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A Review of Commuter Exposure to Ultrafine Particles and Its Health Effects

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Determining a Commuters' Exposure to Particle and Noise Pollution on Double-decker Buses

Erik VelascoThis email address is existence protected from spambots. You demand JavaScript enabled to view information technology. i, Elvagris SegoviaThis email address is being protected from spambots. Yous need JavaScript enabled to view it. 2

i Independent Inquiry Scientist, 118719, Singapore
ii Department of Geography, National University of Singapore, 117568, Singapore

Received: July 5, 2021
Revised: September 28, 2021
Accustomed: October 4, 2021

CopyrightThe Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original writer and source are cited.


Download Commendation: ||https://doi.org/10.4209/aaqr.210165
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Cite this article:

Velasco, East., Segovia, E. (2021). Determining a Commuters' Exposure to Particle and Noise disturbance on Coach Buses. Aerosol Air Qual. Res. 21, 210165. https://doi.org/x.4209/aaqr.210165

HIGHLIGHTS

  • Higher concentration of fine particles tin be expected on upper decks.
  • Lower decks often show slightly higher concentrations of ultrafine particles.
  • Noise levels are lower on upper decks.
  • Intake of outdoor air and air infiltration in terminals decide particle loads.
  • The presence of particles increases sharply every time the doors are opened.

ABSTRACT

This written report evaluates a passenger'south exposure to particles and noise on Singapore's motorbus buses when choosing to travel on the upper or lower deck. Clean and calm journeys are vital to make public buses a choice style of transport. In the instance of Singapore, equally in many other cities, double-deckers are in common utilize and contain a big fraction of the total public transport ridership. Exposure to noise levels and concentrations of fine particles (PM2.5), equivalent black carbon (eBC) and number of particles (as a proxy of ultrafine particles, UFP) were simultaneously measured on both decks. Concentrations of particle-bound polycyclic hydrocarbons and particles' agile surface area were too measured to investigate the combustion fingerprint of the particles and their average size. Concentrations of PM2.v on the upper deck exceeded up to 50% (18 µg m–3) those on the lower deck. Concentrations of eBC were higher on the upper deck up to 12% (0.6 µg m–3), but on occasions were 40% (0.ix µg g–3) lower. In contrast, UFP oftentimes measured in slightly higher concentrations (~8%, 1000 # cm–three) on the lower deck. Noise levels were e'er ~5 dBA higher downstairs. Particle loads responded to the intake of outdoor air by the air workout system, while sharp increases were observed on both decks every time the doors were opened to pick upwardly and driblet off passengers. In the case of the assessed road, which started and ended in fully enclosed terminals, the infiltration of particles emitted past arriving and departing buses contributed critically to the loads of PMtwo.5 on the upper deck. Improvements in the ventilation system on both decks, changes in engine operation at bus terminals, and faster boarding at bus stops through amend coordination of bus services volition reduce the load of particles to which passengers are exposed on double-decker buses.


Keywords: Urban transport pollution, Personal exposure, Public buses, Ultrafine particles, PM2.v

1 INTRODUCTION

Motorbus buses are widely used for public mass transport in many cities effectually the world not only because of their large seating capacity, but also due to their shorter length that allows for like shooting fish in a barrel functioning through narrow streets and tight corners (Hancock and Woodcock, 1988; Vuchic, 2002).

Commuters usually cull the upper deck for longer trips, while preferring to remain in the lower deck for shorter rides, thus avoiding climbing stairs and rushing down for alighting (Sun et al., 2014). This is especially true for senior commuters, who sometimes claim that struggling up the stairs causes them to feel lightheaded, which has been proved past assessments of the ability of passengers to maintain balance inside moving buses (Turner and Griffin, 1999; Kerekla and Tyler, 2019).

Passengers choosing to stay on the lower deck may experience college loads of road traffic pollution when doors open. Still, poor ventilation may cause an aggregating of pollutants in the upper deck, which makes staying downstairs more advisable. Similarly, with no driver guarding the upper deck, commuters tend to be louder and more raucous when they are out of earshot. Hence, sitting downstairs may exist more pleasant.

To find out whether or not passengers sitting on the upper deck are exposed to less traffic pollution and experience calmer trips, nosotros evaluated and compared the characteristics and concentration of particles, as well as dissonance levels on both the upper and lower decks of double-decker buses in Singapore. We focused on particles and not on gaseous pollutants such every bit carbon monoxide (CO) and nitrogen dioxide (NO2), which are as well toxic species embedded in exhaust plumes, because studies suggest that adverse health effects are mostly associated most strongly with particles (Knibbs et al., 2011, and references therein).

