j-value 

in atmospheric chemistry
https://doi.org/10.1351/goldbook.J03359
An effective first order rate constant for a photochemical reaction (photodecomposition, etc.) of light-absorbing impurities in the atmosphere.
Source:
PAC, 1990, 62, 2167. (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2198 [Terms] [Paper]