AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Coalescence Sampling of Ambient Aerosol Using Aerosol Optical Tweezers
JIM WALKER, Allen E. Haddrell, Lilly Manzoni, Jonathan P. Reid, Rebecca Hopkins, University of Bristol
Abstract Number: 268 Working Group: Instrumentation and Methods
Abstract We present the first exploratory study to assess the sensitivity and suitability of aerosol optical tweezers (AOT) as a technique for sampling and detecting free-flowing accumulation- and coarse-mode ambient aerosol. A subpicoliter aqueous droplet is captured in an optical trap and used as a sampling volume to accrete mass from free-flowing aerosol generated nearby using a medical nebulizer or atomizer.
Collection of the ambient aerosol by the sampling droplet is identified by real-time changes to the radius, refractive index and relative proportions of chemical species within the sampling droplet. These quantities are retrieved using Raman spectroscopy and are shown to be measured with high accuracy and low noise. Typical standard deviations in the initial (pre-sampling) 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 measured intensity ratio between two spontaneous Raman bands, used to infer the OH:CH ratio of the sampling droplet.
When sampling coarse-mode aerosol, mass changes of <10 pg can be detected by the sampling droplet as individual coalescence events indicated by discrete step-changes to the measurement quantities. 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, identified by more gradual changes to the measured quantities.
We will present the sensitivity limits of this sampling approach and demonstrate the future potential of the technique for sampling and identifying secondary organic aerosol, among other ambient aerosols.