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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/amt-2019-370
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2019-370
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 07 Oct 2019

Submitted as: research article | 07 Oct 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

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

Leigh R. Crilley1,a, Ajit Singh1, Louisa J. Kramer1, Marvin D. Shaw2, Mohammed S. Alam1, Joshua S. Apte3, William J. Bloss1, Lea Hildebrandt Ruiz3, Pingqing Fu4,5, Weiqi Fu5, Shahzad Gani3, Michael Gatari6, Evgenia Ilyinskaya7, Alastair C. Lewis2, David Ng'ang'a6, Yele Sun5, Rachel C. W. Whitty7, Siyao Yue5, Stuart Young2, and Francis D. Pope1 Leigh R. Crilley et al.
  • 1School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, UK
  • 2National Centre for Atmospheric Science, Wolfson Atmospheric Chemistry Laboratories, University of York, York, UK
  • 3Department of Civil, Architectural and Environmental Engineering, The University of Texas at Austin, Austin, Texas, USA
  • 4Institute of Surface-Earth System Science, Tianjin University, Tianjin, China
  • 5Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
  • 6Institute of Nuclear Science and Technology, University of Nairobi, Nairobi, Kenya
  • 7Schoolof Earth and Environment, University of Leeds, Leeds, UK
  • anow at: Department of Chemistry, York University, Toronto, Canada

Abstract. There is considerable interest in using low-cost optical particle counters (OPC) to supplement existing routine air quality networks that monitor particle mass concentrations. In order to do this, low-cost OPC data needs to be cross-comparable with particle mass reference instrumentation, and as yet, there is no widely agreed methodology. Aerosol hygroscopicity is known to be a key parameter to consider when correcting particle mass concentrations derived from a low-cost OPC, 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 a low-cost OPC (OPC-N2, Alphasense), collected in four cities in three continents, to explore the performance of this correction factor. We report evidence that the elevated particle mass concentrations, reported by the low-cost OPC relative to reference instrumentation, is due to bulk aerosol hygroscopicity under different RH conditions, which is determined by aerosol composition and in particular the levels of hygroscopic aerosols (sulphate and nitrate). We exploit measurements made in volcanic plumes in Nicaragua, that are predominantly composed of sulphate aerosol, as a natural experiment to demonstrate this behaviour in the ambient atmosphere, with the observed humidogram closely resembling the calculated pure sulphuric 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 corrected OPC-N2 PM2.5 mass concentrations to be within 33 % of reference measurements at all sites. The results indicated that an in situ derived κ (using suitable reference instrumentation) would lead to the most accurate correction relative to co-located reference instruments. Applying literature κ in the correction factor also resulted in improved performance of OPC-N2, to be within 50 % of reference. 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 RH, 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.

Leigh R. Crilley et al.
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Short summary
There is considerable interest in using low-cost optical particle counters (OPC) for particle mass measurements, however there is no agreed method for calibration. Here we exploit a number of datasets globally to demonstrate that particle composition and relative humidity are key factors affecting measured concentrations by a low-cost OPC, and present a simple correction methodology that corrects for this influence.
There is considerable interest in using low-cost optical particle counters (OPC) for particle...
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