This article focuses on the mechanism and molecular dynamics of light-initiated atmospheric reactions. The availability of photons of the correct wavelengths, the response to collisions, and the effect of heterogeneous environments will be considered in this discussion. A new class of light-initiated reactions occurring in the ground electronic state by concerted mechanisms with significant rates are highlighted with examples such as sulfuric acid dehydration and pyruvic acid decarboxylation. The unprecedented mechanism investigated in pyruvic acid initiates reaction by hydrogen atom chattering on very fast time scales. The dynamics suggests significant consequences for this chemistry in photoprocessing acids and alcohols in the atmosphere. An important observation is that water may potentially affect these reactions' energetics, rates, and mechanisms. Examples from our work on light-initiated reactions of atmospheric targets in the gas phase and as affected by water are discussed to point to future areas for physical chemistry research in this field.