In reaction equations, why are j and k sometimes capitalized to J and K?
The entry in Wikipedia misses to define the entries in the formula.  But note equations 1 and 2 in a paper by Griffin et al. (doi 10.1029/2006JD007604, open access) in the Journal of Geopphysical Research, Atmospheres. To establish context of the paper by Griffin et al., the authors introduce the reader to the topic citing earlier work.

«In the troposphere, a molecule of $\ce{NO2}$ is degraded photochemically to form a molecule of $\ce{NO}$ and an oxygen atom ($\ce{O}$). It should be noted that this process depends on the sufficient flux of photons associated with light of the appropriate wavelength to photolyze $\ce{NO2}$. The $\ce{O}$ atom quickly reacts with molecular oxygen ($\ce{O2}$) in the presence of a third body to form $\ce{O3}$. However, the $\ce{O3}$ formed can react rapidly with the $\ce{NO}$ to reform $\ce{NO2}$ and $\ce{O2}$. The net result of these reactions is a null cycle, and when steady state is assumed, the $\ce{O3}$ concentration can be predicted as [Seinfeld and Pandis, 1998]
$$ [\ce{O3}] = \frac{j_\ce{NO2} [\ce{NO2}]} {k_1 [\ce{NO}]}$$
where the bracket notation represents a concentration or mixing ratio (here, parts per billion by volume, ppbv), $j_\ce{NO2}$ ($\pu{s^{-1}}$) is the photolysis rate of $\ce{NO2}$, and $k_1$ ($\pu{ppbv^{-1} s^{-1}}$) is the temperature-dependent rate constant for the reaction between $\ce{NO}$ and $\ce{O3}$.»

The article continues by

«This is referred to as the photostationary state for $\ce{O3}$. If each side of equation (1) is divided by $[\ce{O3}]$, the right hand side is termed the Leighton ratio, $\Phi$ [Leighton, 1961]:
$$\Phi = \frac{j_\ce{NO2} [\ce{NO2}]} {k_1 [\ce{NO}] [\ce{O3}]} $$
[...]»