Can we set performance targets for low cost sensors?
Presented by: John Saffell, Alphasesne Ltd.
Summary: Air quality (AQ) networks and personal monitors using low cost sensors are generating results at a rapidly expanding rate. But can these results be accepted as valid measurements? We can rephrase this question: do air quality sensors and sensor systems meet their performance targets?
We first review performance targets that have been set for gases, particles and VOCs. Targets can be based on national or international concentration limits, sensor technology capabilities, typical concentrations, limits of detection, or laboratory and field validation capabilities. Performance targets should also depend on the application: reference and equivalent measurements, fixed site urban networks, mobile monitoring, IAQ, personal exposure or citizen science. We consider how the total measurement error should reflect the application.
Users of low cost AQ sensors often request a performance certificate to a national or international standard; there are test standards for reference and equivalent analytical systems, but to date there are no test standards for low cost AQ sensors. CEN 264, Working Group 42 has been working since 2015 on a classification and validation performance standard for low cost gas and particle sensors. The first draft is available March 2020 and allows classification of low cost sensors from near-equivalence capability and simpler classification of citizen science AQ boxes. EDF and ASTM in North America are also working towards standards for AQ networks. We discuss progress.
Work with UNEP over the last years in Nairobi and other LMIC locations has reminded us of other issues that must be included when considering performance targets. The sensor system alone can be tested and validated in the lab, but field validation of performance must also include siting and deployment, sensor drift and regular field validation. A final consideration is the geographical location and diurnal and seasonal patterns which strongly affect field measurement quality.