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. We developed an online instrument for measuring the oxidative potential (OP)of ambient particulate matter (PM) using the dithiothreitol (DTT) assay. Theinstrument uses a mist chamber (MC) to continuously collect the ambientPM 2.5 in water, and then determines its DTT activity using an automated syringe pump system. The instrument wasdeployed at an urban site in the University of Illinois campus, and its fieldperformance was evaluated by comparing the results with the offline DTTactivity measurements of simultaneously collected PM-laden filters. Theonline DTT activity measurements correlated well with the offlinemeasurements but were higher than both methanol (slope  =1.0 8, R2=0.93 ) and Milli-Q water (slope  =1.86 , R2=0.86 ) extracts of the PMfilters, indicating a better efficiency of the MC for collecting thewater-insoluble fraction of PM. The hourly measurements of ambient PM 2.5 OP were obtained by running the online instrument intermittently for 50 dayswith minimal manual assistance. The daytime DTT activity levels weregenerally higher than at night. However, a 4-fold increase in the hourlyaveraged activity was observed on the night of 4 July (Independence Dayfireworks display). The diurnal profile of the hourly averaged OP duringweekdays showed a bimodal trend, with a sharp peak in the morning (around07:00 LT), followed by a broaderafternoon peak which plateaus around 14:00 LT and starts subsiding at night(around 19:00 LT). To investigate the association of the diurnal profile of DTT activity withthe emission sources at the site, we collected time-segregated composite PMfilter samples in four different time periods of the day (morning,07:00–10:00 LT; afternoon, 10:00–15:00 LT; evening, 15:00–19:00 LT; andnight, 19:00–07:00 LT) and determined the diurnal variations in the redoxactive components (i.e., water-soluble Cu, Fe, Mn, organic carbon, elementalcarbon, and water-soluble organic carbon). Based on this comparison, weattributed the daytime OP of ambient PM 2.5 to the vehicular (bothexhaust and non-exhaust) emissions and resuspended dust, whereas secondaryphotochemical transformation of primary emissions appear to enhance the OP ofPM during the afternoon and evening period.

atmospheric measurement techniques

Development and field testing of an online instrument for measuring the real-time oxidative potential of ambient particulate matter based on dithiothreitol assay