Abstract: This invention is directed to an air purification method capable of removing small aerosols and a wide size range of aerosols, with minimum aerosol radius smaller than 0.01 micron. The method humidifies the air in an enclosed space and produces supersaturation at a level of at least 12 percent in at least a portion of the enclosed space so that water vapor condenses on aerosols to form water droplets. The method also coalesces the water droplets formed by supersaturation using coalescence devices that can coalesce droplets with minimum radius smaller than 0.01 micron, thereby lowering the equilibrium supersaturation, enabling water droplets to grow faster, be more stable and easier to be removed compared to when coalescence is absent. The method dehumidifies the air so that the water droplets with captured aerosols are removed. Coalescence enables droplets to grow under a lower ambient supersaturation level, thus saving a considerable amount of energy.