Low ozone in the marine boundary layer of the tropical Pacific Ocean: Photochemical loss, chlorine atoms, and entrainment

Aircraft measurements of ozone, its key precursors, and a variety of chemical tracers were made in the troposphere of the western and central Pacific in October 1991. These data are presented and analyzed to examine the occurrence of low ozone concentrations in the remote marine boundary layer of the tropical and equatorial Pacific Ocean. The data from these flights out of Guam, covering an area extending from the equator to 20°N and from south of the Philippines to Hawaii, show average O 3 concentrations as low as 8–9 ppb (ppb = 10 −9 v/v) at altitudes of 0.3–0.5 km in the boundary layer. Individual measurements as low as 2–5 ppb were recorded. Low O 3 concentrations do not always persist in space and time. High O 3 , generally associated with the transport of upper tropospheric air, was also encountered in the boundary layer. In practically all cases, O 3 increased to values as large as 25–30 ppb within 2 km above the boundary layer top. Steady state model computations are used to suggest that these low O 3 concentrations are a result of net photochemical O 3 destruction in a low NO environment, sea surface deposition, and low net entrainment rates (3.6 ± 1.7 mm s −1 ) from the free troposphere. Day/night measurements of select organic species (e.g., ethane, propane, C 2 Cl 4 ) suggest that Cl atom concentrations in the vicinity of 10 5 molecules cm −3 may be present in the marine boundary layer in the early morning hours. This Cl atom abundance can only be rationalized if sea‐salt aerosols release active chlorine (Cl 2 or HOCl) to the gas phase when exposed to sunlight. These Cl atom concentrations, however, are still insufficient, and halogen chemistry is not likely to be an important contributor to the observed low O 3 .