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Air pollution exposure is associated with a range of adverse health effects, including cardiovascular and respiratory diseases. Particle-bound ROS has been recognised as one of the prevailing parameters to indicate the toxic potential of airborne particulate matter (PM). The temporal variability of particle-bound ROS is a very important metric crucial for the improvement of public health and risk assessment policies. To our knowledge this is the first study aiming to investigate diurnal ROS profiles in both the particle and gas phase and associate them with diurnal variations of important pollutants. For that purpose, we have successfully applied a new instrument to continuously monitor diurnal ROS profiles at two locations in Hong Kong: a busy roadside and an urban background. Data was collected over both the working week and during weekends. We have observed a high correlation between particle-bound ROS and lower concentrations of black carbon (BC) at the roadside during the working week. These associations were less significant over the weekend and at all times with ozone. Our results suggest that most of the particle-bound ROS from both the particle and the gas phase arises from fresh emission sources directly from the traffic. A very interesting observation came out as a result of this study where measured ROS concentration was decreasing with the rise of ozone in conjunction with particle number, suggesting potential role of ROS in particle growth and aging. • First study investigating diurnal ROS profiles in particle and gas phase and association with diurnal pollutant profiles. • ROS from both the particle and the gas phase mostly arises from fresh emission sources directly from the traffic. • ROS decreased with the rise of ozone and particle number, suggesting potential role of ROS in particle growth and aging. • Quantitative and qualitative ROS gas phase analysis are crucial for assessing the health impacts of atmospheric pollution. • Nature of anthropogenic gaseous emissions is a very relevant metrics for health implications of various emissions. [ABSTRACT FROM AUTHOR]