Chemical branching fraction ($f_j=k_j/{\sum }_l{k}_l$)

For competing irreversible pseudo-first-order channels with rates $k_j$, the fraction following channel $j$ is $f_j=k_j/{\sum }_l{k}_l$ when those conditions stay fixed during the intermediate's lifetime. A photochemical destruction rate must count only branches that actually remove the chosen species after fast recycling. For excited oxygen, collisional deactivation followed by ozone reformation is different from reaction with water to make hydroxyl radicals.