Coalescence Sampling and Analysis of Aerosols using Aerosol Optical Tweezers

We present a first exploratory study to assess the use of aerosol optical tweezers as an instrument for sampling and detecting accumulation- and coarse-mode aerosol. A subpicoliter aqueous aerosol droplet is captured in the optical trap and used as a sampling volume, accreting mass from a free-flowing aerosol generated by a medical nebulizer or atomizer. Real-time measurements of the initial stability in size, refractive index, and composition of the sampling droplet inferred from Raman spectroscopy confirm that these quantities can be measured with high accuracy and low noise. Typical standard deviations in size and refractive index of the sampling droplet over a period of 200 s are <±2 nm and <±0.0005, respectively, equivalent to <±0.04% in both measured quantities. A standard deviation of <±1% over a 200 s period is achieved in the spontaneous Raman intensity measurement. When sampling coarse-mode aerosol, mass changes of <10 pg can be detected by the sampling droplet as discrete coalescence events. With accumulation-mode aerosol, we show that fluxes as low as 0.068 pg s^-1 can be detected over a 50 s period, equivalent to ∼3 pg of sampled material.