Evaluation of a low-cost optical particle counter (Alphasense OPC-N2) for ambient air monitoring
Abstract. A fast-growing area of research is the development of low-cost sensors for
measuring air pollutants. The affordability and size of low-cost particle
sensors makes them an attractive option for use in experiments requiring a
number of instruments such as high-density spatial mapping. However, for
these low-cost sensors to be useful for these types of studies their accuracy
and precision need to be quantified. We evaluated the Alphasense OPC-N2, a
promising low-cost miniature optical particle counter, for monitoring ambient
airborne particles at typical urban background sites in the UK. The precision
of the OPC-N2 was assessed by co-locating 14 instruments at a site to
investigate the variation in measured concentrations. Comparison to two
different reference optical particle counters as well as a TEOM-FDMS enabled
the accuracy of the OPC-N2 to be evaluated. Comparison of the OPC-N2 to the
reference optical instruments shows some limitations for measuring mass
concentrations of PM 1 , PM 2.5 and PM 10 . The OPC-N2 demonstrated
a significant positive artefact in measured particle mass during times of
high ambient RH (> 85 %) and a calibration factor was
developed based upon κ -Kohler theory, using average bulk particle
aerosol hygroscopicity. Application of this RH correction factor resulted in
the OPC-N2 measurements being within 33 % of the TEOM-FDMS, comparable to
the agreement between a reference optical particle counter and the TEOM-FDMS
(20 %). Inter-unit precision for the 14 OPC-N2 sensors of
22  ±  13 % for PM 10 mass concentrations was observed. Overall,
the OPC-N2 was found to accurately measure ambient airborne particle mass
concentration provided they are (i) correctly calibrated and (ii) corrected
for ambient RH. The level of precision demonstrated between multiple
OPC-N2s suggests that they would be
suitable devices for applications
where the spatial variability in particle concentration was to be determined.