Solar Occultation Flux (SOF)
The Solar Occultation Flux (SOF) method is used to map and quantify gas emissions from the large industrial conglomerates down to individual sources.
The method is able to efficiently screen large areas of the site and identify significant sources. The SOF technique is dependent on direct sunlight. SOF monitors solar radiation over a broad spectral IR region for the detection and quantification of emissions of the specified gases. The SOF technique uses IR spectrometry to analyze sunlight passing through the atmosphere and detected from a mobile system. By driving downwind of the sources, and intersecting the plume with the measurement path, an integrated concentration profile is obtained. The measurements are generally carried out by measuring around the perimeter of the emission sources, making it possible to subtract the upwind component from the downwind measurement after combining the respective path integrated concentrations with wind information to determine gas fluxes.
SOF is patented by FluxSense and is Best Available Technology in Europe for refineries and chemical industry. The method is included in the European standard EN 17628 which will be available in April 2022.
| Physical Basis of measurements | Measurement of slant gas column data combined with local wind data and positional data to derive the emission flux from the sources. |
| Main Purpose | Detection and quantification of point source and diffuse area-specific emissions and total site emissions. Localization of the sources. |
| 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) | Integrated slant column mass (mg/m2) above reference/start value. |
| Secondary Quantities (units) | Emission flux (kg/h) and geolocated (lat/lon) vertical columns (mg/m2). |
| Complementary data | Geolocation of the measurement vehicle (latitude, longitude). Local wind direction (deg N) and wind speed (m/s). Optional; mass ratio of additional gases (mg/m3) to total alkanes (mg/m3) for indirect emission measurements of complementary gas species. |
| Compounds measured | Simultaneous measurement of a wide range of VOC compounds including alkanes (C2-C20), alcohols (C1-C8), alkenes (C2-C4), amines, dienes, aldehydes and vinyl chloride. Other, non-VOCs, include NH3, CO, HCl and HF. |
| Spatial resolution | The method is able to locate the emission from an isolated source, even in the presence of background sources, as long as the source is dominating and can be encircled. For compact and complex units, different sources can be separated and located by measurements in different wind directions and the measured plume composition, typically with a resolution between 20 m to 50 m. This is highly dependent on perimeter access, source strength, background emissions and source type. |
| Detection limits (flux and/or concentration) | The flux detection limit values of a typical SOF measurement is about 1 kg/h for near-field measurements of an isolated source. However, the actual sensitivity varies with compound, wind speed, plume dispersion and distance to the source. Typical slant column detection limits: 1-5 mg/m2 depending on species |
| Typical range | Flux: 1 kg/h to several tons per hour (no defined upper limit) Slant column: 1 mg/m2 to around 2 000 mg/m2 |
| Time resolution/sampling frequency | Sampling frequency of column measurements: 1 s – 5 s. The time resolution of a complete emission measurement varies from 10 min to 1 h depending on the physical size of the source, repeated over 3 days. |
| Typical expanded uncertainty in emission rate | 20 % to 40 % |
