Linking particle number concentration (PNC), meteorology and traffic variables in a UK street canyon

Ambient particle number concentration (PNC) has been linked with adverse health outcomes such as asthma, reduced lung function and cardiovascular disease. To investigate the relationship between PNC,meteorology and traffic we measured size segregated respirable particles in a busy commuter street in Swansea, UK for ten months using a Dekati Electrical Low Pressure Impactor (ELPI). The ELPI segregates particles into 12 size fractions between 7 nm and 10 μm. The median PNC for the sampling period was 31,545 cm-3. For the ultrafine particles (7-93 nm), the highest PNC was found inwinter (46,615 cm-3; 15 minute average) and the lowest for that size fraction in summer (29,696 cm-3). For the particles below 93 nm there was a trimodal distribution to weekdays (particularlyMonday toWednesday), with PNC peaks at 09:00, 16:00 and 23:00. Wind direction had a significant influence on PNC and differed between particles in the fine range (below 2.5 μm) and more coarse particles (up to 10 μm). For fine particles, winds parallel to the canyon were associated with higher PNCs which were attributed to the replenishment of traffic particles. For coarse particles, PNCswere higher fromwinds perpendicular to the canyon and this was linked to source distribution around the sampling site and the recirculation of pollutants within the canyon. During times when vehicle volumes were high and vehicles were exhibiting stop-start behaviour, if thiswas combined with low wind speeds, ultrafine PNC was highest. This effect was generally observed during themorning rush hour. Current mass-based legislation does not take into account exposure to the number of particles or the change in population exposure diurnally