Evidence for cloud venting of mixed layer ozone and aerosols

Observations are presented which substantiate the hypothesis that significant vertical exchange of ozone (O₃) and aerosol pollutants occurs between the mixed layer and the free troposphere during cumulus cloud convective activity. Flight experiments conducted in July 1981 utilized the airborne UV-DIAL (Ultra-Violet Differential Absorption Lidar) system developed by NASA. This system provides simultaneous range resolved O₃ concentration and aerosol backscatter profiles with high spatial resolution. Data were obtained during the afternoon along east-west and south-north intersecting transects over North Carolina in the presence of active, non-precipitating cumulus clouds. Evening transects were obtained in the area indicated by trajectory calculations to be the current position of the air mass sampled earlier in the day. Space-height cross-section analyses for the evening flight show the cloud ‘debris’ as patterns of aerosol and O₃ in excess of the ambient free tropospheric background. The O₃ excess was approximately the value of the concentration difference between the afternoon mixed layer and free troposphere measured in the afternoon from independent in-situ vertical soundings made by another aircraft.