissions.

Click here to view the Source Inventory of Bay Area Greenhouse Gas Emissions.