of primary pollutants per unit activity (i.e., from
improvements in vehicle technology and penetration of exhaust treatments such as catalytic converters through the vehicle fleet); and changes in
the overall activity level, that is, increasing economic prosperity and population levels, particularly in many megacities in developing nations.
A further external driver which may affect future
urban composition is the interaction between
atmospheric composition processes and global
climate.
For the gaseous pollutants, the species of particular concern on an ongoing basis are nitrogen
oxides (specifically NO 2 as this is the NO x component against which legislation is commonly
framed) and ozone. Of the other regulated pollutants (Table 2), sulfur dioxide levels are falling as
the intensive sources (power and heavy industry)
have emissions controlled at source and carbon
monoxide levels have greatly reduced with the
advent of catalytic converters.
Urban NO x levels show a downward trend in
most developed cities when viewed on a 2–3
decade timescale, driven by improved emission
control technologies in the vehicle fleet; for example, across a range of UK urban sites, annual mean
NO x levels have fallen by 40% over the period
1993–2005, from 135 to 80 mg m
À3 [18]. Within
this overall trend, there are some significant local
variations, for example, data from London suggests that the fraction of NO x emitted as NO 2 has
risen in recent years, possibly due to the introduction of diesel particulate filters, while traffic control measures such as the London Congestion
Zone have altered the average fleet composition
in central London to favor diesel vehicles with
increased NO x emissions, thought to be responsible for the 7% increase in NO 2 observed in central
London over the period 2003–2008. More widely
however, NO x levels in most developed cities
have fallen dramatically over the past 2–3
decades [18].
Globally, background ozone levels have risen
over the past century – the earliest instrumental
data thought to be reliable, the Montsouris measurements recorded on the outskirts of Paris from
1876, indicate average background ozone levels
of 22 mg m
À3 over the period 1876–1910
[31]. The equivalent values for the past decade
from background locations are much higher – for
example, ca. 80 mg m
À3 for background air at
Mace Head, Ireland (value for 2006 from Derwent
et al. [32]); background levels have increased over
the past 2–3 decades at up to 10 mg m
À3 per
decade (5 ppb per decade) [33]. On top of this
hemispheric trend, urban ozone levels respond to
regional pollution events, and to the extent of
local titration of O 3 to NO 2 through NO x
(strictly, NO) emissions. Emissions controls have
reduced the prevalence and extent of ozone episodes over the past 1–2 decades, reducing this
contributor to urban ozone; however reductions
in NO x emissions have offset some of this change,
reducing the ozone urban decrement. These
effects are clearly seen in the UK over the period
1990–2006 (e.g., AQEG [34]), where the maximum hourly mean values (associated with
regional ozone pollution episodes) have
decreased substantially due to European controls
on emissions of VOCs and NO x , while at intermediate percentiles, urban sites show a positive
trend in ozone with time due to the reduced urban
decrement. The lowest hourly mean ozone levels
show no change as these essentially correspond to
total titration of O 3 to NO, consequently ozone
levels are zero across the time period considered.
Primary sources (vehicle exhaust) of PM have
been substantially reduced in may developed
nations, and PM levels (measured as PM 10 ) have
fallen substantially over the past few decades.
Recently however, the downward trend appears
to have been arrested, at least for PM 10 in Europe,
and the sulfate and nitrate components of this
PM 10 [35]. It is unclear if this is a temporary
departure from the general trend or reflects an
ongoing leveling off, possibly reflecting nonexhaust sources, changing meteorology, and the
interaction between sulfate and nitrate via ammonia availability to form ammonium nitrate.
Future Directions
In many developing megacities, the outcome of
the opposing effects of increasing population density and economic activity, and adoption of
226
Urban Atmospheric Composition Processes
improvements in vehicle technology and penetration of exhaust treatments such as catalytic converters through the vehicle fleet); and changes in
the overall activity level, that is, increasing economic prosperity and population levels, particularly in many megacities in developing nations.
A further external driver which may affect future
urban composition is the interaction between
atmospheric composition processes and global
climate.
For the gaseous pollutants, the species of particular concern on an ongoing basis are nitrogen
oxides (specifically NO 2 as this is the NO x component against which legislation is commonly
framed) and ozone. Of the other regulated pollutants (Table 2), sulfur dioxide levels are falling as
the intensive sources (power and heavy industry)
have emissions controlled at source and carbon
monoxide levels have greatly reduced with the
advent of catalytic converters.
Urban NO x levels show a downward trend in
most developed cities when viewed on a 2–3
decade timescale, driven by improved emission
control technologies in the vehicle fleet; for example, across a range of UK urban sites, annual mean
NO x levels have fallen by 40% over the period
1993–2005, from 135 to 80 mg m
À3 [18]. Within
this overall trend, there are some significant local
variations, for example, data from London suggests that the fraction of NO x emitted as NO 2 has
risen in recent years, possibly due to the introduction of diesel particulate filters, while traffic control measures such as the London Congestion
Zone have altered the average fleet composition
in central London to favor diesel vehicles with
increased NO x emissions, thought to be responsible for the 7% increase in NO 2 observed in central
London over the period 2003–2008. More widely
however, NO x levels in most developed cities
have fallen dramatically over the past 2–3
decades [18].
Globally, background ozone levels have risen
over the past century – the earliest instrumental
data thought to be reliable, the Montsouris measurements recorded on the outskirts of Paris from
1876, indicate average background ozone levels
of 22 mg m
À3 over the period 1876–1910
[31]. The equivalent values for the past decade
from background locations are much higher – for
example, ca. 80 mg m
À3 for background air at
Mace Head, Ireland (value for 2006 from Derwent
et al. [32]); background levels have increased over
the past 2–3 decades at up to 10 mg m
À3 per
decade (5 ppb per decade) [33]. On top of this
hemispheric trend, urban ozone levels respond to
regional pollution events, and to the extent of
local titration of O 3 to NO 2 through NO x
(strictly, NO) emissions. Emissions controls have
reduced the prevalence and extent of ozone episodes over the past 1–2 decades, reducing this
contributor to urban ozone; however reductions
in NO x emissions have offset some of this change,
reducing the ozone urban decrement. These
effects are clearly seen in the UK over the period
1990–2006 (e.g., AQEG [34]), where the maximum hourly mean values (associated with
regional ozone pollution episodes) have
decreased substantially due to European controls
on emissions of VOCs and NO x , while at intermediate percentiles, urban sites show a positive
trend in ozone with time due to the reduced urban
decrement. The lowest hourly mean ozone levels
show no change as these essentially correspond to
total titration of O 3 to NO, consequently ozone
levels are zero across the time period considered.
Primary sources (vehicle exhaust) of PM have
been substantially reduced in may developed
nations, and PM levels (measured as PM 10 ) have
fallen substantially over the past few decades.
Recently however, the downward trend appears
to have been arrested, at least for PM 10 in Europe,
and the sulfate and nitrate components of this
PM 10 [35]. It is unclear if this is a temporary
departure from the general trend or reflects an
ongoing leveling off, possibly reflecting nonexhaust sources, changing meteorology, and the
interaction between sulfate and nitrate via ammonia availability to form ammonium nitrate.
Future Directions
In many developing megacities, the outcome of
the opposing effects of increasing population density and economic activity, and adoption of
226
Urban Atmospheric Composition Processes