Chemistry Reference
In-Depth Information
specific monitoring aims. The European Directive [
17
] approaches site definitions
from the other direction, by defining the monitoring aim and describing the site types
needed; distances to sources are stipulated and specific siting criteria have to be
fulfilled. It is also important to note that sampling and siting is more stringently
defined in the European Directive compared with the USA requirements.
• Microscale siting requirements and pollutants
Even though the major air pollutants and limit values appear very similar, signifi-
cant differences may arise from different siting requirements. CFR 40 [
13
] states
that near road measurements, which are only required for NO
2
and CO, are to be
conducted within 50 m of the target road segment. No near-road measurements are
required for, e.g. particulate matter PM
2.5
and PM
10
in the USA. By contrast, the
European Directive [
17
] requires near-road measurements in a maximum distance
of 10 m from the road edge for a larger variety of pollutants (SO
2
, NO, NO
2
,PM
10
,
PM
2.5
, lead, benzene and CO). Various studies have shown the rapid decrease in
primary pollutant concentrations with distance from roads; concentrations can be
20-40% lower for PM
2.5
,PM
10
and particle number (both
m
in diameter) at 50 m away from a road compared with concentrations at 10 m [
19
,
20
]. This means that the US limit value of 100 ppb for NO
2
is not equivalent to its
European counterpart, even though 100 ppb
100 nm and 0.5-20
<
m
g/m
3
. It is actually about 40%
higher (140 ppb) if concentrations in comparable locations are considered.
200
m
• The concept of population exposure
The European Directive [
17
] is alone in having the concept of regulating population
exposure via an average exposure indicator (the AEI for PM
2.5
). This means that an
average urban background concentration for a country is calculated and regulated.
The concentrations determined at this site type are considered to be the most
exposure-relevant for the population. This approach is based on cost-benefit
analyses comparing the benefits of focussing on hot spots (in response to a limit
value approach) with those from efforts affecting larger areas [
21
]. Given the
absence of evidence for a threshold for PM health effects, it is more beneficial for
the general health of the population, and more cost effective, to decrease the
average pollution exposure for the whole population than it is to decrease the
concentration at highlighted locations in a populated area. The “hot-spot approach”
with attainment of limit values at all relevant measurement sites is still pursued, as
in the other countries, in parallel.
3 How Do Air Quality Monitoring Technologies
Influence Strategies?
Robust air quality monitoring is based on the quantification of pollutant concentrations
with a high degree of accuracy, precision, comparability and long-term stability.
Accuracy and precision of measurements are essential for the assessment of limit
value attainment and law enforcement to improve ambient air quality.
