Overall PM concentration during mobile measurements

Figure 1 shows the concentrations of PM size fractions in four commuting modes, with Table 2 showing their detailed summary. The widest range of variability in PM 10 mass concentrations was observed during walk mode (1.2‒975 μg m−3), followed by cycle (1.4‒899 μg m−3), buses (2.6‒717 μg m−3) and car (6.9‒169 μg m−3). Irrespective of the time of the day, average PM 10 concentrations have the following trend: buses > walk > car > cycle (Table 2).

Fig. 1 Concentrations of different PM fractions during morning (MP), off-peak (OP) and evening hours (EP) for the studied commuting modes. The diamond symbols refer to the arithmetic mean concentrations while the median values are shown by horizontal lines within the bars Full size image

Table 2 Descriptive statistics for each of the modes of travel by time span and overall; MP, OP and EP refer to morning peak, midday off-peak and evening peak, respectively Full size table

Average fine and coarse particles have a different trend (Fig. 1). For fine particles, the trend in average concentration was bus > car > walk > cycle (Fig. 2). Fine particles in the bus and walk modes were time-dependent. For example, concentrations were lower for walk mode than for buses during the morning peak (MP) and evening peak (EP) periods, whereas higher concentrations in walk mode during the OP period. PM 1 was always highest for car mode and lowest for cycle mode. The concentration trend of PM 1 in the bus and walk modes was similar to that of PM 2.5 . For coarse particles, the trend was bus > walk > cycle > car (Fig. 2). Besides the infiltration through the bus envelope and, especially, the entrance of outdoor particles during the natural ventilation periods when the doors were open, the highest concentrations in buses may also be affected by high rates of re-suspension of particles due to inflow and outflow of travellers at bus stops.27,34,42,45 Concentrations of coarse particles in the car mode were comparable during the MP, OP and EP periods (Fig. 2). The ratio of in-cabin to ambient particles is a function of whether windows are open, and, if windows are closed, whether air is recirculated or there is fresh air intake from the heating, ventilation, and air conditioning (HVAC) system.10,46 The lowest concentrations in the car with respect to the rest of modes could be explained by the hindered entrance of outdoor sourced pollutants to the cabin owing to the closed windows. Therefore, infiltration of outdoor particles was expected to be low. Natural ventilation was absent, but mechanical ventilation was forced through the activation of air conditioning. Besides the hindered infiltration by closed windows, the lowest concentrations in the car could also be explained by an efficient removal of coarse particles compared with fine particles by the air conditioning filtration system.23,37 However, the low concentrations observed in the cycle mode, even less than the walk mode, could be attributable to forced de-routing on some occasions from busy traffic to quiet areas due to roads work during some runs. In addition, the concentrations usually decay exponentially with height in near-road environments,47,48 the relatively higher sampling height during cycle measurements may have made some contributions to the differences. The differences in meteorological conditions and traffic volume were trivial during the walk (February/March) and cycle (March/April) measurements and hence a modest effect of dispersion conditions could be expected to contribute to concentration differences.

Fig. 2 Average concentrations of coarse and fine particles over the three time periods for commuting modes. The images used are copyright free under the creative common licence (extracted using the Microsoft PowerPoint). Only positive standard deviation values are added to maintain the clarity of the figure Full size image

Comparison of the concentrations during different time periods revealed that the PM 1 and PM 2.5 concentrations exhibited a similar trend in each mode: EP > MP > OP in bus and cycle modes; MP > EP > OP in car; and MP > OP > EP in walk mode (Table 2). The trend for coarse particles was different in all modes: MP > EP > OP in bus; MP > EP ≈ OP in cycle; EP ≈ OP > MP in car; and EP > OP > MP in walk mode. Despite high traffic volume, previous studies have shown lower coarse particles in comparison with fine particles during morning periods during the walk, owing to the wetness of pavement due to overnight dew.49 The walk and car modes followed a similar trend; EP periods had higher concentrations of coarse particles than those in MP and OP periods (Table 2). The bus and cycle modes showed an opposite trend with higher concentrations of coarse particles during MP and EP compared with OP periods (Table 2), presumably because of high re-suspension in these TMEs. Fine particle concentrations were always lower in all modes during OP versus MP and EP periods. These lower concentrations could be attributed to lower traffic volume during OP periods and relatively better dispersion conditions compared with EP/MP periods.47

Fractional contributions of different-sized PM in TMEs

The absolute and fractional contributions of different-sized PM in studied TMEs are shown in Fig. 3a, b. The average total PM mass concentrations, based on the sum of PM 1 , PM 1-2.5 and PM 2.5-10 size ranges, are clearly greater for bus and walk modes than for car and cycle modes. However, the bus and walk mode total concentrations are dominated by the larger PM 2.5-10 size fraction. The fraction of particles that penetrate to the bronchioles and alveoli is smaller for the larger-sized particles. The smaller-sized particles, such as PM 1 and PM 1-2.5 , are therefore typically of more health concern. In this regard, the bus, car and walk modes are approximately similar, with PM 2.5 mass concentrations ranging from approximately 20 to 40 μg m–3, depending on the time of day. There is less variability by time of day for the car and walk modes than for the bus mode. The cycle mode clearly has lower total PM and PM 2.5 mass concentrations than any other mode, regardless of time of day. The car and cycle modes have a relatively little total mass contribution from coarse particles.

