Dobson spectrophotometer 83 was established in 1962 as a standard for total ozone measurements in the United States. In 1980 the instrument was designated by the World Meteorological Organization (WMO) as the primary standard Dobson spectrophotometer for the world. Since the early 1960s, virtually all (∼90) Dobson instruments in the global Dobson instrument network have been calibrated several times, either directly or indirectly, relative to instrument 83. Calibrations of instrument 83 by the Langley method were performed in 1962 at Sterling, Virginia, and during 1972–1987 at Mauna Loa Observatory (MLO), Hawaii. A detailed analysis of these calibration data, as well as calibration data based on standard lamp measurements made since 1962, indicates that the long‐term (25‐year) ozone measurement precision for the instrument is known to within an uncertainty of ±0.5%. On an absolute scale, the ozone measurements made at MLO with instrument 83 are estimated to be too low by about 2.4%, as a result of errors in the A and D wavelength ozone absorption coefficients used with the instrument and their temperature dependence. Cumulative other biases most likely do not exceed ±1%. This documentation of the calibration history of instrument 83 lends credence to the accuracy with which other Dobson instruments have been calibrated in the past and to Dobson and satellite instrument ozone measurement comparisons, thereby increasing confidence in ozone trends determined by these ozone measurement systems in recent years.

Mean monthly total ozone data obtained at South Pole, Antarctica, during 1964‐1985 are presented. Preliminary analysis of the data suggests that total ozone has decreased at South Pole by roughly 20% since the mid‐1960's. Largest decreases have occurred during October and November months, but autumn and winter months also exhibit a decrease. During February months, ozone has increased slightly. Superimposed on the long‐term downward ozone trend are episodic decreases in ozone dating back to 1965 that have occurred at intervals of 4‐6 years. These decreases are shown to be related to delays in stratospheric warmings in Antarctica and the transport of ozone into Antarctica each spring.

Dobson spectrophotometer observations conducted since the early‐to‐mid 1960s at Bismarck, North Dakota; Caribou, Maine; Boulder, Colorado; Wallops Island, Virginia; Nashville, Tennessee; and at Fresno, California, since 1983, have revealed record low total ozone values during 1993. The tendency toward the low ozone values began in May 1992, but accelerated 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 deviations below normals 42% of the time. On average, the January–April 1993 ozone values were 12.6% below normal, with ozone deficits as large as 18% observed at Caribou and Wallops Island in January. Of particular concern are unusually low ozone values that occur in summertime when solar ultraviolet insolation is high. Such record lows occurred at four of the six stations (Caribou, Wallops Island, Fresno, and Nashville). During May–August 1993, ozone was on average 8.5% below normal at these sites. Monthly means at these stations were, furthermore, lower on average by 3.7% than corresponding lowest values observed there in the past. The ozone decrease of 12.6% below normal at the six continental Dobson 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, Wallops Island, Fresno, and Nashville, implies that on average, UV erythemal radiation may have been higher than normal at these stations during the summer of 1993 by 11–17%.

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