Aerosol optical depth measurements acquired through the Canadian sunphotometer network were statistically analyzed for the 1987--1992 period in order to investigate spatial and temporal commonalities between the member stations. Four stations were chosen to yield a spatially representative sampling of atmospheric optical conditions across Canada (East Coast, eastern continental, western continental, and West Coast). All the stations were located in rural, local pollution free areas. The results of aerosol optical depth measurements showed significant differences between eastern and western Canadian stations. The effect of the Pinatubo volcanic eruption was clearly seen in the measurements acquired at Sable Island, Nova Scotia. Air mass relationships for the four stations sampled demonstrated the relevance of applying air mass classification criteria to the analysis and discrimination of atmospheric optical depth. Knowledge of the seasonal trend combined with information concerning air mass type enables a coarse a priori estimation of aerosol optical depth in the absence of traditional optical data. Synoptical air mass analysis facilitates the understanding of the mechanisms involved in the seasonal variations of aerosol optical depth and yields useful information about the atmospheric optical state. The relevance of the synoptical air mass approach was demonstrated in one particular case: a seasonal aerosol optical depth trend for Arctic air masses was observed for the three sunphotometer stations which regularily experience this type of air mass. ¿ American Geophysical Union 1996