1. A method for determining atmospheric profiles using a physical retrieval algorithm, the method comprising the steps of: (a) building a fast monochromatic radiative transfer model (RTM), wherein the fast monochromatic RTM is limited to a response of a theoretical instrument; (b) applying atmospheric data to the fast monochromatic RTM to obtain fast monochromatic atmospheric radiance; (c) converting directly observed instrument channel radiance into a pseudo monochromatic radiance by (i) determining principal component amplitudes of the directly observed instrument channel radiance, and (ii) transforming the principal component amplitudes determined in step (i) into pseudo-monochromatic radiance by applying a transformation matrix; (d) applying the pseudo-monochromatic radiance obtained in step (c) and the fast monochromatic radiance obtained in step (b) to the physical retrieval algorithm; and (e) determining the atmospheric profiles using the physical retrieval algorithm in response to step (d).