Global Distribution Of Tropospheric Ozone

The distribution of tropospheric ozone can be determined from the analyses of satellite data sets obtained independently from two different instruments: The Total Ozone Mapping Spectrometer (TOMS) and the Stratospheric Aerosol and Gas Experiment (SAGE). Between October 1978 and May 1993, TOMS functioned on the Nimbus 7 satellite and provided daily maps of the distribution of total ozone. Additional TOMS were launched in 1991 (on the Russian Meteor satellite) and two in 1996 (see Chapter 21; "Stratospheric Ozone Observations"). The National Aeronautical and Space Administration's (NASA's) Earth Observing System (EOS) now is operational and total ozone will be measured as part of EOS. Total ozone is defined as the integrated amount of ozone between the surface and the top of the atmosphere. A unit of measure for total ozone is a quantity known as the Dobson unit (DU), where 1 DU = 2.69 x 1016 molecules 03/cm2. If this amount of ozone were brought down to standard atmospheric temperature and pressure, the depth of this column would be 1 mm. Thus, another common measure of column ozone is mm-atm, where a mm-atm is equivalent to 1 DU. A typical amount of total ozone found in the atmosphere is 300 DU, and approximately 90% of this ozone is located in the stratosphere.

At middle and high latitudes, the distribution of total ozone is primarily governed by the prevailing large-scale circulation patterns. These patterns can vary substantially on a daily basis, and intense gradients of total ozone have been observed with differences of 200 DU at locations less than a few thousand kilometers apart. At these higher latitudes, total ozone amounts can range between ~225 and ~500 DU. Only recently have values as low as 100 DU been observed during austral spring in conjunction with the Antarctic ozone hole.

At lower latitudes, however, the total ozone distribution patterns exhibit much smaller gradients than at middle and high latitudes. The intense gradients of as much as 200 DU found at the higher latitudes are replaced by much more subtle gradients of no more than 20 to 30 DU. Because the primary intent of the measurement of total ozone was to study the distribution of stratospheric ozone, very little research was conducted using the information provided by TOMS in the tropics. Subsequently, however, it has been shown that the variations in total ozone at low latitudes were

4 TROPOSPHERIC OZONE TRENDS IN NONURBAN TROPOSPHERE 51

primarily the result of variability of ozone in the troposphere even though only ~10% of the total ozone was in the troposphere.

The use of TOMS for tropospheric studies has taken a substantive step further when data from SAGE were used to derive the amount of ozone in the stratosphere (Fishman et al, 1990). Ozone measurements from the SAGE instruments (SAGE was launched in February 1979 and operated through November 1981; SAGE II was launched in November 1984 and is still operating) provide the vertical distribution of ozone in the stratosphere. From these profiles, the amount of ozone in the stratosphere can be integrated and then subtracted from the co-located total ozone amount derived independently from the TOMS on the same day.

The distribution of the integrated amount of tropospheric ozone as a function of season is shown in Figure 1 (Fishman et al, 2002). These seasonal depictions show that there is considerably more ozone in the Northern Hemisphere than in the Southern Hemisphere, especially during the summer. During most of the seasons, distinct plumes that seem to result from pollution originating in North America, Asia, Africa, and Europe can be observed. In the three northern continents, the plumes originate over the eastern portions of each landmass and are transported by the prevailing westerly winds for several thousand kilometers. At low latitudes, the highest concentrations of pollution are off the west coast of Africa and is most pronounced during austral spring (September-November). At these latitudes, the prevailing low-level winds are trade winds (easterlies), which would carry the emissions from central and western Africa to the eastern tropical South Atlantic Ocean. The prevailing upper level winds are westerlies, so any ozone that gets to altitudes of ~5 km or higher are transported long distances to the east. Evidence of the long-range transport of emissions from biomass burning in Africa and South America to Australia is evident in long-term Australian data sets of not only ozone but also carbon monoxide and elemental carbon, two other products of widespread burning.

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