Long-term trends of black carbon and particle number concentrations and their vehicle emission factors in Stockholm.

Black carbon (BC) and particle number (PN) concentrations are usually high in cities due to traffic emissions. European mitigation policies, including Euro emission standards, have been implemented to curb these emissions. We analyzed BC and PN (particle diameter Dp > 4 nm) concentrations in Stockholm spanning the years 2013-2019 (BC) and 2009-2019 (PN) measured at street canyon and rooftop sites to assess the effectiveness of the implemented policies. Combining these data with inverse dispersion modeling, we estimated BC and PN emission factors (EFBC and EFPN) for the mixed fleet, reflecting real-world driving conditions. The pollutants showed decreasing trends at both sites, but PN concentrations remained high at the canyon site considering the World Health Organization (WHO) recommendations. BC concentrations declined more rapidly than PN concentrations, showing a -9.4% and -4.9% annual decrease at the canyon and -7.2% and -0.5% at the rooftop site in the years 2013-2019. The EFBC and EFPN trends showed that the mitigation strategies for reducing particulate emissions for on-road vehicles were successful over the study period. However, the introduction of biofuels in the vehicle fleet -ethanol and later rapeseed methyl ester (RME)- increased the concentrations of particles with Dp < 10 nm before the adoption of particulate filters in the exhausts. Stricter Euro emission regulations, especially with diesel particulate filters (DPF) in Euro 5, 6, and VI vehicles, led to 66% decrease in EFBC and 55% in EFPN. Real-world EFBC surpassed HBEFA (Handbook Emission Factors for Road Transport) database values by 2.4-4.8 times; however, direct comparisons between real-world and HBEFA EFPN are difficult due to differences in lower cut-off sizes and measurement techniques. Our results underscore the necessity for revising the HBEFA database, updating laboratory testing methods and portable emission measuring systems (PEMS) measurements to account for liquid condensate contributions to PN measurements.

Significant differences in black and brown carbon concentrations at urban and suburban sites.

Light-absorbing carbonaceous particles (LAC) may cause and/or exacerbate non-communicable diseases, interfere with the Earth's radiative balance, darken urban buildings and impair vistas. In this study, we explored the temporal behaviour of LAC concentrations measured at wavelengths of 370 nm (brown carbon, BrC) and 880 nm (black carbon, BC) at two sites of a mid-sized city in Brazil. We observed sharp changes in LAC concentrations at the city centre site in response to variations in traffic volume. The highest concentrations were observed when winds originated from both the city core and from the direction of the bus terminal. The suburban site exhibited a notably uniform diurnal pattern and consistently lower LAC concentrations throughout the day. Nevertheless, substantial increases during the evening led to mean BrC and BC concentrations (2.6 and 2.2 µg m-3, respectively) comparable to daytime peaks observed in the city centre (3 µg m-3 and 2.5 µg m-3). This phenomenon was attributed to the burning of residential waste and overgrown vegetation in nearby vacant lots. Moreover, the highest concentrations coincided with periods of low wind speeds, usually linked to non-buoyant plumes from point sources. BrC concentrations surpassed BC concentrations, even at the city centre site. Not only was the Ångström absorption exponent (Å370/880) larger at the suburban site compared to the city centre (95th percentiles of 1.73 and 1.38, respectively), but it also exhibited a wider span. Overall, the combined LAC and Å370/880 data indicated that i) biomass burning is a major source of LAC at the suburban site; ii) at the city centre, bare BC particles may become internally mixed with BrC from biomass or fossil fuel emissions and enhance absorption at lower wavelengths. The occurrence of LAC peaks outside the evening rush hours suggests that other sources but on-road vehicular emissions may contribute to the deterioration of the air quality in the urban core. Tackling air quality across the urban perimeter requires targeting other potential sources but traffic emissions.

Ambient concentrations and dosimetry of inhaled size-segregated particulate matter during periods of low urban mobility in Bragança, Portugal.

