Passive area sources usually and area have no defined flow or a weak flow. Examples include a waste dump, clarifier or pond. According to the VDI3880, a passive source has an average flow velocity over the source area as less than 30m/h. According to the VDI3880, the number of measurement points for a homogeneous passive area source with up to a ground area of 100m2 is at least three points, and with up to 1000m2 is at least five points.
Essentially, two methods for measuring odour concentration exist: micro meteorological conditions and using ‘hoods’ to cover an area of the source for sampling. A static flux hood or wind tunnel can be used to sample passive sources. Both devices use odourless neutral gas to create gas movement from the passive source surface. The difference between both is the air flow dynamics and the sweep/ventilation flow rate.