Abstract
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.
Highlights
Solar ultraviolet (UV) radiation initiates much of the chemistry of Earth’s troposphere by photo-dissociating relatively stable molecules into highly reactive fragments such as oxygen atoms and hydroxyl or organic radicals
All the measurements used in this work were taken during the INTEX-NA 2004 campaign with two Scanning Actinic Flux Spectroradiometers (SAFS) developed at the National Center for Atmospheric Research (NCAR, USA)
Examination of three cameras installed on board the DC-8 and flight track overlays on GOES-10 or GOES-12 imagery indicate that this day was mostly free of clouds, except for brief periods around 16:15 and 20:15 UTC
Summary
Solar ultraviolet (UV) radiation initiates much of the chemistry of Earth’s troposphere by photo-dissociating relatively stable molecules into highly reactive fragments such as oxygen atoms and hydroxyl or organic radicals. In practice a distinction is often made for the contributions of the direct solar beam (F o), and the diffuse radiation down-welling (F ↓) and up-welling (F ↑) incident respectively from the upper and lower hemispheres. This diffuse radiation arises from molecular (Rayleigh) scattering which is effective at UV wavelengths due to its λ−4 dependence, from reflections at the Earth’s surface (the albedo of land or water), and from scattering by aerosols and clouds. The calculation of actinic fluxes is a critical component of chemistry-transport models (CTMs) used to study environmental problems such as photochemical smog, regional
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