Variability of ozone in the marine boundary layer of the equatorial Pacific Ocean

This study examines the processes controlling the diurnal variability of ozone (O-3) in the marine boundary layer of the Kwajalein Atoll, Republic of the Marshall Islands (latitude 8 degrees 43' N, longitude 167 degrees 44' E), during July to September 1999. At the study site, situated in the equatorial Pacific Ocean, O-3 mixing ratios remained low, with an overall average of 9-10 parts per billion on a volume basis (ppbv) and a standard deviation of 2.5 ppbv. In the absence of convective storms, daily O-3 mixing ratios decreased after sunrise and reached minimum during the afternoon in response to photochemical reactions. The peak-to-peak amplitude of O-3 diurnal variation was approximately 1-3 ppbv. During the daytime, O-3 photolysis, hydroperoxyl radicals, hydroxyl radicals, and bromine atoms contributed to the destruction of O-3, which explained the observed minimum O-3 levels observed in the afternoon. The entrainment of O-3-richer air from the free troposphere to the local marine boundary layer provided a recovery mechanism of surface O-3 mixing ratio with a transport rate of 0.04 to 0.2 ppbv per hour during nighttime. In the presence of convection, downward transport of O-3 richer tropospheric air increased surface O-3 mixing ratios by 3-12 ppbv. The magnitude of O-3 increase due to moist convection was lower than that observed over the continent (as high as 20-30 ppbv). Differences were ascribed to the higher O-3 levels in the continental troposphere and weaker convection over the ocean. Present results suggest that moist convection plays a role in surface-level O-3 dynamics in the tropical marine boundary layer.