Aerosols represent the second largest contributor to atmospheric heating after CO2. Aerosol optical properties are poorly understood, resulting in the largest source of uncertainty in modelling atmospheric warming and hinder the extent to which models can interpret climate phenomena. The quality of aerosol optical data is hindered by the lack of aerosolized materials with known properties (absorption and scattering), prohibiting instrument calibration and making quantitative aerosol measurements a challenge.
This project has three goals to address challenges associated with making quality aerosol optical measurements. First, is the development of techniques to enable mass traceable absorption and extinction measurements of an aerosol stream. Second, apply these techniques to atmospherically relevant aerosol systems. And lastly, to develop an aerosolizable material with known composition and morphology to enable reproducible and transferrable aerosol optical calibration.
We have recently developed a technique that can select an aerosol with known size and mass from a disperse aerosol distribution. The aerosol absorption and extinction (absorption + scattering) is measured in parallel and the particles are then counted. From these measurements we can measure aerosol optical properties as a function of particle density, independent of particle size and chemical composition. Using this technique for aerosols with known composition allows for quantitative optical measurements, and for the first time, truly quantitative comparisons of aerosol as a function of composition and size. We have recently added the ability to make absorption measurements across the full visible solar spectrum and to generate aerosol spectra in a flowing stream. This gives us the capability to quantitatively calculate aerosol radiative forcings.
In the second area we have applied the techniques described above to atmospherically relevant materials. We have explored laboratory generated soot and brown carbon, both important contributors to atmospheric absorption. These experiments require manipulation of experimental parameters at the nanoscale, which in turn, allow us to influence chemical composition, morphology, and particle size. Through careful control of the aerosol stream the optical dependence of particle morphology can be determined for particles of the same mass, to allow for direct comparison of aerosol optical properties. In addition to chemical composition, we can study optical properties as a function of relative humidity, thereby better capturing the state of aerosol in the terrestrial environment where water plays an important role.
To date no materials exist for aerosol optical calibration. This inhibits quantitative and comparative measurements to be made and represents a major challenge for technical advancement of the field. We have been addressing this need by researching and developing materials that are easy to prepare, have reproducible properties, and can be readily generated and used across a wide variety of experimental conditions. We have recently studied reduced graphene oxide, fullerene, graphene, and water soluble carbon black aerosol. Using the system described previously, we can tune aerosol morphology, size, and, in some systems, the chemical composition of the analyzed material.