Measuring Spectral Actinic Flux and Irradiance: Experimental Results from the Actinic Flux Determination from Measurements of Irradiance (ADMIRA) Project

Abstract

Results are presented from the Actinic Flux Determination from Measurements of Irradiance (ADMIRA) campaign to measure spectral global UV irradiance and actinic flux at the ground, beneath an atmosphere well defined by supporting measurements. Actinic flux is required to calculate photolysis rates for atmospheric chemistry, yet most spectral UV measurements are of irradiance. This work represents the first part of a project to provide algorithms for converting irradiances to actinic fluxes with specified uncertainties. The campaign took place in northern Greece in August 2000 and provided an intercomparison of UV spectroradiometers measuring different radiation parameters, as well as a comprehensive radiation and atmospheric dataset. The independently calibrated spectroradiometers measuring irradiance and actinic flux agreed to within 5%, while measurements of spectral direct irradiance differed by 9%. Relative agreement for all parameters proved to be very stable during the campaign. A polarization problem in the Brewer spectrophotometer was identified as a problem in making radiance distribution measurements with this instrument. At UV wavelengths actinic fluxes F were always greater than the corresponding irradiance E by a factor between 1.4 and 2.6. The value of the ratio F : E depended on wavelength, solar zenith angle, and the optical properties of the atmosphere. Both the wavelength and solar zenith angle dependency of the ratio decreased when the scattering in the atmosphere increased and the direct beam proportion of global irradiance decreased, as expected. Two contrasting days, one clear and one with higher aerosol and some cloud, are compared to illustrate behavior of the F : E ratio.

Twilight tropospheric and stratospheric photodissociation rates derived from balloon borne radiation measurements

“…As shown by e.g. Webb et al (2002) the ratio of actinic flux spectra to irradiance spectra depend on wavelength, solar zenith angle and aerosol amount. However, the same spectral lines are present in both irradiance and actinic flux spectra.…”

“…4. During the Photochemial Acticity and Solar Ultraviolet Radiation (ADMIRA) campaign in August 2000 at Nea Michaniona, Greece (Webb et al, 2002), the NILU-CUBE was collocated with a Bentham DTM300 scanning spectroradiometer. Following the procedure outlined above and detailed in Dahlback (1996), calibration factors were calculated for one of the heads.…”

Technical note: The libRadtran software package for radiative transfer calculations - description and examples of use

“…The NILU-CUBE instrument measures the irradiance on the six faces of a cube in two channels centered approximately at 312 and 340 nm (Kylling et al, 2003a). During the ADMIRA campaign in August 2000 at Nea Michaniona, Greece (Webb et al, 2002), the NILU-CUBE was mounted about 3 m above a roof with one head pointing up, one down, and the others to the east, south, west and north. Thus, the NILU-CUBE measured the down-and upwelling irradiances in addition to the irradiance on the vertical detector heads facing east, south, west and north.…”

“…It is interesting to note that for 340 nm for these particular measurements, the maximum irradiances for the east and west facing detectors are larger than the maximum value for the south facing detector. (Webb et al, 2002). The downwelling irradiance is shown in red, the upwelling in yellow.…”

Technical Note: A new discrete ordinate first-order rotational Raman scattering radiative transfer model – implementation and first results

“…The aerosol case was based on a situation that occurred during the Actinic flux determination from measurements of irradiance (ADMIRA) measurement campaign, August 2000 at Nea Michaniona, Greece (Webb et al, 2002). Two irradiance spectra on the 5th of August were simulated.…”