Double-decker buses are in common use throughout Singapore. They operate since 1977 (Menon and Kuang, 2006). Out of 5860 public buses plying the roads, around fifty per cent are omnibus buses. All of them are equipped with rear-mounted air conditioning units, while most are powered by diesel fuel engines under Euro V or Six emission standards. It is of import to signal out that buses are the nearly widely used mode of public transport in Singapore, comprising 53 per cent of the total public transport ridership (LTA, 2020).

A previous study in Singapore reported higher concentrations of particles, specially of ultrafine particles (≤ 100 nm in size, UFP), inside buses than in a site not direct affected by traffic emissions, 25 and 21 × 103 # cm–3, respectively; but lower than at double-decker stops and sidewalks along a busy road, 46 and 38 × 103 # cm–3, respectively (Tan et al., 2017). The same written report also found an increase in the load of particles when doors open and shut to pick up passengers at omnibus stops, equally it has too been reported elsewhere (e.g., Targino et al., 2017; Lim et al., 2015; Tsai et al., 2008). Assessing the concentration of particles to which commuters are exposed to within buses is relevant because commuters are probable to experience exposure to a big proportion of air pollution during daily commuting trips (Kumar et al., 2018; de Nazelle et al., 2017; Cepeda et al., 2017; Knibbs et al., 2011).

Cross-sectional epidemiological studies take plant that long-term exposure to both traffic noise and air pollution increases the risk of cardiovascular disease, hypertension, and mild cognitive impairment (e.thou., Fucks et al., 2017; Tzivian et al., 2016; Kälsch et al., 2014; Beelen et al., 2009); while a controlled report establish that short-exposure to diesel frazzle and traffic noise causes oxidative stress (Hemmingsen et al., 2015). Although a synergistic mechanism betwixt both exposures has even so to be found, their independent impacts take been well documented, therefore noise and air pollution must be considered when assessing the chance to public health (Stansfeld, 2015).

To the best of our cognition, no studies have previously evaluated a passenger's exposure to particle pollution and noise on both decks of omnibus buses simultaneously. Thus, the findings of this study should be of special interest to cities with an intense service of these type of public buses, such equally London, Dublin, Hong Kong, and Singapore. Similarly, the results should also exist useful for cities planning to innovate or expand the utilize double-decker buses like Cairo, Addis Ababa, Penang, and Vancouver, as well as in those in which they are part of fleets of Bus Rapid Transit (BRT) systems like Mexico City and Ottawa. The results of this study aim to provide information to achieve cleaner systems of public transport because the wellness of the passengers a priority.


ii METHODOLOGY

Bus service 7 was selected for this study as information technology is a major body route bridging the east and w sides of Singapore (see Fig. SM3). It covers a route of 23 km in approximately 90–100 min with an operation frequency of 12 min. It connects the residential states of Bedok and Clementi with many suburban areas and the city center, and passes along the iconic retail boulevard of Orchard Road and the shopping area of Bugis. Much of this route has been retained since the 1970s because of its high demand.

This route plies avenues of v- to eight-lane single- or double-carriageway with several intersections with minor and major roads. The speed limit of all vehicles is fifty or 60 km h– ane depending on the road. One lane of the route is dedicated to buses, and depending on the route's section, it may accommodate up to fifteen passenger vehicle services. The measurements were conducted effectually the evening rush hour (17–21 h), when traffic gets worse. In the most congested sections, similar Orchard Road and Bugis area, betwixt 1700 and 2400 vehicles per 60 minutes circulate at that time (Velasco and Tan, 2016; Tan et al., 2017). Private cars stand for 65% of Singapore's vehicular fleet, heavy and light skilful vehicles, and motorbikes account for 17% and fifteen%, respectively, while taxis and buses less than 2% each. Diesel engines are mostly reserved for goods vehicles and buses, private cars run on gasoline, every bit well as half of the taxi armada, while the rest of taxis are hybrid models combining a gasoline engine with an electric motor (Land Transport Authority, 2021). The large majority of diesel fuel and gasoline vehicles encounter at least the Euro 4 emission standards, since 2018 all new vehicles have to meet the Euro VI emission standards, while motorcycles are tightened to Euro Iv standards. The utilise of ultra-low sulfur free (ten ppm) diesel fuel and gasoline is mandatory since 2017 (Singapore Chaser General's Chambers, 2019).

The road was covered 16 times inside 8 days in March 2018 during the bound inter-monsoon season. No day of measurements was afflicted past rain, generally cloudy skies and northeast winds were present during the sampling sessions, ambient temperature and relative humidity ranged between 27 and 31°C, and 66–89%, respectively, every bit reported by the local meteorological service (https://www.nea.gov.sg/weather). No particular event of local or transboundary air pollution was observed. Hourly mass concentrations of fine particles (≤ two.v µm in size, PM2.5) betwixt 10 and 20 µg m–iii were reported by the local air quality monitoring network at ambient level across the city (https://www.haze.gov.sg/). On regular days Singapore's particle pollution originates from local combustion sources with a major office related to vehicular traffic, and another associated with fresh secondary aerosols produced nether the influence of shipping and industrial emission activities (Rivellini et al., 2020).

