Photophoresis used for measurements of light absorption by a single particle

The contribution of light absorption by brown carbon aerosols to the Earth’s energy balance still poses a significant uncertainty in our understanding of climate forcing. As a result, one of the main open questions regarding organic aerosols in atmospheric chemistry is related to the formation and degradation of light-absorbing compounds during aging processes. Towards this goal, we explore the use of photophoresis for high sensitivity measurements of light absorption by a single levitated particle in an experimental setup that facilitates realistic atmospheric gas concentrations and aging time.

Photophoresis occurs when the surface of an illuminated, light-absorbing particle is unevenly heated relative to its surroundings. The temperature difference between the illuminated and the ‘dark’ side of the particle results in an uneven momentum transfer from colliding gas-phase molecules. This leads to a net photophoretic force, acting on the particle in the direction of the momentum transfer gradient. The photophoretic force is related to the complex refractive index of the particle and to its size parameter through the distribution of internal electric fields. As such, it may lead to a net force away from (positive) or towards (negative) the light source.

Using this phenomenon, we were able to retrieve the imaginary part of the complex refractive index (k) of a single particle levitated in an electrodynamic balance (EDB) at 473 nm wavelength. Extremely low values of k from 10^-4 to 10^-5 were successfully retrieved with uncertainty of less than 35% during a photo-bleaching experiment of a slightly absorbing organic particle used as a model for atmospheric brown carbon.    

An advantage of the EDB is that heterogeneous chemistry and photochemistry experiments are performed on a single particle that is levitated for days and weeks allowing for realistic atmospheric gas concentrations and aging times. In such experiments, measurements of the particle’s light absorption properties using photophoresis would add valuable information on the evolution of light absorption by the aging of organic aerosols.

A future study will implement this approach in an EDB-MS system where the EDB will be coupled with a soft ionization mass spectrometer. This will allow the identification of the molecular species responsible for light absorption as it evolves.