formed along the airmass trajectory at rates of the
order of 20–30 ppb per day (daytime chemical
production, part of which is offset by night-time
chemistry and deposition [25]). If a typical episode
is characterized by ozone levels of the order of
90 ppb compared with the rural background of
around 30–40 ppb, several days are clearly
required to achieve this level. In more extreme
conditions, ozone production proceeds more rapidly: during the 2003 Western Europe heatwave
event, a photochemical model constrained by in
situ observations of long-lived species from
suburban North-East London determined the
peak ozone production to be 17 ppb h
À1
, with an
average production rate during midsummer at
midday of 7.2 ppb h
À1 [26]. Observations in Mexico City have led to calculated ozone production
rates of up to 50 ppb h
À1 [27] during stagnant
meteorological conditions.
Exposure to ozone has substantial human
health consequences, being associated with irritation to eyes and nose, inflammatory reactions and
reduced lung function. Ozone is also harmful to
vegetation and ecosystems, and certain materials
(erosion of polymers through reaction with component double bonds), and is an important greenhouse gas in its own right; consequently reduction
in ozone levels is a major goal of air quality
policy. The long lifetime of ozone limits the
impact that local/national air quality legislation
can have upon boundary layer ozone levels, and
dictates that ozone is treated as a transboundary
pollutant.
The secondary nature of ozone further complicates attempts to reduce its abundance: The precursor species, NO x and VOCs, must be
controlled; however the response of the chemical
ozone production rate to the level of, for example,
NO x and CO is not straightforward: As can be
seen from inspection of Fig. 3, at high NO x levels,
the reaction of OH with NO 2 , forming HNO 3 , can
compete with the reaction of OH with VOCs. In
this instance, reducing NO x levels (e.g., through
emission controls) would have the effect of
increasing the abundance of OH, and the rate of
the OH+VOC reactions, and hence would lead to
an increase in ozone production. At lower NO x
levels, where NO 2 is not competing effectively
with VOCs for OH, reduction in NO x leads to a
reduction in the ozone production rate, by reducing the rate of the peroxy reaction (or strictly, by
allowing other competing processes, not shown
on Fig. 3, to occur – [24]). Two overall regimes for
ozone production can be identified: VOC-limited,
where NO x levels are high, and ozone production
rates will rise or fall with increase/decreases in
VOC levels respectively, and NO x -limited, where
VOCs are relatively more abundant, NO x levels
low, and ozone levels rise and fall with increases/
decreases in NO x . Reduction of NO x in a VOClimited regime may lead to increased ozone production. Many urban environments correspond to
VOC-limited conditions (i.e., higher NO x levels);
as air is advected from the urban centre to surrounding rural regions, with reduced emission
sources, NO x levels fall and the ozone production
regime undergoes a transition to a NO x -limited
state. The complex dependence of the ozone production rate upon the abundance of primary pollutants between urban and rural areas, which is
compounded by the reduction in ozone levels on
the street canyon scale within urban areas by the
titration of O 3 to NO 2 by fresh vehicle NO emissions, presents a major challenge to the development of effective policy for mitigation of ozone
pollution – see below.
Different organic compounds have differing
potentials for ozone production (reflecting their
chemical identity/size, and the rates of their reactions with oxidants such as OH), and of course are
emitted in varying quantities. Metrics to rank species in terms of their contribution to ozone levels
(such as Photochemical Ozone Creation Potentials,
POCP values) have been developed to account for
the integrated effect of these factors; in the case of
the UK, the highest POCP values are assigned to
tri-methylbenzene, however considering the massweighted emissions, the largest contributors to the
ozone levels experienced are toluene and n-butane
[28]. These values are specific to the UK case, that
is, to the emissions, and in particular NO x levels,
typically experienced by an air parcel advected
from continental Europe to southern Britain.
