Bus journeys were undertaken by all four individuals as part of their commute to work. The routes are named after the different individuals (routes A = individual A, routes B = individual B, etc.) Individual A took two buses to work, which alter their routes in some places on the way back. They are therefore presented here as four different bus routes (A1 – A4, numbered in the sequence they are taken during a day). Individuals B, C, and D took one bus route each (individual B and D in both directions, individual C in one direction). All buses run predominantly along major roads. The bus route taken by individual C was however only taken once, results are therefore not discussed. In order to monitor concentrations in a bus a Dustmate monitor in a small rectangular bag was used for recordings during bus commutes. The monitor was stabilised in the bag by polystyrene, the inlet was sticking out at the top of the bag through a custom-cut hole. While standing in the bus, the bag was held in the hand (at least above knee height, usually higher), when seated in the bus, the bag was placed on the lap or on the neighbouring seat. The locations of the individuals in the buses varied. All buses were Diesel buses.
110
Figure 21: Central London, showing bus routes taken during personal exposure monitoring. Road Data Source: Meridian.
Summary statistics are presented in Table 20 for PM2.5 and PM10. The highest median concentrations were recorded on route A1 for both particle size fractions with median PM2.5 concentrations of 16.61µg/m³ and median PM10 concentrations of 69.63µg/m³. Route A4 is the same bus route number as route 1, but taken on the return journey (with about ¼ along a different road in a one way system). Results for A4 show much lower concentrations compared to A1, as well as compared to the other routes. For PM2.5 the median concentration for route A4 is 9.89µg/m³, and for PM10 the median concentration is 30.55µg/m³. The IQR are also lower in the evening compared to the morning, especially for PM10 (route A1 = 55.84µg/m³, route A4 = 32.46µg/m³). A potential explanation could be the increased traffic or the increased passenger number during the more pronounced morning rush-hour. According to Abt et al. (2000) the concentration increase from people’s movement is more pronounced with larger particles, as it was observed here.
Main Individual Indiv. B Indiv. C Indiv. D
Bus A1 Bus A2 Bus A3 Bus A4 Bus B Bus C Bus D
No. of repeats (minutes) 16 (65) 16 (668) 14 (572) 12 (41) 2 (123) 1 (17) 4 (83)
PM2.5 Median (Mean) 16.61 (18.30) 12.21 (13.50) 8.27 (10.46) 9.89 (10.53) 5.98 (6.05) 13.92 (13.01) 3.87 (4.46) IQR (SD) 8.50 (11.34) 8.90 (8.18) 5.65 (7.99) 7.78 (6.21) 1.84 (1.42) 7.66 (5.14) 2.15 (2.31) PM10 Median (Mean) 69.63 (83.38) 43.45 (53.54) 31.96 (39.31) 30.55 (43.27) 23.20 (24.35) 56.40 (51.85) 19.80 (21.75)
IQR (SD) 55.84 (54.28) 35.31 (52.71) 30.85 (6.37) 32.46 (36.19) 7.40 (6.63) 29.65 (21.04) 11.10 (8.24) Table 20: PM2.5 and PM10 concentrations (both in µg/m³) of bus routes visited as part of the personal exposure monitoring campaign, based on minute concentrations
111 Bus A2 and A3 are as well the same bus route (with one-way alterations) taken in different directions, with A2 is taken during morning rush hour, and A3 taken in the evening. As for routes A1 and A4, PM concentrations are higher in the morning, but the difference is not as pronounced as along route A1/A4. For PM2.5 median levels in the morning is 12.21µg/m³ compared to 8.27µg/m³ in the afternoon; median PM10 concentrations in the morning is 43.45µg/m³ and only 31.96µg/m³ in the afternoon.
Route A2/A3 runs along some of the busiest roads in London (Euston Road, Marylebone Rd. etc.) with DAT of on average around 39,000 (length weighted average along LAEI road segments). Along routes A1/A4 in comparison DAT is just above 15,000. Several studies have shown that in-vehicle concentrations on high traffic roads are higher than on low traffic roads (Vijayan & Kumar 2010; Zuurbier et al. 2010). Despite the higher DAT, routes A2/A3 show however lower particle concentrations. Other factors therefore may be of importance, such as window openings or passenger numbers (see chapter 2). Concentrations for bus B are the second lowest recorded along the different bus routes. The median PM2.5 concentration is 5.98µg/m³ and median PM10 is 23.2µg/m³. Additionally the IQR are low, and show only 1.84µg/m³ for PM2.5 and 7.4µg/m³ for PM10. Individual B chose however to sit upstairs on all journeys, which may potentially have had a reduction effect on the PM levels due to vertical displacement. Bus D has the lowest median values with median PM2.5 concentration of only 3.87µg/m³ and of 19.8µg/m³ for PM10. The DAT along the route is also one of the lowest compared to other bus routes with DAT of just above 15,000 (weighted by segment lengths).
Several other studies have collected data during bus journeys, generally finding higher concentrations than described above for this thesis. Mean PM2.5 concentrations of in-bus studies presented in chapter 2 lie between 34.5µg/m³ and 128.16µg/m³ for in-bus exposure (Adams et al. 2001; Kaur et al. 2005b; McNabola et al. 2009a; Zuurbier et al. 2010). In comparison mean PM2.5 concentrations presented in Table 20 lie between 4.46µg/m³ and 18.30µg/m³. Only two studies provide results for PM10 inside buses. One study by Praml & Schierl (2000) also collected PM10 concentrations inside buses, finding mean concentrations of 110.08µg/m³ and 164.95µg/m³ for two different buses in Munich, Germany. Again, results by this thesis are lower with a mean concentration of between 21.75µg/m³ and 83.38µg/m³ (see Table 20). Results from a study, by Zuurbier et al. (2010) show median PM10 concentration of 46.7µg/m³ for a Diesel bus route. These findings lie below the median concentration for route A1, but higher than median concentrations for all other routes in this thesis. Many explanations for the difference to other studies are possible as in-bus concentrations change with location and time, and different study set ups may have captured different ranges of concentrations. Conclusions are therefore difficult.
112