Potential Factors Contributing to Ozone Production in AQUAS-Kyoto Campaign in Summer 2020: Natural Source-Related Missing OH Reactivity and Heterogeneous HO 2 /RO 2 Loss.

This study presents total OH reactivity, ancillary trace species, HO 2 reactivity, and complex isoprene-derived RO 2 reactivity due to ambient aerosols measured during the air quality study (AQUAS)-Kyoto campaign in September, 2020. Observations were conducted during the coronavirus disease (COVID-19) pandemic (associated with reduced anthropogenic emissions). The spatial distribution of missing OH reactivity highlights that the origin of volatile organic compounds (VOCs) may be from natural-emission areas. For the first time, the real-time loss rates of HO 2 and RO 2 onto ambient aerosols were measured continuously and alternately. Ozone production sensitivity was investigated considering unknown trace species and heterogeneous loss effects of XO 2 (≡HO 2 + RO 2 ) radicals. Missing OH reactivity enhanced the ozone production potential by a factor of 2.5 on average. Heterogeneous loss of radicals could markedly suppress ozone production under low NO/NO x conditions with slow gas-phase reactions of radicals and change the ozone regime from VOC- to NO x -sensitive conditions. This study quantifies the relationship of missing OH reactivity and aerosol uptake of radicals with ozone production in Kyoto, a low-emission suburban area. The result has implications for future NO x -reduction policies. Further studies may benefit from the combination of chemical transport models and inverse modeling over a wide spatiotemporal range.