title: Comparison of the accuracy of aerosol refractive index measurements from single particle and ensemble techniques.
abstract: The ability of two techniques, aerosol cavity ring down spectroscopy (A-CRDS) 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.
authors.affiliation: School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK.
authors.forename: Bernard J
authors.initials: BJ
authors.lastname: Mason
authors.forename: Simon-John
authors.initials: SJ
authors.lastname: King
authors.forename: Rachael E H
authors.initials: RE
authors.lastname: Miles
authors.forename: Katherine M
authors.initials: KM
authors.lastname: Manfred
authors.forename: Andrew M J
authors.initials: AM
authors.lastname: Rickards
authors.forename: Jin
authors.initials: J
authors.lastname: Kim
authors.forename: Jonathan P
authors.initials: JP
authors.lastname: Reid
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Orr-Ewing