Antarctic ozone drop much less than in 1987.

A springtime "ozone hole" which has appeared in Antarctica in recent years was considerably less deep in 1988 than it was in 1987, and even shallower than in 1986, the National Oceanic and Atmospheric Administration (NOAA) reports. Measurements with balloon-borne instruments launched by NOAA

Average early October values in 1986 and 1987 were 165 and 135 Dobson units, respectively; remarkably low ozone values for anywhere on earth, Komhyr said. If present trends continue, he noted ozone values during the latter half of October would approach those last observed at South Pole prior to 1980 when values on average during mid-to-late October were in excess of 250 Dobson units.

Ozone has been decreasing in the spring in Antarctica since the mid-to-late 1970's. Typically, the decrease each year begins in early September with ozone reaching minimum values in early October. The general downward trend has exhibited slight temporary ozone recovery in alternate years, with the largest recovery prior to 1988 occurring in 1986. The 1988 recovery was even larger.

The ozone hole occurs within the Antarctic polar vortex, a belt of strong west-to-east winds that circle the Antarctic during winter and spring months. Within the vortex in 1988, stratospheric temperatures at the South Pole in September and early October were 5 [degrees] -10 [degrees] C warmer than they were in 1987, and 2 [degrees] -5 [degrees] C warmer than in 1986. The warmer temperatures, Komhyr said, did not favor the formation of polar stratospheric clouds in 1988, as did the colder temperatures in previous years.

Stratospheric clouds promote the photochemical destruction of ozone by chlorine compounds derived from man-made chlorofluorocarbons. Polar stratospheric clouds form at temperatures colder than about 780C. The ozone readings this 1988 austral spring in the Antarctic do not mean that the threat of reduced ozone levels globally no longer exist, Komhyr emphasized, nor does it portend the end of the Antarctic ozone hole. Rather, he said, the readings indicate that large year-to-year changes in ozone can occur from natural variations in atmospheric processes. The ozone monitoring program at South Pole is conducted by NOAA in cooperation with the National Science Foundation which is the agency responsible for the United States Antarctic Program.