| Jordi Vilà-Guerau de Arellano Institute For Marine and Atmospheric Research University Utrecht P.O. Box 80.005 3508 TA Utrecht, The Netherlands |
Peter G. Duynhkirke | Michiel van Weele |
A large amount of chemical reactions in the atmosphere are initiated by photochemical processes. The photodissociation rate of these reactions depends on the absorption cross section, the quantum yield and the actinic flux. Whereas the two first are properties of the chemical species and are dependent on the wave length and the temperature, the actinic flux is a radiative quantity which is defined as the radiance integrated over all solid angles.
In the frame of the Atlantic STratocumulus EXperiment (ASTEX), more than 34-tethered balloon soundings were carried out. The main purpose of the soundings was to measure vertical profiles of actinic flux under different meteorological conditions. The actinic flux was measured with a photoelectrical detector with a spectral response that ranges from 330 nm to 390 nm. It is in this region where the photodissociation of the main precursor of ozone in the troposphere, nitrogen dioxide, takes place.
Several vertical profiles of actinic flux under clear and cloud conditions will
he presented. These profiles show the different behaviour of the actinic flux
below, in and above clouds. These observations are completed with detailed
measurements of cloud characteristics. An almost constant value for the
actinic flux in the whole atmospheric boundary layer was observed under clear
sky conditions. The data is also used to evaluate a radiative transfer model
based on the delta-Eddington approximation method. A good agreement is found
between modeI results and tethered balloon measurements.