Mario Ouellet

Thesis Aproved:

Effect of Atmospheric Conditions on Ultraviolet-B Fluxes

Stratospheric ozone acts as a shield against the damaging ultraviolet-B (UV-B) radiation (defined as the band from 280 nm to 320 nm). It has been demonstrated that certain atmospheric conditions, such as the increased presence of haze, can act to offset the decrease in ozone by lengthening the pathlength of the radiation through the atmosphere, thus increasing UV-B absorption. Clouds are another factor influencing the levels of ultraviolet reaching the surface. Overexposure of organic material to UV-B can result in skin cancer, adverse effects on the immune system, damage to cells and disruption of the radiation induced synthesis of vitamin D. The unprotected eye is also susceptible to the negative effects of the radiation. There is growing evidence that there exists a correlation between eye exposure to UV-B radiation and the formation of certain types of cataracts. While some attempts have been made at estimating the actual amount of radiation reaching the eye, there appears to be a need to model this quantity for a range of atmospheric conditions.

In the first part of the work a mapping technique which produces hemispheric distributions of upwelling and downwelling UV-B radiance is presented. The radiation transmittance model LOWTRAN 7 is used to obtain radiance values over a user-defined grid covering the sky, for a variety of atmospheric conditions and solar positions. The maps are presented in the form of polar contour plots, which give a visual representation of the diffuse UV-B radiance field. Integration of the data over the hemisphere provides the total upwelling and downwelling UV-B flux. The altitude of the simulated observer can be specified. Estimates of the incident UV-B flux on the eye are calculated from the hemispheric distribution maps. An increase in the surface UV-B from a 10% decrease of ozone in the stratosphere is shown to be offset by the absorption of UV-B by haze and planetary boundary layer (PBL) ozone in a polluted environment where the horizontal visibility is reduced to 5 km and the PBL ozone concentration is 5 times the average value. Haze is shown to be more efficient than PBL ozone at offsetting increased UV-B levels. PBL ozone is shown to be more efficient than stratospheric ozone at absorbing UV-B radiation, on a per molecule basis. The hemispheric distribution of UV-B radiance was found to be greatly affected by the scattering properties of the particulates present in the atmosphere, stratus producing featureless radiance fields and haze producing sharp radiance maxima. Broken cirrus and cumulus fields were shown to enhance surface UV-B irradiance by as much as 32%. The incident flux of radiation to the eye is a strong function of surface albedo. Minimum eye exposure is achieved by looking 180 away from the sun, towards the ground for a surface albedo of 0.1, and towards the horizon for a surface albedo of 0.9.

Mario is now working for the Atmospheric Environment Branch, Gander, Newfoundland.