Chemistry Reference
In-Depth Information
For these reasons, during the last 20 years NO
x
emissions have decreased less
than would have been expected given the systematic tightening of the emission
limits. A direct implication of this is that NO
2
concentrations still frequently exceed
health-based limits in many urban areas. According to the European Environment
Agency, in 2004 more than 20% of the European urban population were exposed
to ambient NO
2
concentrations above the annual mean limit value of 40
g/m
3
[
47
].
Furthermore, NO
2
concentrations at many monitoring sites are not decreasing
[
15
,
47
,
48
]. Although NO
2
is only a fraction of the total NO
x
emitted from vehicles,
analyses have indicated that a significant proportion of ambient NO
2
is actually
primary exhaust from vehicles, and that the road traffic contribution to ambient
NO
2
has increased in recent years [
22
,
49
-
52
]. Two contributing factors have
been cited:
m
1. Diesel vehicles emit more NO
x
than petrol vehicles, and with a larger proportion
of primary NO
2
. In parallel, the market share of diesel vehicles has increased in
many European countries. For example, the share of first registrations of diesel
passenger cars in Finland increased from 17% in 2005 to 52% in 2008 [
53
].
2. The average proportion of primary NO
2
in diesel exhaust is increasing
with changes in technology. This appears to be linked to the growth in the use
of specific after-treatment technologies in modern diesel vehicles which involve
in situ generation of NO
2
, such as catalytically regenerative DPFs and DOCs
[
22
,
45
].
This increase in primary NO
2
emissions is compounded by atmospheric
processes; background concentrations of ozone are also increasing [
54
], which
increases the amount of atmospheric NO converted to NO
2
.
It is clear that effectively addressing the NO
x
emission problem involves not
only more stringent emission limits over the current type approval procedure,
but also a more effective regulatory policy as a whole. Measures that can be
taken include:
• A revised type approval procedure that introduces a more realistic driving cycle
covering a wider area of engine operation.
• The selection of random engine modes during type approval emission testing,
and ensuring that emission limits are not exceeded (the “not-to-exceed” limit
approach).
• The direct regulation of NO
2
emissions, independently of NO
x
.
• Revised and advanced in-use compliance testing/inspection and maintenance
schemes, involving OBD checks to control emissions over the lifetime of the
vehicle.
• Extension of PEMS-based regulation in all vehicle categories, so as to effec-
tively measure emissions in the field.
Several of these items are already being discussed within the UNECE working
group on a Worldwide Harmonized Light vehicle Test Procedure.
Beyond the 2020-2030 horizon, reductions in NO
x
will depend heavily on more
effective regulations and actions, as well as the rate of introduction of low-carbon
technologies such as electric and hydrogen vehicles. Vehicles operating on
