Real-time quantification of the total HO2 reactivity of ambient air and HO2 uptake kinetics onto ambient aerosols in Kyoto (Japan)
HO2 radicals play important roles in tropospheric chemistry. The large discrepancies between field measurements and sophisticated model predictions for the overall HO2 concentrations may due to the HO2 uptake coefficients onto ambient aerosols (γ) have not yet been properly quantified. This study presents the first online measurement of the total HO2 reactivity caused by the ambient gas phase (k_g^') and aerosol phase (k_a^') in summer 2018 in Kyoto, Japan, using a technique combining laser-flash photolysis and laser-induced fluorescence (LFP–LIF), coupled with a versatile aerosol concentration enrichment system (VACES) that enriches ambient aerosols by ~10 times to compensate for its relatively low concentration. Results show that k_g^' ranged from 0.1 s−1 (25th percentile) to 0.32 s−1 (75th percentile) with an average value of 0.22 ± 0.16 s−1 (1), which can be mostly explained by the reaction of HO2 with NO2. With the application of VACES and the auto-switching aerosol filter, k_a^' ranged from 0.004 s−1 (25th percentile) to 0.028 s−1 (75th percentile) with an average value of 0.017 ± 0.015 s−1. When converted to ambient conditions (by dividing with the enrichment factor), this result was ~10 times higher than the HO2 reactivity caused by its self-reaction under ambient concentration levels (~ 5 ppt) at 298 K. The related γ ranged from 0.08 (25th percentile) to 0.36 (75th percentile), with an average value of 0.24, which is comparable with the values used in previous modeling studies (~0.2) but with a large variation of ±0.20 (1) within the measurement time. This suggests that a large bias may exist for the estimation of HO2 concentrations when using a constant γ value. We suggest that different γ values should be applied in modeling studies depending on the environment conditions, and the γ value obtained here can be used as the upper limit values in urban areas in summer due to the possible aerosol phase change. The analysis of ambient air backward trajectories indicated that the predominant NO2 emission sources came from the mainland of Japan. However, no significant differences regarding HO2 uptake coefficients were found when air masses came through the mainland or from the coastal direction. This study provides more reliable γ which could promote the accuracy of the modeling of heterogeneous processes in tropospheric chemistry. However, a combination of online and offline methods appears to be more suitable for addressing variation and model discrepancies in further studies.