their determination of  air quality on an airshed scale or
Criteria Pollutants
at a particular location can be limited.
Criteria pollutants are those which are widespread, com-
Emissions inventories are a compilation of  estimates of
mon to or associated with many anthropogenic activities
the amounts and types of  pollutants emitted by various
and for which there are established national ambient
sources over a given period within a certain geographical
quality standards or objectives for the protection of
region.  Emission inventories are comprised of  informa-
human health and the environment.
tion from point (or stationary) sources, area sources and
mobile sources.  Point sources are generally large
In the United States, the Criteria pollutants, as designated
industrial or public sector facilities (e.g. coke operations,
under the Clean Air Act, are: Ground Level Ozone;
petroleum refineries, municipal refuse incinerators).
Sulphur Dioxide; Nitrogen Dioxide; Carbon Monoxide;
Area sources are an aggregation of  several smaller
Particulate Matter; and Lead.  The Canadian list is similar,
commercial operations (e.g. dry cleaning, auto body
but does not include lead.  Although not considered a
shops) within a select geographic region.  Individually,
criteria pollutant, a discussion of  Total Reduced Sulphur
these latter sources may emit only small amounts of
is included in this report as it has been a prevalent
pollutants, but collectively their contribution can be
pollutant in the Windsor and Sault Ste. Marie areas and
significant.  Mobile sources are separated into two
has been part of  the calculation of  the AQI in Ontario.
categories, on-road and off-road.  On-road sources
includes cars, buses, trucks, and motorcycles.  Off-road
In the United States, the Clean Air Act, last amended in
sources include railway locomotives, aircraft, ships and
1990, also required the U.S. EPA to set National Ambient
boats, and construction equipment.
Air Quality Standards (NAAQS) for pollutants considered
harmful to human health and the environment.  The
Models use mathematical formulas to link a regionís
Clean Air Act established two types of  air quality stan-
meteorological conditions, physical features and emis-
dards (Ref. 3).  Primary standards were established to
sion sources and estimate regional air quality.  Models
protect public health, including the health of  sensitive
provide estimates of  the ambient air quality characteris-
populations such as asthmatics, children, and the elderly.
tics, including prevailing concentrations.  Models should
Secondary standards were set to protect public welfare,
be validated with ambient measurements.  Air quality
including protection against decreased visibility and
modeling is a necessary tool for estimating current and
damage to animals, crops, vegetation, and buildings.  Both
future air quality trends and devising appropriate control
standards will be discussed in this document and are listed
in Table 1.
Ambient air quality monitoring is crucial in order to
A geographic area that meets the NAAQS for a particular
provide information needed to assess whether a region
pollutant is referred to as being in attainment: those that
is currently complying with the applicable ambient air
do not meet the NAAQS are called nonattainment areas.
quality criteria, largely established to protect human
States containing nonattainment areas are required to
health.  Emission inventories and modeling can provide
submit air quality plans, known as State Implementation
insight on air quality on larger scales and predict air
Plans (SIPs) for approval by the U.S. EPA  detailing how
quality or estimate it in areas where the ambient moni-
they intent to attain the standard.  Failure by a state to
toring network is not adequate to entirely determine the
meet these and other requirements is to be penalized in
prevailing air quality.  All of  these tools are necessary for
accordance with the Clean Air Act.  Penalties can include
effective air quality management.
a reduction of  federal support to the nonattainment areas,
such as funding of  highway construction.
Pollutants of  Interest
In Canada, the provincial and territorial governments are
primarily responsible for many air quality issues.  As a
For the purpose of  this report, air contaminants will be
result, these governments adopt their own air quality
divided into two groups:  criteria or common air con-
objectives which may or may not be consistent with those
taminants and airborne toxic or hazardous air pollutants.
promulgated by the federal government.  The Ambient