The airborne particulate matter (PM) is known to cause adverse pulmonary and cardiovascular health effects. This study investigated the diurnal characteristics of PM induced PM-bound ROS from real time measurements in typical roadside and urban background sites in Hong Kong, and identified potential influencing factors to its variation in atmosphere. A nitroxide probe (BPEAnit) was employed in the real-time measurement of separate PM and gas phase induced reactive oxygen species (ROS) with a Particle-into-Liquid-Sampler. Measurements were carried in 2 h intervals over a 24 h period, and repeated for 6 days at each site. The total ROS (particle + gas phase) measurements resulted in multiple peaks at 08:00, 16:00 and 20:00 h time periods at background site, compared to non-intermittent decrease/increase trends at roadside. The total ROS generation was noted significantly higher by 50–100% (n = 6 p < .05) at roadside compared to the background site. While the contribution of PM2.5- and gas phase-induced ROS to the total ROS was on average 48 ± 8% and 52 ± 8% across 24 h at background site, the ratio of gas phase-induced ROS increased to 79 ± 4% and that of particle phase decreased to 21 ± 4% at roadside site. The total ROS well correlated with black carbon (BC) and particle-bound PAHs (p-PAHs), which are 4–8 fold and 10–20 fold higher at roadside than background. The ratio of p-PAHs/BC was high at roadside during peak hours compared to late nights suggesting fresh PM may have induced more total ROS generation compared to aged aerosols using this specific ROS probe.