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. A fast-growing area of research is the development of low-cost sensors formeasuring air pollutants. The affordability and size of low-cost particlesensors makes them an attractive option for use in experiments requiring anumber of instruments such as high-density spatial mapping. However, forthese low-cost sensors to be useful for these types of studies their accuracyand precision need to be quantified. We evaluated the Alphasense OPC-N2, apromising low-cost miniature optical particle counter, for monitoring ambientairborne particles at typical urban background sites in the UK. The precisionof the OPC-N2 was assessed by co-locating 14 instruments at a site toinvestigate the variation in measured concentrations. Comparison to twodifferent reference optical particle counters as well as a TEOM-FDMS enabledthe accuracy of the OPC-N2 to be evaluated. Comparison of the OPC-N2 to thereference optical instruments shows some limitations for measuring massconcentrations of PM 1 , PM 2.5 and PM 10 . The OPC-N2 demonstrateda significant positive artefact in measured particle mass during times ofhigh ambient RH (> 85 %) and a calibration factor wasdeveloped based upon κ -Kohler theory, using average bulk particleaerosol hygroscopicity. Application of this RH correction factor resulted inthe OPC-N2 measurements being within 33 % of the TEOM-FDMS, comparable tothe agreement between a reference optical particle counter and the TEOM-FDMS(20 %). Inter-unit precision for the 14 OPC-N2 sensors of22  ±  13 % for PM 10 mass concentrations was observed. Overall,the OPC-N2 was found to accurately measure ambient airborne particle massconcentration provided they are (i) correctly calibrated and (ii) correctedfor ambient RH. The level of precision demonstrated between multipleOPC-N2s suggests that they would besuitable devices for applicationswhere the spatial variability in particle concentration was to be determined.

Evaluation of a low-cost optical particle counter (Alphasense OPC-N2) for ambient air monitoring