Passive sources are area sources that typically have no defined flow or a weak flow. Examples include a waste dump, clarifier or even a pond.  According to VDI3880, a passive source has an average flow velocity over the source area as less than 30m/h. 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.

How to Measure Passive Sources

Essentially, two methods for measuring odour concentration exist: micrometeorological 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 airflow dynamics and the sweep/ventilation flow rate.

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An active source means to have a defined gas flow emitted from a source. As an example, both a biofilter or an aerated pond are both active sources. According to VDI3880, an active source is defined by an average flow velocity of 30m/h or greater over the area of the source. Active sources can be sampled using a sampling hood.

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