A system is provided for determining emission rate of a gas from a source. The system includes a selector for designating an in-plume area extending across a plume of gas. The in-plume area includes a length dimension greater than a width dimension. The system also includes a processor for determining an area-integrated concentration value for the in-plume area. The system further includes a wind vector received by the processor. The processor determines the emission rate of the gas based on the area-integrated concentration value and a vector component of the wind vector.