 A second set of processes internal to a given volume of gas include particle formation (nucleation), evaporation, chemical reaction, and coagulation.A differential equation called the "Aerosol General Dynamic Equation" (GDE) characterizes the evolution of the number density of particles in an aerosol due to these processes.Change in time = Convective transport + brownian diffusion + gas-particle interactions + coagulation + migration by external forces

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