concentrations in the different microenvironments
are not known. The term “average exposure” is
therefore commonly used, and this is normally
taken to mean the pollutant concentration over a
specified period (e.g., annual mean) at an outdoor
location which is broadly representative of where
people are likely to spend time. This approach is
used in the regulation of ambient air quality.
Once the pollutant has crossed a physical
boundary (e.g., alveolar epithelial cells), the concept of “dose” is used [25]. The dose is the mass of
material absorbed or deposited in the body during
an interval of time, and depends on the respiratory
activity of the individuals concerned. Responses
to doses – the actual health effects – can also vary
from person to person, depending on physiological conditions.
Emission Processes
Combustion
Most vehicles are powered by internal combustion engines (ICEs). Notable exceptions include
vehicles powered by electricity drawn from an
external grid (as in the case of rail locomotives),
from on-board batteries (as in the case of road
vehicles and waterborne vessels) or, more
recently, from in situ generation involving fuel
cells.
The principles of ICEs apply equally to road
vehicles, rail locomotives, and waterborne vessels, the main differences being the size of the
engine and the type of fuel used. In an ICE energy
is derived from the burning of hydrocarbon fuel in
air, with the main products being CO 2 and water
vapor. Incomplete combustion results in the production of CO, PM (especially in the case of diesel
engines), and many different organic compounds.
In addition, at the high temperatures and pressures
found in the combustion chamber, some of the
nitrogen in the air is oxidized, forming mainly
nitric oxide (NO) with some NO 2 . By convention,
the sum total of oxides of nitrogen (i.e., NO +
NO 2 ) is termed NO x .
NO formation proceeds via two main mechanisms: the “thermal” (Zel’dovich) and “prompt”
mechanisms [26], with the former being responsible for more than 90% of emissions [27]. NO 2 is
predominantly a secondary pollutant, its major
source being the atmospheric oxidation of
combustion-derived NO. The NO 2 emitted
directly from vehicles is referred to as “primary
NO 2 ,” and its importance will be described later.
Particles in diesel exhaust cover a range of
sizes, and the shape of the size distribution
depends on whether the weighting is by number
or mass (Fig. 6). There are three distinct size
modes: the nucleation mode (sometimes referred
to as nuclei or nanoparticles), the accumulation
mode, and the coarse mode. The nucleation mode
has traditionally been defined as particles with a
diameter of less than 50 nm. Accumulation mode
particles range in size from around 50 nm to
around 1 mm, with particles smaller than 0.1 mm
being referred to as ultrafine particles. When a
nucleation mode is observed (this is not always
the case in engine exhaust), it contains many more
particles than the accumulation mode, although
because each particle is so small the total mass
will not increase substantially. The coarse mode
consists of particles larger than around 1 mm.
Emissions
Activity data
Emission factors
Ambient
concentrations
Atmospheric conditions
Human activity
Exposure
Respiratory conditions
Dose
Health effects
Physiological conditions
Air Quality, Surface Transportation Impacts on,
Fig. 5 Simplified pathway from transport emission to
health effect. (Adapted from [23])
Air Quality, Surface Transportation Impacts on
53
are not known. The term “average exposure” is
therefore commonly used, and this is normally
taken to mean the pollutant concentration over a
specified period (e.g., annual mean) at an outdoor
location which is broadly representative of where
people are likely to spend time. This approach is
used in the regulation of ambient air quality.
Once the pollutant has crossed a physical
boundary (e.g., alveolar epithelial cells), the concept of “dose” is used [25]. The dose is the mass of
material absorbed or deposited in the body during
an interval of time, and depends on the respiratory
activity of the individuals concerned. Responses
to doses – the actual health effects – can also vary
from person to person, depending on physiological conditions.
Emission Processes
Combustion
Most vehicles are powered by internal combustion engines (ICEs). Notable exceptions include
vehicles powered by electricity drawn from an
external grid (as in the case of rail locomotives),
from on-board batteries (as in the case of road
vehicles and waterborne vessels) or, more
recently, from in situ generation involving fuel
cells.
The principles of ICEs apply equally to road
vehicles, rail locomotives, and waterborne vessels, the main differences being the size of the
engine and the type of fuel used. In an ICE energy
is derived from the burning of hydrocarbon fuel in
air, with the main products being CO 2 and water
vapor. Incomplete combustion results in the production of CO, PM (especially in the case of diesel
engines), and many different organic compounds.
In addition, at the high temperatures and pressures
found in the combustion chamber, some of the
nitrogen in the air is oxidized, forming mainly
nitric oxide (NO) with some NO 2 . By convention,
the sum total of oxides of nitrogen (i.e., NO +
NO 2 ) is termed NO x .
NO formation proceeds via two main mechanisms: the “thermal” (Zel’dovich) and “prompt”
mechanisms [26], with the former being responsible for more than 90% of emissions [27]. NO 2 is
predominantly a secondary pollutant, its major
source being the atmospheric oxidation of
combustion-derived NO. The NO 2 emitted
directly from vehicles is referred to as “primary
NO 2 ,” and its importance will be described later.
Particles in diesel exhaust cover a range of
sizes, and the shape of the size distribution
depends on whether the weighting is by number
or mass (Fig. 6). There are three distinct size
modes: the nucleation mode (sometimes referred
to as nuclei or nanoparticles), the accumulation
mode, and the coarse mode. The nucleation mode
has traditionally been defined as particles with a
diameter of less than 50 nm. Accumulation mode
particles range in size from around 50 nm to
around 1 mm, with particles smaller than 0.1 mm
being referred to as ultrafine particles. When a
nucleation mode is observed (this is not always
the case in engine exhaust), it contains many more
particles than the accumulation mode, although
because each particle is so small the total mass
will not increase substantially. The coarse mode
consists of particles larger than around 1 mm.
Emissions
Activity data
Emission factors
Ambient
concentrations
Atmospheric conditions
Human activity
Exposure
Respiratory conditions
Dose
Health effects
Physiological conditions
Air Quality, Surface Transportation Impacts on,
Fig. 5 Simplified pathway from transport emission to
health effect. (Adapted from [23])
Air Quality, Surface Transportation Impacts on
53