We developed an online instrument for measuring the oxidative potential (OP) of ambient particulate matter (PM) using dithiothreitol (DTT) assay. The instrument uses a mist chamber (MC) to continuously collect the ambient PM 2.5 in water, followed by its DTT activity determination using an automated syringe pump system. The instrument was deployed at an urban site in the University of Illinois campus, and its field performance was evaluated by comparing the results with the offline DTT activity measurements of simultaneously collected PM-laden filters. The online DTT activity measurements correlated well with the offline measurements but were higher than both methanol (slope = 0.86, R 2 = 0.93) and Milli-Q water (slope = 0.52, R 2 = 0.86) extracts of the PM filters, indicating a better efficiency of MC for collecting the water-insoluble fraction of PM. The hourly measurements of ambient PM 2.5 OP were obtained by running the online instrument intermittently for 50 days with minimal manual assistance. The daytime DTT activity levels were generally higher than at night. However, a four-fold increase in the hourly averaged activity was observed on the night of July 04 (Independence Day fireworks display). Diurnal profile of the hourly averaged OP during weekdays showed a bimodal trend, with a sharp peak in the morning (around 7:00 AM), followed by a broader afternoon peak, which plateaus around 2:00 PM, and starts subsiding at night (around 7:00 PM). To investigate the association of the diurnal profile of DTT activity with the emission sources at the site, we collected time-segregated composite PM filter samples in four different time periods of the day [morning (7:00 AM–10:00 AM), afternoon (10:00 AM–3:00 PM), evening (3:00 PM–7:00 PM) and night (7:00 PM–7:00 AM)] and determined the diurnal variations in the redox active components [i.e. water soluble Cu, Fe, Mn, organic carbon, elemental carbon and water soluble organic carbon]. Based on this comparison, we attributed the daytime OP of ambient PM 2.5 to the water-soluble Cu from both exhaust and non-exhaust emissions, whereas secondary particles formed by the photochemical transformation of primary emissions appear to enhance the OP of PM during the afternoon and evening period.