Measurements of total ozone reactivity in a suburban forest in Japan

To investigate tropospheric ozone, O₃, and secondary organic aerosols, SOAs, reactivities of biogenic volatile organic compounds, BVOCs, with atmospheric oxidants have been focused on. In this study, as an ensemble monitor covering all ozone-reactive BVOCs, the analyzer of total ozone reactivity, RO₃, was modified and applied to the ambient observation of RO₃ in a forest. The analyzer was improved to realize the RO₃ measurements of ambient BVOCs in low concentration. After improvement, the typical limit of detection of present RO₃ analyzer was determined as 1.5 × 10⁻⁵ s⁻¹ (S/N = 3, 600-s average) under laboratory conditions. For the atmospheric observation, it is important to consider coexisting compounds in the ambient air. It was confirmed that the configurations of dual chemiluminescence detectors (CLDs) could reasonably cancel out the contributions of ambient ozone. It is also necessary to distinguish the contributions of NO (RNO) from total ozone reactivity (RALL). The method for quantifying RNO was also established. Contributions of VOCs, RO₃, to RALL were acquired by subtracting those of NO and NO₂ from measured RALL. It was confirmed that the present analyzer had capability to quantify RO₃ down to 4 × 10⁻⁵ s⁻¹ as the overall limit of detection under ambient conditions. Then, trial observations of RO₃ were conducted in a suburban forest in Japan. RO₃ data were acquired successfully for a total of 8 days, including high temperature days over 35 °C. It was confirmed that RO₃ could be significantly quantified utilizing the present analyzer when temperature was high enough and NO level was insignificantly low. Fractional contributions of VOC, NO, and NO₂ to O₃ loss reactions were also investigated. As a result of regression analysis between RO₃ and temperature, a clear dependence of RO₃ on temperature was observed in the afternoon. When the temperature dependent algorithm was considered, the temperature sensitivity was acquired as 0.23 ± 0.03 K⁻¹ (26% K⁻¹), which was comparable to those in previous studies on monoterpene emission from vegetation. The RO₃ analyzer might also capture the early morning peak of monoterpene emission in a forest though the uncertainties were large due to fluctuations of NO. Consequently, it was confirmed that the present RO₃ analyzer be useful for investigating BVOCs in a forest. Especially, monoterpene emission under elevated temperature conditions up to 36.2 °C was significantly captured during the RO₃ observation in the forestal atmosphere.