This included establishing common methods and
criteria for assessment of air quality, and setting
concentration limits for the pollutants. These
limits were then tightened by a following directive
in 1999. Current legislation and regulations are
outlined in more detail in section “Current Air
Quality Legislation and Metrics”.
Air Quality Pollutants
Air pollution has many different components that
differ in concentration depending on the superimposition of local, regional, and global emission sources. These emissions are frequently
separated into anthropogenic (“man-made”) and
biogenic (“natural”) sources. The components of
air pollution can be roughly separated into gaseous and particulate (or aerosol) fractions.
Within both gas and particulate pollution there
can be primary and secondary pollutants, organic
and inorganic compounds. Primary pollutants are
those emitted directly into the atmosphere from a
source, such as sulfur dioxide or black carbon
(BC) (a significant light absorbing component of
soot). Secondary pollutants are those formed in
the atmosphere from chemical and/ or physical
transformations of primary pollutants, such as
ozone (O 3 ) and secondary organic aerosol
(SOA).
Regional air pollution results from the combination of emissions, which vary both spatially and
temporally and meteorological factors (that allow
the pollutants to build up in an area or transport
pollutants into an area where they can mix with
local emissions) causing poor regional air quality.
Figure 2 shows different emissions sources and
meteorological components that have an impact
on regional air quality. In addition to wind transporting pollutants, sunlight plays an important
role in photochemical reactions in the atmosphere
that create some of the secondary pollutants. Air
pollutants are then removed from the atmosphere
through dry deposition (e.g., settling) or wet deposition (e.g., rain scavenging). The following two
sections will discuss the two main components of
air pollution – gaseous pollutants and particulate
pollutants in terms of their sources, sinks, atmospheric transport, and transformations.
Gaseous Pollutants
There are myriad different gaseous species that are
emitted into the atmosphere from various air pollution sources. Some of the major gaseous pollutants
are carbon monoxide (CO), nitrogen compounds
(e.g., NO, NO 2 , HNO 3 ), sulfur compounds (e.g.,
SO 2 ), hydrocarbons (HCs) including methane and
nonmethane hydrocarbons (NMHCs), and photochemical oxidants (e.g., O 3 ). Other gaseous pollutants, such as carbon dioxide (CO 2 ) and methane
(CH 4 ), will not be addressed here because they are
not a major focus for regional air quality concerns
because their longer lifetimes qualify them as global
pollutants.
Carbon monoxide’s primary source is from
incomplete combustion. In addition to being an
important primary pollutant, it is also an important
precursor compound for the formation of ozone, as
well as a secondary pollutant itself, formed from
the oxidation of methane and other NMHCs by the
OH radical [5]. It has a lifetime of a couple months
which makes it a regional and global scale pollutant that can be transported significant distances
from its emission source. Carbon monoxide is primarily removed from the atmosphere by reaction
with OH, a small amount being removed by deposition. Ambient levels of 0.15–10 ppmV are common in urban areas mainly owing to road
transport–related sources [6, 7]. In the USA,
mobile sources (including non-road mobile
sources) make up 80% of national CO emissions,
while in the UK 47% of CO emissions are attributed to road transport sources. Figure 3 shows how
CO emissions largely follow the road network in
the UK, with the highest emissions in urban areas
[8]. Furthermore, the high correlation of CO with
population density in Asia is shown in Fig. 4
[9]. Carbon monoxide emissions have shown significant reductions over the past 2 decades
[7]. These emission reductions are largely credited
to the increased use of catalytic convertors in cars.
Total CO emissions in the USA have decreased
68% since the 1990s until 2008, and currently all
monitoring stations in the USA show that no areas
are in nonattainment for the 8-h CO standard of
9 ppmV [10]. Similarly, CO emissions in the UK
have been reduced by 71% from 1990 to 2005 in
the UK. Elevated levels of CO observed in rural/
350
Regional Air Quality
criteria for assessment of air quality, and setting
concentration limits for the pollutants. These
limits were then tightened by a following directive
in 1999. Current legislation and regulations are
outlined in more detail in section “Current Air
Quality Legislation and Metrics”.
Air Quality Pollutants
Air pollution has many different components that
differ in concentration depending on the superimposition of local, regional, and global emission sources. These emissions are frequently
separated into anthropogenic (“man-made”) and
biogenic (“natural”) sources. The components of
air pollution can be roughly separated into gaseous and particulate (or aerosol) fractions.
Within both gas and particulate pollution there
can be primary and secondary pollutants, organic
and inorganic compounds. Primary pollutants are
those emitted directly into the atmosphere from a
source, such as sulfur dioxide or black carbon
(BC) (a significant light absorbing component of
soot). Secondary pollutants are those formed in
the atmosphere from chemical and/ or physical
transformations of primary pollutants, such as
ozone (O 3 ) and secondary organic aerosol
(SOA).
Regional air pollution results from the combination of emissions, which vary both spatially and
temporally and meteorological factors (that allow
the pollutants to build up in an area or transport
pollutants into an area where they can mix with
local emissions) causing poor regional air quality.
Figure 2 shows different emissions sources and
meteorological components that have an impact
on regional air quality. In addition to wind transporting pollutants, sunlight plays an important
role in photochemical reactions in the atmosphere
that create some of the secondary pollutants. Air
pollutants are then removed from the atmosphere
through dry deposition (e.g., settling) or wet deposition (e.g., rain scavenging). The following two
sections will discuss the two main components of
air pollution – gaseous pollutants and particulate
pollutants in terms of their sources, sinks, atmospheric transport, and transformations.
Gaseous Pollutants
There are myriad different gaseous species that are
emitted into the atmosphere from various air pollution sources. Some of the major gaseous pollutants
are carbon monoxide (CO), nitrogen compounds
(e.g., NO, NO 2 , HNO 3 ), sulfur compounds (e.g.,
SO 2 ), hydrocarbons (HCs) including methane and
nonmethane hydrocarbons (NMHCs), and photochemical oxidants (e.g., O 3 ). Other gaseous pollutants, such as carbon dioxide (CO 2 ) and methane
(CH 4 ), will not be addressed here because they are
not a major focus for regional air quality concerns
because their longer lifetimes qualify them as global
pollutants.
Carbon monoxide’s primary source is from
incomplete combustion. In addition to being an
important primary pollutant, it is also an important
precursor compound for the formation of ozone, as
well as a secondary pollutant itself, formed from
the oxidation of methane and other NMHCs by the
OH radical [5]. It has a lifetime of a couple months
which makes it a regional and global scale pollutant that can be transported significant distances
from its emission source. Carbon monoxide is primarily removed from the atmosphere by reaction
with OH, a small amount being removed by deposition. Ambient levels of 0.15–10 ppmV are common in urban areas mainly owing to road
transport–related sources [6, 7]. In the USA,
mobile sources (including non-road mobile
sources) make up 80% of national CO emissions,
while in the UK 47% of CO emissions are attributed to road transport sources. Figure 3 shows how
CO emissions largely follow the road network in
the UK, with the highest emissions in urban areas
[8]. Furthermore, the high correlation of CO with
population density in Asia is shown in Fig. 4
[9]. Carbon monoxide emissions have shown significant reductions over the past 2 decades
[7]. These emission reductions are largely credited
to the increased use of catalytic convertors in cars.
Total CO emissions in the USA have decreased
68% since the 1990s until 2008, and currently all
monitoring stations in the USA show that no areas
are in nonattainment for the 8-h CO standard of
9 ppmV [10]. Similarly, CO emissions in the UK
have been reduced by 71% from 1990 to 2005 in
the UK. Elevated levels of CO observed in rural/
350
Regional Air Quality