actinic flux, \(S_{\lambda}\)
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 @[email protected] \(L\left (\lambda,\,\theta,\,\varphi \right )\) over all directions of incidence of the light, \(E(\lambda) = \int _{\theta}\, \int _{\phi} L\left (\lambda,\theta,\varphi \right )\, \text{cos}\,\theta \: \text{sin}\,\theta\: \text{d}\theta\: \text{d}\varphi\). If the @[email protected] is expressed in \(\text{J m}^{-2}\ \text{s}^{-1}\ \text{st}^{-1}\ \text{nm}^{-1}\) and \(hc/\lambda\) is the energy per quantum of light of @[email protected] \(\lambda\), the @[email protected] flux has units of \(\text{quanta cm}^{-2}\ \text{s}^{-1}\ \text{nm}^{-1}\). 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 @[email protected] flux is determined by the solar radiation entering the atmosphere and by any changes in this due to atmospheric gases and particles (e.g. @[email protected] absorption by stratospheric ozone, @[email protected] and absorption by aerosols and clouds), and reflections from the ground. It is therefore dependent on the @[email protected] of the light, on the altitude and on specific local environmental conditions. The @[email protected] flux has borne many names (e.g. flux, flux density, beam irradiance, @[email protected] irradiance, integrated intensity) which has caused some confusion. It is important to distinguish the @[email protected] flux from the @[email protected], which refers to energy arrival on a flat surface having fixed spatial orientation (\(\text{J m}^{-2}\ \text{nm}^{-1}\)) given by: \[E(\lambda) = \int _{\theta}\, \int _{\phi} L\left (\lambda,\theta,\varphi \right )\, \text{cos}\,\theta \: \text{sin}\,\theta\: \text{d}\theta\: \text{d}\varphi\] The @[email protected] flux does not refer to any specific orientation because molecules are oriented randomly in the atmosphere. This distinction is of practical relevance: the @[email protected] 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 @[email protected] is suggested for the quantity herein called @[email protected] flux.
See also:
flux density
PAC, 1990, 62, 2167. (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2170 [Terms] [Paper]