UNPRECEDENTED 1993 OZONE DECREASE OVER THE UNITED-STATES FROM DOBSON SPECTROPHOTOMETER OBSERVATIONS

Dobson spectrophotometer observations conducted since the early-to-mid 1960s at Bismarck, North Dakota; Canbou, Maine; Boulder, Colorado; Wa llops Island, Virginia; Nashville, Tennessee; and at Fresno, Californi a, since 1983, have revealed record low total ozone values during 1993 . The tendency toward the low ozone values began in May 1992, but acce lerated in early 1993. During January-August 1993, ozone monthly means at the stations were more than 2 standard deviations below long-term normal monthly means 72% of the time and more than 3 standard deviatio ns below normals 42% of the time, On average, the January-April 1993 o zone values were 12.6% below normal, with ozone deficits as large as 1 8% observed at Caribou and Wallops Island in January. Of particular co ncern are unusually low ozone values that occur in summertime when sol ar ultraviolet insolation is high. Such record lows occurred at four o f the six stations (Caribou, Wallops Island, Fresno, and Nashville). D uring May-August 1993, ozone was on average 8.5% below normal at these sites. Monthly means at these stations were, furthermore, lower on av erage by 3.7% than corresponding lowest values observed there in the p ast. The ozone decrease of 12.6% below normal at the six continental D obson instrument stations during the winter and spring months of 1993 implies a possible average increase in UV erythemal radiation at that time of 16-25% above normal. The 8.5% decrease in ozone at Caribou, Wa llops Island, Fresno, and Nashville, implies that on average, UV eryth emal radiation may have been higher than normal at these stations duri ng the summer of 1993 by 11 - 17%.