Effective measurement techniques are required to assess the environmental impacts of various emission sources in industry and agriculature. Open-path Fourier-transform-infrared- spectroscopy is an efficient method especially for diffuse source monitoring. Based on measurments with the high-resolution K300 spectrometer operating inside a mobile laboratory for air pollution remote sensing, path-averaged concentrations of CO2, N2O, CH4, NH3, CO, NO, and HCNO in extended area plumes are retrieved. The analysis software uses radiative transfer line-by-line calculations and least-square fit procedures. Emission rates are calculated from these values and simultaneously recorded meteorological data using a Gaussian plume dispersion model. The methodology is discussed and examples are given for emission rate estimates including industrial and agriculatural areas, highways, waste disposal sites, and forest fires.