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 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.
Citation: Palancar, G. G., Shetter, R. E., Hall, S. R., Toselli, B. M., and Madronich, S.: Ultraviolet actinic flux in clear and cloudy atmospheres: model calculations and aircraft-based measurements, Atmos. Chem. Phys., 11, 5457-5469, doi:10.5194/acp-11-5457-2011, 2011.