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
The emission inventory is divided
into stationary, mobile, and biogenic sources.
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
- 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
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.
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.
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 Districtís jurisdiction
for the following gases: carbon dioxide, methane, nitrous oxides, hydrofluorocarbons,
perfluorocarbons, and sulfur hexafluoride.
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).
to view the Source Inventory of Bay
Area Greenhouse Gas Emissions (2002 Base Year).
Amir Fanai firstname.lastname@example.org