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Volume 11, issue 11
Atmos. Chem. Phys., 11, 5457–5469, 2011
https://doi.org/10.5194/acp-11-5457-2011
© Author(s) 2011. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmos. Chem. Phys., 11, 5457–5469, 2011
https://doi.org/10.5194/acp-11-5457-2011
© Author(s) 2011. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 14 Jun 2011

Research article | 14 Jun 2011

Ultraviolet actinic flux in clear and cloudy atmospheres: model calculations and aircraft-based measurements

G. G. Palancar1,2, R. E. Shetter2, S. R. Hall2, B. M. Toselli1, and S. Madronich2 G. G. Palancar et al.
  • 1INFIQC-CONICET, Departamento de Físico Química, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Centro Láser de Ciencias Moleculares, 5000, Córdoba, Argentina
  • 2Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, Colorado, USA

Abstract. 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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