The Sky Differential Optical Absorption Spectroscopy
The Sky Differential Optical Absorption Spectroscopy (SkyDOAS) method is used to map and quantify gas emissions from the large industrial conglomerates down to individual sources. It is able to efficiently screen large areas of the site and identify significant sources. The SkyDOAS technique is dependent on zenith scattered sunlight (sky-light). SkyDOAS monitors solar radiation in the UV region for the detection and quantification of emissions of the specified gases. 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.
| 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 vertical 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 typical combustion gases including SO2, NO2 and HCHO |
| Detection limits (flux and/or concentration) | The flux detection limit values of a typical SkyDOAS measurement is about 1 kg/h for near-field measurements of an isolated main equipment. However, the actual sensitivity varies with compound, wind speed, plume dispersion and distance to the source. Typical vertical column detection limits: 1-5 mg/m2 depending on species |
| 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. |
| Typical range | Flux: 1 kg/h to several tons per hour (no defined upper limit) Vertical 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 (at least valid 4 transects) varies from 10 min to 1 h depending on the physical size of the source. |
| Typical expanded uncertainty in emission rate | 20 - 40 % |
