ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-5457-2011 Ultraviolet actinic flux in clear and cloudy atmospheres: model calculations and aircraft-based measurements Palancar G. G. 1 2 Shetter R. E. 2 Hall S. R. 2 Toselli B. M. 1 Madronich S. 2 INFIQC-CONICET, Departamento de Físico Química, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Centro Láser de Ciencias Moleculares, 5000, Córdoba, Argentina Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, Colorado, USA 14 06 2011 11 11 5457 5469 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/11/5457/2011/acp-11-5457-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/5457/2011/acp-11-5457-2011.pdf

Ultraviolet (UV) actinic fluxes measured with two Scanning Actinic Flux Spectroradiometers (SAFS) aboard the NASA DC-8 aircraft are compared with the Tropospheric Ultraviolet-Visible (TUV) model. The observations from 17 days in July-August 2004 (INTEX-NA field campaign) span a wide range of latitudes (28° N–53° N), longitudes (45° W–140° W), altitudes (0.1–11.9 km), ozone columns (285–353 DU), and solar zenith angles (2°–85°). Both cloudy and cloud-free conditions were encountered. For cloud-free conditions, the ratio of observed to clear-sky-model actinic flux (integrated from 298 to 422 nm) was 1.01±0.04, i.e. in good agreement with observations. The agreement improved to 1.00±0.03 for the down-welling component under clear sky conditions. In the presence of clouds and depending on their position relative to the aircraft, the up-welling component was frequently enhanced (by as much as a factor of 8 relative to cloud-free values) while the down-welling component showed both reductions and enhancements of up to a few tens of percent. Including all conditions, the ratio of the observed actinic flux to the cloud-free model value was 1.1±0.3 for the total, or separately 1.0±0.2 for the down-welling and 1.5±0.8 for the up-welling components. The correlations between up-welling and down-welling deviations are well reproduced with sensitivity studies using the TUV model, and are understood qualitatively with a simple conceptual model. This analysis of actinic flux observations illustrates opportunities for future evaluations of photolysis rates in three-dimensional chemistry-transport models.

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