Tropical tropospheric ozone derived using Clear-Cloudy Pairs (CCP) of TOMS measurements

Abstract. Using TOMS total-ozone measurements over high-altitude cloud locations and nearby paired clear locations, we describe the Clear-Cloudy Pairs (CCP) method for deriving tropical tropospheric ozone. The high-altitude clouds are identified by measured 380 nm reflectivities greater than 80% and Temperature Humidity InfraRed (THIR) measured cloud-top pressures less than 200 hPa. To account for locations without high-altitude clouds, we apply a zonal sine fitting to the stratospheric ozone derived from available cloudy points, resulting in a wave-one amplitude of about 4 DU. THIR data is unavailable after November 1984, so we extend the CCP method by using a reflectivity threshold of 90% to identify high-altitude clouds and remove the influence of high-reflectivity-but-low-altitude clouds with a lowpass frequency filter. We correct ozone retrieval errors associated with clouds, and ozone retrieval errors due to sun glint and aerosols. Comparing CCP results with Southern Hemisphere ADditional OZonesondes (SHADOZ) tropospheric ozone indicates that CCP tropospheric ozone and ozonesonde measurements agree, on average, to within 3 ± 1 DU standard error of the mean. The most significant difference between CCP and ozonesonde tropospheric ozone can be explained by the low Total Ozone Mapping Spectrometer (TOMS) version-7 retrieval efficiency of ozone in the lower troposphere.