Photophoretic spectroscopy in atmospheric chemistry high-sensitivity measurements of light absorption by a single particle

Light-absorbing organic atmospheric particles, termed brown carbon, undergo chemical and photochemical aging processes during their lifetime in the atmosphere. The role these particles play in the global radiative balance and in the climate system is still uncertain. To better quantify their radiative forcing due to aerosol-radiation interactions, we need to improve process-level understanding of aging processes, which lead to either "browning" or "bleaching" of organic aerosols. Currently available laboratory techniques aim to simulate atmospheric aerosol aging and measure the evolving light absorption, but they suffer from low sensitivity and precision. This study describes the use of electrodynamic balance photophoretic spectroscopy (EDB-PPS) for high-sensitivity and high-precision measurements of light absorption by a single particle. We demonstrate the retrieval of the time-evolving imaginary part of the refractive index for a single levitated particle in the range of 10(-4) to 10(-5) with uncertainties of less than 25% and 60 %, respectively. The experimental system is housed within an environmental chamber, in which aging processes can be simulated in realistic atmospheric conditions and lifetimes of days to weeks. This high level of sensitivity enables future studies to explore the major processes responsible for formation and degradation of brown carbon aerosols.