Schematic presentation of the various chemical pathways of production and destruction of [CI], [OI], and [NI] metastable states in the cometary coma. Solid arrows, dashed arrows, and solid curved arrows represent the production channels of the excited species via photon and electron impact dissociative excitation of cometary neutrals, loss of excited species via radiative decay, and loss of excited species due to collisional quenching with neutrals, respectively. In the non-collisional region of the coma, all these photochemical reactions, except the collisional quenching reactions (curved arrows), determine the intensities of forbidden emission lines.