[en] CHARM is a computer model for the behavior of a one component aerosol in a single region with time-varying external conditions. It treats particle agglomeration due to Brownian motion, gravity, turbulence, thermophoresis and diffusiophoresis. Turbulence properties are estimated for flow through a region of arbitrary cross-sectional shape; with aerodynamically rough or smooth walls at any Reynolds number. The gas can be of any composition. The time-varying external conditions allowed for are the temperature, pressure and velocity of the gas, wall temperatures, and the rate, mass median radius and geometric standard deviation of the source. The model is simply modified to enable this list to be extended if needed. A new method of solving the governing equations, based on the finite element collocation method, enables the time-varying conditions to be treated accurately and economically. The models, the numerical methods, the execution of the computer code (including how to write the input data file and interpret results), and how to make simple modifications to the model are described in detail. Model implementation as a submodel of a larger one and what further work needs to be done to enable it to efficiently treat multicomponent aerosols, and condensation onto and evaporation from particles are also discussed