The diurnal characteristics of PM-bound ROS and its influencing factors at urban ambient and roadside environments

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 PM_2.5- 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.