A new experiment is presented for the measurement of single aerosol particle extinction efficiencies, Qext, combining cavity ring-down spectroscopy (CRDS, λ = 405 nm) with a Bessel beam trap (λ = 532 nm) in tandem with phase function (PF) measurements. This approach allows direct measurements of the changing optical cross sections of individual aerosol particles over indefinite time-frames facilitating some of the most comprehensive measurements of the optical properties of aerosol particles so far made. Using volatile 1,2,6-hexanetriol droplets, Qext is measured over a continuous radius range with the measured Qext envelope well described by fitted cavity standing wave (CSW) Mie simulations. These fits allow the refractive index at 405 nm to be determined. Measurements are also presented of Qext variation with RH for two hygroscopic aqueous inorganic systems ((NH4)2SO4 and NaNO3). For the PF and the CSW Mie simulations, the refractive index, nλ, is parameterised in terms of the particle radius. The radius and refractive index at 532 nm are determined from PFs, while the refractive index at 405 nm is determined by comparison of the measured Qext to CSW Mie simulations. The refractive indices determined at the shorter wavelength are larger than at the longer wavelength consistent with the expected dispersion behaviour. The measured values at 405 nm are compared to estimates from volume mixing and molar refraction mixing rules, with the latter giving superior agreement. In addition, the first single-particle Qext measurements for accumulation mode aerosol are presented for droplets with radii as small as ∼300 nm.

General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. (0)117 9287672 e-mail: a.orr-ewing@bristol.ac.uk AbstractThe extinction cross sections of individual, optically confined aerosol particles with radii of a micron or less can, in principle, be measured using cavity ring-down spectroscopy (CRDS). However, when the particle radius is comparable in magnitude to the wavelength of light stored in a high-finesse cavity, the phenomenological cross-section retrieved from a CRDS experiment depends on the location of the particle in the intra-cavity standing wave and differs from the Mie scattering cross section for plane-wave irradiation. Using an evaporating 1,2,6-hexanetriol particle of initial radius ~1.75 m confined within the 4.5-m diameter core of a Bessel beam, we demonstrate that the scatter in the retrieved extinction efficiency of a single particle is determined by its lateral motion, which spans a few wavelengths of the intra-cavity standing wave used for CRDS measurements. Fits of experimental measurements to Mie calculations, modified to account for the intra-cavity standing wave, allow precise retrieval of the refractive index of 1,2,6-hexanetriol particles (with relative humidity, RH < 10%) of 1.47824  0.00072.

A single horizontally-propagating zeroth order Bessel laser beam with a counter-propagating gas flow 9 was used to confine single fine-mode aerosol particles over extended periods of time, during which 10 process measurements were performed. Particle sizes were measured by the analysis of the angular 11 variation of light scattered at 532 nm by a particle in the Bessel beam, using either a probe beam at 12 405 nm or 633 nm. The vapour pressures of glycerol and 1,2,6-hexanetriol particles were determined 13 to be 7.5 2.6 mPa and 0.20 0.02 mPa respectively. The lower volatility of hexanetriol allowed better 14 definition of the trapping environment relative humidity profile over the measurement time period, 15 thus higher precision measurements were obtained compared to those for glycerol. The size evolution 16 of a hexanetriol particle, as well as its refractive index at wavelengths 532 nm and 405 nm, were 17 determined by modelling its position along the Bessel beam propagation length while collecting phase 18 functions with the 405 nm probe beam. Measurements of the hygroscopic growth of sodium chloride 19 and ammonium sulfate have been performed on particles as small as 350 nm in radius, with growth 20 curves well described by widely used equilibrium state models. These are the smallest particles for 21 which single-particle hygroscopicity has been measured and represent the first measurements of 22 hygroscopicity on fine mode and near-accumulation mode aerosols, the size regimes bearing the most 23 atmospheric relevance in terms of loading, light extinction and scattering. Finally, the technique is 24 contrasted with other single particle and ensemble methods, and limitations are assessed. 25 26 2

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.

During the SEAC 4 RS campaign in 2013, inflight measurements of light-absorption by aerosol in biomass burning and agriculture fire plumes were collected along with concomitant measurements of aerosol extinction, scattering, and black carbon mass concentration. Here, we compare three measurements of aerosol absorption coefficients: from a photoacoustic spectrometer (PAS), a particle soot absorption photometer (PSAP), and a continuous light absorption photometer (CLAP). Each of these absorption measurements was collected in three visible spectral regions: red, green, and blue (although the precise wavelength and bandwidth vary with each instrument). The absorption measurements were compared during the plumes, in the boundary layer, and in the free troposphere. The slopes from the comparison ranged from 0.6 to 1.24. For biomass burning plumes, the uncertainty in the absorption measurements translates into a range in single scattering albedos of 0.93-0.94 at a wavelength of 660 nm, 0.94-0.95 at 532 nm and 0.92-0.95 at 405 nm. Overall, the aerosol absorption instruments agreed within their stated accuracies. Comparisons with simultaneous measurements of refractive black carbon mass concentration (collected by a single particle soot photometer), were used to derive the mass absorption coefficients (MAC). For all wavelengths, the MAC was high by greater than a factor of three compared to the expected MAC for black carbon.