Sizing response of the Ultra-High Sensitivity Aerosol Spectrometer (UHSAS) and Laser Aerosol Spectrometer (LAS) to changes in submicron aerosol composition and refractive index

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.

Ambient aerosol properties in the remote atmosphere from global-scale in situ measurements

“…11k). The biomass burning particles are found mostly in the upper end of the accumulation-mode volume, consistent with the larger diameters typically found near wildfire sources (Radke et al, 1977;Moore et al, 2021) compared to secondary particles from natural and anthropogenic sulfur and organic sources.…”

“…As described by Moosmüller and Ogren (2017), practical values of g i in the atmosphere range from 0 (symmetrically scattered light) to +1 (purely forward-scattered light). Typical values for accumulation-mode-dominated size distributions for mid-visible wavelengths are ∼ 0.4-0.6, with larger values possible for size distributions with a substantial coarse fraction (e.g., Andrews et al, 2006;Fiebig and Ogren, 2006).…”

Cloud droplet formation at the base of tropical convective clouds: closure between modeling and measurement results of ACRIDICON–CHUVA

“…Aerosol size distributions were measured using an ultrahigh-sensitivity aerosol spectrometer (UHSAS; Droplet Measurement Technologies, Inc., Longmont, CO, USA) (Cai et al, 2008;Moore et al, 2021). The UHSAS combines a high-power infrared laser (λ = 1054 nm) and a large solidangle range in a sideways direction for the detection of light scattered by individual particles (Andreae et al, 2018).…”

“…The UHSAS was calibrated with monodisperse polystyrene latex (PSL) spheres of known size. Typical uncertainties of UHSAS measurements are both 15 % in diameter and concentration (Cai et al, 2008;Moore et al, 2021).…”

Reconciling Assumptions in Bottom‐Up and Top‐Down Approaches for Estimating Aerosol Emission Rates From Wildland Fires Using Observations From FIREX‐AQ

“…Refractory BC mass concentrations are provided by a Single Particle Soot Photometer (SP2, Droplet Measurement Technologies) (Gao et al, 2007). The 50% geometric transmission diameter for the ToF-AMS is ∼600 nm, which sufficiently captures the size range for the majority of biomass burning derived particles, with the exception of supermicron ash particles (Adachi et al, 2021;Moore et al, 2021). Total carbon is calculated as the sum of CO 2 , CO, CH 4 , organic carbonaceous aerosol (OC), and BC aerosol.…”