Ultraviolet Actinic Flux in Clear and Cloudy Atmospheres: Model Calculations and aircraft-based measurements

G. G. PALANCAR; R. E. SHETTER; S. R. HALL; B. M. TOSELLI; S. MADRONICH

ATMOSPHERIC CHEMISTRY AND PHYSICS

COPERNICUS PUBLICATIONS

Lugar: Gottingen; Año: 2011 vol. 11 p. 5457 - 5469

1680-7316

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 (28º N - 53º N), longitudes (45º W - 140º W), altitudes (0.1 - 11.9 km), ozone columns (285 - 353 DU), and solar zenith angles (2º - 85º). 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.