The restrictive measures in place during the COVID-19 pandemic provided a timely scenario to investigate the effects of human activities on air quality, and the extent to which mobility reduction strategies can impact atmospheric pollutant levels. Real-time concentrations of PM1, PM2.5 and PM10 were measured using a mobile platform in a small city of Portugal, during morning and afternoon rush hours, in two distinct phases of the pandemic: emergency phase (cold period, lockdown) and calamity phase (warm period, less restricted). The Multiple-Path Particle Dosimetry Model (MPPD) was used to calculate the PM deposition for adults. Large spatio-temporal variabilities and pronounced changes in mean PM concentrations were observed, with lower concentrations in the calamity phase: PM1 = 2.33 ± 1.61 µg m-3; PM2.5 = 5.15 ± 2.77 µg m-3; PM10 = 23.30 ± 21.53 µg m-3 than in the emergency phase: PM1 = 16.85 ± 31.80 µg m-3; PM2.5 = 30.92 ± 31.93 µg m-3; PM10 = 111.27 ± 104.53 µg m-3. These changes are explained by a combination of meteorological factors and local emissions, mainly residential firewood burning. Regarding regional deposition, PM1 was the main contributor to deposition in the tracheobronchial (5%) and pulmonary (12%) regions, and PM10 in the head region (92%). In general, total deposition doses were higher for males than for females. This work quantitatively demonstrated that even with a 38% reduction in urban mobility during the lockdown, the use of firewood for residential heating is the main contributor to the high concentrations of PM and the respective inhaled dose.

Compilation of a city-scale black carbon emission inventory: Challenges in developing countries based on a case study in Brazil.

Black carbon (BC) inventories for cities are scarce, especially in developing countries, despite their importance to tackle climate change and local air pollution. Here, we draw on results from a case study in a Brazilian city to discuss the challenges of compiling a BC inventory for different activity sectors. We included traditionally inventoried sectors, such as industries and on-road transportation, other less reported sectors (food establishments and aviation), and open burning of household solid waste (HSW), typically found in developing countries. We present a machine-learning technique (Random Forest) as a novel approach to obtain HSW burning activity using a set of spatial predictors. The BC inventory was based on PM2.5 emissions weighted by the fraction of PM2.5 emitted as BC and developed for the year 2018. We also reported the disaggregated spatial PM2.5 emissions for the same combustion sources, and documented the databases used for activity data and emission factors (EF). The total estimated BC and PM2.5 emissions amounted to 57.88 and 234.75 tons, respectively, with on-road vehicle exhaust emissions and industrial combustion as the main BC sources (63 and 22%, respectively). For PM2.5 emissions, on-road transportation (exhaust and non-exhaust) contributed 48%, followed by industrial combustion (21%) and food establishments (20%). Population density, number of vacant lots, and property tax values were identified as the most important features to predict the HSW fire activity. A comparison with other inventories revealed that the BC emission profile of Londrina is similar to the profile reported for Greater Mexico City, another Latin American city. Thus, the methodology used in this study could be extended to other cities with similar local BC sources. Finally, we highlight that the lack of local activity data, representative EF, and even methodology may undermine the development of reliable BC inventories, and intensive research should be conducted to characterize the emission sources.

Fine-scale modeling of the urban heat island: A comparison of multiple linear regression and random forest approaches.

Characterizing the spatiotemporal variability of the Urban Heat Island (UHI) and its drivers is a key step in leveraging thermal comfort to create not only healthier cities, but also to enhance urban resilience to climate change. In this study, we developed specific daytime and nighttime multiple linear regression (MLR) and random forest (RF) models to analyze and predict the spatiotemporal evolution of the Urban Heat Island intensity (UHII), using the air temperature (Tair) as the response variable. We profited from the wealth of in situ Tair data and a comprehensive pool of predictors variables - including land cover, population, traffic, urban geometry, weather data and atmospheric vertical indices. Cluster analysis divided the study period into three main groups, each dominated by a combination of weather systems that, in turn, influenced the onset and strength of the UHII. Anticyclonic circulations favored the emergence of the largest UHII (hourly mean of 5.06 °C), while cyclonic circulations dampened its development. The MLR models were only able to explain a modest percentage of variance (64 and 34% for daytime and nighttime, respectively), which we interpret as part of their inability to capture key factors controlling Tair. The RF models, on the other hand, performed considerably better, with explanatory power over 96% of the variance for daytime and nighttime conditions, capturing and mapping the fine-scale Tair spatiotemporal variability in both periods and under each cluster condition. The feature importance analysis showed that the meteorological variables and the land cover were the main predictors of the Tair. Urban planners could benefit from these results, using the high-performing RF models as a robust framework for forecasting and mitigating the effects of the UHI.

