The mass emission (kg/h), also denoted mass flux, from a source can be measured by multiplying integrated atmospheric mass columns (mg/m2) along the plume by the orthogonal plume wind speed (m/s). The method is used to quantify gas emissions of sources at a large range of spatial scales but also for concentration/column mapping and leak search. A wide range of target gases can be measured depending on the spectral region and resolution of the sensor. Flux measurements are carried out at both the fence line and inside the industrial plants with no disruption to normal operational activities. Direct flux is measured by SOF and SkyDOAS instrument methods.