Using miniaturised scanning mobility particle sizers to observe size distribution patterns of quasi-ultrafine aerosols inhaled during city commuting.

Portable miniaturised scanning mobility particle sizer (SMPS) instruments measuring atmospheric particles within the 10-241 nm size range were used to track particle number size distributions and concentrations during near-simultaneous pedestrian, bicycle, bus, car, tram and subway commuting journeys in Barcelona, Spain on 4th-6th July 2018. The majority of particles in this size range were <100 nm, with k-means cluster analysis identifying peaks at 15-22 nm, 30-40 nm, and 45-75 nm. Around 10-25% of the particles measured however were >100 nm (especially in the subway environment) and so lie outside the commonly defined range of "ultrafine" particles (UFP, or <100 nm particles). The study demonstrated in detail how personal exposure to quasi-UFP (QUFP, <241 nm), most of which present in the city streets are produced by road traffic, varies greatly depending on the transport mode and route chosen. Proximity to fresh traffic exhaust sources, such as in a car with open windows, on-road cycling, walking downwind of busy roads, or in a subway station contaminated by roadside air, enhances commuter exposure to particles <30 nm in size. In contrast, travelling inside air-conditioned bus or tram offers greater protection to the commuter from high concentrations of fresh exhaust. Ultrafine number size distributions in traffic-contaminated city air typically peak in the size range 30-70 nm, but they can be shifted to finer sizes not only by increased content of fresh proximal exhaust emissions but also by bursts of new particle formation (NPF) events in the city. One such afternoon photochemical nucleation NPF event was identified during our Barcelona study and recognised in different transport modes, including underground in the subway system. The integration of static urban background air monitoring station information with particle number concentration and size distribution data obtained from portable miniaturised SMPS instruments during commuting journeys opens new approaches to investigating city air quality by offering a level of detail not previously available.