Performance of low-cost sensors to measure PM10: do they also measure coarse particles?

Low-cost sensor can be key additional tools to fixed air quality monitoring stations (AQMS)for more extended AQ assessment. Sensors can be deployed at a higher density due to their lower cost. However, the data quality of sensors is still unknown and can be function of location and meteorological conditions.

One of the issues that PM sensors are dealing with is the ability to measure coarse fractions of PM. It is known that some low-cost sensors calculate PM₁₀ concentrations based on the measured concentrations of PM_2.5. The main issue with the evaluation of PM sensors for PM₁₀ in the field, is that PM_10-2.5 fractions at most AQMS are relatively small, and relying on only field test for PM₁₀ would not identify the problem, whereas the sensor would largely underestimate the PM₁₀ concentrations when deployed at areas or specific events with high coarse fractions.

Coarse particles are defined here as PM_10-2.5 = PM₁₀ – PM_2.5. A laboratory test was developed for PM sensors (as part of the CEN/TS 17660-2:2024) and will evaluate the potential of the PM₁₀ sensor system to correctly measure the coarse fraction. This presentation presents the lab test to evaluate if sensor systems can measure also coarse PM fractions and can measure PM₁₀ rather than calculating it from the PM_2.5 signal.

The sensor systems under test are placed in the test chamber in close vicinity of the optical monitor (as equivalent method) and particles are generated and mixed so that sensor systems and optical monitor are exposed to the same PM concentrations and fractions.

The sensor systems are exposed to two different PM fractions over two tests (‘Coarse test’ and ‘Fine test’) to evaluate their response to PM. The size fractions generated inside the test chamber will fulfil the following requirements:

• Coarse test: >80% PM_10-2.5 in PM₁₀
• Fine test: <20% PM_10-2.5 in PM₁₀

For each test, a monodisperse aerosol with respectively coarse and fine size restrictions (and with the same composition) is used to generate these conditions. Based on these tests, the sensor response (ratio of the output of the sensor versus the equivalent method) is calculated for PM_10-2.5 (coarse test) and PM_2,5 (fine test). When the sensor measures the PM₁₀ concentration it is assumed that the sensor response will not change significantly between the two conditions. An example to illustrate this approach will be given using the AirsensEUR (version 3.0) sensor system. The AirsensEUR sensor system has two sensors included.  Laboratory test results will also be compared to field observations for this sensor system; data collected as part of the sensEURcity project.