The absorption of solar radiation by atmospheric species and the corresponding photoinduced processes, define the chemistry of a planet's atmosphere. While many of these photoinduced processes and their effects are well- known, even in Earth's atmosphere there are gaps in our understanding. The focus of this thesis is the theoretical study of the absorption of sunlight by some important atmospheric species where no or minimal experimental results are available. We simulate electronic absorption spectra of atmospherically relevant sulfur-containing molecules, including sulfuric acid, which is present in Earths' atmosphere and OSSO, which was recently identified in the atmosphere of Venus. We also simulate electronic absorption spectra for a series of N₂O complexes thought to be present in Earth's atmosphere, namely Ar-N₂O, N₂O-N₂, O₂-N₂O and H₂O-N₂O. We consider how complexation affects the absorption cross section of N₂O monomer in these complexes and also undertake photodissociation dynamics simulations for the H₂O-N₂O complex. Where possible, we compare and validate our theoretical results with experiment, and in the absence of experimental data, we suggest that our results may be of use to the atmospheric modeling community.