Ubiquity of hazardous airborne substances on passenger ferries.

The ferry service of the city of Rio de Janeiro (Brazil) is one of the busiest in the world. However, a disadvantage of this mass transportation is the large emissions of hazardous substances from diesel combustion. We measured fine particulate matter (PM2.5), equivalent black carbon (eBC), particle number (PN) and total volatile organic compounds (TVOCs) while commuting by double-decker ferries. The particulate concentrations were larger in the lower than in the upper decks, attributed to the infiltration of smoke when ferries were docked and leakage through openings around the door frames during cruising. Boarding/alighting were the most polluted phases (eBC, PM2.5 and PN were 3.3-, 1.4- and 2.7-fold larger than during cruising), due to the high engine load to keep the ferries locked in position, while TVOCs showed no statistically significant differences. Particulate concentrations on naturally ventilated vessels were between 2.5- and 3.5-fold larger than on the air-conditioned ones, but TVOCs were 150-fold higher in the latter, attributed to emissions from furniture and cleaning products. Mean eBC and PM2.5 concentrations on-board the ferries surpassed those at the kerbside. Modernising or retrofitting the vessels could diminish the emissions of hazardous substances, while jet bridges could reduce the commuters' exposure during boarding.

Long-term trends in nitrogen oxides concentrations and on-road vehicle emission factors in Copenhagen, London and Stockholm.

Road transport is the main anthropogenic source of NOx in Europe, affecting human health and ecosystems. Thus, mitigation policies have been implemented to reduce on-road vehicle emissions, particularly through the Euro standard limits. To evaluate the effectiveness of these policies, we calculated NO2 and NOx concentration trends using air quality and meteorological measurements conducted in three European cities over 26 years. These data were also employed to estimate the trends in NOx emission factors (EFNOx, based on inverse dispersion modeling) and NO2:NOx emission ratios for the vehicle fleets under real-world driving conditions. In the period 1998-2017, Copenhagen and Stockholm showed large reductions in both the urban background NOx concentrations (-2.1 and -2.6% yr-1, respectively) and EFNOx at curbside sites (68 and 43%, respectively), proving the success of the Euro standards in diminishing NOx emissions. London presented a modest decrease in urban background NOx concentrations (-1.3% yr-1), while EFNOx remained rather constant at the curbside site (Marylebone Road) due to the increase in public bus traffic. NO2 primary emissions -that are not regulated- increased until 2008-2010, which also reflected in the ambient concentrations. This increase was associated with a strong dieselization process and the introduction of new after-treatment technologies that targeted the emission reduction of other species (e.g., greenhouse gases or particulate matter). Thus, while regulations on ambient concentrations of specific species have positive effects on human health, the overall outcomes should be considered before widely adopting them. Emission inventories for the on-road transportation sector should include EFNOx derived from real-world measurements, particularly in urban settings.

A global observational analysis to understand changes in air quality during exceptionally low anthropogenic emission conditions.

