diesel prevalence and a greater contribution from
small two-stroke (oil burning) powered two
wheelers. Domestic heating and cooking, where
this is primarily through wood burning, is a major
contributor to urban atmospheric composition (and
indeed to overall pollutant exposure, through
indoor effects within the home).
Characteristics of Motor Vehicle and Domestic
Fuel Emissions
Within internal combustion engine vehicles, emissions of a range of pollutants reflect the composition
of the fuel and oil burnt, and the high-temperature
decomposition of atmospheric oxygen and nitrogen
leading to the production of nitrogen oxides. NO x
(NO + NO 2 ) at the tailpipe is conventionally
assumed be partitioned (on average) into 95% NO
and 5% NO 2 , although there is some evidence that
the direct fraction of NO 2 may be greater than this,
and increasing across the fleet on average – in
particular, for diesel-engined HGVs (heavy goods
vehicles) [18]. Incomplete combustion leads to the
production of carbon monoxide (CO), a vast range
of hydrocarbons which may be gases (if they are
sufficiently small and volatile) or may condense to
form the organic component of exhaust particulates,
as the exhaust gases cool. The introduction of catalytic converters has greatly reduced emissions of
NO, CO and hydrocarbons (which are converted
to N 2 , CO 2 and H 2 O) for petrol-engined vehicles,
while diesel oxidation catalysts have reduced CO
and hydrocarbon emissions from diesel engines.
Increasingly stringent emissions regulations, such
as the EURO-1 to EURO-5 EU series of emissions
standards for various vehicle classes, have substantially reduced emissions per unit activity (at least
within the parameters of the defined driving cycles),
and (disregarding CO 2 ) the principal emission concerns from modern vehicles are NO x and particulate
matter, for which in both cases diesel engine emissions tend to be higher emitters. Exhaust PM in
particular is known to incorporate toxic and carcinogenic components such as PAH and heavy
metals, and (as new particles are formed through
condensation/nucleation) to contribute substantially
to the population of the smallest particles particularly associated with adverse health effects [19]. The
introduction of biofuels (ethanol) in some markets
(notably, South America) has been associated with
increased emissions of aldehydes such as formaldehyde and acetaldehyde [20] which are toxic, and
also potent radical precursors leading to photochemical smog formation (see below). Biofuel (wood and
charcoal) combustion for domestic heating and
cooking is a substantial source of pollutants within
many homes in developing nations, leading to
indoor air pollutant exposure, and forming a major
emission source within urban regions. The relatively
low temperatures commonly found lead to substantial emissions of carbon monoxide, black carbon
(soot) and a range of trace compounds VOCs, oxygenated compounds and acids, including toxic substances such as polycyclic aromatic hydrocarbons
(PAHs), particularly within the organic component
of the emitted particulate matter [21].
Urban Atmospheric Composition
Processes
Gas-Phase Composition Processes
Gas-phase processes in urban environments are
dominated by the oxides of nitrogen, (primarily)
NO and NO 2 , emitted directly from the exhausts of
high-temperature (i.e., internal combustion engine)
vehicles. Following emission, a number of gas-phase
processes follow. In close proximity (within a few
meters) to the tailpipe, NO levels may be sufficiently
high before mixing and dilution that the NO-selfreaction, converting NO into NO 2 , may occur:
NO þ NO þ O 2 ! 2NO 2
ð1Þ
However the dominant chemistry of NO x in the
urban environment is the interaction between NO,
NO 2 , and ozone. In daylight, these three species
are very closely coupled by the NO + O 3 reaction,
and the solar decomposition (photolysis) of NO 2 :
NO þ O 3 ! NO 2 þ O 2
ð2Þ
NO 2 þ hv ! NO þ O
ð3Þ
The O atoms formed through NO 2 photolysis
rapidly and near-exclusively recombine with molecular oxygen (O 2 ) to form ozone (reaction 4), where
220
Urban Atmospheric Composition Processes
small two-stroke (oil burning) powered two
wheelers. Domestic heating and cooking, where
this is primarily through wood burning, is a major
contributor to urban atmospheric composition (and
indeed to overall pollutant exposure, through
indoor effects within the home).
