First and foremost, IAQM Position Statement touched on the very important point related to accuracy of measurements when using small sensors for carrying out professional assessments.
Airly is using the principle of light scattering, functioning as an Optical Particle Counter (OPC). Here is a brief overview of the process:
- An internal fan draws an air sample through a measurement chamber.
- A focused laser beam illuminates the particles within this air stream.
- As each individual particle passes through the beam, it scatters the laser light.
- A photodetector measures the intensity and pattern of this scattered light.
This data is used to determine the particle’s size (aerodynamic diameter) and to count the number of particles in different size “bins”. The mass concentrations (PM1, PM2.5, PM10) are then calculated based on this particle count and size distribution. This calculation requires assuming a particle density and a spherical shape. This method is exceptionally accurate for fine and ultrafine particles (PM1 and PM2.5), which are typically more spherical and have a more uniform density.
However, its accuracy for the coarse fraction of PM10 (particles with diameters between 2.5μm and 10μm) can be lower. Coarse particles, especially from mechanical sources like construction and trenching (e.g., mineral dust, soil), are often irregularly shaped and have varying densities. This can lead to an underestimation of their mass by light-scattering OPCs.
We believe it’s worth mentioning that:
- Airly instruments are MCERTS certified for indicative monitoring for PM10, which confirms Airly meets a required standard of performance. Certification documents show that the Airly sensor has a lower Measurement Uncertainty for PM10 at 21.5%, compared to other suppliers (Aeroqual – 23.5%, Bettair (no corrected) – 142.4%, Earthsense – 24.3%, EMSOL – 32.2%, AQ Mesh with heated inlet – 22.5%, Kunak Air Pro (no corrected) – 81.1%, Met One – 29.2%, Oizom – 48.8%, DustTrack – 34.2%, Osiris from Turnkey – 29.2%). Note that some companies claiming MCERTS they’re not tested for PM10. All certificates are available here.
- Airly sensors do not use any humidity-related correction which can remove peaks when measuring PM10. Additionally Airly confirms that there are no algorithms to remove peaks as part of cleaning the ‘noise’ in the data, or add additional equipment (as described by IAQM). Airly provides signal from the sensor with slope correction applied after 2-weeks calibration process in our office (against GRIMM EDM-180 – certified reference analysers that meet the Fixed Measurements criteria outlined in Directive (EU) 2024/2881 of the European Parliament on ambient air quality.
- The air sample in Airly sensors is conditioned before measurement by heating the air by 5°C relative to the ambient temperature using LED lights. The slightly elevated temperature (>=5°C) ensures that the dew point is not exceeded (condensation does not occur). The special design of the housing and the air inlet, as well as the heated air intake path with a special mesh that condenses water vapor on its surface, eliminates the influence of air humidity on the overestimation of PM10 concentration measurements.
- Based on a few years of co-locating sensors against reference instruments, we can confirm that our systems are capable of measuring PM10 above recommended site action level (190 µg/m3).
- As recommended by IAQM – any variability and rapid increases in PM2.5 and PM10 concentrations should be investigated in the context of the adjusted (lowered) site-specific action levels appropriate to the LCS system. Comparing upwind and downwind monitoring location data can help determine whether the observed increases are attributable to site activities or to regional or background sources.
