Environmental Engineering Reference
In-Depth Information
The measurements are characterized by an important number of high peaks
coinciding with motorized traffic on the road and alternating with much lower values.
Values around 10,000 particles per cm
3
are most common while frequent peaks over
100,000 particles per cm
3
have been recorded. The highest values are in excess of
the P-TRAK range (500,000 cm
−3
). P1 (closest to the road) frequently records
higher values than P2 does (only 1 m further away from the road). Although occa-
sionally P2 values are marginally higher than P1, values of P1 are often three to five
times as high during peak UFP events (right axis in Fig.
4
).
On average, 1,150 (stdev: 722) more particles cm
−3
were measured closest to the
road. Although there is a lot of variation and data is obviously not distributed nor-
mally, non-parametric tests reveal that the average P1 values for each trajectory are
significantly higher (Wilcoxon matched pairs signed ranks test: p < 0.005 one-sided
test). The average P1/P2 ratio is 1.06 (stdev 0.04).
We could not demonstrate an effect of wind direction on the measured concen-
trations. Measured particles numbers are not significantly different and indepen-
dent of whether the wind is blowing emissions towards the cycling track (trips in
northern direction) or to the opposite side of the road (away from the cyclist, trips
in southern direction).
Discussion
These limited sets of measurements resulted in two surprising conclusions. Firstly,
our validation measurements revealed that UFP concentrations at the cycling track
are independent of height. The exposure of children should therefore not be higher
than that of adults, although the opposite is often claimed. Future studies should
however confirm if heavier particles such as TSP (Total Suspended particles) or
PM10 (e.g. from re-suspension) and exhaust UFP behave differently.
Our results also highlight the surprising fact that concentrations of UFP may
decrease significantly over a small distance from the emission source even though this
distance is of the same order of magnitude as, e.g. the width of the cycling track or
the variation in the exact position of the exhaust pipes relative to the cycling track.
We can see three distinct reasons why particle numbers can be different for both
instruments. The first possibility is “dispersion and dilution” where particles are
transported by wind and at increasing distances are diluted in a larger volume of
air. This process certainly occurs at larger scales but at this scale level we do not
see a consistent difference between both measurements that would be in line with
this explanation. Possibly this pattern may emerge during measurements at higher
wind speeds.
The second possibility is that particles are not yet uniformly mixed and erratic
turbulence (caused by traffic itself) causes the exposure of one of the instruments
(but not the other) to the emission plume. This would explain why differences are
occasionally very large and differences in the other direction (although smaller) can
also occur. The fact that there appears to be no significant effect of cycling direction
