Direct comparison of the hygroscopic properties of ammonium sulfate and sodium chloride aerosol at relative humidities approaching saturation.

Holographic optical tweezers are used to make comparative measurements of the hygroscopic properties of single component aqueous aerosol containing sodium chloride and ammonium sulfate over a range of relative humidity from 84% to 96%. The change in RH over the course of the experiment is monitored precisely using a sodium chloride probe droplet with accuracy better than ±0.09%. The measurements are used to assess the accuracy of thermodynamic treatments of the relationship between water activity and solute mass fraction with particular attention focused on the dilute solute limit approaching saturation vapor pressure. The consistency of the frequently used Clegg-Brimblecombe-Wexler (CBW) treatment for predicting the hygroscopic properties of sodium chloride and ammonium sulfate aerosol is confirmed. Measurements of the equilibrium size of ammonium sulfate aerosol are found to agree with predictions to within an uncertainty of ±0.2%. Given the accuracy of treating equilibrium composition, the inconsistencies highlighted in recent calibration measurements of critical supersaturations of sodium chloride and ammonium sulfate aerosol cannot be attributed to uncertainties associated with the thermodynamic predictions and must have an alternative origin. It is concluded that the CBW treatment can allow the critical supersaturation to be estimated for sodium chloride and ammonium sulfate aerosol with an accuracy of better than ±0.002% in RH. This corresponds to an uncertainty of ≤1% in the critical supersaturation for typical supersaturations of 0.2% and above. This supports the view that these systems can be used to accurately calibrate instruments that measure cloud condensation nuclei concentrations at selected supersaturations. These measurements represent the first study in which the equilibrium properties of two particles of chemically distinct composition have been compared simultaneously and directly alongside each other in the same environment.

Using optical landscapes to control, direct and isolate aerosol particles.

We demonstrate the ability to direct the flow of aerosol droplets through a trapping cell using a tailored optical landscape generated by spatial light modulation. Using an optical barrier, droplets held in an optical trap can be effectively isolated from other droplets within the aerosol. To illustrate the effective isolation we compare the influence of different optical landscapes on the flow of free aerosol around a trapped droplet. We also present spectroscopic evidence of the optical barrier effect and apply the technique to permit controlled loading of different aerosol particles into neighbouring optical traps. This method will enable comparative measurements of aerosol properties to be made and facilitate the study of aerosol chemistry in sub-picolitre droplets. It also facilitates the use of an isolated droplet of known composition as a sensitive probe of the gas phase conditions in an aerosol ensemble.

Spectroscopic characterisation and manipulation of arrays of sub-picolitre aerosol droplets.

Arrays of optically tweezed aerosol droplets, each of sub-picolitre volume, are manipulated by holographic optical tweezers and characterised by cavity enhanced Raman spectroscopy. A spatial light modulator is employed to generate arrays of optical traps from a single laser beam and to control the array dimensions and relative trap positions. Comparative hygroscopicity measurements are performed concurrently on five trapped droplets by monitoring the evolving size of each droplet. This is extended to the controlled coalescence of an array of droplets accompanied by spectroscopic measurements. These data represent the first ever simultaneous measurements of the evolving composition and size of an array of aerosol droplets. We consider the possibility of using aerosol arrays as a platform for studying chemical reactions in sub-picolitre volumes, exploiting the versatility of aerosol arrays for performing optical digital microfluidic operations accompanied by micro-total analysis.

In situ comparative measurements of the properties of aerosol droplets of different chemical composition.

Aerosol optical tweezers can be used to manipulate multiple aerosol particles simultaneously. When coupled with spontaneous and stimulated Raman scattering, the composition, size and phase partitioning of different chemical components within a liquid droplet can be investigated. In combination, these two techniques suggest the possibility of a new strategy for characterising the thermodynamic behaviour of aerosols and the kinetics of mass transfer between the gas and condensed phases. We demonstrate here that two droplets can be characterised simultaneously, examining specifically the variation in wet particle size with relative humidity, recording the changes in size with nanometre accuracy. In a further demonstration, we use the size of a sodium chloride droplet to determine the relative humidity of the gas phase, allowing the variation in hygroscopicity of a second aqueous glutaric acid/sodium chloride droplet to be studied. We suggest that such a comparative approach can provide new insights into aerosol dynamics.