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Content archived on 2024-05-21

Actinic flux determination from measurements of irradiance


Problems to be solved
The challenges of measuring solar ultraviolet (UV) radiation have received considerable attention since recognition of stratospheric ozone depletion, with a resulting increase in availability and quality of UV data. Spectral measurements allow the data to be applied to any biological or chemical photoreaction with a known action spectrum. However, the standard UV measurement is of irradiance incident on a flat horizontal surface. This single geometry is not applicable to all the targets that may be affected by the radiation. One important example is the atmospheric chemistry of the boundary layer, which is strongly dependent on the UV radiation but where the spherical target molecules are subject to radiation from all directions. The appropriate measure of UV in this case is actinic flux, but such measurements are not made on a routine basis. We address the problem of how to expand the use of existing spectral UV irradiance data by deriving from it the associated actinic flux.
Scientific objectives and approach
The objective of the project is to develop tools to convert existing and future spectral UV irradiance data into spectral actinic fluxes. The problem is approached through a combination of measurement and modelling techniques. Horizontal irradiance and actinic flux measurements are made at the same site and supported by comprehensive ancillary measurements of ozone and aerosol in the atmosphere. The resulting data sets are used to develop and test conversion algorithms, both empirical and based on a high performance radiative transfer model. A series of algorithms are being developed, depending on the supporting data available with the horizontal irradiance measurements. The horizontal and actinic fluxes are also monitored at a number of contrasting sites to provide data under a greater range of atmospheric conditions and with more realistic (i.e. less specialized) operational supporting data. These longer-term data sets allow more extensive testing of the algorithms. The uncertainty in actinic fluxes calculated with each algorithm (level of supporting data) is also being assessed. Finally, the appropriate algorithms are applied to a selection of irradiance data in the European UV database, then the resulting actinic fluxes, and the algorithms, are made available through the database.
Expected impacts
The programme enables historical estimates of spectral actinic flux (within given uncertainty bounds) to be made from existing databases of global irradiance measurements, and allows conversion of all future irradiance measurements to actinic fluxes. This greatly expands the potential use of spectral irradiance measurements by making the data applicable to a much wider user community, especially those concerned with photochemistry of the atmosphere. Solar UV in the lower atmosphere is closely associated with stratospheric ozone, while air pollution and atmospheric composition are strongly influenced by that UV. Thus the project helps address the integration of two global change issues covered by the Montreal and Kyoto Protocols.

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University of Manchester Institute of Science and Technology (UMIST)
EU contribution
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Sackville Street
M60 1QD Manchester
United Kingdom

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Participants (6)