Measurements of total ozone reactivity in a suburban forest in Japan

Abstract

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.