Bulk Attenuation Properties of Tropospheric Aerosols as Determined by Surface Measurements

Abstract

<p>Cloudless sky values of aerosol attenuation coefficient, scattering, absorption, recovery rate, bulk scattering albedo, and absorption to backscatter ratio were evaluated from surface measurements of global, diffuse, and direct beam irradiances for Montreal and Woodbridge (1968-78), Goose (1968-77), Charlottetown and Vancouver (1977-78), Winnipeg (1977), Hamilton (summer, 1977), and Sal (Cape Verde Islands, West Africa, summer, 1974).</p> <p>The mean value of the coefficient (~0.64) indicates that turbidity of the Saharan dust at Sal is comparable to those of heavily industrialized cities of Europe and Eastern United States but in Canada, the mean values (0.10 - 0.20) for urban areas are less than those for similar cities in the world. The mean value at Goose (~0.05) is typical for the sub-Arctic. For the Canadian locations, maximum values of the coefficient (~ 0.64 for Montreal) occur in summer and the minimum (0.001) in winter but at Sal, maximum values (> 1.0) are reached during the Saharan dust outbreaks that occur once in 4 to 5 days in summer.</p> <p>Comparison of some meteorological parameters with the values of the coefficient for Montreal and Woodbridge show: a slight inverse relationship with windspeed; maximum values that are generally associated with winds from between southwest and southeast and minimum values associated with winds from between northwest and northeast; a direct positive correlation with dew point temperature and also for relative humidity < 70%; that for visibility more than 10 km there is a strong linear relationship between turbidity and the inverse of visibility.</p> <p>Mean annual values of the coefficient indicate significant downward trends in cloudless sky turbidity at Montreal and Woodbridge from 1968 to 1978 and at Goose from 1968 to 1977.</p> <p>The amount of radiation scattered in a forward direction (downward) by aerosol is much larger at Sal (about 42% of global irradiance) than in Canada (6 - 13% of global irradiance). In Canada, aerosol absorption and backscatter are respectively 2-9% and 2-3% of global irradiance compared with 10% absorption and 7% backscatter at Sal. Aerosol recovery rates are least at Montreal. Estimates of bulk scattering albedo for Canadian urban sites (0.6 to 0.8) fall within the range of single scattering albedo values for American locations.</p> <p>Mean values of the absorption to backscatter ratio indicate that, considering the solar radiation balance, aerosols over Montreal, Woodbridge, Hamilton and Goose could induce warming of the Earth-atmosphere system. Aerosols over Vancouver, Winnipeg, Charlottetown and Sal could induce cooling of the system.</p>