**2.3. Particle size distribution**

Particle sizes in the atmosphere are known to distribute with certain frequency modes, namely, nuclei mode, accumulation mode and coarse mode [23]. The nuclei mode, or ultrafine mode is mainly primary emission of vehicle exhaust and is carbonaceous particulate matter. The accumulation mode is responsible for formation of secondary organic aerosols especially through photochemical reactions with VOCs including gas phase PAHs and also for coagulation of particles. The coarse mode (>1.8 um) particles are mostly grown particles in the atmosphere and/or re-suspended road dust, which are reported to be subject to condensation of volatile materials including lighter PAHs. From previous studies, concentrations of PAHs are found to be highly dependent upon the size of particles. In view of association mechanisms and atmospheric processes of PAHs to urban aerosols, those particle size modes are applied to this study [7]. Based on previous studies on PAHs measurement using cascade air samplers (e.g., [24-25]) three particle size modes are defined for this study: ultrafine mode (< 0.18 μm), accumulation mode (0.18-1.8 μm) and coarse mode (1.8μm <) according to the particle cut sizes of the MOUDI.

## **2.4. Traffic and meteorological data**

Road traffic was recorded using a video camera for 24 hours or shorter during the air sampling. The traffic volume was counted manually for 10 minutes in every hour, then multiplied by six to estimate hourly average volumes. At the CC site, hourly meteorological data monitored by the PCD were obtained. The meteorological data included temperature, solar radiation, relative humidity, rain, wind speed and wind direction. At the R6 site, wind speed and wind directions were monitored at 10-minute intervals using KADEC wind monitors (Kona Systems, Japan), and temperature, solar radiation, relative humidity and rainfall were monitored at 5-minute intervals using an AutoMet meteorology monitor (MET ONE Instruments, USA).
