Solar spectral actinic flux and photolysis frequency measurements in a deciduous forest

Bohn, B.

doi:10.1029/2005JD006902

Journal Article

PreJuSER-49902

Solar spectral actinic flux and photolysis frequency measurements in a deciduous forest

Bohn, B.FZJ*

2006
Union Washington, DC

Journal of Geophysical Research 111, D15303 (2006) [10.1029/2005JD006902]

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Please use a persistent id in citations: http://hdl.handle.net/2128/20479 doi:10.1029/2005JD006902

Abstract: [ 1] Within the Emission and Chemical Transformation of Biogenic Volatile Organic Compounds ( ECHO) project the photochemistry of biogenic organic compounds in a forest was investigated. In this context, two spectroradiometers were used to determine solar spectral actinic flux and photolysis frequencies within and above a deciduous forest at Julich, Germany. Locations in the forest and in a small clearing were examined 2 m above ground. Under overcast conditions, photolysis frequencies were within 1 - 3% and 6 - 8% of above canopy values at forest and clearing locations, respectively. In a spectral range below 500 nm the canopy was found to act as a gray filter that diminished spectral actinic flux independent of daytime and wavelength. Under clear-sky conditions, diurnal variations within the forest also followed that of diffuse sky radiation above the forest except for sharp peaks from direct sunlight that was incident sporadically at the selected locations. The results under all conditions were in accord with the foliage being opaque toward UV and gaps in the canopy being responsible for the remaining actinic flux. Sky photographs showed that the distribution and size of gaps in the foliage were extremely heterogeneous with patterns changing rapidly upon changing location. Regarding daytime chemical reactions of biogenic compounds within the forest, it was estimated that degradation by OH was reduced by a factor similar to the photolysis frequencies, while potential degradation by NO3 was increased by a factor of similar to 3 compared with above forest conditions. As a result, it was estimated that minor fractions of 0.8% of isoprene and 3.4% of monoterpenes were degraded by reactions with OH, NO3, and O-3 prior to transport into the layer above the forest.

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