Characteristics of Motor Vehicle and Domestic
Fuel Emissions
Within internal combustion engine vehicles, emissions of a range of pollutants reflect the composition
of the fuel and oil burnt, and the high-temperature
decomposition of atmospheric oxygen and nitrogen
leading to the production of nitrogen oxides. NO x
(NO + NO 2 ) at the tailpipe is conventionally
assumed be partitioned (on average) into 95% NO
and 5% NO 2 , although there is some evidence that
the direct fraction of NO 2 may be greater than this,
and increasing across the fleet on average – in
particular, for diesel-engined HGVs (heavy goods
vehicles) [18]. Incomplete combustion leads to the
production of carbon monoxide (CO), a vast range
of hydrocarbons which may be gases (if they are
sufficiently small and volatile) or may condense to
form the organic component of exhaust particulates,
as the exhaust gases cool. The introduction of catalytic converters has greatly reduced emissions of
NO, CO and hydrocarbons (which are converted
to N 2 , CO 2 and H 2 O) for petrol-engined vehicles,
while diesel oxidation catalysts have reduced CO
and hydrocarbon emissions from diesel engines.
Increasingly stringent emissions regulations, such
as the EURO-1 to EURO-5 EU series of emissions
standards for various vehicle classes, have substantially reduced emissions per unit activity (at least
within the parameters of the defined driving cycles),
and (disregarding CO 2 ) the principal emission concerns from modern vehicles are NO x and particulate
matter, for which in both cases diesel engine emissions tend to be higher emitters. Exhaust PM in
particular is known to incorporate toxic and carcinogenic components such as PAH and heavy
metals, and (as new particles are formed through
condensation/nucleation) to contribute substantially
to the population of the smallest particles particularly associated with adverse health effects [19]. The
introduction of biofuels (ethanol) in some markets
(notably, South America) has been associated with
increased emissions of aldehydes such as formaldehyde and acetaldehyde [20] which are toxic, and
also potent radical precursors leading to photochemical smog formation (see below). Biofuel (wood and
charcoal) combustion for domestic heating and
cooking is a substantial source of pollutants within
many homes in developing nations, leading to
indoor air pollutant exposure, and forming a major
emission source within urban regions. The relatively
low temperatures commonly found lead to substantial emissions of carbon monoxide, black carbon
(soot) and a range of trace compounds VOCs, oxygenated compounds and acids, including toxic substances such as polycyclic aromatic hydrocarbons
(PAHs), particularly within the organic component
of the emitted particulate matter [21].
Urban Atmospheric Composition
Processes
Gas-Phase Composition Processes
Gas-phase processes in urban environments are
dominated by the oxides of nitrogen, (primarily)
NO and NO 2 , emitted directly from the exhausts of
high-temperature (i.e., internal combustion engine)
vehicles. Following emission, a number of gas-phase
processes follow. In close proximity (within a few
meters) to the tailpipe, NO levels may be sufficiently
high before mixing and dilution that the NO-selfreaction, converting NO into NO 2 , may occur:
NO þ NO þ O 2 ! 2NO 2
ð1Þ
However the dominant chemistry of NO x in the
urban environment is the interaction between NO,
NO 2 , and ozone. In daylight, these three species
are very closely coupled by the NO + O 3 reaction,
and the solar decomposition (photolysis) of NO 2 :
NO þ O 3 ! NO 2 þ O 2
ð2Þ
NO 2 þ hv ! NO þ O
ð3Þ
The O atoms formed through NO 2 photolysis
rapidly and near-exclusively recombine with molecular oxygen (O 2 ) to form ozone (reaction 4), where
220
Urban Atmospheric Composition Processes