The paper deals with air mass influence on atmospheric spectral transmittance in a rural, pollution-free area (Sherbrooke, Qu¿bec, in eastern Canada). A statistical analysis of aerosol optical depths (0.04≤&tgr;a≤0.60) and Angstrom parameters (0.20≤α≤2.1) derived from measurements of direct spectral solar radiation in the period from January 1989 up to August 1991 is presented. The analysis incorporates investigations into correlations with air mass type (obtained from synoptic maps) and source type (deduced from back trajectory analysis). Mean monthly values of aerosol optical depth and corresponding information about frequency of air mass occurrence are also presented. The temporal comparison of these two ensembles of data helps to explain the similarities and differences observed in the month to month variations of aerosol optical depth during the 3-year measurement period. The data arrays are then partitioned in such a way as to facilitate the interpretation in terms of determining the optical mechanisms which influence the aerosol optical depths. For air masses of arctic origin the discrimination of seasonal variations of aerosol optical depth is consistent with independent measurements of turbidity made in Alaska. The role of air mass source in defining aerosol optical depth is evaluated in terms of its being potentially a more fundamental influence than air mass type. ¿ American Geophysical Union 1994