Observational study of ultrafine particulate matter exposure under different commuting modes in a typical city of the Yangtze River Delta

<p indent="0mm">Exposure to traffic-related air pollution poses health risks. Commuters in urban areas are exposed to traffic-related air pollutants at concentrations ~3–10 times higher than background levels. Ultrafine particulate matter (UFP), with a diameter of less than <sc>100 nm,</sc> is one of the traffic-related pollutants. Due to its small size but large surface area, UFP adsorbs toxic substances and can efficiently reach the deepest parts of the lungs (alveoli), causing a greater health risk than fine particulate matter (PM_2.5). Nevertheless, a limited number of studies have investigated UFP exposure in different commuting modes. In this study, we assessed the UFP exposure level in Nanjing, a typical city in the Yangtze River Delta. Real-time measurements of particle number concentration (PNC), mean particle size (Diam), lung deposited surface area (LDSA), temperature (<italic>T</italic>) and relative humidity (<italic>RH</italic>) were measured in the microenvironment of taxi, bus, electric vehicle (e-bike), bicycle and subway using a hand-held nanoparticle dosimeter during the morning peak (7:00-9:00), off-peak (12:00-14:00) and evening peak (16:00–18:00) on selected days. During the off-peak hours, the mean PNC of e-bike, bicycle, bus, taxi, and subway commute was 17325±12023, 18423±1294, 21870±14529, 23517±17029, and 18097±12201 pt/cm3, which were 101%, 114%, 123%, 123% and 140% of the mean PNC of the morning peak respectively, and 1–1.3 times the mean PNC of the evening peak. The highest UFP concentration is associated with the passing by of diesel trucks and intersections where traffic gathers. Mean PNC levels were highest in semi-enclosed modes (taxi and bus), followed by open modes (bicycle and e-bike), and lowest in closed modes (subway). Analysis of meteorological factors revealed that temperature was positively correlated with PNC within the commuter microenvironment, and relative humidity had a statistically significant negative correlation with PNC. LDSA, an indicator of the hazard associated with the surface area of particles, were observed highest with a mean of 48.40±27.44 μm²/cm³ in taxi microenvironment which is 1.1–1.3 times higher than the mean LDSA of bus, bicycle, e-bike and subway. The contribution of different particle modes to the LDSA varies considerably. The Aitken mode dominated LDSA within each commuting microenvironment, with the number of particles distributed in the Aitken mode in five commuting modes being 77%‒87% of the total and having the highest mean LDSA concentrations <sc>(40.1–50.3 μm²/cm³).</sc> Nucleation mode particles, despite their small proportion, corresponded to a mean LDSA that was only slightly lower than that of the Atiken mode, suggesting that nucleation mode poses a greater health risk than the Atiken mode for an equal number of particles. The lowest mean LDSA concentrations were found in the accumulation mode <sc>(18.8–32.4 μm²/cm³),</sc> and the number of particles in the accumulation mode in the five microenvironments accounted for 2%–9% of the total number of particles. Based on this result, the commuting environment can be improved in the future by increasing green facilities on roads, providing more lanes for public transport, and optimising the energy source of motor vehicle drives. Commuters can reduce the exposure level of semi-enclosed modes by improving the internal ventilation of the compartments (by closing the windows or by simultaneous mechanical ventilation) or by opting for lower-exposure commuting modes, such as the subway, bicycles, or e-bike for shorter commuting distances.