Friday, 7 June 2002
An Approach to Estimate Surface Actinic Flux from Satellite Measurements
Photodissociation of atmosphere trace gases by solar UV radiation plays a fundamental role in the chemistry of the terrestrial atmosphere. In the troposphere, photochemical smog or ground-level O3 results partially from photochemical reaction, rendering severe air pollution in many large cities. The energy driving the photochemical reaction is determined by the so-called actinic flux, the amount of radiation, mainly in the UV spectrum, received by a unit spherical surface. Due to experimental difficulties, the existing knowledge of surface actinic flux is both temporally and spatially very limited. In this study, a remote sensing approach is proposed to estimate surface actinic flux from satellite measurements in order to provide surface actinic flux on a global scale. The development of the method takes advantage of comprehensive studies on retrieving surface UV irradiance from satellite and the relationship between UV irradiance and actinic flux. The actinic flux is proportional to UV irradiance for cloudless and overcast sky conditions weighted by cloud fraction. The method will be validated in three ways, namely, against simulations with a DISORT-based model for various atmospheric conditions, against the existing but limited ground-based measurements of actinic flux or photolysis rate in some locations, and against actinic fluxes converted from measured UV irradiances with ground-based instruments. By doing so, the accuracy and uncertainties of the approach are thoroughly investigated. Global mapping of actinic flux will be attempted following this approach.