The Reactive Plume Model (RPM-IV) is a photochemical plume-segment model that simulates the evolution of a photochemical plume by representing the plume as a series of cells across the horizontal extent of the plume. The RPM is a Lagrangian model that represents an air parcel of pollutants as it moves downwind, simulating the entrainment of ambient (background) air and the resulting chemical transformations within the plume. Each run of the RPM simulates one air parcel trajectory as it travels downwind. The RPM estimates short-term concentrations of primary pollutants (reactive or inert emissions) and secondary pollutants (those formed within point-source plumes). Different chemical mechanisms can be used within and outside the plume in different model runs, such as first-order reaction rate approximations to nonlinear chemical systems, different levels of condensation of a master mechanism, or different representations of chemical reactions (e.g., alternative reaction rate constants).