MOBILE EXTRACTIVE DIFFERENTIAL ABSORPTION SPECTROSCOPY (MeDOAS)
The Mobile extractive Differential Optical Absorption Spectroscopy (MeDOAS) method is used to map the gas concentrations (ppb or mg/m3) at ground level from various sources. It is able to efficiently screen large areas for hotspot or leaks. The MeDOAS technique is essentially an internal lamp, a multi-reflection cell and an UV-spectrometer. MeDOAS monitors UV-radiation over a broad spectral IR region for the detection and quantification of BTEX (Benzene, Toluene, Ethylbenzene, Xylene) and other gases inside the cell.
MeDDAS optical cell
| Physical Basis of measurements | Measurement of local gas concentration inside a reflection cell, combined with local wind data and positional data. |
| Main Purpose | Concentration mapping and leak search. Emission quantification if used with the Tracer Correlation method or Indirect Flux. |
| Spatial information -Scale -Resolution | From the scale of industrial conglomerates (50 km by 50 km) down to sections of a site (50 m by 50 m) or isolated single sources. 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 in extracted air (mg/m3) and (ppb) above reference/start value. |
| Secondary Quantities (units) | 1) Mass concentration ratio of ground plume combined with SOF and MeFTIR gives Indirect Flux [kg/h] and plume height information [m] 2) Mass flux [kg/h] via Tracer Correlation method. |
| Complementary data | Geolocation of the measurement vehicle (latitude, longitude). Local wind direction (deg N) and wind speed (m/s) are needed for leak search. |
| Compounds measured | Target species are BTEX (Benzene, Toluene, Ethylbenzene and Xylene) but also other gases including; Phenol, Paracresol, Trimethylbenzene, Furfural, Naphtalene, Styrene and SO2 are detectable at significant concentrations. |
| Detection limits (conc) | The detection limits are species dependent but typically in the 1-20 ppb range. |
| Typical range (conc) | From detection limit to saturation (no defined upper limit) |
| Time resolution/ sampling frequency | Sampling frequency varies between measurement modes/species in the range: 1 s – 10 s |
| Typical uncertainties (ppb) or (mg/m3) | 10 % |
