Abstract
Abstract. We evaluate the sensitivity of the size calibrations of two commercially available, high-resolution optical particle sizers to changes in aerosol composition and complex refractive index (RI). The Droplet Measurement Technologies Ultra-High Sensitivity Aerosol Spectrometer (UHSAS) and the TSI, Inc. Laser Aerosol Spectrometer (LAS) are two commonly used instruments for measuring the portion of the aerosol size distribution with diameters larger than nominally 60–90 nm. Both instruments illuminate particles with a laser and relate the single-particle light scattering intensity and count rate measured over a wide range of angles to the size-dependent particle concentration. While the optical block geometry and flow system are similar for each instrument, a significant difference between the two models is the laser wavelength (1054 nm for the UHSAS and 633 nm for the LAS) and intensity (about 100 times higher for the UHSAS), which may affect the way each instrument sizes non-spherical or absorbing aerosols. Here, we challenge the UHSAS and LAS with laboratory-generated, mobility-size-classified aerosols of known chemical composition to quantify changes in the optical size response relative to that of ammonium sulfate (RI of 1.52+0i at 532 nm) and NIST-traceable polystyrene latex spheres (PSLs with RI of 1.59+0i at 589 nm). Aerosol inorganic salt species are chosen to cover the real refractive index range of 1.32 to 1.78, while chosen light-absorbing carbonaceous aerosols include fullerene soot, nigrosine dye, humic acid, and fulvic acid standards. The instrument response is generally in good agreement with the electrical mobility diameter. However, large undersizing deviations are observed for the low-refractive-index fluoride salts and the strongly absorbing nigrosine dye and fullerene soot particles. Polydisperse size distributions for both fresh and aged wildfire smoke aerosols from the recent Fire Influence on Regional to Global Environments Experiment and Air Quality (FIREX-AQ) and the Cloud, Aerosol, and Monsoon Processes Philippines Experiment (CAMP2Ex) airborne campaigns show good agreement between both optical sizers and contemporaneous electrical mobility sizing and particle time-of-flight mass spectrometric measurements. We assess the instrument uncertainties by interpolating the laboratory response curves using previously reported RIs and size distributions for multiple aerosol type classifications. These results suggest that, while the optical sizers may underperform for strongly absorbing laboratory compounds and fresh tailpipe emissions measurements, sampling aerosols within the atmospherically relevant range of refractive indices are likely to be sized to better than ±10 %–20 % uncertainty over the submicron aerosol size range when using instruments calibrated with ammonium sulfate.
Highlights
The size distribution and optical properties of atmospheric aerosols are important primary inputs to radiative transfer calculations of direct climatic effects
We assess the instrument uncertainties by interpolating the laboratory response curves using previously reported refractive index (RI) and size distributions for multiple aerosol type classifications. These results suggest that, while the optical sizers may underperform for strongly absorbing laboratory compounds and fresh tailpipe emissions measurements, sampling aerosols within the atmospherically relevant range of refractive indices are likely to be sized to better than ±10 %– 20 % uncertainty over the submicron aerosol size range when using instruments calibrated with ammonium sulfate
Recent work during the Atmospheric Tomography Mission (ATom) and Observations of Aerosols Above Clouds and their Interactions (ORACLES) field campaigns suggests that the undersizing of the absorbing aerosol particles by the Ultra-High Sensitivity Aerosol Spectrometer (UHSAS) may be due to particle vaporization by the high instrument laser power similar to that observed in the single-particle soot photometer, SP2 (Kupc et al, 2018; Howell et al, 2020)
Summary
The size distribution and optical properties of atmospheric aerosols are important primary inputs to radiative transfer calculations of direct climatic effects. Most recent field campaigns include measurements of the number size distribution and its higher moments from some combination of mobility, optical, and aerodynamic sizing. Of these techniques, modern optical particle sizers are ideal for covering the size range of interest, with both high size resolution and the high time resolution needed for airborne observations. Zimmerman et al (2015) attempted to apply this method using combined scanning electrical mobility particle classification followed by UHSAS and LAS sizing of laboratory-generated aerosols of varying complex RI but found limited dynamic range in particle size changes for purely scattering particles smaller than 550 nm diameter and significant undersizing for absorbing species. That explanation would not explain the similar undersizing reported by Zimmerman et al (2015) for the LAS with much lower laser power
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