Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> Optical particle counters (OPC) are widely used to measure the aerosol particle number size distribution at atmospheric ambient conditions and over a large size range. Their measurement principle is based on the dependence of light scattering on particle size. However, this dependence is not monotonic at all sizes and light scattering also depends on the particle composition (i.e., the complex refractive index, <em>CRI</em>) and morphology. Therefore, the conversion of the measured scattered intensity to the desired particle size depends on the microphysical properties of the sampled aerosol population and might not be unique at all sizes. While these complexities have been addressed before, corrections are typically applied ad-hoc and are not standardised. This paper addresses this issue by providing a consistent and extended database of pre−computed correction factors for a wide range of complex refractive index values representing the composition variability of atmospheric aerosols. These correction factors are calculated for five different commercial OPCs (USHAS, PCASP, FSSP, GRIMM and its airborne version Sky− GRIMM, CDP) by assuming Mie theory for homogeneous spherical particles, and by varying the real part of the <em>CRI</em> between 1.33 and 1.75 in steps of 0.01 and the imaginary part between 0.0 and 0.4 in steps of 0.001. Correction factors for mineral dust are provided at the <em>CRI</em> of 1.53 – 0.003i and account for the asphericity of these particles. The datasets described in this paper are distributed at open-access repository: https://doi.org/10.25326/234 (license CC BY, Formenti et al., 2021) maintained by the French national center for Atmospheric data and services AERIS to data users/geophysicists who number size distribution measurements from OPC for their research on atmospheric aerosols. Application and caveats of the <em>CRI</em>-corrections factors are presented and discussed. The dataset presented in this paper is not only useful for correcting the size distribution from an OPC when the particle refractive index is known, but even when only assumptions can be made. Furthermore, this dataset can be useful in calculating uncertainties or sensitivities of aerosol volume/mass/extinction from OPCs given no or limited knowledge of refractive index.
Highlights
Latex spheres (PSL) or equivalent scattering material of known CRI at the wavelength of light used in the instrument
We describe a set of standardized corrections of particle sizing by Optical particle counters (OPCs)
Instruments in order to account for the dependence of angular scattering on particle composition, as represented by the particle complex refractive index CRI. This dataset of corrections is based on the simple assumption of homogeneous spherical particles and the use of Mie theory, and considers nominal OPC characteristics in terms of scattering angles of the sensing volume and wavelengths of the light sources
Summary
Aerosol particles are some of the more elusive, but highly climate−relevant components of the atmosphere (Boucher et al, 2013). The scattering cross-section Csca measured by an OPC can be converted into an optical equivalent diameter (DEO) based on calibration with non−absorbing spherical particle latex spheres (PSL) or equivalent scattering material of known CRI at the wavelength of light used in the instrument. This is complicated by the fact that atmospheric aerosols have different compositions (CRI) than the calibration material, and that the intensity of scattered light depends on particle morphology (Dubovik et al, 2006; Huang et al, 2021).
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