Effect of aerosol composition on the performance of low-cost optical particle counter correction factors
Authors: Crilley Leigh R. , Singh Ajit , Kramer Louisa J. , Shaw Marvin D. , Alam Mohammed S. , Apte Joshua S. , Bloss William J. , Hildebrandt Ruiz Lea , Fu Pingqing , Fu Weiqi , Gani Shahzad , Gatari Michael , Ilyinskaya Evgenia , Lewis Alastair C. , Ng'ang'a David , Sun Yele , Whitty Rachel C. W. , Yue Siyao , Young Stuart , Pope Francis D. 
Published: March 2020

Abstract. There is considerable interest in using low-cost optical
particle counters (OPCs) to supplement existing routine air quality networks
that monitor particle mass concentrations. In order to do this, low-cost OPC
data need to be comparable with particle mass reference
instrumentation; however, there is currently no widely agreed upon methodology to accomplish this. Aerosol
hygroscopicity is known to be a key parameter to consider when correcting
particle mass concentrations derived from low-cost OPCs, particularly at
high ambient relative humidity (RH). Correction factors have been developed
that apply κ-Köhler theory to correct for the influence of water
uptake by hygroscopic aerosols. We have used datasets of co-located
reference particle measurements and low-cost OPC (OPC-N2, Alphasense) measurements, collected in four cities on three continents, to explore the performance of
this correction factor. We provide evidence that the elevated particle mass
concentrations, reported by the low-cost OPC relative to reference
instrumentation, are due to bulk aerosol hygroscopicity under different RH
conditions, which is determined by aerosol composition and, in particular, the
levels of hygroscopic aerosols (sulfate and nitrate). We exploit
measurements made in volcanic plumes in Nicaragua, which are predominantly
composed of sulfate aerosol, as a natural experiment to demonstrate this
behaviour in the ambient atmosphere; the observed humidogram from these measurements closely
resembles the calculated pure sulfuric acid humidogram. The results
indicate that the particle mass concentrations derived from low-cost OPCs
during periods of high RH (>60 %) need to be corrected for
aerosol hygroscopic growth. We employed a correction factor based on κ-Köhler theory and observed that the corrected OPC-N2 PM2.5 mass
concentrations were within 33 % of reference measurements at all sites.
The results indicated that a κ value derived in situ (using suitable reference
instrumentation) would lead to the most accurate correction relative to
co-located reference instruments. Applying a κ values from the literature in the
correction factor also resulted in improved OPC-N2 performance, with the measurements being within 50 % of the reference values. Therefore, for areas where suitable reference
instrumentation for developing a local correction factor is lacking, using a
literature κ value can result in a reasonable correction. For
locations with low levels of hygroscopic aerosols and low RH values, a simple
calibration against gravimetric measurements (using suitable reference
instrumentation) would likely be sufficient. Whilst this study generated
correction factors specific for the Alphasense OPC-N2 sensor, the
calibration methodology developed is likely amenable to other low-cost PM
sensors.