This global study, which has been coordinated by the World Meteorological Organization Global Atmospheric Watch (WMO/GAW) programme, aims to understand the behaviour of key air pollutant species during the COVID-19 pandemic period of exceptionally low emissions across the globe. We investigated the effects of the differences in both emissions and regional and local meteorology in 2020 compared with the period 2015-2019. By adopting a globally consistent approach, this comprehensive observational analysis focuses on changes in air quality in and around cities across the globe for the following air pollutants PM2.5, PM10, PMC (coarse fraction of PM), NO2, SO2, NOx, CO, O3 and the total gaseous oxidant (OX = NO2 + O3) during the pre-lockdown, partial lockdown, full lockdown and two relaxation periods spanning from January to September 2020. The analysis is based on in situ ground-based air quality observations at over 540 traffic, background and rural stations, from 63 cities and covering 25 countries over seven geographical regions of the world. Anomalies in the air pollutant concentrations (increases or decreases during 2020 periods compared to equivalent 2015-2019 periods) were calculated and the possible effects of meteorological conditions were analysed by computing anomalies from ERA5 reanalyses and local observations for these periods. We observed a positive correlation between the reductions in NO2 and NOx concentrations and peoples' mobility for most cities. A correlation between PMC and mobility changes was also seen for some Asian and South American cities. A clear signal was not observed for other pollutants, suggesting that sources besides vehicular emissions also substantially contributed to the change in air quality. As a global and regional overview of the changes in ambient concentrations of key air quality species, we observed decreases of up to about 70% in mean NO2 and between 30% and 40% in mean PM2.5 concentrations over 2020 full lockdown compared to the same period in 2015-2019. However, PM2.5 exhibited complex signals, even within the same region, with increases in some Spanish cities, attributed mainly to the long-range transport of African dust and/or biomass burning (corroborated with the analysis of NO2/CO ratio). Some Chinese cities showed similar increases in PM2.5 during the lockdown periods, but in this case, it was likely due to secondary PM formation. Changes in O3 concentrations were highly heterogeneous, with no overall change or small increases (as in the case of Europe), and positive anomalies of 25% and 30% in East Asia and South America, respectively, with Colombia showing the largest positive anomaly of ~70%. The SO2 anomalies were negative for 2020 compared to 2015-2019 (between ~25 to 60%) for all regions. For CO, negative anomalies were observed for all regions with the largest decrease for South America of up to ~40%. The NO2/CO ratio indicated that specific sites (such as those in Spanish cities) were affected by biomass burning plumes, which outweighed the NO2 decrease due to the general reduction in mobility (ratio of ~60%). Analysis of the total oxidant (OX = NO2 + O3) showed that primary NO2 emissions at urban locations were greater than the O3 production, whereas at background sites, OX was mostly driven by the regional contributions rather than local NO2 and O3 concentrations. The present study clearly highlights the importance of meteorology and episodic contributions (e.g., from dust, domestic, agricultural biomass burning and crop fertilizing) when analysing air quality in and around cities even during large emissions reductions. There is still the need to better understand how the chemical responses of secondary pollutants to emission change under complex meteorological conditions, along with climate change and socio-economic drivers may affect future air quality. The implications for regional and global policies are also significant, as our study clearly indicates that PM2.5 concentrations would not likely meet the World Health Organization guidelines in many parts of the world, despite the drastic reductions in mobility. Consequently, revisions of air quality regulation (e.g., the Gothenburg Protocol) with more ambitious targets that are specific to the different regions of the world may well be required.

Occurrence of pyrethroids in the atmosphere of urban areas of Southeastern Brazil: Inhalation exposure and health risk assessment.

The occurrence of organochlorine pesticides (OCPs) used decades ago for vector control in urban areas is still reported as a threat to human health. Pyrethroids emerged as a replacement for OCPs in sanitary campaigns and are currently the main insecticides used for vector control worldwide, with prominent use as agricultural and household insecticides, for veterinary and gardening purposes, and as wood preservative. This study aimed to assess the occurrence, seasonal variation, and potential sources of pyrethroids in ambient air of two urban regions of Southeastern Brazil, along with the potential health risks to local populations via inhalation exposure. Pyrethroids were sampled by polyurethane foam passive air samplers and their concentrations were determined by gas chromatography coupled with electron capture negative ionization mass spectrometry (GC/ECNI-MS). Atmospheric pyrethroid concentrations (hereinafter reported in pg m-3) were considerably higher than those reported by previous studies worldwide. Cypermethrin (median: 2446; range: 461-15 125) and permethrin (655; 19-10 328) accounted for 95% of the total measured pyrethroids in ambient air. The remaining fraction comprised smaller amounts of bifenthrin (46;  urban-industrial > background areas. HQs increased with decreasing age group, but deterministic and probabilistic estimates did not identify direct health risks for any group. Nevertheless, since only inhalation exposure was considered in this work, other pathways should be investigated to provide a more comprehensive risk assessment of the human exposure to pyrethroids.

