The mass emission (kg/h), also denoted mass flux, from a source can be measured by multiplying integrated atmospheric mass columns (mg/m^{2}) 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.

# Direct Flux

Q_{N}^{j}[kg/s]= \bar{v}_{\tau}[m/s]\cdot\int\limits_{P}CD_{l}^j[kg/m^{2}]\cdot cos(\theta_{l})\cdot sin(\alpha_{1})dl[m] \\
Where,

\bar{v}_{T} = \textrm{the average wind speed at plume height for the transect},

CD_{l}^{j} = \textrm{the measured column densities for the species j.}\\ \textrm{SOF measures slant column and SkyDOAS vertical columns}

\theta_{l}=\textrm{the angles of the light path from zenith (} cos(\theta_{l})\textrm{ gives vertical columns).}\\ \theta_{l}=0 \textrm{ for SkyDOAS.}

\alpha_{l} = \textrm{the angles between the wind directions and driving directions}

dl = \textrm{the driving distance across the plume}