Photochemical ozone production at a subtropical island of Okinawa, Japan: Implications from simultaneous observations of HO₂ radical and NOx

Yugo Kanaya, Jun Matsumoto, Hajime Akimoto

Journal of Geophysical Research, 107(D19),4368, doi:10.1029/2001JD000858, 2002.


During the Observations at a Remote Island of Okinawa (ORION99) field campaign in summer 1999, HO2 radical and NOx concentrations were simultaneously measured with other important parameters relevant to photochemistry. The observed HO2 concentration levels, relationship between HO2 and NOx, and NO/NO2 balance were well reproduced by model calculations based on current photochemistry theory, indicating the photochemistry taking place at the island seemed to be understandable within known chemistry. Thus, the photochemical ozone production rate was calculated using the calculated peroxy radical concentrations. Isoprene concentrations were high at the site and isoprene peroxy radical + NO reaction contributed to ozone production by similar degree to HO2 + NO reaction. The net ozone production rate during daytime was calculated to be 1-4 ppbv hr-1, due to moderate NO concentrations (36-244 pptv) there, which were higher than NO compensation point of ~ 25 pptv. Sensitivity studies showed that the ozone production rate is calculated within NOx limited regime. Thus ozone production rate seemed to be relatively uniform throughout the boundary layer, even if isoprene concentration had a steep vertical gradient. Up to 20 ppbv of ozone was expected to be photochemically produced in one day, and the clear diurnal variations of ozone with daytime buildup of 15 ppbv observed there in 1996 seems to be explained by photochemistry.