Open waste burning causes fast and sharp changes in particulate concentrations in peripheral neighborhoods.

The open burning of municipal solid waste (MSW) -frequently observed in developing countries- emits harmful pollutants, including fine particulate matter (PM2.5) and black carbon (BC), and deteriorates the air quality in urban areas. This work reports on PM2.5 and BC measurements (fixed and mobile) conducted in a residential neighborhood on the outskirts of a Brazilian city (Londrina), complemented by a public opinion survey to understand the open burning in the context of waste management. Mean (± standard deviation) BC concentration (1.48 ± 1.40 µg m-3) at the fixed sites of the neighborhood was lower than downtown, while PM2.5 (9.68 ± 8.40 µg m-3) concentration was higher. The mobile monitoring showed higher mean PM2.5 concentrations but lower BC/PM2.5 ratios than downtown, with sharp and fast spikes (up to 317.87 and 565.21 µg m-3 for BC and PM2.5, respectively). The large spatial heterogeneity of particulate concentrations was associated with the occurrence of MSW burning events. Our observations were verified by the survey respondents who identified poor waste management practices: garbage in streets, waste burning, and illegal dump sites. Even though the area has a municipal waste collection service, the majority of the respondents (87%) had seen waste burning close to their homes on a weekly basis, and think that people burn waste out of habit (54%) and because they are not patient to wait for the collection services (67%). To combat this illegal practice, we suggest raising the public awareness through campaigns at local level, adopting education initiatives and economic incentives for correct waste segregation, and enforcing regular inspection of burning events by the authorities. Our research method proved to be a time- and cost-effective approach for mapping particulate concentrations and for identifying undesirable waste practices, and could be effectively applied to other global cities.

Cyclists' exposure to air pollution under different traffic management strategies.

We characterized the air pollution exposure of cyclists in the city center of Curitiba (Brazil) and then systematically analyzed the influence of several traffic management strategies (bus lanes, bicycle lanes, traffic calming area, traffic lights, and cleaner vehicle technologies) on the exposure. We focused on concentrations of particulates monitored on-board bicycles: PM2.5, black carbon mass (BC) and particle number concentration (PNC), and also reported on total volatile organic compound concentrations (TVOC). Overall, mean (± standard deviation) exposure was moderate compared to other cities around the world (BC: 6.98 ± 11.53 µg m--3, PM2.5: 33.22 ± 25.64 µg m-3, PNC: 3.93 × 104 ± 4.17 × 104 cm-3, TVOC: 361 ± 99 ppb). Concentrations were higher in the morning rush hour than in the afternoon traffic peak, and exhibited a large spatial variability. Bus stops and signalized traffic intersections emerged as hotspots when compared to the rest of the journey, increasing all particulate concentrations. Lower exposure was found on streets with low traffic (particularly, small number of heavy-duty vehicles) and within shallow canyon structures. The impact of traffic calming areas on cyclists' exposure is still inconclusive and further experimental and modelling studies are needed. Simple emission calculations based on traffic activity and real-world emission factors suggested that replacing the diesel bus fleet with hybrid electric buses might largely decrease (64%) the exposure to BC in the city center. Urban planners could use this valuable information to project new cycleways, which would lead to healthier active transportation. Synchronizing traffic signals might further reduce exposure at intersections.

Bus commuter exposure and the impact of switching from diesel to biodiesel for routes of complex urban geometry.

We report on commuters' exposure to black carbon (BC), PM2.5 and particle number (PN, with aerodynamic diameter, da, in the range 0.01

Surface ozone climatology of South Eastern Brazil and the impact of biomass burning events.

