Effect of aerosol composition on the performance of low-cost optical particle counter correction factors

Abstract

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.

Assessing the sources of particles at an urban background site using both regulatory instruments and low-cost sensors – a comparative study

“…The present study investigates the ability of low-cost sensors to provide measurements that can be used to identify the sources of pollution at a background site in Birmingham, UK, using clustering of particle size profiles. This method was successfully used in a number of previous studies, though with the use of measurements from research-grade instruments (Beddows et al, 2009(Beddows et al, , 2015Von Bismarck-Osten and Weber, 2014;Dall'Osto et al, 2011Sabaliauskas et al, 2013). To support the clustering method, chemical composition data from both research-grade and low-cost sensor instruments were used, as well as meteorological data from a closely located measurement station.…”

Calibration Method for Particulate Matter Low-Cost Sensors Used in Ambient Air Quality Monitoring and Research

“…They report that the elevated particle mass concentration found in LCS is due to the bulk aerosol hygroscopicity under different RH conditions. Crilley et al (2020) [54] conclude that a factor based on Koehler's theory (k-factor) derived from in situ measurements, as they did, offers better calibration and improves the performance of the OPC-N2. Nevertheless, in conditions where in situ measurements are impractical, then Crilley et al (2020) [54] suggest using a "literature-based k-factor".…”

“…Crilley et al (2020) [54] checked the validity of the calibration method described in Crilley et al (2018) [50] in four cities and on three different continents. They report that the elevated particle mass concentration found in LCS is due to the bulk aerosol hygroscopicity under different RH conditions.…”

“…Therefore, it was decided to use the correction factor based on Köhler's theory for the entire dataset. The value of ĸ is assumed to be 0.4 since the measurements were carried out in an urban area similar to that of the study conducted in Crilley et al (2020) [54]. The URBMOBI 3.0 data is corrected for relative humidity using the following Equations ( 3) and (4).…”

Evaluation of optical particulate matter sensors under realistic conditions of strong and mild urban pollution

“…Furthermore, densities appearing in equation 4are not known a priori. That explains why it is difficult to calibrate OPS for realistic ambient PM concentration measurements: any laboratory calibration may or may not be applicable to the changing outdoor conditions (Tryner et al, 2020;Crilley et al, 2020).…”