Abstract View
Ammonium Sulfate: Good for Optical Diagnostics but Not Quite Good Enough for Calibration
JAMES RADNEY, Christopher Zangmeister, Chun-Ning Mao, Akua Asa-Awuku, National Institute of Standards and Technology
Abstract Number: 312
Working Group: Aerosol Standards
Abstract
Ammonium sulfate – (NH4)2SO4, AS – has seen significant use in the aerosol community as a test particle because of its ease of use, non-toxicity, nearly spherical morphology and well-known physical and chemical properties, e.g. thermal decomposition, hygroscopicity and refractive index to name a few. Historically, our lab used AS as a reference material for the co-calibration of extinction coefficient (αext) and number density (N) measurements by a cavity ring-down spectrometer and condensation particle counter, respectively, by comparing the derived extinction coefficients (Cext = αext/N) to those calculated using Mie theory and some form of effective medium approximation. Here we will present multiple years’ worth of optical extinction data to highlight the variability in the measured Cext of AS. This variability demonstrates that AS can be used for diagnostic measurements with errors and uncertainties on the order of ± 10 %. To improve upon these errors and uncertainties, requires perfectly spherical nanomaterials with a better-defined refractive index, e.g. polystyrene nanospheres (PSL). Using PSL, we can co-calibrate the CRD and CPC to have precision and accuracy better than 2 %. While these values are still orders of magnitude higher than often found in gas-phase spectroscopy, it demonstrates that nanoscale heterogeneity may represent a physical limit for the use of aerosol as a calibrant.