Measurements were limited to weekdays. On each day, two sets of measurements were conducted. We started recording the first set of measurements at the Clementi bus interchange around 17:00 h and stopped at the Bedok bus interchange 99 ± 11 min (geometric mean ± 1 standard deviation) later. The 2nd set of measurements were recorded post-obit the contrary direction, starting around 19:30 h and finishing 86 ± 5 min later on. The beginning trip was longer than the 2d past xiii minutes on average due to worsening traffic during the elevation of the evening blitz hr.


2.one Instrumentation

We measured in-situ the mass concentration of PM2.five and equivalent blackness carbon (eBC), as well as the particle number concentration (PN) and equivalent continuous audio level (L eq) in both decks using two sets of portable battery-operated sensors. The mass concentration of particle-spring polycyclic aromatic hydrocarbons (pPAHs) and the agile surface area (ASA) of the particles were measured on alternate trips on each deck because we counted with only 1 prepare of sensors for measuring these 2 parameters. The particle number concentration and ASA were measured as a means to quantify UFP. In microenvironments affected by traffic emissions nearly of the particles are constitute in the ultrafine fraction (Morawska et al., 2008). The small size and chemical limerick of these particles exacerbate the health risk they pose. In terms of toxicology, ASA is the metric that explains improve the variability in pulmonary inflammation triggered by particle pollution (Schmid and Stoeger, 2016; Oberdöster et al., 2005), which is specially truthful for UFP, since they correspond the highest surface expanse per mass. Measurements of air temperature (T) and relative humidity (RH) were also included.Tabular array 1summarizes the characteristics of each instrument.

Ane gear up of instruments was operated by a researcher sitting on the upper deck, whilst a second researcher monitored the other assault the lower deck. The sensors were placed over seats in the heart sections of both decks, two rows behind the stairs on the upper deck, and one row after the exit door on the lower deck. The sensors' position within the cabin is patently less important, at least when the bus is moving with the doors closed, equally shown by a study in Germany that found that particle exposure is not affected by seat placement in modernistic and well-ventilated buses (Bauer et al., 2018). Both researchers took notes on the events on board as a reference for the data analysis.

All sensors were programmed for 1 southward readings, with the exception of those measuring pPAHs and ASA, which were programmed for ten south readings. Our previous studies of personal exposure on transport modes showed that such frequencies are needed to capture the variability that characterizes particle pollution and noise in microenvironments affected past vehicular traffic (Tan et al., 2017; Velasco et al., 2016, 2019).

The instruments' configuration and training, besides as corrections practical and data postprocessing are described in the articles cited to a higher place. Details are provided in the Supplementary Material (SM). In previous studies, we evaluated the responses of the instruments for measuring PM2.v and eBC confronting reference instrumentation, as a means of indirect calibrations. The pairs of instruments, including the sensors for monitoring particles, noise, T and RH, were run side-by-side for five minutes before and subsequently each trip at locations not affected past traffic emissions outside the bus terminals. The hateful differences betwixt pairs of sensors during both testing periods were used to build linear regressions to remove any trends during the measurements and adjust the data sampled on both decks.


2.2 Combustion Origin and Average Size of the Particles

To gain farther insight on the origin of the particles, the ratio obtained from the concurrent measurements of pPAHs and ASA was used as a fingerprint of the type of combustion particles (Bukowiecki et al., 2002). Following the acronyms of the instruments used to measure both metrics (Photoelectric Charger (PC) for measuring pPAHs, and Diffusion Charger (DC) for measuring ASA) the fingerprint is known as PC/DC ratio. It depicts distinctive values co-ordinate to the type of fuel and combustion. For case, diesel exhaust particles yield ratios of ~1 ng mm– 2, while those from gasoline frazzle yield ratios < 0.6 ng mm– 2 (Ott and Siegmann, 2006). Ratios of > 1 ng mm– 2 are related to high emissions of pPAHs during periods of hard acceleration due to incomplete combustion (Tang et al., 2001). Particles from non-combustion sources or those that are already coated by condensable species such every bit semi-volatile hydrocarbons or molecules of h2o yield a PC/DC » 0.

Similarly, bold spherical particles, the average size of traffic particles can be determined by the diameter of boilerplate surface (D Aver,S) as proposed by Kittelson et al. (2000) from the concurrent and contained measurements of PN concentration and ASA. D Aver,South represents the diameter of a hypothetical monodisperse particle that has the aforementioned ASA every bit the measured polydisperse particle.


