principal lachrymator (tear inducing) component
of photochemical smog. PAN is formed from the
reaction between peroxy acetyl radicals, CH 3 C
(O)OO, and NO 2 , and in terms of the chemical
cycling acts as a temporary reservoir for the HO x
and NO x radicals involved in ozone production.
A key feature of the equilibrium between PAN
and its precursor species is the strong temperature
dependence, with lower temperatures favoring
PAN stability. A scenario which arises then is
that PAN is formed in a polluted urban area,
transported aloft (where temperatures are lower
and PAN is stable) over long distances, before
airmass descent and warming leads to PAN
decomposition and release of the component
NO x in otherwise unpolluted regions.
Sulfur dioxide (SO 2 ) is possibly the classic air
pollutant formed from the combustion of sulfurcontaining fuels. SO 2 emissions have been successfully reduced in many regions, through a
combination of the use of low sulfur fuels, and
application of flue gas desulphurization technologies on major plants (e.g., power stations). Within
the atmosphere, SO 2 is processed to H 2 SO 4 on a
timescale of days, both in the gas phase, initiated
by reaction with the OH radical, and in the condensed phase, within water droplets, where the
oxidation reactions are driven predominantly by
hydrogen peroxide, H 2 O 2 . Gas phase sulfuric acid
has a very low vapor pressure, and is readily
incorporated into aerosol and condensation, raising PM acidity. More widely, co-condensation of
H 2 SO 4 with water vapor and/or ammonia leads to
the production of new particles in the atmosphere.
Condensed Phase Composition Processes
Particulate matter is an important component of
urban atmospheric composition, in particular
where health effects are concerned. Sources of particulate matter include the local primary emissions
from vehicles and combustion described above,
including mechanical sources related to vehicle
brake pad/disc, tyre and road surface wear, wider
atmospheric inputs such as wind-blown dust and
crustal material, and secondary contributions arising
from the condensation of low-volatility gases to
form new or (predominantly) contribute to the
growth of existing particles.
Levels of particulate matter or aerosol are usually expressed as a mass concentration below a
certain size; the size ranges commonly selected
relating loosely to the extent to which particles
penetrate the human respiratory system. PM 10 and
PM 2.5 define the fractions of suspended particulate matter with (aerodynamic) diameters below
10 and 2.5 mm respectively (the aerodynamic
diameter is a concept used to allow for the varying
morphology or shape of particles; a particle with
an aerodynamic diameter of 1 mm will exhibit the
same inertial properties as a sphere with a diameter of 1 mm and a density of 1 g cm
À3
–
irrespective of the actual size, shape or density
of the particle). PM 10 and PM 2.5 correspond
approximately to size ranges which may be
inhaled into the thoracic cavity (lungs) and into
the alveolar regions respectively. The size distribution of urban particles typically exhibits several
modes, reflecting the varying dominance of different processes responsible for particle formation
and growth: The nucleation mode, below approximately 0.1 mm in diameter, where secondary
particles are formed from the condensation of
low-volatility gases (or, more commonly, condenses onto pre-existing particles); the accumulation mode (0.1–2.5 mm), where particles grow by
condensation and coagulation (collisions), but
within which the mass is too small for gravitational setting to be a major sink (hence the size
range where particles accumulate, until removal
primarily by precipitation) and the coarse mode,
which incorporates most mechanically generated
particles, and within which gravitation settling
begins to be a significant loss process. As simple
geometric considerations show, the largest number of particles is found in the nucleation mode,
while the mass is dominated by the coarse and
accumulation modes. It is perhaps unfortunate
then that most current air quality metrics focus
upon particle mass concentration (PM 10 or
PM 2.5 ), while scientific evidence suggests that
many human health impacts are related to exposure to the fine-mode particles which can penetrate most deeply into the alveolar structure.
