Extinction Cross Section Measurements for Single Aerosol Particles Confined to a Linear Electrodynamic Quadrupole Trap

Theme: Environmental Aerosols

Start date: Cohort 1: 2019

Supervisors: Prof Andrew Orr-Ewing (Bristol) and Dr Adam Squires (Bath)

The contribution of organic aerosol to the warming of the Earth’s atmosphere remains uncertain because particle composition and morphology affect the absorption of sunlight. Using a recently developed spectroscopic apparatus, this project will measure precise optical properties of single, trapped aerosol particles.


Aerosol particles can both scatter and absorb solar and terrestrial radiation, with scattering having a cooling effect on the atmosphere and absorption a warming effect. The interaction of radiation with absorbing aerosol particles is currently one of the largest uncertainties in global climate models. Therefore, a new technique will be developed allowing accurate determination of the effect of irradiation of both weakly and strongly absorbing aerosol particles on global climate. 

Cavity ring-down spectroscopy (CRDS) measurements reported by Cotterellet al.determined refractive index (RI) values for non-absorbing aerosol particles to an accuracy of 0.1% for particles in the 0.5-1μm size range. However, confinement of absorbing aerosol particles proved difficult, owing to photophoretic forces acting on the particles created by the Bessel beam (BB) optical trap employed. During this project, a CRDS set-up utilising an linear electrodynamic quadrupole (LEQ) trap will be developed allowing precise trapping of absorbing particles. 

The accuracy of the new technique will be validated by comparison with well established measurements of evaporating organic and hygroscopic inorganic species. Work will then begin to determine both the scattering and absorption componentsof the RI of absorbing aerosol particles, namely brown carbon (BrC) species. Later stages of the project may allow exploration of the mixing state, morphology and processing (e.g. oxidative ageing) dependency of aerosol particle light extinction.