A 25-year data record of atmospheric ozone in the Pacific from Total Ozone Mapping Spectrometer (TOMS) cloud slicing: Implications for ozone trends in the stratosphere and troposphere

[1] The newly reprocessed solar backscatter ultraviolet (SBUV) and Total Ozone Mapping Spectrometer ( TOMS) version 8 data from 1979 to 2003 are used to estimate the seasonal cycle, latitude dependence, and long-term trends in ozone averaged over the Pacific region (120 degrees W to 120 degrees E) in three broad layers of the atmosphere: upper stratosphere ( 32 hPa and above), lower stratosphere ( 32 hPa to tropopause), and the troposphere. The ozone amount in these layers is derived by first determining stratospheric column ozone in the Pacific from TOMS using deep convective clouds, which are numerous in the region. Tropospheric column ozone (TCO) for the Pacific is then determined by taking the difference between total column ozone and stratospheric column ozone. This "cloud-slicing'' technique is extensively tested from the tropics extending to +/-60 degrees latitude using stratospheric ozone data from the Stratospheric Aerosol and Gas Experiment II instrument. The validity of the cloud-slicing technique in obtaining TCO is also tested using data from ozonesondes over a wide range of latitude. SBUV ozone profiles are used to measure upper stratospheric column ozone for the Pacific region. Lower stratospheric column ozone is then derived from the difference between stratospheric column ozone and upper stratospheric column ozone. This process yields a unique 25-year record of Pacific mean ozone in three atmospheric layers covering all latitudes and seasons. The analysis of the data shows that the seasonal cycles, latitude dependence, and trends in these layers are substantially different. Over the 25-year record most ozone depletion has occurred in the lower stratosphere below similar to 25 km altitude. In middle and high latitudes, ozone losses are 3 - 4 times larger in the lower stratosphere compared with the upper stratosphere, even though the ozone amounts in the two regions are about the same. For the troposphere, TCO shows a statistically significant upward trend in the midlatitudes of both hemispheres but not in the tropics.