3 RESULTS AND Word

The concentration of particles and noise level on both decks forth the sampling trips were examined and compared beginning to those observed in previous studies in microenvironments affected past traffic emissions in Singapore. The main features of the particles along the trips were also analyzed in terms of the road and frequency of motorcoach stops. Then, the concurrent measurements of ASA and pPAHs were used to investigate the combustion fingerprint of the particles, and the ASA and PN information to guess the average size of the particles.

The Mood'southward median test and Kruskal-Wallis test were performed at 95 per cent conviction level (p ≤ 0.05) to evaluate and verify the significance of differences between decks with respect to concentrations of PM2.5, eBC and PN, noise levels as well equally T and RH, respectively. The Anderson-Darling examination showed that the observed variables were not normally distributed but positively skewed, hence the demand to use nonparametric tests to investigate the equality of results obtained betwixt decks.

The results presented here were obtained from more than 24 hours of sampling inside the buses, in which almost 90-m readings were nerveless for metrics measured every 1 due south and 9-yard for metrics measured at every ten s. The readings nerveless during sixteen sampling sessions were expected to provide enough prove on the difference in particle concentrations between both decks based on experiences from previous personal exposure studies on public ship (Tan et al., 2017; Velasco et al., 2019). Every bit is the case in most field campaigns, practical choices had to exist fabricated to best meet the study objectives. A larger number of sampling sessions during more days would have allowed to collect more than data and test other routes, thus providing more than robust bear witness, simply field piece of work expenses would have increased.


iii.1 Particle Concentrations and Principal Features

Fig. aneshows the time series of PM2.5, PN, eBC and noise measured simultaneously in the upper and lower decks during a representative sampling trip. The means and variabilities observed in each trip are presented in the SM (Tables SM1–SM11 and Fig. SM4).

Fig. 1. Time series of mass concentration of PM2.5 and equivalent black carbon, particle number concentration and noise level measured on the upper (blue) and lower (black) decks during a representative sampling trip (6 Mar 2018). The shaded sections at the beginning and end of the trip correspond to the time spent at the boarding and alighting berths at the bus terminals.Fig. 1. Time serial of mass concentration of PM2.5 and equivalent blackness carbon, particle number concentration and noise level measured on the upper (blue) and lower (black) decks during a representative sampling trip (6 Mar 2018). The shaded sections at the beginning and end of the trip correspond to the time spent at the boarding and alighting berths at the bus terminals.

Fig. iishows the difference in concentrations of PM2.five, PN and eBC, dissonance levels, and T and RH measured simultaneously on both decks during each sampling trip. The differences were statistically meaning for each private trip for all measured variables, even though the variation between decks was modest on occasions, specially for PN, eBC and RH. There were not statistically pregnant differences in PN concentrations measured during ane trip only.

Fig. 2. Differences in concentrations of (a) PM2.5, (b) particle number, (c) equivalent black carbon, (d) temperature, (e) relative humidity, and (f) noise levels between the lower deck and upper deck (i.e., lower – upper) for each set of measurements. Records above zero (blue thick line) indicate higher concentrations or values in the lower deck, and the opposite is true for negative records. In each box the middle line, top and bottom are arithmetic means, upper and lower quartiles (75th and 25th percentiles), respectively. Blue dots indicate medians. Whiskers extend to the 95th and 5th percentiles. Fig. 2. Differences in concentrations of (a) PM2.five, (b) particle number, (c) equivalent blackness carbon, (d) temperature, (due east) relative humidity, and (f) noise levels between the lower deck and upper deck (i.due east., lower – upper) for each ready of measurements. Records in a higher place nil (blue thick line) indicate college concentrations or values in the lower deck, and the opposite is true for negative records. In each box the middle line, top and lesser are arithmetic ways, upper and lower quartiles (75thursday and 25thursday percentiles), respectively. Blue dots signal medians. Whiskers extend to the 95th and fivethursday percentiles.

Nosotros institute that concentrations of PMii.5 were consistently higher on the upper deck. Concentrations of eBC were also higher on the upper deck, but less consistently. In contrast, concentrations of UFP were somewhat less abundant upstairs during most trips. Accounting for all trips, concentrations (median, 75th–25th percentiles, time to come unless otherwise stated) of 35, 42–27 (nineteen, 22–15) µg grand–three of PM2.5 were observed upstairs (downstairs). Similarly, mean concentrations of 5.2, 7.2–ii.2 (4.6, six.6–2.nine) µg m–3 of eBC, and xi.vii, 17.4–vii.0 (12.8, 16.2–8.i) ×103 # cm–3 of UFP were observed.

Although sharp increases were recorded on both decks every time the bus opened the doors to pick up and drop off passengers, these increases were more than evident on the lower deck as shown inFig. 1. Precipitous increases in PMtwo.5 were hardly registered on the upper deck. Even so, such increases in PN and eBC (particle metrics strongly associated with vehicle exhaust) were ever visible on both decks.

