Photodissociation in the atmosphere: 1. Actinic flux and the effects of ground reflections and clouds

The actinic flux must be distinguished from other radiometric quantities such as the irradiance. This distinction shows that (1) a fraction of the recent atmospheric chemistry literature contains improperly calculated rates of photodissociation, and (2) photodissociation rate coefficients inside clouds, and particularly inside cloud droplets, can frequently exceed the clear‐sky values, in contrast to current usage in cloud chemistry models. Both of these findings are traceable to the ∼2 cos θ factor incurred in the actinic flux (but not in the irradiance) when the solar beam is scattered and diffused into nearly isotropic light.