The long-term trend of background O-3 in surface air over the United States from 1980 to 1998 is examined using monthly probability distributions of daily maximum 8-hour average O-3 concentrations at a large ensemble of rural sites. Ozone concentrations have decreased at the high end of the probability distribution (reflecting emission controls) but have increased at the low end. The cross-over takes place between the 30th and 50th percentiles in May-August and between the 60th and 90th percentiles during the rest of the year. The increase is statistically significant at a 5% level in spring and fall, when it is 3-5 ppbv. The maximum increase is in the Northeast. A possible explanation is an increase in the O-3 background transported from outside the United States. Better understanding of the causes of the increase is needed because of its implications for meeting O-3 air quality standards. [References: 20]
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