Air Pollution Exposure in an Unventilated Tunnel in Leeds, UK - Highway and Urban Environment - page 118

Fig. 3 External peak and inter-peak PNC concentrations
As displayed in Fig. 3 , irrespective of the location, average PNCs are higher during
the peak period, with the greatest actual variation between sampling periods evident
within the two tunnel bores, showing decreases of between 42,000 and 45,000
particles cm −3 . This clearly shows the influence of road traffic on UFP levels.
Levels within the eastbound bore are 4.86 and 4.76 magnitudes greater that the
rest of the non-tunnel section of the route and greater by a slightly smaller factor
of 4.11 and 3.86 for the westbound bore. This demonstrates that absence of any
ventilation and the restricted topography has resulted in more than fourfold increase
in UFP concentration. A comparison between the two bores shows that PNC levels
within the eastbound bore are 1.2 magnitudes greater than those of the westbound
bore for the peak and inter-peak periods, with an average difference of 48,400
particles cm −3 . Interrogating the CAN data showed that the average speed of the
monitoring vehicle through both bores was recorded as being similar; however,
during sampling, the driver commented that whilst traversing the westbound bore
it was not necessary to use the accelerator to maintain the vehicle's momentum.
Interrogation of the CAN data showed that the accelerator pedal is further depressed
whilst ascending the eastbound bore and 'hardly depressed' whilst in the west-
bound bore. It is therefore probable that an increased load on the engine and associ-
ated increase in emissions is the contributing factor. During the inter-peak, traffic
flows are the same in each bore and this explanation therefore still holds.
The highest maximum particle concentration level was within the eastbound
bore during the peak period, measuring 397,604 particles cm −3 (Table 1 ). This can
be directly compared to [18] , who also used a TSI Model 3007 CPC, and recorded a
maximum value of 800,000 particles cm −3 ; however this was sampled along highways
and attributed to high polluting diesel vehicles, not tunnels. Within the Westgate
tunnel, highest mean concentrations are seen during the peak period, measuring