Reverse to increases in PN, increases of eBC were expected to add up to a similar amount to PM2.5 mass concentration, simply it was not ever the case. Equally shown inFig. 1, not all eBC peaks were reflected by PMtwo.5 peaks to the same extent. This anomaly may answer to inaccurate readings to fast changes in particle loads past the micro-aethelometer and DustTrak droplets monitor used to measure eBC and PM2.5 mass concentrations, respectively. The former instrument is based on aerosol lite absorption, and contributions from non-carbonaceous light-arresting aerosol components like mineral dust, or not-black carbon light-absorbing carbonaceous thing (i.east., brownish carbon) may affect its readings (Petzold, et al., 2013). Also note that the micro-aethalometer tin can yield irregular peaks and negative values when sampling at a high frequency or at depression concentrations. These spurious readings were corrected during information postprocessing every bit described in the SM, but they can still exist incorrect at times, especially when concentrations are particularly low. This consequence represents a drawback to our report and must be considered when interpreting the results. Similarly, the PMtwo.5 monitor is based on aerosol light handful, and temporal changes in particle properties such as size distribution and chemic composition may affect its readings (Zhang et al., 2018). Therefore, nosotros caution that the accented readings from both instruments should be observed carefully, particularly during transition periods with desperate changes in aerosol abundance.

For the times serial shown inFig. 1, frequent stops along the commercial boulevard of Orchard Road and the city center (18:20–18:50 h) yielded particle peaks every 3–4 min when the doors opened for periods of 40–60 south to pick up passengers. The number of commuters forth these sections of the road was large enough to pack the lower deck with over 40 seated and continuing passengers, and over 35 seated passengers on the upper deck. Several routes ply this sector of the city, and it is usual to discover two or iii buses queuing at each terminate, whose emissions contribute to fill the cabin with polluted air.

The number of passengers and loads of UFP and eBC showed similar patterns along the unabridged trips, although correlations between them were modest (r ii = 0.25–0.28). More than commuters means more time with doors open, and therefore a major infiltration of dirty air. This tendency can be observed in the box plots of Fig. SM4 summarizing the statistics of each measured parameter, including number of commuters, for each trip.

Plumes from braking and accelerating cars are ubiquitous during the evening rush hour, contributing also to the particle loads inside buses, especially at bus stops adjacent to traffic lights and road intersections (Tan et al., 2017). Going back to the time serial shown inFig. 1, the large particle tiptop registered at 19:fifteen h occurred in a bus stop next to a train station (Paya Lebar MRT station) and an 8-laned busy double-carriageway. Many passengers alighted at this stop and doors were kept open for no less than one infinitesimal, allowing muddy air to enter the passenger vehicle.

In one case the doors were opened, the particle loads peaked apace, reaching maximum concentrations within ten–35 seconds on the lower deck and 25–55 seconds upstairs based on the observations of the 16 trips. Particle peaks declined slowly during the side by side 5–7 minutes before returning to pre-top concentrations on both decks. In cases of nearby stops (i.due east., closer than 5 minutes), particle peaks did not render to pre-peak levels, and the infiltration of new polluted plumes built new peaks over already existing ones. Similar findings have been reported past other authors looking also into particle exposure inside single-decker buses (e.g., Tan et al., 2017; Targino et al., 2017; Lim et al., 2015; Tsai et al., 2008).

For the sampling trip shown inFig. ane, the effect of frequent stops tin can exist observed in the serial of four consecutive peaks between 18:25 and 18:35 h. Although PM2.5 and eBC registered these peaks, the pattern was more evident in the case of UFP (i.e., PN). In cities with strong vehicular controls, the bulk (> 95%) of traffic particles represent to ultrafine aerosols (Morawska et al., 2008; Vu et al., 2015).

Taking as reference the time in which the loads of UFP increased speedily afterwards the doors were opened, and the associated changes in concentrations, we establish that the plumes rich in fresh particles diluted by 65–xc% on boilerplate in one case they reached the upper deck. However, on a few occasions, spikes up to 2 times larger were observed on the upper deck, possibly as a upshot of the internal flow inside the cabin. The temporal increases in PM2.5, as already mentioned, were by and large negligible on the upper deck.

No aggregating of particles was observed on either deck along the journeys. In contrast, a gradual decrease was consistently observed on both decks during a menstruum of 10–20 min later leaving the bus terminals. Few and quick stops limited the infiltration of particles throughout the beginning section of the route in both directions. However, the infiltration of dirty air at the bus terminals was of import, and resulted in high concentrations of particles during the first few minutes of the trips.


