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Comparison of the accuracy of aerosol refractive index measurements from single particle and ensemble techniques

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Published copy (DOI)


  • Bernard J. Mason
  • Simon John King
  • Rachael E.H. Miles
  • Katherine M. Manfred
  • Andrew M.J. Rickards
  • Jin Kim
  • Jonathan P. Reid
  • Andrew J. Orr-Ewing


Publication details

JournalJournal of Physical Chemistry A
DatePublished - 23 Aug 2012
Issue number33
Number of pages10
Pages (from-to)8547-8556
Original languageEnglish


The ability of two techniques, aerosol cavity ring down spectroscopy (ACRDS) and optical tweezers, to retrieve the refractive index of atmospherically relevant aerosol was compared through analysis of supersaturated sodium nitrate at a range of relative humidities. Accumulation mode particles in the diameter range 300-600 nm were probed using A-CRDS, with optical tweezer measurements performed on coarse mode particles several micrometers in diameter. A correction for doubly charged particles was applied in the A-CRDS measurements. Both techniques were found to retrieve refractive indices in good agreement with previously published results from Tang and Munkelwitz, with a precision of ±0.0012 for the optical tweezers and ±0.02 for the A-CRDS technique. The coarse mode optical tweezer measurements agreed most closely with refractive index predictions made using a mass-weighted linear mixing rule. The uncertainty in the refractive index retrieved by the A-CRDS technique prevented discrimination between predictions using both mass-weighted and volume-weighted linear mixing rules. No efflorescence or kinetic limitations on water transport between the particle and the gas phase were observed at relative humidities down to 14%. The magnitude of the uncertainty in refractive index retrieved using the A-CRDS technique reflects the challenges in determining particle optical properties in the accumulation mode, where the extinction efficiency varies steeply with particle size.

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