scope:

The transformation of trace gases in the atmosphere is determined by radical chain reactions initiated by photolysis processes. More detailed assessment and prediction of air quality, therefore, require knowledge of photolysis frequencies, for example the calculation of the NO/NO2 conversion in accordance with VDI 3783 Part 19, but also more complex models of atmospheric chemistry as described in VDI 3783 Part 5.

This standard provides photolysis frequencies for tropospheric conditions. To this end, spectral actinic photon flux densities were calculated for a large number of meteorological situations so as to cover typical conditions for Central Europe, with 200 m above sea level taken as the average altitude. The output of the calculation is done for the altitudes 0 m and 1000 m above ground. The actinic photon flux densities are then used to calculate the photolysis frequencies of tropospheric trace gases. These values depend on the solar zenith angle, the cloud cover, and other atmospheric properties. Appropriate parameterisations, which simplify the modelling of chemical reaction systems, are also derived.

The calculations of the actinic photon flux densities are based on a radiation transport model that considers all radiation processes. Since all the relevant factors are stated, the described photolysis frequencies can also be used as reference values for the validation of less complex modelling approaches for the calculation of photolysis frequencies, as they are still used sometimes in chemical transport models.

The calculated photolysis frequencies and the provided parameterisations are accessible via a website [28] of the Jülich Research Centre. Since the input variables used in the calculations are described in detail and the sources cited, it is possible for the user to follow the calculation and where necessary also to calculate photolysis frequencies not covered here. The required spectral actinic photon flux densities are also provided.

For the underlying radiation physics, we refer the reader to DIN 5031-1 and DIN 5031-11; the latter in particular deals with the actinic radiation variables important for photolysis. The metrology of radiation variables, especially global irradiance, can be found in VDI 3786 Part 5.