Tracer Correlation Method
The Tracer Correlation method is used for 2-dimensional mapping of concentrations and leak search on an industrial site and quantification of gas emissions from individual components, main equipment (e.g. tanks) and in some cases subareas on a site, if spatially separated. TC relies on the controlled release of a known rate of a tracer gas, for example C2H2 or N2O. The concentration of the tracer is measured downwind, usually with a mobile monitor, with simultaneous measurement of the source gas concentration. From the known release rate of the tracer gas (kg/h) and the measured mass concentration (mg/m3) in the emission plume of both he the tracer gas and the source gas, the emission rate of the source gas can be retrieved. The technique assumes the tracer is subject to the same dispersion and transport in the atmosphere as the source gas emissions. The tracer technique is typically used to quantify emissions from known or suspected sources (source areas), on a source by source basis.
Applying the TC method, the source flux is retrieved by integrating the cross-plume mass concentrations (above background baseline) of the source gas and tracer gas respectively. The mass concentration ratio together with the known tracer gas release rate gives the source emission.
The method is included in the European standard EN 17628 which will be available in April 2022.
|Physical Basis of measurements||Measurement of gas concentration data from source gas and tracer gas.|
|Purpose||Detection and quantification of specific emissions from components and smaller process units. Localization of the sources.|
|Spatial information -Scale -Resolution||From maximum working range of between 10 m to 2 km. Provides spatial concentration information in one dimension along the measurement path. 5 m to 25 m in the horizontal, dependent on the driving speed.|
|Measured Quantity (unit)||Local concentration of tracer gas and source gas in µg/m3 at intake.|
|Secondary Quantities (units)||Emission flux in kg/h and concentrations (lat/lon) in µg/m3.|
|Complementary data||Geolocation of the vehicle (latitude, longitude). Wind direction (deg N), for concentration mapping (source identification). Wind data not required for the flux retrieval.|
|Compounds measured||Various sensors are available to measure the concentration of source and tracer gas species. Infrared absorption spectrometers can for example be used to detect a wide range of VOC compounds including methane, other alkanes (C2-C20), alcohols (C2OH-C8OH), alkenes (C2-C4), amines, dienes, aldehydes, vinyl chloride and aromatic VOCs. Other, non-VOC species, include NH3, CO, HCl and HF. In the infrared, N2O and acetylene are suitable tracer gas species.|
|Detection limits (flux and/or concentration)||The flux detection limit values vary from 0,1 to 1 kg/h for an isolated source depending on compound, wind speed, plume dispersion and distance to the source. Detection limit requirement <20 µg/m3 for the source gas and <10 µg/m3 for the tracer gas.|
|Typical range||Flux: 0,1 kg/h to several tons per hour (no defined upper limit) Concentration: 1-1000 µg/m3|
|Time resolution/sampling frequency||Sampling frequency of concentration measurements: 1 s – 10 s The time resolution of a complete plume transect depends on the size of the source. An actual measurement consists of at least 4 successful plume transects that are averaged: Point sources: 1 min – 5 min, diffuse sources: 10 min – 20 min|
|Typical uncertainty in emission rate||Accuracy: 30 % – 40 % (emission, SOF) + 10% (ratio, MeFTIR/MeDOAS) Precision: 10 % – 30 % (emission, SOF) + 5% (ratio, MeFTIR/MeDOAS)|