In the austral spring, biomass fires affect a vast area of South America each year. We combined in situ ozone (O3) data, measured in the states of São Paulo and Paraná, Brazil, in the period 2014-2017, with aerosol optical depth, co-pollutants (NOx, PM2.5 and PM10) and air backtrajectories to identify sources, transport and geographical patterns in the air pollution data. We applied cluster analysis to hourly O3 data and split the investigation area of approximately 290,000 km2 into five groups with similar features in terms of diurnal, weekly, monthly and seasonal O3 concentrations. All groups presented a peak in September and October, associated with the fire activities and enhanced photochemistry. The highest mean O3 concentrations were measured inland whilst, besides having lower concentrations, the coastal group was also associated with the smallest diurnal and seasonal variations. The latter was attributed to lower photochemical activity due to frequently occurring overcast weather situation. The mean annual regional contribution of O3 over the area was 61 µg/m3, with large seasonal and intersite variabilities (from 35 to 84 µg/m3). The long-range transport of smoke contributed with between 23 and 41% of the total O3 during the pollution events. A pollution outbreak in September 2015 caused many-fold increases in O3, PM2.5 and PM10 across the investigation area, which exceeded the World Health Organisation recommendations. We show that the regional transport of particulates and gas due to biomass burning overlays the local emissions in already highly polluted cities. Such an effect can outweigh local measures to curb anthropogenic air pollution in cities.

Modelling urban cyclists' exposure to black carbon particles using high spatiotemporal data: A statistical approach.

This is a pioneering work in South America to model the exposure of cyclists to black carbon (BC) while riding in an urban area with high spatiotemporal variability of BC concentrations. We report on mobile BC concentrations sampled on 10 biking sessions in the city of Curitiba (Brazil), during rush hours of weekdays, covering four routes and totaling 178 km. Moreover, simultaneous BC measurements were conducted within a street canyon (street and rooftop levels) and at a site located 13 km from the city center. We used two statistical approaches to model the BC concentrations: multiple linear regression (MLR) and a machine-learning technique called random forests (RF). A pool of 25 candidate variables was created, including pollution measurements, traffic characteristics, street geometry and meteorology. The aggregated mean BC concentration within 30-m buffers along the four routes was 7.09 µg m-3, with large spatial variability (5th and 95th percentiles of 1.75 and 16.83 µg m-3, respectively). On average, the concentrations at the street canyon façade (5 m height) were lower than the mobile data but higher than the urban background levels. The MLR model explained a low percentage of variance (24%), but was within the values found in the literature for on-road BC mobile data. RF explained a larger variance (54%) with the additional advantage of having lower requirements for the target and predictor variables. The most impactful predictor for both models was the traffic rate of heavy-duty vehicles. Thus, to reduce the BC exposure of cyclists and residents living close to busy streets, we emphasize the importance of renewing and/or retrofitting the diesel-powered fleet, particularly public buses with old vehicle technologies. Urban planners could also use this valuable information to project bicycle lanes with greater separation from the circulation of heavy-duty diesel vehicles.

Spatial variability of on-bicycle black carbon concentrations in the megacity of São Paulo: A pilot study.

In 2015, a controversial bicycle lane was installed on Paulista Avenue -a thoroughfare in the heart of the megacity of São Paulo with a high rate of motorised vehicles. For the first time, on-bicycle air pollution concentrations were assessed along this lane using black carbon (BC) as an indicator of fossil fuel combustion. We measured BC concentrations with a hand-held microaethalometer at a high temporal resolution, enabling the capture of fine spatial gradients along the route. Although this new link expanded the city's cycling network, our pioneering study showed that BC concentrations were large (mean 8.5 µg m-3) with extreme values reaching 24.0 µg m-3, comparable to concentrations found in many megacities. In agreement with other studies, we observed that concentrations decreased about 1.6 times on a section of the bicycle lane running through a calmer neighbourhood, which could indicate the potential to safeguard the health of cyclists by installing lanes with greater separation from main roads, such as Paulista Avenue. This pilot work paves the way to more detailed studies aiming to map out the spatial distribution of other traffic-related pollutants across the city's 458-km long bicycle network.

