"Cloud slicing": A new technique to derive upper tropospheric ozone from satellite measurements

A new technique called cloud slicing has been developed for measuring upper tropospheric O3. Cloud slicing takes advantage of the opaque property of water vapor clouds to ultraviolet wavelength radiation. Measurements of above‐cloud column O3 from the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) instrument are combined together with Nimbus 7 temperature‐humidity and infrared radiometer (THIR) cloud‐top pressure data to derive O3 column amounts in the upper troposphere. In this study, tropical TOMS and THIR data for the period 1979–1984 are analyzed. By combining total tropospheric column ozone (referred to as TCO) measurements from the convective cloud differential (CCD) method with 100‐ to 400‐hPa upper tropospheric column O3 amounts from cloud slicing, it is possible to estimate 400‐ to 1000‐hPa lower tropospheric column O3 and evaluate its spatial and temporal variability. Results for both the upper and lower tropical troposphere show a year‐round zonal wave number 1 pattern in column O3 with the largest amounts in the Atlantic region (up to ∼15 DU in the 100‐ to 400‐hPa pressure band and ∼25–30 DU in the 400‐ to 1000‐hPa pressure band). Upper tropospheric O3 derived from cloud slicing shows maximum column amounts in the Atlantic region in the June‐August and September‐November seasons which are similar to the seasonal variability of CCD‐derived TCO in the region. For the lower troposphere, the largest column amounts occur in the September‐November season over Brazil in South America and also southern Africa. Localized increases in the tropics in the lower tropospheric O3 are found over the northern region of South America around August and off the west coast of equatorial Africa in the March‐May season. Time series analysis for several regions in South America and Africa show an anomalous increase in O3 in the lower troposphere around the month of March which is not observed in the upper troposphere. The eastern Pacific indicates weak seasonal variability of upper, lower, and total tropospheric O3 compared with the western Pacific, which shows the largest TCO amounts in both hemispheres around spring months. O3 variability in the western Pacific is expected to have greater variability caused by strong convection, pollution and biomass burning, land‐sea contrast, and monsoon developments.