Photolysis (or photo dissociation) of trace gases initiates most chemical reactions that take place in the atmosphere. Photolysis splits gas-phase chemical species with energy from sunlight. Photolysis is involved in the formation of smog, an air pollution problem that affects human, animal, and plant health. Simulating photochemical reactions accurately is therefore a key issue that strongly influences air quality model performance.

CCTM uses state-of-the-science techniques to simulate photolytic reactions in the Phot module. Photolysis reactions and their rates of reaction are driven by sunlight. Similar to kinetic reaction rates for non-photochemical reactions, the photolysis rate quantifies how much reactant is produced from a photolytic reaction in a given amount of time. The rate of photolysis is a function of the amount of solar radiation (called actinic flux), which varies based on the time of day, season, latitude, and terrestrial features. The amount of solar radiation is also affected by the amount of cloudiness and by aerosol absorption and scattering in the atmosphere. The photolysis rate also depends on species-specific molecular properties like the absorption cross-section (the effective molecular area of a particular species when absorbing solar radiation, which results in a shadow region behind the particle) and quantum yield (the number of molecules that dissociate for each light photon incident on the atmosphere). These molecular properties depend on the wavelength of the incident radiation and the temperature (and hence, on the available photon energy). Thus, estimating the photolysis rate is further complicated by these temperature and wavelength dependencies. As discussed in Section 2.2.3, the CMAQ modeling system includes an advanced photolysis model (JPROC) to calculate temporally varying photolysis rates for use in simulating photolysis in CCTM.