Using video-microscopic techniques, single layers (monolayers) of charged colloidal particles of 1- and 3-mum diameter were studied at an air-water interface. The particles normally arranged themselves in an ordered lattice-like array. However, occasionally, the monolayer formed loosely bonded crystallite clusters, here called mesostructures. The monolayer was exposed to a number of external influences: ions, electric current, and ultraviolet light. Depending on the perturbation applied, the lattice-like structure formed fractal aggregates, clusters (mesostructures), and two other particular structures, here referred to as striations and loops. The mechanisms of formation and implications of the different patterns are discussed.