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 regularly experience this type of air mass.

Satellite remote sensing, a powerful monitoring tool, often lacks the high spatial resolution and operational flexibility needed for research in Earth Sciences. In order to highlight the concept of linking ground data and remote sensing imagery, a review of tremendous recent advances in low-altitude remote sensing, as well as in supporting systems such as light airborne platforms is presented. Airborne digital photography and videography provide near real-time data at the local and regional scales. The integration of portable multi-spectral digital sensors, navigation systems and small slow-flying aircraft creates a modern do-it-yourself data acquisition tool for everyday field work and ground truthing of satellite imagery. This approach closes the existing gap between the advanced spaceborne remote sensing methods and the precise but inefficient ground studies. It is particularly advantageous for geomorphology and Earth Sciences.

We evaluated the potential of a digital ground-penetrating radar (GPR) for soil moisture determination. Ground-penetrating radar offers a fast and nondestructive way for estimating the soil dielectric constant and may be an interesting tool for low-cost mapping of soil moisture. Two modes of operation were considered: (i) the ground mode where signal propagation between the antennas placed on the surface was studied; (ii) the airborne mode where the antennas are raised 5-15 m above the soil surface. Variables describing the GPR response were defined and related to the soil moisture. It is shown from field experiments that the GPR measurements performed at 200 MHz in both modes are mainly affected by the moisture in the 10- to 20-cm topsoil layer. In the ground mode, the obtained soil moisture error after calibration was lower than 0.03 m(3)/m(3). In the airborne mode, soil moisture estimations were less accurate (0.046 m(3)/m(3)). The wave spreading loss characterization has to be improved to get an altitude-independent GPR measurement.

The paper deals with air mass influence on atmospheric spectral transmittance in a rural, pollution-free area (Sherbrooke, Quebec, in eastern Canada). A statistical analysis of aerosol optical depths (0.04 less than or equal to tau(a) less than or equal to 0.60) and Angstrom parameters (0.20 less than or equal to alpha less than or equal to 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.