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Very rapid increases in the concentrations of ozone and ozone precursors, in the late afternoon, have been observed at a rural and an urban site in southern Ontario. Ozone concentration increases of 30 ppbv in a few minutes have been observed. These increases occur simultaneously with the arrival of a Lake Ontario lake breeze front as identified from meteorological measurements and visible satellite imagery. This indicates that polluted air masses from over Lake Ontario are being transported inland by the lake breeze. Aircraft measurements of ozone, NO.., and hydrocarbons show such an air mass moving inland. Chemical measurements at the sites show that the polluted air masses are not of local origin, but are of similar age to those regularly encountered in rural areas. I

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