The chart shows the average number of days exceeding the state 1-hour nitrogen dioxide standard for the period 1976-1995 for the seven sites with complete data throughout the period. The three-year average number of days exceeding the state standard at the seven trend sites between 1976 and 1995 decreased by 100% from an average of 12 days in 1976-1978 to zero days in 1993-1995. The average nitrogen dioxide concentration for the seven trend sites decreased 31% between 1976-1978 and 1993-1995. There were no exceedances of the federal annual nitrogen dioxide standard in the Basin in 1995.
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