Fig. 3 a Absolute and b fractional contributions of average PM mass concentrations Full size image

The PM 1 /PM 2.5 ratio was nearly identical (≈0.73) for cycle and walk modes. Usually smaller the size of particles, more risk such particles poses to human health. About two-third of PM 2.5 being PM 1 clearly suggest exposure to such high health-damaging particles. The similarity in the particle size fractions coupled with the differences in exposure concentration implies that the cyclists were exposed to more diluted particles than were the pedestrians. However, the PM 2.5 /PM 10 ratio of the cycle mode (0.68 ± 0.04) was different from the walk mode (0.37 ± 0.04; Fig. 3b). The coarse particles, predominantly resulting from re-suspension of dust and tyre wear37 in near-road environments, contain larger settling velocity compared with their smaller counterparts. A distinct reasoning was unavailable but the differences in height of measurements may have partly attributed to higher coarse PM exposure to pedestrians versus cyclists.

Fine particles dominate (PM 2.5 /PM 10 ≈ 90%) the PM concentrations in car mode. The windows were closed (low infiltration) and air conditioning system was on (removing more efficiently coarse particles) during our car measurements (Description of travel modes). Coarse particles are more efficiently filtered out by the air conditioning system of the car compared to fine particles,23,42,50 explaining the relatively low fraction of coarse particles inside the car.

Normalised PM mass concentrations

We normalised the MP, OP and EP concentrations measured on cycle, car and bus modes with respect to those observed in the walk mode to assess differences between the transport modes. PM 10 , PM 2.5-10 , PM 2.5 and PM 1 concentrations were significantly lower for cycle mode compared to the walk mode (Fig. 4). In the car mode, during all three observation periods (i.e., MP, OP and EP), the total PM 10 and coarse particles concentrations were low, but the average concentrations of fine and submicron (PM 1 ) particles were higher compared to walk mode (Fig. 4). In this regard, commuting in cars could be more harmful than by walk from the viewpoint of exposure to fine and PM 1 , which can penetrate deeper into the bronchiole and alveoli regions and cause adverse health effects. Compared to walk, bus concentrations were approximately comparable for all size ranges for OP and approximately twice as high for EP, with MP values closer to OP than EP.

Fig. 4 Normalised (with respect to walk mode) concentrations of different-sized PM in various TMEs assessed in this study. The error bars represent minimum and maximum values of the average ratios Full size image

As compared to the walk mode higher levels of coarse particles were observed in the buses, while higher concentrations of fine and submicron particles were observed in the cars and, sometimes, also in the buses. The cycle mode had the lowest average levels of PM 1 , PM 2.5 and PM 10 . Fine particle abundance in vehicle modes, including car and bus, compared to walk mode is consistent with earlier observations of a higher in-vehicle versus walk exposure concentrations in Central London.51 However, the fine particle concentration ratio for cycle versus walk mode of only 0.41 is much lower than a ratio of 1.2 reported from Central London, presumably due to street canyon effects compared with mostly open road conditions in our study.51 In the highly polluted city of Delhi (India), the mass ratios of fine particles between modes were 1.5 for cycle-to-walk, 0.3 for car-to-walk and 0.9 for bus-to-walk.52

Concentrations of PM 2.5-10 as compared to PM 2.5 were relatively high in the bus and walk modes (SI Fig S3). The average mass concentration ratio of PM 2.5-10 /PM 2.5 in bus mode varied from 1.6 to 3.0, whereas in walk mode it ranged from 1.4 to 2.0, as high as by a factor of 2 to 3 (SI Fig S3). Conversely, PM 2.5 predominates in the car and cycle modes: PM 2.5-10 /PM 2.5 in cars ranged from 0.05 to 0.1 (i.e., 5 to 10% of fine particles), whereas it varied from ~0.4 to 0.55 on cycle mode (i.e., 40 to 55%). Re-suspension effect appears to be the main cause for elevated levels of coarse particles in the bus and in walk modes.

Exposure assessment

RDD account for differences in physical activity and the time spent in a microenvironment.53,54 We estimated RDD for both fine and coarse particles using inhalation rates for males (Fig. 5) and females (SI Fig S4). Since the trend of results for male and female RDD are similar, the discussion in the subsequent text are valid for both genders. Here we only refer to male doses in the discussion for the brevity reasons. The RDD for coarse particles (Fig. 5a) were the highest during the walk mode, with mean RDD ranging from 40 to 66 μg h−1 among the MP, OP and EP periods. These were followed by buses (28‒32 μg h−1), cycle (10‒15 μg h−1) and cars (0.8‒1.3 μg h−1). However, fine particle-based RDD were comparable for walk (5‒6 μg h−1) and cycle (4‒6.5 μg h−1) and were higher than for bus (3.5‒5 μg h−1) and car (1.8‒2 μg h−1) modes. The fine particle-based RDD for walk mode were comparable to that reported by Kumar and Goel37 for the same study area (7 μg h−1). Likewise, Rivas et al.42 reported that mean PM 2.5 -based RDD decreased when comparing walk to bus and car modes. A Barcelona-based study also found that walk and cycle modes had similar RDD, with walk at 6.8 µg h−1 and cycle at 6.7 µg h−1, and that RDD were lower for bus (5.4 µg h−1) and car (5.6 µg h−1) modes.8

Fig. 5 Estimated RDD for a coarse and b fine particles for males during sitting (bus and car), heavy exercise (cycling) and light exercise (walk). The whiskers in the figure represent 5th and 95th percentiles, whereas the box plot represents for 25th, 50th (median) and 75th percentiles. The arithmetic mean for the data is represented by diamond symbols Full size image

Relative to fine particles, the mean RDD for coarse particles were 8‒12, 6.5‒8.3 and 1.8‒3.7 times higher during walk, bus and cycle modes, respectively, but only between 0.3 and 0.8 for car mode. This shows the effect of traffic-related sources such as re-suspension in the walk, bus and cycle modes while the in-car exposure to coarse particles is hindered by the closed windows and the filtration system. The difference in RDD between coarse and fine particles can be explained by the difference in fine and coarse PM mass concentrations, particle mean diameter and size-dependent deposition fractions (DFs).