Unsworth and Monteith's aerosol attenuation coefficient τA was calculated with hourly cloudless sky data at Goose Bay, Montreal and Woodbridge (near Toronto) for the period 1968 to 1978. Turbidity was less on average (τA < 0.11) than for British urban locations (0.25 < τA < 0.55). Values of τA were largest in summer and in southerly flow, increased with relative humidity and varied inversely with visibility. Turbidity decreased over the 1968–78 period at the three stations. The decrease was a feature of air flow from most directions. It coincides with decreases in air quality indices reported by Ricci et al. for Canada between 1971 and 1975.