Bimodal distribution of free tropospheric ozone over the tropical western Pacific revealed by airborne observations

A recent airborne field campaign over the remote western Pacific obtained the first intensive in situ ozone sampling over the warm pool region from oceanic surface to 15-km altitude (near 360-K potential temperature level). The new data set quantifies ozone in the tropical tropopause layer under significant influence of convective outflow. The analysis further reveals a bimodal distribution of free tropospheric ozone mixing ratio. A primary mode, narrowly distributed around 20-ppbv, dominates the troposphere from the surface to 15-km. A secondary mode, broadly distributed with a 60-ppbv modal value, is prominent between 3 and 8-km (320-K to 340-K potential temperature levels). The latter mode occurs as persistent layers of ozone-rich drier air and is characterized by relative humidity under 45%. Possible controlling mechanisms are discussed. These findings provide new insight into the physical interpretation of the "S"-shaped mean ozone profiles in the tropics.