The quantity of light available to molecules at a particular point in the atmosphere
and which, on absorption, drives photochemical processes in the atmosphere. It is
calculated by integrating the
spectral radiance
overall directions of incidence of the light,
.
If the
radiance is expressed in
and
is the energy per quantum of light of
wavelength
,
the
actinic flux has units of
.
This important quantity is one of the terms required in the calculation of
j-values, the first order rate coefficients for photochemical processes in the sunlight-absorbing,
trace gases in the atmosphere. The
actinic flux is determined by the solar radiation entering the atmosphere and by any changes
in this due to atmospheric gases and particles (e.g.
Rayleigh scattering absorption by stratospheric ozone,
scattering and absorption by aerosols and clouds), and reflections from the ground. It is therefore
dependent on the
wavelength of the light, on the altitude and on specific local environmental conditions. The
actinic flux has borne many names (e.g. flux, flux density, beam irradiance
actinic irradiance, integrated intensity) which has caused some confusion. It is important
to distinguish the
actinic flux from the
spectral irradiance, which refers to energy arrival on a flat surface having fixed spatial orientation
(
)
given by:
.
The
actinic flux does not refer to any specific orientation because molecules are oriented randomly
in the atmosphere. This distinction is of practical relevance: the
actinic flux (and therefore a
j-value) near a brightly reflecting surface (e.g. over snow or above a thick cloud)
can be a factor of three higher than that near a non-reflecting surface. The more
descriptive name of
spectral spheradiance is suggested for the quantity herein called
actinic flux.
Source:
PAC, 1990, 62, 2167
(Glossary of atmospheric chemistry terms (Recommendations 1990))
on page 2170