3.two Particle Infiltration in Bus Terminals

The assessed road starts and ends in bus terminals fully enclosed inside buildings serving as integrated transport hubs. Although drivers plough off the engine afterwards parking the double-decker, doors are usually left open up, and dirty air easily infiltrates the cabin in the ~xxx minutes that the charabanc remains parked. The dirty and warm air in the terminal replaces the cool and less polluted air inside the bus. The vehicular concourse and parking areas are poorly ventilated, and the particles emitted past arriving and departing buses accumulate and pollute the interior of parked buses. Both terminals are ever busy, ane serving 14 different routes, and the other 26.

The infiltration of particles in aggregating mode (50 nm–1 µm) apparently determines the loads of larger particles (i.due east., PM2.5) to which commuters are exposed to along the entire trip on upper decks. These are carbonaceous particles straight produced in the engine that are after coated with sulfate and semi-volatile organic species. They contribute largely in terms of mass rather than number. Yet, the larger number of particles emitted by diesel fuel buses falls into the nucleation way (< 50 nm) (Alam et al., 2016 and references therein). The smallest particles (< x nm) form at elevated temperatures in the engine and tailpipe, and consist of highly not-volatile cloth (Rönkkö and Timonen, 2019). The remainder of nucleation fashion particles are rapidly formed within the exhaust feather as combustion gases cool and condense, and consist mainly of volatile organic and sulfur containing compounds (Morawska et al., 2008). These particles coagulate quickly with other particles, which favors the increase of larger particles, at the price of reducing the full number of particles (Maricq, 2007). This procedure contributes to accumulate larger particles on the upper deck.

Although passenger concourses are air-conditioned and boarding and alighting berths are equipped with automatic sliding glass doors that open simply when the omnibus reaches the berth, commuters are exposed to brief particle spikes while boarding and alighting. These spikes represent the highest particle loads to which passengers tin can be exposed during a terminal to terminal trip. Short-lived spikes (10–30 s) of 98 ± 44 × 103 # particles cm–3, 66 ± 28 µg m–three of PM2.5 and xl ± 28 g chiliad–3 of eBC (geometric hateful ± 1 standard divergence, hereafter unless otherwise stated) were observed, and represented concentrations amounting to 5–8, ix–twenty and 3–8 times higher than those experienced at the rider concourse, respectively.


3.three Infiltration of Outdoor Air

Subsequently the initial stretch of trips, the lack of trend in the abundance of PM2.v, eBC and UFP on both decks suggests the infiltration of outdoor air past the ac system (come acrossFig. 1). Such air dilutes temporal peaks caused by infiltration of muddied air at omnibus stops, and maintains relatively constant loads of particles.

We were not able to verify if the air conditioning systems were set to recirculation mode in the sampled buses. Setting the air-workout to recirculation mode on the lower deck would be useless due to constant infiltration of street air when doors were opened. On the other hand, such a setting may have served to clean the air inside the upper cabin. The goose egg reduction or accumulation of particles after the initial stretches of the trips suggests that UFP and eBC concentrations inside the passenger vehicle corresponded to those outside forth the road. For the case of PMtwo.5, the concentrations on the lower deck must have corresponded to those outside, while those on the upper deck (which were consistently higher than those measured downstairs) corresponded to concentrations reached while the motorbus was parked in the enclosed terminals.

Conversely, whilst concentrations of PM2.five on the lower deck were smaller than those measured at Singapore bus stops and forth sidewalks of Orchard Road in previous studies (32 ± x and 36 ± ix µg m–3, respectively) PM2.v concentrations on the upper deck corresponded to such prior study measurements (Velasco and Tan, 2016; Tan et al., 2017). The concentrations of eBC on both decks were somewhat lower than those previously observed forth sidewalks (6.7 ± 4.0 µg m–3), and clearly lower than those at double-decker stops (eighteen.3 ± 8.1 µg g–three). For UFP, the number of particles was college at bus stops and along sidewalks (73.one ± 32.8 × x3 and 44.0 ± 6.1 # particles cm–3, respectively) than on both decks.

The higher concentrations of PM2.five, eBC and UFP reported along sidewalks and at bus stops in previous studies tin can be explained past the fleet modernization during the last five years. When such studies were carried out, public buses were already subject to Euro V emission standards, merely it was still possible to discover buses falling under Euro I, Ii and III emission standards.

We cannot explicate why the differences in concentrations of PMii.5 were much larger than those of eBC and UFP between decks. Instead, we hypothesize that equally a event of the design and operation of the air-conditioning and ventilation system a small force per unit area difference develops between the decks in a matter of minutes at the get-go of a trip, which in turn hinders a vigorous exchange of air between them. It is also possible that the internal air menstruum, especially when the doors are open up, limits the exchange of air between decks. The initial minutes of a trip with few and quick stops are manifestly enough to mix the loads of UFP and eBC accumulated in the terminal, merely not of larger particles (i.e., PM2.5). The infiltration of anile aggregating mode particles in enclosed terminals is patently more critical than the infiltration of freshly emitted particles on the upper deck along the trip.


