## actinic flux, $$S_{\lambda}$$

https://doi.org/10.1351/goldbook.A00086
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 @S05824@ $$L\left (\lambda,\,\theta,\,\varphi \right )$$ overall 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 @R05037@ 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 @W06659@ $$\lambda$$, the @AT07314@ 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 @AT07314@ flux is determined by the solar radiation entering the atmosphere and by any changes in this due to atmospheric gases and particles (e.g. @R05160@ absorption by stratospheric ozone, @S05487@ and absorption by aerosols and clouds), and reflections from the ground. It is therefore dependent on the @W06659@ of the light, on the altitude and on specific local environmental conditions. The @AT07314@ flux has borne many names (e.g. flux, flux density, beam irradiance @AT07314@ irradiance, integrated intensity) which has caused some confusion. It is important to distinguish the @AT07314@ flux from the @S05817@, which refers to energy arrival on a flat surface having fixed spatial orientation ($$\text{J m}^{-2}\ \text{nm}^{-1}$$) given by: The @AT07314@ flux does not refer to any specific orientation because molecules are oriented randomly in the atmosphere. This distinction is of practical relevance: $E(\lambda) = \int _{\theta}\, \int _{\phi} L\left (\lambda,\theta,\varphi \right )\, \text{cos}\,\theta \: \text{sin}\,\theta\: \text{d}\theta\: \text{d}\varphi$ the @AT07314@ 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 @S05832@ is suggested for the quantity herein called @AT07314@ flux.