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. We evaluate the sensitivity of the size calibrations oftwo commercially available, high-resolution optical particle sizers tochanges in aerosol composition and complex refractive index (RI). TheDroplet Measurement Technologies Ultra-High Sensitivity Aerosol Spectrometer (UHSAS) and the TSI, Inc. Laser Aerosol Spectrometer (LAS) aretwo commonly used instruments for measuring the portion of the aerosol sizedistribution with diameters larger than nominally 60–90 nm. Both instrumentsilluminate particles with a laser and relate the single-particle lightscattering intensity and count rate measured over a wide range of angles tothe size-dependent particle concentration. While the optical block geometryand flow system are similar for each instrument, a significant differencebetween the two models is the laser wavelength (1054 nm for the UHSAS and633 nm for the LAS) and intensity (about 100 times higher for the UHSAS), whichmay affect the way each instrument sizes non-spherical or absorbingaerosols. Here, we challenge the UHSAS and LAS with laboratory-generated,mobility-size-classified aerosols of known chemical composition to quantifychanges 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 arechosen to cover the real refractive index range of 1.32 to 1.78, whilechosen light-absorbing carbonaceous aerosols include fullerene soot,nigrosine dye, humic acid, and fulvic acid standards. The instrumentresponse is generally in good agreement with the electrical mobilitydiameter. However, large undersizing deviations are observed for thelow-refractive-index fluoride salts and the strongly absorbing nigrosine dye and fullerene soot particles. Polydisperse size distributions for both freshand aged wildfire smoke aerosols from the recent Fire Influence on Regionalto 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 andcontemporaneous electrical mobility sizing and particle time-of-flight massspectrometric measurements. We assess the instrument uncertainties byinterpolating the laboratory response curves using previously reported RIsand size distributions for multiple aerosol type classifications. Theseresults suggest that, while the optical sizers may underperform for stronglyabsorbing laboratory compounds and fresh tailpipe emissions measurements,sampling aerosols within the atmospherically relevant range of refractiveindices are likely to be sized to better than ±10 %–20 % uncertainty over the submicron aerosol size range when using instruments calibrated withammonium sulfate.

atmospheric measurement techniques

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