3.iv Particle-bound Polycyclic Aromatic Hydrocarbons and Agile Surface Surface area

The measurements of pPAHs and ASA on both decks during alternate trips did not make it possible to make up one's mind differences in levels observed upstairs and downstairs. However, both metrics showed similar patterns to those observed for UFP and eBC. Correlations between ASA confronting PN and eBC were in general low (r 2 < 0.20). Withal, moderate or strong correlations were observed for the example of pPAHs confronting both metrics (0.20–0.98, and 0.lx–0.99, respectively).

Polycyclic effluvious hydrocarbons are good tracers of vehicle exhaust particles. When the exhaust gases absurd, effluvious species of four or more rings condense on the surface of existing aerosols, and form the so chosen pPAHs (Ravindra et al., 2008). Spikes over 300 ng m–iii of pPAHs were observed on both decks in phase with spikes of UFP and eBC. Considering the entire fix of measurements, concentrations of 85, 100–82 (81, 125–59) ng m–3 were recorded upstairs (downstairs). The college variability downstairs responds to higher temporal increases when doors were opened.

In traffic microenvironments, where particle burdens are dominated by UFP, high levels of ASA can be expected since such particle size fraction represents the highest surface area per mass of particles. Therefore, spikes over 350 mmii m–iii in concert with spikes of pPAHs and UFP were often observed on both decks. Concentrations of 108, 130–91 (127, 148–112) mm2 k–3 were recorded on the upper (lower) decks.

Similar to concentrations of eBC and number of particles, concentrations of pPAHs and ASA were in general lower on both decks than in bus stops and along the sidewalks of Orchard Route, equally previously reported. In the case of pPAHs and ASA, hateful concentrations of 225 ± 84 ng m–3 and 172 ± 68 mm2 g–three, respectively were measured at Singapore's coach stops, whilst measurements of 97 ± 17 ng chiliad–3 and 134 ± 23 mm2 m–three, respectively were registered along Orchard Road's sidewalks (Velasco and Tan, 2016; Tan et al., 2017).


3.five Combustion Particles Fingerprint

The pPAHs and ASA measurements on alternate trips yielded hateful PC/DC ratios of 0.83 ± 0.16 on the upper deck and 0.77 ± 0.28 on the lower deck. The large variability and limited number of samples did not allow to make up one's mind a divergence between decks. Notwithstanding, the ratios observed on both decks were similar to that observed along the sidewalks of Orchard Road (0.74), and substantially lower than those reported at double-decker stops (1.43) in our previous studies. This finding suggests that the intake of outdoor air past the air conditioning system could have a greater influence on the load of UFP on both decks than the air that infiltrates every time the doors are opened. The PC/DC ratio on double-decker buses falls between those reported for frazzle plumes of gasoline and diesel vehicles, which is expected for measurements taken along the roads of Singapore (where all cars run on gasoline, and diesel consumption is express to taxis, heavy duty vehicles and public buses).


3.half-dozen Average Size of Particles

On boilerplate, particles of 52 ± 9 and 61 ± 21 nm were observed on the upper and lower decks, respectively. Similar to the PC/DC ratio, the number of samples taken were too small to decide a deviation between decks, nevertheless the particle size was apparently more variable downstairs. The boilerplate size of particles barbarous into the college range of the nucleation mode and the lower range of aggregating mode particles. As already explained, particles in the former mode are formed within the exhaust feather and tend to coagulate and condensate on larger particles, while accumulation fashion particles are generated directly from the engine, and are mostly agglomerates of carbonaceous material (Morawska et al., 2008; Rönkkö and Timonen, 2019). Larger particles (i.eastward., > 1 µm) are primary emitted past mechanical abrasion processes, including wear emissions from brake linings, tires, and route pavement, as well as route grit resuspension (Thorpe and Harrison, 2008).

In our previous study at bus stops, the average size of particles ranged from 15 to 40 nm with a mean of 27 nm, essentially the range reported for diesel particles from modern engines equipped with catalytic traps. The slightly larger D Aver,Due south found in this report, suggests that the particles to which commuters are exposed on double-decker buses take already gone through a cursory aging process resulting in an increased size. Moreno et al. (2021) used a miniaturized portable scanning mobility particle sizer to measure directly the particles' size distribution inside buses in Barcelona, Spain, also finding a size range (35–50 nm) representative of aged traffic particles.


3.7 Noise Level

In relation to noise, the sounds generated on the lower deck turned out to exist louder. The equivalent continuous sound level along the entire trips was 66.7, 66.9–66.1 (71.eight, 72.half-dozen–71.4) dBA upstairs (downstairs). The difference between decks was statistically meaning (i.e., p ≤ 0.01). Instantaneous racket levels over 80 dBA were scarce. Maximum readings of 70 (79) dBA were recorded in all sampling trips. No special trend was observed forth the route. No substantial increases were recorded at bus stops. From the notes taken past the researchers about elevated or dissonant readings during the measurements, it was plant that peaks responded merely to noisy people, especially passengers using their smartphones for watching videos or playing video games.