Measurements of the composition of urban particulate matter are less common than total mass
concentration, but a number of composition
224
Urban Atmospheric Composition Processes
of photochemical smog. PAN is formed from the
reaction between peroxy acetyl radicals, CH 3 C
(O)OO, and NO 2 , and in terms of the chemical
cycling acts as a temporary reservoir for the HO x
and NO x radicals involved in ozone production.
A key feature of the equilibrium between PAN
and its precursor species is the strong temperature
dependence, with lower temperatures favoring
PAN stability. A scenario which arises then is
that PAN is formed in a polluted urban area,
transported aloft (where temperatures are lower
and PAN is stable) over long distances, before
airmass descent and warming leads to PAN
decomposition and release of the component
NO x in otherwise unpolluted regions.
Sulfur dioxide (SO 2 ) is possibly the classic air
pollutant formed from the combustion of sulfurcontaining fuels. SO 2 emissions have been successfully reduced in many regions, through a
combination of the use of low sulfur fuels, and
application of flue gas desulphurization technologies on major plants (e.g., power stations). Within
the atmosphere, SO 2 is processed to H 2 SO 4 on a
timescale of days, both in the gas phase, initiated
by reaction with the OH radical, and in the condensed phase, within water droplets, where the
oxidation reactions are driven predominantly by
hydrogen peroxide, H 2 O 2 . Gas phase sulfuric acid
has a very low vapor pressure, and is readily
incorporated into aerosol and condensation, raising PM acidity. More widely, co-condensation of
H 2 SO 4 with water vapor and/or ammonia leads to
the production of new particles in the atmosphere.
Condensed Phase Composition Processes
Particulate matter is an important component of
urban atmospheric composition, in particular
where health effects are concerned. Sources of particulate matter include the local primary emissions
from vehicles and combustion described above,
including mechanical sources related to vehicle
brake pad/disc, tyre and road surface wear, wider
atmospheric inputs such as wind-blown dust and
crustal material, and secondary contributions arising
from the condensation of low-volatility gases to
form new or (predominantly) contribute to the
growth of existing particles.
Levels of particulate matter or aerosol are usually expressed as a mass concentration below a
certain size; the size ranges commonly selected
relating loosely to the extent to which particles
penetrate the human respiratory system. PM 10 and
PM 2.5 define the fractions of suspended particulate matter with (aerodynamic) diameters below
10 and 2.5 mm respectively (the aerodynamic
diameter is a concept used to allow for the varying
morphology or shape of particles; a particle with
an aerodynamic diameter of 1 mm will exhibit the
same inertial properties as a sphere with a diameter of 1 mm and a density of 1 g cm
À3
–
irrespective of the actual size, shape or density
of the particle). PM 10 and PM 2.5 correspond
approximately to size ranges which may be
inhaled into the thoracic cavity (lungs) and into
the alveolar regions respectively. The size distribution of urban particles typically exhibits several
modes, reflecting the varying dominance of different processes responsible for particle formation
and growth: The nucleation mode, below approximately 0.1 mm in diameter, where secondary
particles are formed from the condensation of
low-volatility gases (or, more commonly, condenses onto pre-existing particles); the accumulation mode (0.1–2.5 mm), where particles grow by
condensation and coagulation (collisions), but
within which the mass is too small for gravitational setting to be a major sink (hence the size
range where particles accumulate, until removal
primarily by precipitation) and the coarse mode,
which incorporates most mechanically generated
particles, and within which gravitation settling
begins to be a significant loss process. As simple
geometric considerations show, the largest number of particles is found in the nucleation mode,
while the mass is dominated by the coarse and
accumulation modes. It is perhaps unfortunate
then that most current air quality metrics focus
upon particle mass concentration (PM 10 or
PM 2.5 ), while scientific evidence suggests that
many human health impacts are related to exposure to the fine-mode particles which can penetrate most deeply into the alveolar structure.
Measurements of the composition of urban particulate matter are less common than total mass
concentration, but a number of composition
224
Urban Atmospheric Composition Processes