Table 1 AQ sensor system data processing classification (adapted from Schneider et al.17), differentiating independent and non-independent measurements, as well as predictions
Levela | Main signal | Correctionsb | Signals provenance | Model architecturec | ISM? | Definitione | |
|---|---|---|---|---|---|---|---|
0 | raw signal | --------- | --------- | --------- | --------- | --------- | Output signal produced by the sensing element (e.g., voltage, current, resistance, etc.), representing the raw, unprocessed data. |
1 | intermediate geophysical quantities | Pollutant of interest (on-line only) | No compensation | --------- | Independent of local data | Yes | Independent measurements derived from Level 0 data, without compensation schemes, mostly using—but not constrained to—basic physical principles or simple calibration equations to obtain the measurand. |
2A | standard geophysical quantities | Pollutant of interest (on-line only) | Demonstrated artefact (on-line only) | Internal to the system | Independent of local data | Yes | Independent measurements derived from Level 0 or Level 1, using on-board, on-line parameters for demonstrated artefact correction, and basic physical principles or simple calibration equations. More complex calibration models can be used, however on-site model building and/or re-training with local environmental data is not allowed. |
Independent / Non-Independent measurement boundary | |||||||
2B | standard geophysical quantities (extended B) | Pollutant of interest (on-line only) | Demonstrated artefact (on-line only) | Internal to the system | Requires model building/re-training using local data | No | Non-independent measurements derived from Level 0, combined with on-board on-line parameters for artefact correction. Level-1 or level-2 data may also be used as an input. These measurements rely on mostly complex algorithms developed for a specific context and/or requiring on-site re-training for final application (i.e., relying on local co-located data for calibration). |
2C | standard geophysical quantities (extended C) | Pollutant of interest (on-line only) | Demonstrated artefact (on-line only) | External to the systemd | Not constrained | No | Non-independent measurements generated using one or more external, on-line data sources for demonstrated artefact correction. These rely on simple or complex algorithms that may have been developed under various data contexts but do not necessarily require on-site re-training. Since these measurements depend on external-to-the-system signals, they remain non-independent. |
Measurement / Prediction boundary | |||||||
3 | advanced geophysical quantities | Not constrained | Unrelated to the measurement principle | Not constrained | Not constrained | No | Prediction generated using parameters (internal or external, on-line or off-line) unrelated to the measurement principle. Data products at this level may use simple or complex algorithms developed in various data contexts, which may or may not require on-site retraining. |
4 | spatially continuous geophysical quantities | Not constrained | Not constrained | Not constrained | Not constrained | No | Similar to Level 3 but providing a spatially continuous map. This data product may be derived from sensor networks alone or in combination with satellite data and/or model outputs (e.g., chemical transport models). |
