Abstract. Total hydroxyl radical (OH) reactivity was measured at the PROPHET (Program for Research on Oxidants: PHotochemistry, Emissions, and Transport) forested field site in northern Michigan during the 2009 Community Atmosphere–Biosphere INteraction EXperiment (CABINEX). OH reactivity measurements were made with a turbulent-flow reactor instrument at three heights from the forest floor above (21 and 31 m) and below (6 m) the canopy at three different time periods during the CABINEX campaign. In addition to total OH reactivity measurements, collocated measurements of volatile organic compounds (VOCs), inorganic species, and ambient temperature were made at the different heights. These ancillary measurements were used to calculate the total OH reactivity, which was then compared to the measured values. Discrepancies between the measured and calculated OH reactivity, on the order of 1–24 s−1, were observed during the daytime above the canopy at the 21 and 31 m heights, as previously reported for this site. The measured OH reactivity below the canopy during the daytime was generally lower than that observed above the canopy. Closer analysis of the measurements of OH reactivity and trace gases suggests that the missing OH reactivity could come from oxidation products of VOCs. These results suggest that additional unmeasured trace gases, likely oxidation products, are needed to fully account for the OH reactivity measured during CABINEX.

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Total hydroxyl radical (OH) reactivity was measured at the PROPHET (Program for Research on Oxidants: PHotochemistry, Emissions, and Transport) forested field site in northern Michigan during the 2009 Community Atmosphere–Biosphere INteraction EXperiment (CABINEX). OH reactivity measurements were made with a turbulent-flow reactor instrument at three heights from the forest floor above (21 m, 31 m) and below (6 m) the canopy. In addition to total OH reactivity measurements, collocated measurements of Volatile Organic Compounds (VOCs), inorganic species, and ambient temperature were made at the different heights. These ancillary measurements were used to calculate the total OH reactivity, which was then compared to the measured values. Discrepancies between the measured and calculated OH reactivity, on the order of 1–20 s⁻¹, were observed during the daytime above the canopy at the 21 and 31 m heights, as previously reported for this site. In contrast, the measured OH reactivity during the day was in better agreement with the calculated reactivity below the canopy at the 6 m level during the day. These results suggest that emissions of isoprene and monoterpenes and some measured oxidation products from these primary emissions can explain the measured daytime OH reactivity at the 6 m height, while additional unmeasured trace gases, likely oxidation products, are needed to account for the measured OH reactivity at the 21 m and 31 m heights as well as the nighttime OH reactivity for the 6 m height

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