Explore AQMesh

A few thoughts about the WHO’s report on AQ methods

21-Nov-2023Data validity | Hybrid networks | Modelling | Networks | WHO

A few thoughts about the WHO’s report on AQ methods

This a great report, full of common sense and helpful advice, applicable globally, particularly where air quality monitoring is currently limited. Limited by budgets, logistics, and many factors discussed so eloquently at the ASIC Ghana conference last month.

Central to the report is the position that “air quality monitoring is the first step for understanding a population’s exposure and taking action” and the report lays out clearly the monitoring methodologies available, from passive sampling to modelling.

Low cost sensors are put in their place – but very much given a place – offering the cheapest and simplest way to understand spatial and temporal air pollution variability. To the report’s plea that “ideally, every nation should have access to at least one reference-grade monitor” we would add “well-maintained”. It can be a huge challenge maintaining a reference station in many parts of the world – accessing calibration gases, skills, parts and budgets – but, as the report says, they do indeed open the door to many other air quality methods. We have been involved in many projects where our pods came out of the box and performed well, but that the project ‘failed’ because the objective was comparison with reference standard and that could not be achieved because the reference station could not be brought up to spec within the project timescale. Valuable real-time information, showing minute-by-minute, hourly, daily and weekly trends in air quality across a target area, can be discarded for this reason, which is a shame.

Multiple monitoring methods, or hybrid networks as we refer to them, used effectively, can most effectively achieve the baselining and measurement of progress discussed in the report. It was difficult to fully recognise the generic Low Cost Sensor described in the report, but that is not too surprising, given that the sector is so dynamic. Sources referred to, dated 2017, would be based on fieldwork from preceding years, and an awful lot has changed in Low Cost Sensor world in the last six years or so. Are they really low capital and medium operating costs? That may be true of some, but there are also slightly more expensive options with lower operating costs. We genuinely do not see drifting baselines, but that’s maybe because we have never rated metal oxide sensors. Also, since a step change in the electrochemical sensors making their way into the market in 2016, there has been continuous improvement in temperature correction of electrochemical output, and temperature effects did look like drift over certain time periods.

We would certainly agree with the comment that low cost sensors are “not yet suitable for replacing reference-grade monitors” but the remote calibration techniques and diagnostics offered by the latest generation of systems goes a long way to address the highlighted need for data quality assurance and quality control.

AQMesh complements air quality modelling, reference equipment and indicative data sources

15-May-2017Modelling | Performance | Product

AQMesh complements air quality modelling, reference equipment and indicative data sources

Leading small sensor air quality monitor, AQMesh, has recently been shown to work alongside passive samplers and air quality models, as well as complementing reference station networks. A recent study shows AQMesh calibration against diffusion tubes and two 2017 conferences have highlighted the potential of such systems used in an air quality network.

At the Dispersion Modellers User Group Meeting (DMUG, London, April 2017), CERC showed that the model optimisation of 7-day average NO2 concentrations using AQMesh readings as well as reference data affects concentration contours, giving a general reduction, but increase in some areas. The study shows the potential to achieve a higher level of local air quality accuracy from this combined approach.

The CERC study uses near real-time NO2 data from 20 AQMesh pods and 5 reference stations across Cambridge, UK over three months. The presentation acknowledged the potential from large networks of low cost sensors installed across a city: accuracy and reliability is generally lower than reference monitors, but larger spatial coverage is possible. The study addressed how these sensor data can best be used in modelling.

See the full presentation here.

It is important to distinguish between small air quality sensors and sensor systems. The best sensor systems offer optimisation of sensor output through quality control, platform design and sensor output processing and correction for cross-gas effects and environmental effects. Teams which want to invest in similar air quality studies or city-wide monitoring projects can get a head start by using AQMesh – the most developed such system.

See the Cambridge case study of latest AQMesh performance.

International air quality experts meeting at RIVM in the Netherlands in February reviewed a range of studies using small sensor systems to measure air pollution, particularly NO2 and particulate matter. Whilst highlighting the need for good characterisation of sensors and the pace of development of this new technology, discussions focused on how to make the best use of ‘low cost’ sensors.

Two speakers highlighted the ways in which a small sensor system can be characterised. The best method is by regularly co-locating with a reference station and comparing readings; another method is similar but one sensor system is co-located against reference and then moved around the other units to allow comparison. Other options include comparison with passive samplers or by comparing co-located sensor systems against each other.

AQMesh NO2, PM10 and PM2.5 readings from the Citi-Sense project (2015-16, AQMesh v3.5) – a superseded version of AQMesh without standard sensor quality control or characterisation – were compared with maps of reference data using data fusion and showed encouraging results. An even earlier AQMesh project in Asia (2014, AQMesh v3.0) compared source apportionment plots generated using reference data to those from co-located AQMesh pods and the conclusions to be drawn about pollution sources were the same.

AQMesh continues to build on years of global studies and continued development, offering the most ‘project-ready’ small sensor system which can be used in an air quality network – including reference stations, passive samplers and modelling – to proven effect.

AQMesh can also be scaled using diffusion tubes

27-Apr-2017Diffusion tubes | Modelling | Scaling

AQMesh can also be scaled using diffusion tubes

When three AQMesh pods and four NO2 diffusion tubes were co-located against a reference station, the diffusion tube readings were so consistent with the reference average reading for the period that the diffusion tube average could be used to apply a slope correction to AQMesh data.

By careful management of the co-location phasing it is possible to record sufficient data points to also correct any offset.

Passive samplers for key pollutants such as NO2 are widely available and are often used to reach areas not covered by more expensive and bulky reference stations. Small sensor systems or so-called ‘low cost’ sensor systems can help to fill the data gap between reference real-time readings, passive sampling single average measurements and model output. Calibration of small sensors systems, such as AQMesh, against certified and validated measurements provides a trail of data validity.