The recorded noise level classifies as 'moderately annoying' by well-nigh scales of reference (eastward.m., Ouis, 2001), having a pocket-size impact on people's mood during short periods of time. According to Singapore'due south legislation (NEA, 2020), the dissonance level on both decks was always below the maximum permitted L eq of 75 dBA in commercial premises for periods no longer than 5 minutes. Notably, noise levels on the lower deck slightly exceeded the 12-h 50 eq of 65 dBA legal limit imposed in residential areas during daytime, possibly posing a threat to the health of autobus drivers, who work viii to 10 h daily.


iii.eight Temperature and Humidity

Slightly cooler temperatures were experienced on the upper deck. The constant infiltration of street air somewhat limits cooling downstairs. Temperatures of 23.7, 24.five–23.0 (24.9, 25.seven–23.v)°C were recorded during the whole prepare of trips upstairs (downstairs). When leaving the bus concluding, both decks unremarkably marked an equal temperature, simply along the trip a slightly stronger gradual decrease was attained upstairs. Temperature drops of 4–6°C and 2–five°C were observed on the upper and lower decks, respectively. The cooling inside buses depends on the air-conditioning strength and the time the doors remain open while picking upwardly passengers along the route.

During the final stretches of the trips, temperatures beneath 23°C were frequently recorded on the upper deck, and in ii trips below 22°C. Such temperatures contrast with the typical 28–31°C observed at passenger vehicle stops in the evening (Velasco and Tan, 2016). The quick and drastic modify in temperature when boarding a bus in the hot and humid climate of Singapore creates an unpleasant sensation (negative alliesthesia, de Honey, 2011) and ironically forces many commuters to carry sweaters with them.

Relative humidity ranged generally between 45% and 60% on both decks. Although significant differences were observed for individual trips, a mean humidity of 52 ± 5% was found on both decks when considering all sampling trips. In our previous study, we reported a RH of sixty–75% at passenger vehicle stops. A similar RH was observed at both omnibus terminals in this study. One time on the jitney, the RH decreased chop-chop, reaching values of twoscore–45% in 5 ± 3 minutes. Afterwards, RH levels fluctuated 50–60% during the residue of the trip. For sections with distant omnibus stops, increases of ~5% were often experienced.


4 SUMMARY AND CONCLUSIONS

Our study reveals that choosing to sit down on the upper deck of a double decker bus would upshot in a more peaceful trip (i.e., lower levels of noise), only higher exposure to big particles (i.e., PM2.5). The opposite is true when choosing the lower deck. Consistent differences of 4–7 dBA and 15–xx µg m–iii can be expected between decks.

With respect to ultrafine particles, lower concentrations are often constitute upstairs, but concentrations of equivalent black carbon can exist higher there at times. However, there was little difference in the measurements for these ii components of particle pollution between decks.

Although concentrations of PM2.5 observed on the upper deck fell inside the range rated as normal by the local environmental authorities for 1-h average ambient concentration (< 55 µg m–three,https://www.haze.gov.sg/), information technology could be considered somewhat high compared to concentrations measured on buses in other cities with mod fleets (e.grand., London, Hong Kong, Sacramento, come across Fig. vi in Velasco et al., 2019). However, PM2.5 concentrations on the lower deck, as well as those of ultrafine particles and equivalent black carbon on both decks are in the lower range of those reported for public buses in such cities.

Our results propose that particle loads on double-decker buses respond to the intake of outdoor air past the air-conditioning organization. Sharp increases on both decks tin can be expected every time the doors are opened to pick upwardly and drop off passengers, peculiarly in double-decker stops next to busy roads. In the case of the assessed route, which started and ended in terminals fully enclosed within buildings, the infiltration of particles emitted by arriving and departing buses contributed critically to the loads of PM2.five on the upper deck. Keeping doors closed while buses are parked will forestall the accumulation of large particles on the upper deck, thus reducing their affluence on the next trip.

In closing, commuters should choose the upper deck for calmer trips, but stay downstairs for cleaner trips.


ACKNOWLEDGMENTS

The field piece of work for this study was supported by the National Research Foundation Singapore through the Singapore MIT Brotherhood for Inquiry and Applied science'south CENSAM laboratory. CENSAM stopped activities and airtight its doors on Dec. 2017. Data assay and manuscript grooming were self-financed by the authors. The authors acknowledge the effective comments of two anonymous reviewers and Armando Retama (Independent Enquiry Scientist, Mexico), the editorial assistance of Nuraziah Binte Abdul Aziz (National Academy of Singapore, NUS), and financial back up from the Department of Geography of NUS for the publication of the commodity.

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