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. Light-absorbing organic atmospheric particles, termedbrown carbon, undergo chemical and photochemical aging processes duringtheir lifetime in the atmosphere. The role these particles play in theglobal radiative balance and in the climate system is still uncertain. Tobetter quantify their radiative forcing due to aerosol–radiationinteractions, we need to improve process-level understanding of agingprocesses, which lead to either “browning” or “bleaching” of organicaerosols. Currently available laboratory techniques aim to simulateatmospheric aerosol aging and measure the evolving light absorption, butthey suffer from low sensitivity and precision. This study describes the use ofelectrodynamic balance photophoretic spectroscopy (EDB-PPS) for high-sensitivity and high-precision measurements of light absorption by a singleparticle. We demonstrate the retrieval of the time-evolving imaginary part ofthe refractive index for a single levitated particle in the range of10−4 to 10−5 with uncertainties of less than 25 % and 60 %,respectively. The experimental system is housed within an environmentalchamber, in which aging processes can be simulated in realistic atmosphericconditions and lifetimes of days to weeks. This high level of sensitivityenables future studies to explore the major processes responsible forformation and degradation of brown carbon aerosols.

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