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 30.7 +/- 5.1 s(-1) was observed, while the OH reactivity calculated from CO, NO + NO2 (NOx), ozone (O-3), sulfur dioxide (SO2), and 14 volatile organic compounds (VOCs) was 11.8 +/- 1.0 s(-1). An analysis of 346 peaks from the PTR-ToF-MS accounted for an additional 6.0 +/- 2.2 s(-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.