Total ozone trends in the tropics

Measurements with Dobson spectrophotometers are used to study ozone trends at Cachoeira Paulista (22.7 degrees S, 45.0 degrees W) and Natal (5.8 degre es S, 35.2 degrees W) in Brazil. The time series are from 1974 to 1997 for Cachoeira Paulista and from 1978 to 1997 at Natal. Using a model that accou nts for the quasi-biennial oscillation, seasonal variation, and solar cycle , a trend in total ozone of-2.0+/-1.2% per decade was found at Cachoeira Pa ulista for the period 1974 to 1997. At Natal the trend was -0.8+/-0.8% per decade over the period 1978 to 1997. The annual values of column ozone repo rted at Cachoeira Paulista are lower in the 1990's than in the previous two decades: total ozone amounts are 12.5 Dobson units (DU), or 4.6%, lower in the 1990s than those observed during the 1970s. From a monthly analysis it is shown that these ozone decreases have occurred in all months of the yea r. There was no statistically significant change between the 1980s and 1990 s at Natal.