Peroxy acetyl nitrate (PAN, CH 3 C(O)OONO 2 )
is the simplest example of a range of acetyl nitrate
compounds, secondary pollutants which are the
Urban Atmospheric Composition Processes
223
order of 20–30 ppb per day (daytime chemical
production, part of which is offset by night-time
chemistry and deposition [25]). If a typical episode
is characterized by ozone levels of the order of
90 ppb compared with the rural background of
around 30–40 ppb, several days are clearly
required to achieve this level. In more extreme
conditions, ozone production proceeds more rapidly: during the 2003 Western Europe heatwave
event, a photochemical model constrained by in
situ observations of long-lived species from
suburban North-East London determined the
peak ozone production to be 17 ppb h
À1
, with an
average production rate during midsummer at
midday of 7.2 ppb h
À1 [26]. Observations in Mexico City have led to calculated ozone production
rates of up to 50 ppb h
À1 [27] during stagnant
meteorological conditions.
Exposure to ozone has substantial human
health consequences, being associated with irritation to eyes and nose, inflammatory reactions and
reduced lung function. Ozone is also harmful to
vegetation and ecosystems, and certain materials
(erosion of polymers through reaction with component double bonds), and is an important greenhouse gas in its own right; consequently reduction
in ozone levels is a major goal of air quality
policy. The long lifetime of ozone limits the
impact that local/national air quality legislation
can have upon boundary layer ozone levels, and
dictates that ozone is treated as a transboundary
pollutant.
The secondary nature of ozone further complicates attempts to reduce its abundance: The precursor species, NO x and VOCs, must be
controlled; however the response of the chemical
ozone production rate to the level of, for example,
NO x and CO is not straightforward: As can be
seen from inspection of Fig. 3, at high NO x levels,
the reaction of OH with NO 2 , forming HNO 3 , can
compete with the reaction of OH with VOCs. In
this instance, reducing NO x levels (e.g., through
emission controls) would have the effect of
increasing the abundance of OH, and the rate of
the OH+VOC reactions, and hence would lead to
an increase in ozone production. At lower NO x
levels, where NO 2 is not competing effectively
with VOCs for OH, reduction in NO x leads to a
reduction in the ozone production rate, by reducing the rate of the peroxy reaction (or strictly, by
allowing other competing processes, not shown
on Fig. 3, to occur – [24]). Two overall regimes for
ozone production can be identified: VOC-limited,
where NO x levels are high, and ozone production
rates will rise or fall with increase/decreases in
VOC levels respectively, and NO x -limited, where
VOCs are relatively more abundant, NO x levels
low, and ozone levels rise and fall with increases/
decreases in NO x . Reduction of NO x in a VOClimited regime may lead to increased ozone production. Many urban environments correspond to
VOC-limited conditions (i.e., higher NO x levels);
as air is advected from the urban centre to surrounding rural regions, with reduced emission
sources, NO x levels fall and the ozone production
regime undergoes a transition to a NO x -limited
state. The complex dependence of the ozone production rate upon the abundance of primary pollutants between urban and rural areas, which is
compounded by the reduction in ozone levels on
the street canyon scale within urban areas by the
titration of O 3 to NO 2 by fresh vehicle NO emissions, presents a major challenge to the development of effective policy for mitigation of ozone
pollution – see below.
Different organic compounds have differing
potentials for ozone production (reflecting their
chemical identity/size, and the rates of their reactions with oxidants such as OH), and of course are
emitted in varying quantities. Metrics to rank species in terms of their contribution to ozone levels
(such as Photochemical Ozone Creation Potentials,
POCP values) have been developed to account for
the integrated effect of these factors; in the case of
the UK, the highest POCP values are assigned to
tri-methylbenzene, however considering the massweighted emissions, the largest contributors to the
ozone levels experienced are toluene and n-butane
[28]. These values are specific to the UK case, that
is, to the emissions, and in particular NO x levels,
typically experienced by an air parcel advected
from continental Europe to southern Britain.
Peroxy acetyl nitrate (PAN, CH 3 C(O)OONO 2 )
is the simplest example of a range of acetyl nitrate
compounds, secondary pollutants which are the
Urban Atmospheric Composition Processes
223