Variations in individuals' exposure to black carbon particles during their daily activities: a screening study in Brazil.

Black carbon (BC) is a fraction of airborne PM2.5 emitted by combustion, causing deleterious effects on human health. Due to its abundance in cities, assessing personal exposure to BC is of utmost importance. Personal exposure and dose of six couples with different working routines were determined for 48 h based on 1-min mobile BC measurements and on ambient concentrations monitored simultaneously at home (outdoor) and at a suburban site. Although couples spent on average ~ 10 h together at home, the routine of each individual in other microenvironments led to 3-55% discrepancies in exposure between partners. The location of the residences and background concentrations accounted for the differences in inter-couple exposure. The overall average exposure and dose by gender were not statistically different. The personal exposure and dose calculated with datasets from fixed sites were lower than the calculations using data from mobile measurements, with the largest divergences (between four and nine times) in the transport category. Even though the individuals spent only 7% of the time commuting, this activity contributed to between 17 and 20% of the integrated exposure and inhaled dose, respectively. On average, exposure was highest on bus trips, while pedestrians and bus passengers had lower doses. Open windows elevated the in-car exposure and dose four times compared to settings with closed windows.

Commuter exposure to black carbon particles on diesel buses, on bicycles and on foot: a case study in a Brazilian city.

Commuting in urban environments accounts for a large fraction of the daily dose of inhaled air pollutants, especially in countries where vehicles have old technologies or run on dirty fuels. We measured black carbon (BC) concentrations during bus, walk and bicycle commutes in a Brazilian city and found a large spatial variability across the surveyed area, with median values between 2.5 and 12.0 µg m-3. Traffic volume on roadways (especially the number of heavy-duty diesel vehicles), self-pollution from the bus tailpipe, number of stops along the route and displacement speed were the main drivers of air pollution on the buses. BC concentrations increased abruptly at or close to traffic signals and bus stops, causing in-cabin peaks as large as 60.0 µg m-3. BC hotspots for the walk mode coincided with the locations of bus stops and traffic signals, whilst measurements along a cycle lane located 12 m from the kerb were less affected. The median BC concentrations of the two active modes were significantly lower than the concentrations inside the bus, with a bus/walk and bus/bicycle ratios of up to 6. However, the greater inhalation rates of cyclist and pedestrians yielded larger doses (2.6 and 3.5 µg on a 1.5-km commute), suggesting that the greater physical effort during the active commute may outweigh the reduction in exposure due to the shift from passive to active transport modes.

Hotspots of black carbon and PM2.5 in an urban area and relationships to traffic characteristics.

Three instrumented bicycles were used to measure black carbon (BC) and PM2.5 concentrations in a midsized city in southern Brazil. The objective of this study was to map the spatial distribution of BC and PM2.5, to identify air pollution hotspots and to assess factors that may affect the concentrations of these pollutants, e.g. traffic volume, number of heavy-duty diesel vehicles (HDDV), position of traffic signals and street incline. The cyclists collected data in the city centre along streets of different traffic density during nine sampling sessions in the weekday morning and afternoon rush hours, between March 13 and April 28, 2015. The sampling by bicycle covered an area of 2.70 km2, over variable elevation, and travelled a total distance of 215 km. BC and PM2.5 exhibited a large spatial variability on a scale of tens of metres and the concentrations were positively correlated with traffic counts, but exhibited a stronger relationship with the number of HDDV. These results imply that older buses and diesel-powered trucks may be the main driver behind the high pollution levels in the city's inner core. We observed a strong relationship between BC concentrations at junctions managed by traffic signals and the quantity of HDDV. The mean BC concentration was found to be 8.10 µg m-3 near traffic signals located on an inclined street (HDDV > 100 vehicles h-1) compared to traffic signals on flat terrain (6.00 µg m-3), which can be attributed to the higher acceleration required at the start of motion. This pattern was less evident for PM2.5 concentrations.