| One-minute and daily cloudless sky global, direct and diffuse irradiance measurements at Environment Canada's Baseline Surface Radiation Network Bratt's Lake Observatory in Saskatchewan, throughout 2003, are presented, analyzed and compared to model estimations. The magnitude of broadband irradiances proves highly responsive to changes in optical air mass and aerosol optical depth as shown by the clearness index, transmissivity and diffuse ratio. During summer, wildfire activity in southern British Columbia and the western United States produced aerosols that caused a large increase in optical depth at the Observatory. Attenuation from pyrogenic aerosols caused a negative forcing in broadband global irradiance equal to 3.4 percent.; Spectral global irradiance data collected under summertime cloudless conditions shows increased reductions during periods with pyrogenic aerosol. Both the spectral clearness index and the aerosol optical depth reveal that attenuation is not constant over the spectral range 0.28 to 0.80 micrometres; instead, attenuation increases towards shorter wavelengths. |
