The indirect flux method is used for emission assessments of gases that cannot be directly measured by (SOF or SkyDOAS or other sensors). Indirect flux relies on measured gas-ratios in the same plume as quantified by SOF. The mass concentrations (mg/m3) are measured downwind, usually with a mobile monitor (MeFTIR or MeDOAS). From the plume integrated mass ratio of the “tracer-gas” (measured by SOF) and the “target-gas”, the emission rate of the “target-gas” can be retrieved. The method assumes that the target gas and the tracer gas have the same origin and are dispersed similarly. Parallel measurements of SOF and MeFTIR/MeDOAS are advantageous.