LAUSR

Abstract This study aimed to assess the effects of viaduct settings, meteorological and traffic conditions on the vertical and horizontal distributions of particle number concentrations (PNCs) and particle number size distributions (PNSDs) for accumulation and coarse mode particles around a viaduct in street canyons with different aspect ratios (building height/street width, H/W = 1 and 1.8). Two field measurements were conducted in a built-up block in Guangzhou, China, including vertical locations and various traffic hotspots. The PNCs in the wide street canyon decayed monotonously with increasing height, while those in the deep street canyon decreased first and then increased above the viaduct due to the impact of the viaduct structure and an extra elevated vehicle emission. The frequent stop-and-go behavior of vehicles resulted in 22.8% higher PNCs at the bus stop (BS) compared to the averaged PNCs far from the BS in the deep street canyon. The PNCs at the traffic intersection (TI) with favorable ventilation were slightly lower than those in the wide street canyon. The PNSDs at all traffic hotspots were similar in shape with the same peak values (0.28 and 1.6 μm) in accumulation and coarse modes. Under low wind speeds, the hourly averaged vertical profiles were consistent with those of daily averaged PNCs, while at higher wind speeds, no obvious rule was observed due to unsteady flow field. These findings are useful for understanding exposure in urban areas.