Measurements of NO, NOy, CO and Os and estimation of the ozone production rate at Oki Island, Japan, during

Measurements of NO, NOy, CO, and 03 were conducted at Oki Island, 65 km west of the Japanese mainland during September-October 1991. The results show that the Oki Is- land site is relatively clean with mean (and median) CO and NOy concentrations of 137 (130) ppbv and 713 (505) pptv, respectively. These relatively low concentrations reflect the frequent occurrence of winds from the less populated regions to the north during this campaign. During several periods, elevat-l CO, NOy, and 03 concentrations were associated with transport of pollutants from Japan and Korea. CO and NOy are significantly correlated in the entire data set, reflecting the overall influence of anthropogenic emissions. Mean CO and N Oy concentra- tions show small diurnal cycles with a maximum in the morning and early evening hours. Hourly mean 03 concentrations exhibit a diurnal cycle of amplitude 3.5 parts per billion by volume, with a maximum in the early afternoon attributed to photochemical 03 production. The source of this afternoon 03 enhancement is analyzed using regression analysis of 03 and N Oy and of 03 and CO. Both NOy and CO are significantly correlated with 03 during the daytime but are poorly or insignificanfiy correlated with 03 at night, indicating that upwind photochem- istry operating within the previous ---1 day was responsible for the observed afternoon 03 en- hancement. The NOy-O3 regression analysis indicates formation of 9 03 molecules per NOy molecule reaching Oki Island. The mean and median midday NO concentrations of 55 and 23 parts per trillion by volume, respectively, were sufficient to support net production of 03. Us- ing the results of these measurements and prior modeling studies (Lin et al., 1988), we estimate a "lifetime-averaged" 03 production efficiency of 10 molecules 03 per NOx molecule. Com- bined with estimated East Asian NOx emissions for 1987, this indicates an annual 03 produc- tion of 1.0 x 10 TM g O: from photochemistry which is approximately 2 times the annual strato- spheric flux in this region. This value must be considered as a rough estimate but is probably accurate to within a factor of 2.