10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Resolving the Factors Governing Particle Phase Photochemistry
BRYAN R. BZDEK, Lara Lalemi, University of Bristol
Abstract Number: 111 Working Group: Aerosol Chemistry
Abstract Whereas gas phase photochemistry and its role in producing new particles and determining gas phase composition are relatively well understood, aerosol phase photochemistry is understudied in part because of the complexity of the system. For example, nanofocussing of light into aerosol particles results in nonhomogeneous light distributions that are highly size dependent. Moreover, photochemistry at the solution-air (or particle-air) interface has been implicated in production of atmospherically significant volatile organic compounds that may even produce new atmospheric particles. Finally, aerosol particles can access supersaturated and highly viscous solute states, potentially changing the rates of photochemical reactions as well as the ability for the products to diffuse throughout the particle. The goal of this work is to resolve the factors that govern aerosol photochemistry (i.e. particle size, surface composition, and bulk properties) and compare them to bulk systems. These experiments will utilise holographic optical tweezers, where a laser beam is focussed through a microscope objective to produce one or several optical traps that will capture airborne picolitre volume droplets. Droplet size and refractive index can be precisely retrieved through collection of a Raman spectrum. Droplet surface and bulk properties are quantified through precise measurement of the coalescence dynamics of two optically trapped droplets.
The photolysis of nitrate will be used as a model system to probe photochemistry in aerosol particles. In a first set of experiments, the loss of the nitrate signal in the droplet Raman spectrum will be monitored as a function of exposure to UV light and particle size. A second set of experiments will examine how photolysis rates change as the droplet viscosity is increased by adding a known amount of sucrose to the droplet. A third set of experiments will resolve the significance of the aerosol-air interface to photochemistry. These experiments will involve incorporating a photoactive surfactant (e.g. nonanoic acid) into the droplet and monitoring changes to droplet properties as the surface coverage is changed. Together, these experiments will identify and quantify which factors are most significant to determining photochemical rates in aerosol droplets.