Contributions to OH reactivity from unexplored volatile organic compounds measured by PTR-ToF-MS – a case study in a suburban forest of the Seoul metropolitan area during the Korea–United States Air Quality Study (KORUS-AQ) 2016

We report OH reactivity observations by a chemical ionization mass spectrometer–comparative reactivity method (CIMS-CRM) instrument in a suburban forest of the Seoul metropolitan area (SMA) during the Korea–United States Air Quality Study (KORUS-AQ 2016) from mid-May to mid-June of 2016. A comprehensive observational suite was deployed to quantify reactive trace gases inside of the forest canopy including a high-resolution proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS). An average OH reactivity of inline-formula30.7±5.1 inline-formulas⁻¹ was observed, while the OH reactivity calculated from CO, inline-formulaNO+NO₂ (NOinline-formula_x), ozone (inline-formulaO₃), sulfur dioxide (inline-formulaSO₂), and 14 volatile organic compounds (VOCs) was inline-formula11.8±1.0 inline-formulas⁻¹. An analysis of 346 peaks from the PTR-ToF-MS accounted for an additional inline-formula6.0±2.2 inline-formulas⁻¹ of the total measured OH reactivity, leaving 42.0 % missing OH reactivity. A series of analyses indicate that the missing OH reactivity most likely comes from VOC oxidation products of both biogenic and anthropogenic origin.